Advances in Agronomy, Volume 57

DVANCES IN gronomyy 8- V O L U M5 7E ...- Advisory Board Martin Alexander Eugene J. Kamprath Cornell University ...

Author: Donald L. Sparks

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DVANCES IN

gronomyy

8-

V O L U M5 7E ...-

Advisory Board Martin Alexander

Eugene J. Kamprath

Cornell University

North Carolina State University

Kenneth J. Frey

Larry P. Wilding

Iowa State University

Texas A&M University

Prepared in cooperation with the American Society of Agronomy Monographs Committee P. S. Baenziger J. Bartels J. N. Bigham L. P. Bush

M. A. Tabatabai, Chairman R. N. Carrow W. T. Frankenberger, Jr. D. M. Kral S. E. Lingle

G. A. Peterson D. E. Rolston D. E. Stott J. W. Stucki

D V A N C E S I N

onomy VOLUME 57 Edited by

Donald L. Sparks Department of Plant and Soil Sciences University of Delaware Newark, Delaware

ACADEMIC PRESS San Diego New York Boston London Sydney Tokyo Toronto

This book is printed on acid-free paper. @ Copyright 0 1996 by ACADEMIC PRESS, INC. All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Academic Press, Inc. A Division of Harcourt Brace & Company 525 B Street, Suite 1900, San Diego, California 92101-4495 Uniied Kingdom Edition published by Academic Press Limited 24-28 Oval Road. London NW I 7DX

International Standard Serial Number: 0065-2 I 13 International Standard Book Number: 0-12-000757-6 PRINTED IN THE UNITED STATES OF AMERICA 96 97 9 8 9 9 00 01 BB 9 8 7 6 5

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Contents CONTRIBUWRS .............................................. PREFACE ...................................................

ix xi

SOILSCIENCEAND ARCHAEOLOGY S. J. Sadder, J. E. Foss, and M. E. Collins Introduction. ....................................... The Archaeological Context .................................... ............. 111. Soils Data Useful in Archaeological Interpretations ............. Paleosols.. . . . . . ............................... .. oil-Archaeological Investig V. Case Studies of Soil. VI. Summary .............................. ................................................. References

I. 11.

w.

2 4 12 21 24 67 68

PHOSPHATE ROCKSFOR DIRECT APPLICATION TO SOILS S. S. S. Rajan, J. H. Watkinson, and A. G. Sinclair Introduction. . . . . . . . . . . Reactivity of PRS . . . . . . 111. Measurement of Phospha Factors Affecting Phosphate Rock Dissolution in Soil and Availability to Plants ..................................... V. Modeling the Rate of Phosphate Rock Dissolution in Field Soil. ......... VI. Agronomic Effectiveness of Phosphate Rock . . . . . . . . . . . . VII. Economics of Using Phosphate Rock Fertilizers ....................... ............. VIII. Soil Testing Where Phosphate Rocks Are Used Ix. Amendments to Phosphate Rocks. ................................... ............. X. Concluding Remarks ................... .............................. . . . . . References

I. 11.

w.

78 79 86 91 111 118 130 133 142 146 146

BREEDINGAND IMPROVEMENT OF FORAGESORGHUMS FOR THE TROPICS R. R. Duncan I.

Introduction..

...........................

................................ ................... 111. Breeding.. . . . .................................. IV. Germplasm.. . ......................... V. Conclusions ....................... . 11. 11.

Genetic Param Parameters tienetlc

References

...

Y

161 162 171 173 175 178

vi

CONTENTS

NITROGEN MINERALIZATION IN TEMPERATE AGRICULTURAL SOILS:PROCESSES AND MEASUREMENT Stephen C. Jarvis, Elizabeth A. Stockdale, Mark A. Shepherd, and David S. Powlson ........................... I. Introduction.. . . ............... ...... 11. 111. Process Controls.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

w. Measurement and Prediction of Mineralization v

The Impact of Mineralization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VI. Conclusions and Future Progress

References . . . . . . . . . . . . . . . . . . .

188 189 201 208 219 224 226

THEBUFFERINGPOWEROF PLANT NUTRIENTS AND EFFECTSON AVAILABILITY I. 11.

K. P. Prabhakaran Nair Introduction. . . . . . . . . . . . . . . . . . . . . . . . .

Efficient Plant Nutrient Management in Sustainable Soil Management. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. The Buffer Power and Effect on Nutrient Availability . . . . . . . . . . . . . . . . . . Quantifymg the Buffer Power of Soils and Testing Its Effect on Nutrient Availability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v. The Role of Electro-Ultrafiltration in Measurin K ty for the Construction of Buffer Power Curves.. ... .. VI. Quantifymg the Buffer Power for Precise Availa Prediction - Heavy Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII. Influence of Heavy Metal Contamination on Buffering of Major Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VIII. Possible Buffering Effect on Plant Acqu on of Heavy Metals . . . . . . . . . . Concluding Comments and Future Imperatives. . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

w.

Ix.

238 239 242 247 267 270 277 277 278 281

OVERVIEW OF VERTISOLS: CHARACTERISTICS AND IMPACTS ON SOCIETY

I. 11. 111.

Clement E. Coulombe, Larry P. Wilding, and Joe B. Dixon Introduction. . . . . . . . . Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Formation of Vertisols ...............

w. Morphological Propert v.

VI. VII. VIII.

Pedogenic Processes in Classification: From Marbut to Soil Taxonomy . . . . . . . . . . . . . . . . . . . . . . . . Mineralogical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

290 292 294 301 307 3 16 322 329

CONTENTS M. X. XI . XI1.

Biological Properties .............................................. Physical Properties ................................................ Management of Vertisols ........................................... Summary and Concluding Remarks .................................. References .......................................................

vii 333 336 352 363 364

HYBRID RICE S. S. Virmani Introduction...................................................... Heterosis in ................... Genetic Too ........................... . Breeding Procedures for Developing R m Hybrids ..................... V. Accomplishments ......... VI. Agronomic Management ........................................... ........................ VII. Disease/Insect Resistance .............. VIII . Grain Quality ................................ .............. M . Adaptability to Stress Environm .............. X . Hybrid Seed Production ...... XI . Economic Analysis ................................................ XI1. Technology Transfer and Policy Issues ............................... XI11. Future Outlook ............................. ........................................ X W. Conclusions . References . . . . . . . . . . . . . . . . ................

378 379 393 404 411 419 421 424 425 426 438 442 444 448 449

INDEX .....................................................

463

I. I1. 111.

rv

This Page Intentionally Left Blank

Contributors Numbers in parentheses indicate the pages on which the authors’ contributions begin.

M. E. COLLINS (1) Department of Soil and Water Science, University of Florida, Gainesville, Florida 3261 I CLEMENT E. COULOMBE (289) Department of Soil and Crop Sciences, Texas A 6M University, College Station, Texas 77843 JOE B. DIXON (289) Department of Soil and Crop Sciences, Texas A 6M University, College Station, Texas 77843 R. R. DUNCAN (16 1) Department of Crop and Soil Science, The University of Georgia, Grifin, Georgia 30223 J. E. FOSS (1) Department of Plant and Soil Science, University of Tennessee, Knoxville, Tennessee 3 7901 STEPHEN C. JARVIS (187) Institute of Grassland and Environmental Research, Okehampton, Devon EX20 2DG, United Kingdom K. P. PRABHAKARAN NAIR (237) University of Fort Hare, Alice 5700, Republic of South Africa DAVID S. POWLSON (187) Institute of Arable Crops Research, Rothamsted, Harpenden, Her forshire, A L 5 2yQ United Kingdom S. S. S. RAJAN (77) Department of Earth Sciences, The University of Waikato, Private Bug 31 05, Hamilton, N m Zealand S. J. SCUDDER (1) Department of Anthropology, Florida Museum of Natural History, University of Florida, Gainesville, Florida 3261 I MARK A. SHEPHERD (187) ADAS, Gleadthorpe Research Centre, Mansfield, Nottinghamshire, NG20 9PF, United Kingdom A. G. SINCLAIR (77) Invermay Agricultural Center, 50034 Mosgiel, New Zealand ELIZABETH A. STOCKDALE (18 7 ) Institute of Arable Crops Research, Rothamsted, Harpenden, Her fordshire AL5 2 I Q United Kingdom S. S. VIRMANI (377) International Rice Research Institute, Manila, Philippines J. H. WATKINSON (7 7 ) AgResearch, Ruakura Ayimltural Research Center, 3123, Hamilton, New Zealand LARRY P. WILDING (289) Department of Soil and Crop Sciences, Texas A 6M University, College Station, Texas 77843

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Preface Volume 57 contains seven interesting and comprehensive reviews on various agronomic topics. The application and uses of archaeology in soil science, including soil data useful in archaeological interpretations and case studies of soil archaeological investigations, are discussed in the first chapter. Advances in the application of phosphate rocks to soils, including their reactivity, aspects of phosphate rock dissolution, and the economics and agronomic effectiveness of phosphate rock additions is thoroughly covered in the second chapter. The third chapter discusses the breeding and improvement of forage legumes in the tropics, with information on genetic parameters, breeding techniques, and germplasm. The processes and measurement of nitrogen mineralization in temperate agricultural soils are the subjects of the fourth chapter. Nitrogen pools and processes, process controls, and the measurement and prediction of mineralization are comprehensively reviewed. The fifth chapter provides a thorough overview of the buffering power of plant nutrients and effects on nutrient availability. Details on nutrient management, quantification of buffering power, and effects of metal contamination on buffering of major nutrients are discussed. The sixth chapter is a comprehensive overview of Vertisols, including discussions on their genesis, morphology, classification, chemical, physical, and biological properties, and management. The seventh chapter is a detailed review on advances in hybrid rice including discussions on heterosis, genetics and breeding tools, hybrids, cultural and management practices, hybrid seed production, economic analysis, and technology transfer. I appreciate the authors’ first-rate reviews.

DONALD L. SPARKS

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SOILSCIENCE AND ARCHAEOLOGY S. J. Scudder,' J. E. FOSS,~ and M. E. Collins3 'Department of Anthropology, Florida Museum of Natural History, University of Florida, Gainesville, Florida 3261 I *Department of Plant and Soil Science, University of Tennessee, Knoxville, Tennessee 37901 %Departmentof Soil and Water Science, University of Florida, Gainesville, Florida 3261 1

I. Introduction A. New Applications of Soil Science B. Earth Science C. The Land Resource D. Objectives of Article 11. The Archaeological Context A. The Changing Emphasis in Archaeology B. Modem Subfields of Archaeology C. Applications of Soil Science to Archaeology D. Multiple Lines of Evidence E. Consensus and Controversy 111. Soils Data Useful in Archaeological Interpretations A. Soil Surveys and Maps B. Soil Morphology C. Soil Laboratory Analyses D. Landscape Analyses E. Micromorphology IV.Paleosols A. Buried Paleosols: Keys to Archaeological Interpretations B. A Paleosol Case Study V. Case Studies of Soil-Archaeological Investigations A. Alluvial Sequences in the Southeastern United States B. Soil Studies at the El Mirador Bajo C . Chemical Properties of Soils at Hadrian's Villa, ltaly D. Paleosols near Mt. Vesuvius E. Geomorphology and Site Selection a t the Seminole Rest Site, Volusia County, Florida F. Soils and Landscapes: Archaeopedology at the Pineland Site G. Pedoarchaeological Analysis of D Prehistoric Shell-Bearing Island, Florida VI. Summary References

1 Advonces in Agronomy, Volume 57

Copyright 0 1996 by Academic Press, Inc. MI rights of reproduction in any form reserved.

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S. J. SCUDDER, J. E. FOSS, AND M. E. COLLINS

I. INTRODUCTION A. NEWAPPLICATIONS OF SOILSCIENCE Soil science has traditionally been closely associated with agricultural enterprises such as cropping systems, forestry, and other types of land use where plant growth was the dominating interest. Today, however, soil scientists are using principles developed in the past century or more for many applications in addition to the more conventional plant adaptation and response. One of the first major movements toward applications of soil science outside the agricultural realm was in the soil survey program, which began moving into urban planning and interpretations in the 1960s. These activities were highlighted by the publication in 1966 of Soil Surveys and Land Use Planning (Bartelli et al, 1966). Since that time increasing numbers of soil scientists have been working in numerous other areas such as soil archaeology, forensic science, global cycling of nutrients, geographical information systems (GIs), water pollution, environmental hazards and use of waste products, solute transport, and genetically engineered organisms. Potential developments of the major subdisciplines of soil science were outlined in the golden anniversary publication of the Soil Science Society of America (Boersma et al., 1987).

B. EARTHSCIENCE Earth science is a broad study area that includes numerous subdisciplines such as soil science, geology, hydrology, geophysics, geochemistry, oceanography, climatology, archaeology, and meteorology. It is a relatively new concept in comparison to the disciplines listed above, but it is particularly relevant today due to the widespread interest in a more holistic view of the Earth and its many processes. Earth science has many opportunities to contribute to current issues such as environmental change; evaluation of physical resources such as water, soil, minerals, and wetlands; sustainable development; land use decisions, and understanding basic Earth processes. Archaeological studies provide an excellent avenue to approach some of the Earth science issues mentioned above. Combining subdisciplines such as soil science and geology with archaeological investigations results in detailed environmental studies of the Holocene. The Holocene is generally defined as that portion of the Quaternary period from 10,000 or 12,000 years ago to the present time. Archaeological studies are particularly important in developing the chronology, evidence of environmental change, and the impact of human populations on landscapes during the Holocene. Studies of historical land use and the impact

SOIL SCIENCE AND ARCHAEOLOGY

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of human populations on the environment have been especially revealing at archaeological sites associated with the Romans (Foss et al., 1994a; Stiles et al., 1994; Olson, 1981), in Mayan cultural areas in Central America (Olson, 1977; Dunning, 1993; Dahlin et al., 1980), the Middle East (Olson, 1981; Pendall and Amundson, 1990); and Peru and New Mexico (Sandor, 1992).

C. THELANDRESOURCE Significant progress has been made in our appreciation of the value of the land resource in the past half century or more. At one time, land was simply thought to be used, sometimes abused, and then abandoned with minimal regard to environmental impact and future use. Today development projects must pay close attention to the land resource, e.g., many projects require the identification and delineation of prime agricultural lands, wetlands, aquifer recharge area, and archaeological potential. The concept of sustainable development requires that concern for the long-term impact on natural resources be considered in development projects. The land resource includes soil, water, vegetation, animals, man-made features, and archaeological resources (Vink, 1975). Archaeological resources are considered so important that construction projects such as roads, bridges, and buildings that are federally funded must include an archaeological assessment and impact statement. If significant findings are recorded, mitigation procedures are then developed. These activities have resulted in a proliferation of archaeological investigations and have thus provided opportunities for joint study by archaeologists and associated scientists, especially in soil science and geology.

D. OBJECTIVES OF ARTICLE The objectives of this article are threefold. First, we stress the mutual advantages of developing new interdisciplinary efforts in the earth sciences, especially in archaeology. Although soil scientists have aided in the understanding of archaeological sites, we have also been able to effectively study and understand soil weathering processes because of the detailed chronology established at these sites. Table 1, for example, shows the approximate age of diagnostic soil horizons that have been identified in a number of archaeological sites. Second, we summarize some of the unique contributions of pedology to archaeology. Many interdisciplinary soil-archaeological studies have been published in the past two decades. A sampling of those studies, including several that we have been associated with, is presented here in order to illustrate the broad range of applications of soil science to archaeology. We feel that these

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S. J. SCUDDER, J. E. FOSS, AND M. E. COLLINS Table I Length of Time for the Formation of Various Soil Horizons Observed at Some Archaeological Sites (Foss el al., 1995)

Horizon Cambic Argillic

Fragipan

Approx. age (ye=) 2000 250-3000 3500-4000 3000-4000 3500 3000-4500 8600 8800 6500 8200

Archaeological site Shawnee-Minisink R. B. Russell Thunderbird Fifty Site Flint Run R. B. Russell 38LX338 40PK27 Fifty Site 40CH 162

Location Delaware River, Pennsylvania Savannah River, Georgia-South Carolina Shenandoah River, Virginia Shenandoah River, Virginia Shenandoah River, Virginia Savannah River, Georgia-South Carolina Saluda River, South Carolina Polk County, Tennessee Shenandoah River, Virginia Harpeth River, Tennessee

projects typify the approach and contributions that pedology can make to developing archaeological site history. Third, we hope to encourage other pedologists to become involved in archaeological studies. Interdisciplinary studies are rapidly becoming the norm in science, and soil science has a unique opportunity to collaborate with other earth sciences in understanding the numerous earth processes. Archaeological sites provide perhaps the best and most complete look at the Holocene and the impact of the human population on the environment.

11. THE ARCHAEOLOGICAL CONTEXT A. THECHANGING EMPHASIS IN ARCHAEOLOGY The classical concept of archaeology invokes collection and analysis of cultural artifacts, construction of ceramic typologies and cultural chronologies, and tracing the rise and fall of civilizations by cataloging the material evidence of human endeavor. This artifact-based approach has a venerable history, producing painstaking reconstructions of both the broad scope of cultural evolution and the mundane details of daily life. Increasingly, however, questions of resource use, human impact on natural environments, and the application of archaeologically derived information to the reconstruction of past environments are redefining the focus of archaeology. Human ecology is becoming a unifying theme, placing past cultures in the context of natural landscapes, of changing climate and re-


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source bases, of developing (or failing) horticultural or agricultural systems (Butzer, 1982; Marquardt, 1992a). The cultural markers that identify human groups are still important, but they are now more often employed not simply to name or categorize those groups, but to track them through complex relationships with other human groups and with a changing mosaic of landscapes and plant and animal communities on which their lives depended. The emergence of this new brand of archaeology-termed “bioarchaeology” by many (Larsen, 1987)-has resulted in the alignment of many earth science disciplines with social sciences, or anthropology. The products of these new alignments emphasize the multidisciplinary nature of modem archaeology.

B. MODERNSUBFIELDS OF ARCHAEOLOGY 1. Geoarchaeology Geoarchaeology places the site under study in a local and regional geomorphic setting. Geomorphology, sedimentology, structural geology, hydrology, and pedology are used to define the natural landscape elements which surrounded or were incorporated into past human settlements. Those elements, such as alluvial terraces, floodplains, deltas, and others, possess characteristic sequences of sediments, drainage patterns, and soils, which offered unique resources to past human settlers (Ferring, 1992; Gartner, 1992; Segovia, 1985). Disruptions in natural processes of landscape evolution, initiated by human alterations and manifest as erosion or accumulation, can be detected by comparing archaeological stratigraphic sequences with modern regional geomorphology. Such information can be used to infer the scale of human impact on the local environment (Holliday, 1985; Farrand, 1975).

2. Archaeometry Archaeometry employs remote sensing methods, techniques for chronometric dating, and materials identification and sourcing. Geophysical methods of remote sensing such as magnetometry and resistivity detect differences in subsurface concentrations of iron and water or salts, or material density, to locate buried foundations, ditches, metal artifacts, and other physical or chemical interfaces (Carr, 1982). Ground-penetrating radar (GPR) reflects radar pulses off of subsurface discontinuities such as extreme soil textural changes (e.g., sand to clay), rock layers, water tables, and buried architectural features (Collins and Doolittle, 1993). Geochemical “prospecting,” such as soil phosphate testing on a radial grid at arbitrary levels, gives a three-dimensional map of subsurface element distribution (e.g., Hassan, 1985).

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These methods, used in soil mapping and landscape analysis, are readily applicable to archaeological situations, particularly as reconaissance tools to indicate where to locate excavation units, to what depths to dig, and where not to waste valuable time and money. (See Section III.B.2 for a further discussion of GPR.)

3. Archaeopedology The “study of old soils” characterizes and interprets both paleosols (which have been formed by natural processes) and anthrosols (which have been created by human activity) in archaeological contexts. Paleosols may underlie archaeological sites as complete soils with all horizons intact, or may be truncatedusually by erosion. They are excellent indicators of climate change or stability, forming under specific conditions which either contrast to or corroborate modem conditions and the soils which result from them. They have been used as markers of Holocene/Pleistocene boundaries in some areas (see, for example, Goodyear and Foss, 1993) and therefore can be used to predict or prospect for early human sites. Paleosols are discussed more extensively in Section IV. Anthropogenic soils or anthrosols are as diverse as the cultures which produced them. They may take the form of a simple midden-an aboriginal garbage heap mantling a native soil surface-or a thick series of strata reflecting periods of habitation, intermittent abandonment, and resettlement (e.g., Bullard, 1985). The taxonomic categories erected for human-influenced soils reflect augmentation of native soil resources: the plaggen epipedon describes long-term manuring of fields, the anthropic epipedon is one enriched in phosphorus contributed by human activities, the agric subsurface horizon forms under cultivation and contains significant amounts of illuvial silt, clay, and humus (U.S.D.A., 1975). Many anthrosols fit this model of human enrichment of soils, whether purposeful, as in the case of plaggen soils, or fortuitous, as in the case of “black earth” midden soils. Settlements, animal corrals, food preparation areas, aboriginal refuse heaps, and urban dumps all result in increased soil contents of organic carbon, nitrogen, phosphorus, calcium, magnesium, potassium, and other elements (e.g., Eidt, 1985; Griffith, 1980; Lillios, 1992; Smith, 1980). Some soils, such as those formed in spoil from ditching for raised-field agriculture, canalizing, and the formation of monumental earthworks, exhibit “reverse stratigraphy,” with material from excavated subsurface horizons mounded or spread on top of the original land surface. In such cases, particle-size and element distributions, as well as clay-size mineral occurrence, may appear as a reverse weathering sequence with more enriched materials situated above more depleted ones (Birkeland, 1984; Johnson and Collins, 1993). The original soil surface may present an abrupt increase in organic carbon, or a discontinuity in particle size or specific mineral content (see case study on Pineland, Section


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V.F). With the passage of time and continued weathering, these “inverted soils’’ eventually begin to develop a more normal sequence of horizons and element distributions (Sokoloff and Carter, 1952). One end-member of this “anthrosol continuum” is the depleted or eroded soil resulting from cultural factors such as repeated cycles of unfertilized cropping, poor irrigation or tillage practices, and overstocking. Natural drought or climate changes can also result in such soils if humans are unable to adjust management practices to new conditions (Sanchez, 1976). These soils contrast with enriched anthrosols as well as native, non-human-impacted soils, in decreased contents of major and trace elements removed through cropping or overgrazing. In many cases, the structure of these soils is destroyed or weakened by removal of organic matter, by erosion, compaction, or increased acidity or salinity resulting from drainage or improper irrigation. (Hodges and Carlisle, 1979; Jenny 1961). The value of anthrosols to the interpretation of human lifeways and ecology is multifaceted. From an archaeological perspective they chronicle human landuse, recording patterns of settlement and landscape modification on a large scale. On a smaller scale, they identify intrasite features such as hearths, burials, storage pits, and garden plots. Intensity of use or duration of habitation can be inferred by comparing the chemical and physical characteristics of anthrosols with those of local, nonimpacted native soils (Eidt, 1985; Lillios, 1992). Soil inclusions such as pollen, phytoliths, seeds, and microfauna can be extracted from the mineral matrix, identified, and used to interpret past climate, resource use, and postdepositional soil conditions (Pearsall, 1989; Piperno, 1988; Ruhl, 1995; see also case study: Seminole Rest, Section V.E). From a pedologic perspective, anthrosols offer a unique means of studying soil-forming events by “manipulating” one or more of the classically recognized soil forming factors (Jenny, 194I), especially parent material, biological activity, and/or time. Most archaeological deposits have chronologic indicators such as ceramic or tool types characteristic of specific time periods. Styles of shell tool and lithics manufacture serve to relatively date preceramic deposits. Disturbance of site content and contexts has always presented an interpretive challenge to archaeologists, but a site with no indication of relative age is rare. Also, absolute dating of site materials such as bone, shell, and charcoal using radiometric 14C is now routinely accomplished. So the archaeological site presents the pedologist with a deposit of “parent material”-that is, the midden, household ruin, corral, garden plot, etc.-resultant from the biological activity of humans (and their commensal animals and plants) over a specified period of time, under the influence of a definable climate. The study of the effects of time and parent material in the soil-forming equation is further enhanced by the fact that the local modern soil can be compared not only with the anthrosol, but also with the buried soil or paleosol below the archaeological deposit. Catenas, chronosequences, and clay

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mineral changes, have all been defined and studied (e.g., Holliday, 1985, 1992; Goodyear and Foss, 1993; Foss et al., 1993; Scudder, 1993) using sequences of anthrosols created by past humans interacting with specific environments.

C. APPLICATIONS OF SOILSCIENCE TO ARCHAEOLOGY 1. Human-Modified Landscapes and Settlement Patterns Human effects on natural soils and landscapes can be detected using the most basic tools of pedology: soil morphology, particle-size distribution analysis, clay mineralogy, and patterns of chemical element accumulation. Lippi ( 1988), working on the Nambillo ridgetop site in the western rainforest of the Ecuadorian Andes, constructed a “paleotopographic” map based on soil morphological information from augered cores. The abrupt upper boundaries of a series of four superimposed paleosols, developed in volcanic sediments, were plotted to reconstruct changing local paleogeomorphology. Phosphate accumulations and cultural materials, which spanned approximately 3000 years, were also mapped, creating a composite interpretation of land use patterns and ancient topography. Lippi’s subsurface mapping revealed that the ridgetop landform evolved from a sharply peaked, steeply sloped hilltop to a broader-topped, more habitable one. It also chronicled cycles of human settlement, volcanic activity, abandonment, and resettlement of that evolving hilltop. Research by Smith (1980) in the Neotropics also applied the methods of pedology to the detection and interpretation of past human settlements. Smith, working in Brazil, analyzed physical and chemical properties of terra preta, the black earth soils found scattered in a mosaic throughout large areas of the Amazon basin. He found high contents of phosphorus and calcium, higher than normal pH and base saturation, and cultural artifacts scattered throughout the black soil epipedon but missing from the subadjacent horizons. This led him to confirm that the terra preta were indeed anthropogenic and that earlier theories of their origin as volcanic ash, Tertiary lake sediments, or modern pond sediments were inaccurate. Their occurrence in areas of Oxisols, Spodosols, and Ultisols and their wide range of textures-from silty clay to sand-argued against a common pedogenic origin. Smith estimated accumulation rates for terra preta soils and population densities of past village sites based on modern analogy and areal extent of the black earth sites. He then concluded that the strong evidence for large pre-European human populations in the Amazon basin, presented by the widespread occurrence of terra preta soils, should serve as a warning to biologists discussing “virgin” Amazonian ecosystems. [“Potsherds and black earth may lurk under control plots and pristine natural reserves.” (Smith, 1980, p. 566.)]


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In a different timeframe and landscape, Farrand (1975) interpreted the paleoclimate of southwestern France based on sediments in the Abri Pataud rock shelter at Les Eyzies (Dordogne) (see also Movius, 1975). He combined detailed data from particle-size distribution analysis, heavy mineral identification and quantification, clay weathering sequences, and sedimentary geology and stratigraphy with 14C dates and analysis of cultural remains to reconstruct local conditions in Upper Paleolithic times (35,000 to 20,000 B.P). The shelter itself, a reentrant undercut at the base of a bedrock limestone cliff, was determined to have formed primarily by differential solution. Frost-shattering of the walls and ceiling contributed the majority of the floor sediments (a total of 9 m in depth), with minor contributions from aeolian and anthropogenic sources. Because particle-size class ratios were similar between the fine material contributed from the weathered limestone and that derived from dry-season wind-blown fluvial sediments from the river below, Farrand used clay and heavy mineral assemblages to differentiate the two sediment types. He was then able to delineate periods of relatively dry climatic conditions based on increased content of aeolian sediments in the floor deposits. Changes in ratios of montmorillonite to kaolinite also signalled broad changes in annual precipitation: increased montmorillonite indicated drier times with less weathering, increased kaolinite denoted the reverse. Increases in pedogenic clay in the stratigraphic column were used as a marker of relative climatic stability. Farrand’s study also addressed regional changes in temperature. Variations through time in the size of limestone fragments spalled from the ceiling and walls of the shelter allowed inferrences to be made about changes in intensity of the freeze-thaw cycles, and hence, changes in general seasonal temperature ranges.

2. Traces of Daily Life The same techniques used to interpret ancient landscapes and paleoclimates can yield information on intrasite features such as individual post-holes, hearths and storage pits within rooms, burials (with or without bodies), and small-scale land use applications (e.g., garden plots and corrals). Particle-size distribution analysis, pH, clay mineralogy, and chemical element distribution patterns, all used in the more traditional realm of soil classification, are now used with increasing frequency in anthrosol analyses. These methods, coupled with innovative micromorphological interpretations gleaned from fixed soil-column thin section techniques (Courty et al., 1989; Goldberg and Courty, 1993), offer an an exhaustive array of analytic techniques. Soil phosphorus quantification, in particular, offers a reliable means of assessing the impact of human occupation on native soils. Early work by European geographers revealed positive correlations between abandoned settlements and elevated soil phosphorus content (Arrhenius, 1929; Broadbent, 1981; Proudfoot,

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1976). Refinements in field and laboratory techniques have resulted in the development of a quick, relative spot test well-suited for site reconnaisance work (Eidt, 1985) and in sophisticated fractionation procedures designed to sequentially remove loosely bound, occluded (tightly iron- or aluminum-bound) and residual calcium phosphates from the soil (Chang and Jackson, 1957; Eidt, 1985). Ratios of the various soil phosphate fractions from archaeological soils have been compared with those from modern soils under known land-use regimes to infer past practices (Eidt, 1985; Lillios, 1992). Such detailed fractionation analyses are not without their critics, in terms of both theoretical application (Courty et al. , 1989; White, 1978) and practical considerationsof execution time and cost (Conway, 1983), but they will undoubtedly undergo more refinement and reapplication. Research by Conway (1983) illustrates the use of total phosphorus distribution patterns in the analysis of small-scale occupation deposits. Working on a walled Romano-British hut group in northern Wales, Conway sampled both withinstructure areas and adjacent fields on a linear grid system at 1-m intervals. Because the site presented culturally layered multicomponent deposits, he interpreted the resultant data (drawn as distribution contour maps) using Trend Surface Analysis (Unwin, 1975). That technique compared observed phosphorus distributions with a series of mathematically derived distributions based on natural soils, producing two components: a generalized distribution-the “trend surface’’-and a series of residual values-the differences between the derived and observed values. The variance of the residuals from the derived trend surface provided a measure of significance. Conway found that by matching attributes of known archaeological features (e.g., size, shape, spatial arrangement) with patterns of phosphorus distribution, he was able to greatly increase understanding of the function of those features. For example, one building showed evidence of having been demolished and partially reincorporated into the courtyard of a subsequent structure. The floor area of the remnant original structure was protected by a layer of small stones and contained high levels of phosphorus. That portion of the floor which was subsequently converted into a courtyard, unprotected by stones, had less total phosphorus, having lost it by exposure and erosion. Such an analysis provides not only a means of relatively dating associated structures, but also an approximation of duration or intensity of occupation through comparison of intrasite phosphorus accumulation with the phosphorus content of nonanthropogenic local soils.

D. MULTIPLELINESOF EVIDENCE Archaeopedological studies based on standard soils analyses, such as those discussed above, are becoming integral to archaeological investigations. Still


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other studies, focusing on biological soil components, are also contributing to the multidimensional approach evolving in modem archaeology. Botanical residues such as pollen, phytoliths, seeds, charcoal, and macroremains (stems, leaves), embodying the discipline of archaeobotany, are being cross-correlated with soil characteristics to infer horticultural practices. For example, Johnson and Collins (1993) hypothesized that the high soil aluminum content of in-filled aboriginal ditches at the Fort Center site in south Florida would have provided poor growing conditions and thus explained the virtual lack of corn pollen in soils of that area. His findings call into question the speculation that corn horticulture was responsible for supporting the dense populations and high cultural complexity of the Fort Center people. Jacob’s work in the Cobweb Swamp of Belize (1991) combined field archaeology, pedology, palynology, geomorphology, stable isotope analysis, and invertebrate biology to trace the “agroecological evolution” of an agricultural area adjacent to the Mayan cultural center of Colha. Soil cores combined with pollen analysis and identification of soil invertebrate infauna and microfossils revealed a series of changing environments in this region of Mayan civilization: freshwater cattail swamp, mature forest or forest edge, disturbed vegetation and cropland, abandoned wetland. Cultigens in the pollen record and ditched fields provided evidence of Mayan horticulture. Jacobs used soil profiles, characterization data, and stable isotope analysis of marl carbonates to define episodes of massive upland erosion marked by the genesis of a clay horizon at the swamp margin known as the Maya clay. This episode of human-induced erosion was followed by site abandonment-or at least a significant reduction of the human population. Jacobs superimposed his reconstruction of the ecological evolution of Cobweb Swamp onto work conducted at the adjacent cultural center of Colha. At Colha the Maya clay also appeared, at the time of major social stress that preceded the collapse of Mayan civilization. Although his archaeopedological work could not definitively answer the question of what caused the collapse-the social or the ecological turmoil-it provided substantial clues to conditions at the time.

E. CONSENSUS AND CONTROVERSY There is no question that archaeopedology is becoming an integral component of environmental archaeology, or geoarchaeology. The evolution of attitude regarding the treatment of archaeological soils, from an inert matrix to be disposed of to a rich source of environmental and cultural information, is nothing short of revolutionary. The formalization of the subdiscipline and the increasing communication among its practitioners may be signalled by the inception of professional meetings such as the First International Pedo-Archaeology Conference in 1992 in

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Orlando, Florida. Proceedings of this and subsequent meetings gather and present the works of geographers, archaeologists, soil scientists, and geologists, all focusing on the interpretation of anthropogenic soils and landscapes. One of the functions of the pioneers in any field is to define terms, formulate procedures, and set protocols so that investigations done in diverse settings by a variety of people will be comparable and usable. Archaeopedology is fortunate to have the entire body of soil science to draw from (see Section 111) in terms of soil taxonomy and nomenclature, models of soil/landscape relationships, and field and laboratory procedures. However, the application of these principles and practices to anthropogenic soils presents some unique problems. For example, how should samples be taken? By cultural level, or natural horizon? By an arbitrary 10- or 20-cm level? How should cultural inclusions in the soil, such as charcoal, lithic debitage, or ceramic sherds, be treated? Should they be screened out of the 2-mm fraction typically used in soil characterization analyses, or quantified as pebbles, cobbles, or soil separates? (See Stein, 1987 for a discussion of this question.) How should soil carbon be quantified? By the standard Walkley-Black procedure which does not address charcoal; by loss-on-ignition, which is not comparable to standard soil characterization data, or by a weight per volume measure? And what analytical procedures should be used on soils as radically altered as some anthrols are: available elements? total elements? fractionated elements? (See Courty et ul., 1989 for a discussion of the applicability of some soils analyses to anthrosols.) Another question that needs to be addressed to aid in linking archaeology and pedology is what to call these human-influenced soils. The terms anthrosol and anthropogenic soil give the reader a general meaning. But there is a world of difference between the black earth (terra pretu) featured in Smith’s work in Amazonia and the spalled cave ceiling detritus and lithic debitage of Farrand’s Abri Pataud sediments. Conways’s Welsh hut floor samples bear no relationship to Jacob’s Maya Clay, but they are all anthropogenic. Perhaps a serious consideration of anthrosols as a recognized soil order is due. However, though there may be questions of method and theory yet to answer, the range of topics addressed by soil science leaves no doubt that it has become an indispensible tool in archaeology.

111. SOILS DATA USEFUL IN ARCHAEOLOGICAL INTERPRETATIONS A. SOILSURVEYSAND MAPS One of the invaluable resources available for archaeological investigations and interpretations is soil maps. Soil maps are part of a soil survey which is an


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inventory of the soil resources in a specific area. Soil surveys are completed by field soil scientists on a county-wide basis or a particular part of the state, but some soil surveys may include only part of a county or two or more counties. All soil surveys are made by describing and classifying soils in the field and delineating the areas on maps. Aerial photographs have been used as the base map for most of the soil surveys. Soil series are used to name the map units of the soils delineated in the soil surveys. Soil series are defined and differentiated by one or more of the following significant characteristics: kind, thickness and arrangement of soil horizons along with the color, texture, structure, consistence, pH, content of carbonates and other salts, content of coarse fragments, and mineralogy. Each soil map unit is a collection of areas defined and named according to its soil components (Soil Survey Divison Staff, 1993). Each individual area shown on the soil map is a delineation. The areas that can be delineated on a soil survey depend greatly upon the map scale. The map scale chosen depends on projected use. State general soil maps are published at scales of 1:250,000 to 1:3,000,000 (Soil Survey Division Staff, 1993) and are used for regional to state planning board land-use needs. The map scales used in making local soil surveys generally range from 1:12,000 to 1:24,000. A scale of 1:15,840 was very common, but many soil surveys were converted to the 1:24,000 scale to be used more easily in GIs. Soil map scale influences the size of the soil delineation. Some soil maps are used for national planning with minium-size delineations of 252 to 4000 ha. Others are utilized for local decision making with minium-size delineation of 1 ha or less (Soil Survey Division Staff, 1993). Therefore, the archaeological objective(s) for using soil maps must be determined, so that the level of detail is known and the correct map scale is employed. Some soil maps have been stored in a computer (digitized format) and can be used in a GIS format. The GIS can be especially useful, for example, to quickly search for a particular soil map unit throughout a county that may contain cultural features. Duncan and Hurt (1993) used GIS technology to test a predictive archaeological site model. They used digital soil survey spatial data, soil survey attribute data, and digital known archaeological sites in GRASS (Geographical Resources Analysis Support System) to develop a statistical predictive model. Using this model they were able to adaquately predict archaeological site locations.

1. Use of Soil Maps for Archaeological Purposes Although the locations of cultural features are not identified or delineated in soil surveys, many soil properties or characteristics described may be very useful in archaeological investigations. For example, the landform and physiographic position of the soil map units are described in each soil survey. Many investiga-

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tors have noted a relationship between the landform or landscape position and the presence of cultural features. Foss et af. (1995) indicated that 80 to 90% of the known stratified and well preserved archaeological sites in the eastern United States are in alluvial valleys. They also noted that alluvial soils were not shown with much detail in the older soil surveys, but that more recent soil surveys provide much more detailed data on alluvial soils that can be useful in archaeological investigations. Foss et af. (1995) pointed out that the relationship of the location of Holocene alluvial soils and archaeological sites has resulted in a number of cooperative soil-archeological investigations recently. The natural soil drainage of the soil map units is also determined and described in soil surveys. Drainage and the presence of cultural features also may be related. Distribution of secondary carbonates has been useful in reconstructing paleoenvironments. Other important soil characteristics described and interpreted in soil surveys include the parent material which is useful in determining the age of the cultural features. Ferring (1992) noted the importance of understanding alluvial soils in defining and correlating stratigraphic units in archaeology.

B. SOILMORPHOLOGY Pedology is the science of studying soils as natural bodies on the Earth’s surface. Some describe pedology as “the study of weathering of materials” (Morris et af., 1993). To be able to study soil, one must be able to represent the soil continuum as discrete soils. Therefore, the morphology of a soil is the most important diagnostic criterion in soil science. Without an objective description of soil morphology, no correlation can be made among soils. When a soil is properly described, samples are normally taken by horizon designation. These samples are analyzed for specific soil constituents. The results of the laboratory analyses and the morphology allows one to (i) classify the soil at the soil series level, (ii) understand the soil’s genesis, (iii) determine soil variability by comparing its properties to other known soils, and (iv) interpret suitabilities for land use by noting depths to mottles, bedrock, and restrictive horizons, as well as documenting hydric soil indicators for wetland determinations. The soil’s morphology can also be used to determine if the soil has been disturbed or if the features noted are the result of pedogenic processes. Soil morphological descriptions are made best in an excavated pit large enough to examine any soil variations. The pit should be about 1 m in width so that the vertical face can be studied and described. The description begins by noting the depth or location of changes in the soil. These variations include changes in soil color, texture, structure, and stoniness. Boundaries between soil horizons are marked on the face of the pit, and the horizons are described. Master soil


15

horizons and subordinate designation(s) are assigned to a section of the soil. Peds (natural soil aggregates) are removed so that closer observations can be made. All of the information is written on a soil description form that includes information about the soil’s environment. Archaeological descriptions of soils vary considerably from pedological descriptions. Pedological descriptions assumes no human-influenced disturbance except to the soil surface. Archaeologists, of course, are very interested in features in the soil that are the result of human habitation (Fig. 1). In their study of the San Luis Archaeological Site, Collins and Shapiro (1987) discussed morphological features such as abrupt, smooth boundaries between layers; abrupt, laterally discontinuous layers; dark matrix colors extending to depths greater than expected; weakly developed soils as indicated by the lack of argillic (Bt) or cambic (Bw) horizons; and textures high in sand with mixings of clay in lower layers as evidence of human activities. Some archaeologists (such as Bettis, 1992; Scudder, 1993; Johnson, 1991; and Mandel, 1992) use pedologic terminology to describe soil features. Many archaeologists describe soil color according to the Munsell color notation, describe soil texture according to the U.S.D.A. soil textural triangle, and in some cases use horizon nomenclature.

Figure 1 Example of archaeological soil morphology, Ft. San Luis, Leon C o . , FL.

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1. Soil Morphology as a Stratigraphic Marker Holliday (1989) stated that the earliest use of soils in archaeology was probably as stratigraphic markers. Pedologic features such as soil horizons may be most prominent, especially in stratified deposits. Holliday (1989) noted that in North America, archaeological sites in thick, well-stratified deposits with distinct buried soils are relatively common. He provided numerous examples throughout the United States of the importance of the recognition of buried soils in the recovery and interpretation of archaeological records. In the midwestern United States buried soils have been recognized for many years in the study of Quaternary stratigraphy. Follmer (1978) noted that the Sangamon paleosol was the significant stratigraphic marker in loess/till sequences in Illinois in the 1870s, and was considered one of the most significant stratigraphic units in the world. Recognition of the presence of an argillic horizon (horizon with signficant increase in clay content) can be significant in archaeological studies. It is known that it may require thousands of years to form an argillic horizon. In contrast, cambic horizons (horizons with color development but lacking the increase in clay content) may form quite rapidly. Morris et al. (1993) used basic soil science techniques to study the depositional history and pedogenic properties of two archeological sites in Tennessee. They concluded that soil morphology was the key to the interpretation of the validity of the site in the archaeological context. Also, soil morphology helped determine what processes have affected the site since the time of its deposition. In the study by Moms et al. (1993), a backhoe was used to expose the soil profile, and the physical properties of each soil horizon were described, including color, texture, structure, consistence, and presence of clay films, roots, pores, and concretions. Soil samples were collected from each horizon, and the particle size, total carbon, and other chemical analyses were completed. Results of the study showed that although the two sites in their study had a similar depositional history, the soil morphological development at one of the sites was consistent in the archaeological context while the other site was not. One of the sites had a well-developed buried paleosol. The paleosol had a moderate grade of structure and discontinuous clay films in the argillic horizon. The paleosol also contained charcoal and lithic artifacts and the radiocarbon date was consistent with the soil morphology of the paleosol. The other site exhibited a weaker grade of structure in the paleosol and lacked clay films which would be expected to occur in that period of soil development. Morris et al. concluded that the latter soil morphological features indicated that the paleosol was in an early stage of development when buried and, thus, was not as old as the other. Many archaeological investigations have benefited from pedologic interpretations. Foss et al., (1993) outlined a study of archaeological sites with a soil science approach. They discussed a general survey of the site to better under-


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stand the general soil characteristics and stratigraphy, examined soil morphology; interpreted the morphology together with the laboratory analyses; and assembled all the information including archaeological data to develop a site history plan. Foss et al. (1993) stressed that accurate field descriptions are the key to success in pedoarcheological investigations. They emphasized that soil morphology is the basis for interpretations and that laboratory data are needed to enhance and clarify the field soil descriptions.

2. Geophysical Tools Applicable to Soil Science and Archaeology Geophysical tools such as resistivity, electromagnetics, and ground-penetrating radar (GPR) have been used by both soil scienists and archaeologists in their field investigations. An advantage (but also a disadvantage) in using these tools is that no physical samples are taken. This is an advantage when a site should not be disturbed. It is a disadvantage when you need a sample for other purposes, e.g., laboratory analyses. GPR is increasingly being used by archaeologists for nonintrusive detection of archaeologically significant buried assemblages. GPR has been very useful in archaeological investigations because it can quickly detect possible cultural features without disturbing the site. In the past 10 years there has been a tremendous increase in interest and GPR use by archaeologists and others studying archaeological sites (Batey, 1987; Doolittle, 1988; Doolittle and Miller, 1990; Goodman and Nishimura, 1992; Imai et al., 1987; Kong et al., 1992; Mellett, 1992; Sakayama et al., 1988; Unterberger, 1992; Bauman et al., 1994; Bernabini et al., 1994; Papamarinopoulos and Papaicannou, 1994). Collins and Doolittle ( 1993) discussed GPR and soil science applications to archaeological investigations using case studies and future developments in GPR techniques. Many of the future developments will have a direct effect on archaeological investigations. Software programs, high-speed multiple-channel radar systems, variable frequency antennae, and shorter range antennae (possibly in the range of 3 GHz) will enhance the signal for improved penetration that allows for better resolution. Even though GRP is being used by archaeologists, more scientists would utilize this technology if it were more “user-friendly” and less costly.

C. SOILLABORATORY ANALYSES Laboratory analyses must accompany the soil morphological description. If only the field morphology is used to interpret the archaeological site, only part of the site’s story is told. Laboratory results supplement the field morphology. Laboratory analyses of selected soil properties are routinely done in pedology. These analyses can be separated into physical, chemical, mineralogical, and


micromorphological determinations. Soil scientists use the procedures defined in “Soil Survey Laboratory Manual” (Soil Survey Staff, 1992) and in “Methods in Soil Analyses” (SSSA, 1982, 1986). Microbiological analyses are not routinely done on archaeological soil samples. Recently, archaeologists have become interested in particle-size distribution (Timpson and Foss, 1993; Johnson and Collins, 1993), pH (Coultas et al., 1993), organic matter (Stein, 1992), mineralogy (Scudder et al., 1993), and extractable elements (Morris ef al., 1993). An excellent, detailed discussion of analytical methods in soil science that archaeologists may find useful is presented by Courty et al. (1989).

1. Soil Physical Analyses Particle size analysis is a very common procedure in soil science labs. Determining the percent sand, silt, and clay can be done by several procedures (Soil Survey Staff, 1992). These procedures fractionate the sand into very coarse sand (particles 2.0-1 .O mm), coarse sand (1 .O-0.5 mm), medium sand (0.5-0.25 mm), fine sand (0.25-0.10 mm), and very fine sand (0.10-0.05 mm); silt (0.050.002 mm) into very coarse silt to very fine silt; and clay (0.002 mm) into coarse and fine clay. Timpson and Foss (1993) stated that soil particle-size analyses can be one of the most useful laboratory analyses for characterizing soils and parent materials in alluvial systems. Particle-size analyses can be especially useful in determining depositional history and discontinuities in profiles. Bulk density is the weight per unit volume (e.g., g/cc). Bulk density gives an estimation of the pore space within the sample. Changes in bulk density can be attributed to variations in lithology, depositional history, weathering processes, and restrictive layers. Bulk density changes that cannot be explained by pedogenic processes should interest archaeologists.

2. Soil Chemical Analyses Archaeologists have been interested in the chemical environment of their sites as an indicator of site habitation for many years. Soil chemistry is also useful to evaluate settlement patterns and agricultural history. One chemical that has been studied intensively by archaeologists is phosphorus in various forms. The identification of the different forms in phosphorus in soils has been difficult in the past. Determination of total P necessitated the use of dangerous chemicals and special fume hoods, but these procedures have been made easier by the method developed by Dick and Tabatabai (1977). Differentiating between total (TP) and extractable phosphorus (EP) is important. Extractable P content dependents on the soil’s pH and therefore on the ability of the extracting solution to “extract” the P. If a weak acid extractant is used, much of the total soil P may


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remain unextracted and undetected; therefore, it is important to know which laboratory procedure is used to determine P content. Proudfoot (1976) stated that sampling designs need to be developed to better evaluate the variations in P that occur in natural and artificial features that occur horizontally as well as vertically on the sites. He emphasized that archaeological sites provide pedologists with the unique opportunity to examine variations in P contents of soils within a chronological framework. Further work on the phosphorus content of soils in the archaeological context, especially since many P interactions in soils are time dependent, according to Proudfoot (1976), could help pedologists increase their understanding of soil P. Dauncey (1952) and Cook and Heizer (1965) published reviews on the use of P analyses in archaeology. Collins and Shapiro (1987) used EP (Mehlich 1) and TP to understand the settlement history at San Luis. They compared archaeological and buried soils contents of EP and TP to the amounts in the naturally occurring Orangeburg (fine-loamy, siliceous, thermic Typic Kandiudult) soil. Total phosphorus distributions were difficult to interpret because the Organgeburg soil at San Luis was naturally high in TP. Therefore, using TP as an adjunct to artifact distribution in identifying activity areas (see Sjoberg, 1976) could not be done at San Luis. Distribution of EP, though, was a good indicator of past human activity because the mean EP in the Orangeburg was notably lower than that in the archaeological and buried soils. Other chemical constituents are also being used to discriminate features at archaeological sites. Organic carbon or organic matter content have been examined by soil scientists and archaeologists because of the potential for I4C dating. The amount of organics in soils can be determined by using either wet (e.g., Walkley-Black) or dry combustion (e.g., ashed) laboratory methods. If the organic material is associated with a buried horizon or soil, the absolute age of the material above that layer could be determined if the carbon could be dated. Stein (1992), though, discussed some of the limitations in dating organic matter in archaeological sites, but mentioned that “with a clearer understanding of its potential, the study of organic matter can make a significant contribution to the discipline” (of archaeology). Foss (1991), in a study of alluvial soils along the Delaware River in Pennsylvania, found that the elements Ba, Mn, and Sr were useful in verifying the presence of a buried A horizon. Griffith (198 1) found that exchangeable magnesium was the most efficient discriminator of features at an archaeological site in Ontario, Canada. Magnesium is one of the major chemical constituents of wood-ash, and exchangeable magnesium in Griffith’s study was high in the middens and pits and low in village areas where no chemical-enriching activity occurred. Inorganic and organic P were more uniformly enriched throughout the archaeological site and differences in habitation features were obscured. Griffith (198 1) emphasized, however, that

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although chemical data are important in studying the occupation of archaeological sites, other soil properties should not be overlooked. He noted that the physical characteristics (particle size, structure, general morphology ) and particularly the micromorphology of soils are often neglected, but these are areas of research that benefit from knowledge of pedology.

D. LANDSCAPE ANALYSES Analysis of the landscape is a bridge for geomorphologists, geographers, and pedologists. Archaeologists are also using landscape studies to reconstruct paleoenvironments, to develop a better understanding of the archaeological record, and to predict archaeological site locations (Ferring, 1992). Reconstructing landscapes is important for archaeologists because it is on the old landscapes or surfaces that prehistoric people lived (Stein, 1992). Understanding the different processes that occur on various parts of the landscape is important in making archaeological interpretations. Movement of water across and through the soil profile is one of the major reasons for the differences in soils and one of the primary causes for the movement of materials on slopes. A number of researchers have stressed the horizontal as well as vertical movement of substances on the soil landscape and have tried to relate processes and soils to landscape position. Landscapes are classified as having erosional surfaces if their surfaces are dominated by erosional processes and if their shape is a result of what is left after material has been transported. Depositional surfaces are affected by material being deposited, and their shape depends on the size and shape of material deposited. Landscapes are considered stable if weathering is the dominant process and erosional or depositional processes have little affect on the landscape surfaces (Gerrard, 198I). Archaeologists interested in landscape reconstruction generally study areas dominated by depositional processes and search for buried surfaces on old landscapes that were once stable and have well-developed soils. Ruhe ( 1960) described five landscape positions: summit, shoulder, backslope, footslope, and toeslope. The summit is considered the most stable landscape position. The shoulder generally has the most surface runoff and thus is a relatively unstable landscape position. The backslope is also an unstable landscape position and slumping of material may occur along with surface creep, flow or wash. Recognizing the footslope position is important in archaeological studies as the footslope is a concave landscape position and deposition of material from upslope occurs. Soils on footslopes can be quite complex and heterogeneous due to this movement of materials and deposition as well as to irregular seepage. Palesosols are common on footslope as well as toeslope landscape positions. Toeslopes are unstable because these positions are subject to periodic flooding


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and may be receiving materials from multiple sources. This position may have the thickest A horizon. Underwater archaeology and soil science may seem like strange bedfellows but a recent study by Kuehl and Benson (1993) discussed how both disciplines can compliment each other. In a study of a 15.5-km stretch of the Oklawaha River in north-central Florida, divers and archaeologists collaborated with a soil scientist to make archaeological interpretations along and under the river. The multidisciplinary study used soil morphological features to quantify the erosional and depositional processses occurring along the river and the resulting impact of these processes on archaeological sites. The study also illustrated the importance of understanding the relationship of soils to landscapes and landforms. The recognition of the footslope landscape position and the buried soil associated with this position was useful in predicting the location of excellent archaeological sites. The study also produced a good correlation between the total phosphorus level in the soils and the areas in which human activity had occurred.

E. MICROMORPHOLOGY Micromorphology, as the name implies, considers the morphology of the soil at a microscopic scale. It is the study of soils (or sediments) in thin sections (2530pm) (Courty et al., 1989; Buol ef al., 1989; Goldberg, 1992). Courty er al. (1989) in their book “Soils and Micromorphology in Archaeology” go into great detail on (i) basic principles of soils and micromorphology, (ii) processes and features in archaeological contexts, and (iii) case studies in which observations were made from field level to the micromorphological level to reconstruct the history of the area of interest. Micromorphology was initiated by Kubiena in 1938 when he demonstrated how the processes involved in soil genesis could be resolved with a soil sample prepared as a petrographic thin section examined under a microscope.

IV. PALEOSOLS Paleopedology is the historical branch of pedology whose aims include the retracing of the developmental stages of soils, particularly in the Quaternary period, and the study and interpretation of relict pedological characteristics (Ruellan, 1971). Paleopedology includes the study of paleosols, the term “sol” being Latin for solum or soil. Paleosol is a term that is widely used in archaeology and paleopedology, but is often poorly defined (Fenwick, 1985). Definitions include an-

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cient soils, and soils formed in an environment of the past under conditions generally different from today. The term paleosol, as currently used in North America, refers to any soil that formed on a landscape of the past. Three kinds of paleolandscapes and paleosols are generally recognized: buried soils, relict soils, and exhumed soils (Ruhe, 1965; Ruhe et al., 1971; Ruellan, 1971). In all cases, these soils developed on landscapes in the past. Buried soils were formed at the Earth’s surface and subsequently covered by younger sediments. These soils may crop out on slopes of hills, road cuts, or excavations. Relict soils began forming on a preexisting landscape but were never buried by younger materials. These soils can be very significant in paleopedology in that they date from the initiation of the original land surface. The exhumed paleosols formed, were buried at some time, and later were reexposed at the surface following the erosion of the younger surface. Pedologically, a buried soil may be defined as an identifiable soil profile (A horizon and/or underlying B and/or C horizons that underlie a mantle of pedologically differing material(s)). Pedologically differing materials may be C horizons or B horizon material overlying a buried A horizon which developed under different soil forming periods.

A. BURIEDPALEOSOLS: KEYSTO ARCHAEOLOGICAL INTERPRETATIONS Much of the early interest in paleosols in the late 19th century seems to have derived from the use of paleosols as stratigraphic markers in geological sections (Valentine and Dalrymple, 1976). The most extensive stratigraphic application of buried paleosols was by pedologists and geologists in the loess-covered landscapes in the United States as well as in Europe. Archaeology is the other principal discipline in which buried soils have been used as stratigraphic markers. Archaeological research generally involves the study of soils buried at some depth, and thus, it is the buried paleosols that receive the most attention. Properties of the buried paleosol are commonly used to make assumptions about the climate and vegetation of the preexisting landscape. However, numerous researchers who have studied the physical and chemical characteristics of paleosols warn about potential errors that could result from these assumptions (Ruhe, 1965, 1975; Gerasimov, 1971). Scholtes et al. (1951) emphasized that care must be taken to choose soils from similar topographic positions when comparing buried and surface soils. Past and present-day soils are frequently the result of different soil-forming processes. It can be a mistake to equate the color of paleosols with the color of soils developed in present day environments. For example, thick reddish paleosols are often equated with the present-day reddish-


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colored soils that developed in warm, humid environments. Ruhe (1969) studied the buried and relict grayish Yarmouth-Sangamon paleosols in Iowa and stressed that the paleosols developed in a much wetter environment than the present-day soils, and the gray paleosol does not reflect the water table depth of the soils in today’s climate and environment. Once a soil is buried, it may undergo significant chemical and physical changes. Some of the initial soil properties are lost while new properties are acquired. Workers in paleopedology generally agree that the same methods used in pedology must be used in the study of paleosols (Working Group on the Origin and Nature of Paleosols, 1971). Field evidence of pedogenesis, more than one pedogenic feature or diagnostic horizon, and strict adherence to stratigraphic and geomorphic principles are needed for recognizing and validating paleosols. Detailed field descriptions and laboratory analyses are essential in paleopedological studies along with an understanding that past as well as current processes have led to the development of soils and paleosols. Tracing the upper and lower boundaries of a paleosol is essential in making interpretations in paleopedology. Defining and tracing the boundaries can be difficult, however, as the boundaries are frequently gradational and few paleosols are exposed continuously in any cut or excavation (Follmer, 1982). In addition, the physical, chemical, and morphological properties of any pedostratigraphic unit may vary greatly both vertically and laterally across the landscape. Rolph et al. (1994), using seismic refraction, were able to trace the variations in depth of a paleosol underlying loess in China. The absolute dating of paleosols for stratigraphic studies in archaeology has been attempted by radiocarbon dating both organic and inorganic carbon in the soil. Dating carbon in the soil is difficult as there are continual additions of carbon as well as a constant recycling. The true age of the soil, in terms of when organic matter started to accumulate, may be impossible to determine. Instead, in the case of buried soils, the age represents the time elapsed since burial. An additional complication is encountered in dating archaeological sections in which the date is required in calendar years rather than radiocarbon years. One of the major complicating factors in studying paleopedology is climatic changes. It is well-known that climatic changes have occurred since the Pleistocene time period. One major and widespread contributor to climatic change that resulted in landscape changes was the Pleistocene glaciation. The glaciation resulted in the deposition of loess as well as till deposits and the formation of glacial lakes, terraces, and outwash valleys (Flint, 1976). An understanding of the climatic changes since the Pleistocene is essential to the study of the processes involved in paleopedology. There is no clear distinction between the processes involved in pedology and paleopedology, and thus, interdisciplinary studies are important.

24


B. A PALEOSOL CASESTUDY Goodyear and Foss (1993) illustrated the stratigraphic significance of paleosols along the South Carolina coastal plain in their detailed soil morphological study of an archeological site located on the flood plain of the Savannah River. The soil characteristics were described in each soil horizon and comparisons were made between the pedologic features and the archeological time period. Four major lithologic and pedologic discontinuities were noted. Of particular significance was that the Paleoindian occupation during 11,000to 10,000 B.P. is situated in sands that are essentially pedogenically unmodified. In the three soil profiles studied, the Holocene sands overlie two well-developed paleosols that were described as argillic horizons. Artifacts were observed in the Holocene sands, but no cultural features were associated with the older deposits that were presumed to be late-Pleistocene or older. The presence of these paleosols helps to define the early Holocene alluvial deposits which may contain Paleoindian cultural features in other areas of the Savannah River valley as well as other river valleys in the southeast United States. In this study, it was determined that based on the clay content and structure of the argillic horizon in the lower Pleistocene paleosol, the argillic horizon had weathered for a long period of time (>50,000 years; Foss er al., 1981). Goodyear and Foss (1993) indicated that this argillic horizon is a dominating horizon on many landscapes in the Savannah River valley and it provides an excellent index horizon to help interpret the early Holocene and Pleistocence landscapes in this Southeastern region. When the number of cultural features in alluvial deposits is low, the capability of recognizing the initial Holocene deposits and terminal Pleistocene deposits will help expedite the search for deposits that are old enough to support cultural features during this time period.

V. CASE STUDIES OF SOIL-ARCHAEOLOGICAL INVESTIGATIONS A. ALLUVIAL SEQUENCES INTHE SOUTHEASTERN UNITED STATES 1. Introduction Soils in alluvial valleys have been of great interest in soil-archaeological studies because some 80 to 90% of the known stratified and well preserved archaeological sites in the southeastern part of the United States are found in these landscapes. One of the keys to understanding the erosion, depositional, and


2s

weathering sequences in alluvial environments is through interdisciplinary studies in archaeology-geology-pedology. Although archaeologists have been studying alluvial environments for decades, recent interdisciplinary activities with soils and geology have provided further insight into alluvial sedimentation patterns and chronology. Several examples of these types of studies will be given in the discussion below.

2. Methods Profile descriptions of major soils at the archaeological sites mentioned in this section were prepared using nomenclature and methods outlined in the Soil Survey Manual (Soil Survey Division Staff, 1993). Profile descriptions were made in excavation units and by using a bucket auger. Laboratory analyses included particle-size distribution (Kilmer and Alexander, 1949), organic carbon (Nelson and Sommers, 1982), and elemental analyses (Lewis et al., 1993).

3. Results a. Thunderbird Site, Virginia The Thunderbird site is located along the South Fork of the Shenandoah River in Virginia. It was perhaps one of the first Paleo-Indian sites studied by a multidisciplinary team of archaeologists, geologists, pedologists, palynologists, and other scientists working on developing site chronology and history. That study was described in a publication produced by Gardner (1974). The pedologic portion of the study included a model of landscape development, soil distribution and characteristics, and chronology of the Thunderbird Site and surrounding areas (Foss, 1974). Figure 2 illustrates the landscape and soil development sequence along the South Fork of the Shenadoah River Valley near Front Royal, Virginia (Foss, 1974). This stratified site included artifacts from Paleo-Indian to Woodland (1 1,000 to 2000 B.P.) cultural periods; thus, the stratigraphy and soil development sequences provided an opportunity to interpret site history and observe pedologic processes over well-defined periods of time. The soils varied, showing minimal development along the levee, moderate development on the Holocene terrace, and strong argillic horizons in an old terrace and in the limestone residuum. The Paleo-Indian layer was identified as the Clovis clay (an argillic horizon). This soil had developed on an old terrace of the Shenandoah River (perhaps 13,000-20,000 years) and was subsequently covered with Holocene alluvium. The soil sequence above the Paleo-Indian artifacts included an argillic horizon with clay increases and clay-flow surfaces. Figure 3 shows the clay content with depth in three profiles (a-c) located on the Holocene terrace and underlain by the

Figure 2 Cross section showing soils at the Thunderbird site in Virginia (Foss, 1974).

Clay

-

.-+

(%I

20

-

AP -

40

-

BAIBt 1

60 -

0

5 00 : 100

-

120

-

140

\ \

t

.\

Figure 3 Distribution of clay in three soil profiles at the Thunderbird site in Virginia (Foss, 1974).

26

Table II Chronology of soils and Sediments and General Morpbology of Soils in the Savannah River Valley (Adapted from Foss ef uf., 1981)

Unit N -4

Estimated age of soils (years B. P. x 103)

I IIa

0.25 0.25-4

IIb

4-6 6-8 8-10.3 10.3-30 100-250

IIC

III IVa Ivb

Elevation of soils above Savannah River (m)

Soils characteristics

Mean

Diagnostic B

Colora

B-horizon thickness (m)

0-6 3-6

3 4.6

Variable 10YR-7.5YR

0-0.5 0.5-0.8

Historic Historic/ Woodland

3-6 3-6 3-6 4.6-7.6 6-15.2

4.6 4.6 4.6 6

None to weak Cambic or weak argillic Weak argillic Moderate argillic Moderate argillic Strong argillic Strong argillic

0.8-1.2 0.8-1.2 1.O- 1.5 1.0-2.0 1.5-2.5

Archaic Archaic Early ArchaiclPaleo-Indian

Range

Color is for well-drained soils.

9.1

7.5YR-10YR 7.5YR 7.5YR-5YR

5YR-7.5YR 2.5YR-5YR

Expected archaeological components

Recent to Paleo-Indian

28


Clovis clay. Note the uniformity of the clay curves for the three profiles on the Holocene terrace; these soils are much more uniform than generally expected on alluvial terraces. b. Savannah River Studies, Georgia and South Carolina A detailed study of the soils occurring along the Savannah River in the Piedmont of Georgia and South Carolina was made in association with archaeological investigations of the R.B. Russell dam (Foss and Segovia, 1984; Foss et al., 1985). Table I1 shows the range of soils and some of the morphological characteristics associated with the age of the soils. Qpes of diagnostic B horizons, such as argillic or cambic, were closely associated with the age of the terraces. Figure 4 shows clay distribution curves for three soils illustrating the range of soil development in the Savannah River Valley. Soils on the ancient terrace show clay maxima greater than 30%, while soils 4000 years or younger have maximum clay contents of less than 10%. Figure 5 summarizes five of the general characteristics of soils in the Savannah River Valley under different weathering periods. These data were useful in developing strategies for the archaeological investigations and subsequent interpretations of sites.

Clay (70)

nnn

I

-

2.5

Figure 4 Clay distribution curves for a chronosequence in the Savannah River Valley in Georgia and South Carolina (Foss and Segovia, 1984).

29


0

0

4

8 12 1

100200

Time tx 10%

n m e tx lo3)

0

4

8 12 1 n m e tx

100200

lo3)

nme tx l o 3 )

Figure 5 General characteristics of soils in the Savannah River Valley with age or length of weathering (Foss and Segovia, 1984).

c. Hiwasse River, Tennessee A landscape model was developed for the Hiwasse River in Pope County, Tennessee, in conjunction with an archaeological investigation. As noted in Fig. 6 , Pleistocene and Holocene terraces dominate the alluvial landscape, with recent

Figure 6 Cross section of landscape along the Hiwasse River in Pope County, Tennessee (Foss er al., 1995).

30


sediments occurring on the bench terrace next to the Hiwasse River and in the flood chute. Soils on the Pleistocene terrace consisted of strong, well-developed, reddish argillic horizons. The Holocene terrace had soils with a weakly developed argillic; a I4C date of 8800 B.P. was obtained at the base of this argillic horizon. The bench terrace had more than a meter of recent sediment overlying late Holocene alluvium that contained numerous Woodland period artifacts. In contrast, bench terraces next to rivers in other areas of the Southeast are normally composed entirely of recent sediment.

B. SOILSTUDIESAT

THEEL MIRADOR BAJO

1. Introduction A great deal of interest has been generated in Mayan civilization during the century or more that scientists have been studying it in Central America. Soil resources were probably a major factor in the environmental difficulties experienced by the Mayans and in the subsequent decline of their civilization. Because of densely populated cities, they were evidently forced to use soils of bajos (low areas) for some of their agricultural sites as indicated by raised fields and other landscape modifications. The soils in a bajo near El Mirador, a major Mayan site in the Pettn region of Guatemala, were investigated in conjunction with the archaeological studies by Dahlin et al. (1980). Earlier soils studies at the large Mayan City of Tikal by Olson (1977) showed 12% of the 9 km* central area was classified as swamp; these soils would be classified as Vertisols. The Vertisols near El Mirador are classified with the Yaloch series, which, according to Simmons et al. (1959), composes 229,563 hectares or 6.3% of northern Pettn. Cowgill and Hutchinson (1963) studied the chemistry and mineralogy of a single profile in a bajo near Tikal. More recent studies by Hammond (1994) and Coultas et al. (1993) in Belize and by Dunning (1993) in Guatemala have further elucidated the agricultural systems and soil characteristics in areas of Mayan activity.

2. Methods Twenty-four soil profiles were described and sampled in a 473-ha bajo north and west of El Mirador. The profiles were described and sampled according to procedures in the Soil Survey Manual (Soil Survey Division Staff, 1993). Extractable ions were determined using a weak acid extractant (0.05 N HCl and 0.025 N H2S04) and analyzing for each ion on a Technicon auto-analyzer (Bandel and Rivard, 1975). The pH of a 1:l soil-water ratio was made on a Beckman Zeriomatic pH Meter, and organic matter was determined by the


31

Walkley-Black method. The electrical conductivity method of Bower and Wilcox (1965) was used for soluble salt measurements and the acid-neutralization method described by Allison and Moodie (1965) for determining calcium carbonate equivalents.

3. Results Figure 7 shows the landscape of the El Mirador archaeological site and the bajo that was studied. Figure 8 is a soil map developed from observations made at the archaeological excavations. The landscape at El Mirador is composed of the upland area on which the large city Mayan city was built and the bajo that was used for agriculture and water storage, and as a conduit for transportation to other areas near El Mirador. The upland area consists of limestone-derived soils (Rendolls); in the bajo area, soils were developed on gypsiferous clays and marl of Eocene age and sediments derived from the uplands. Causeways were built across the bajos in several localities, using limestone fragments carried from the uplands (Dahlin ef al., 1980). The causeways ranged from perhaps 0.3 to 1.75 m in height above the surface of surrounding areas. The soils developed on these structures had coarse limestone fragments, dark-colored surfaces, neutral to slightly acid pH values, and weakly developed B horizons (cambic or weakly developed slickensides). Soils of the bajo were classified as Vertisols, although some variation in characteristics was observed. Soil units P1, P3, P10, P17, and P29 on the landscape model and map were classified as Vertisols but with differences in the

Flgure 7 Geopedologic relationships at the bajo at El Mirador, Peten, Guatemala (Dahlin er al., 1984).

32


Figure 8 Soil map of the Bajo at El Mirador, Guatemala (Dahlin et al., 1984).

parent materials and development of slickensides, color, depth of organic matter in subsoils, and chemical properties. Table I11 gives the morphological characteristics of several soils included in the legend. Those soils in the central part of the bajo (e.g., P1, P3, P29) were grayish brown to light brownish gray, while soils along the edge of the bajo near the Rendolls were dark-colored and more typical of the classical Vertisols (e.g., P17). The pH values of the Bss horizons of Vertisols developed on gypsiferous clays were less than 3.5 (soil units 1 and 3).

4. Archaeological Implications The soils investigation at the El Mirador bajo provides information on the general characteristics of the soil resource available to the Mayans. While the Mayans built their city on the more productive (though more fragile) landscapes, some agriculture was undoubtedly practiced in the bajo. With the high clay content, difficult physical conditions, high salt potential in some areas, and very low pH values in some cases, the soil environment of the bajos would be very challenging for agricultural production. Combining these soil characteristics with the 6-month dry period would further limit agricultural production potentials. Modifications of landscape, e.g., raised fields and terraces, although not noted at El Mirador, would provide some improvement of drainage, but they would not modify the overall physical and chemical properties of the soils. Profile 1, shown in Fig. 9, had low pH values which would definitely cause problems in growing corn or other crops. The extent of these highly acid soils (soil units 1 and 3) in the El Mirador area would be of great interest in evaluating the agricultural potential of landscapes for the Mayan civilization and for future development as well.

Table 111 Characteristics of Soils Occurring at the El Mirador Bajo

Horizon

Depth (cm)

A1 BA

0-18 18-40

IOYR 312

Bssl

40-62

I O Y R 5/2

Bss2

62-82

2.5Y 512

Bss3

82- 103

2.5Y 512

2Bss4

103- 137

5Y 613

2Bss5

137-168

2.5Y 612

2Bss6

168- 185

2.5Y 612

A1 A2 Bw

0-20 20-3 1 31-52

c1 c2

52-73 73- 120

lOYR 311 IOYR 411-311 IOYR 5/1, 411 lOYR 811 carbonate coatings lOYR 511, 311 2.5Y 512, 311

Color

lOYR 513

Mottling

Texture

Soil Unit P1 (S78G01)a None C cld C 7.5YR 518 cld C 7.5YR 518 c Id C 7.5YR 518 ClP C 2.5YR 418 m3d C 7.5YR 518 mlP C 2.5YR 418 mlP C 10R 418 Soil Unit P2 (S78GU2)b None C None C None C

Structure 2mabk-2fabk 2msbk

Slickensides

Bound

None None

1-2csbk

Mod.devel.

lcsbk

Mod. devel.

Om

Weak devel.

Om

Weak devel.

Om

Strong devel.

Om

Strong devel.

2mgr 3mgr lmsbk-Om

None None None

None None

1

I

lmpr, lmsbk 3mpr

None Weak

None

1

1mpr

None

(10%)

c3

120-166

2.5Y 512, 311 (10%)

(continues)

Table I11 (condnued) ~~

Depth (cm)

Color

2Ab 2Bssb 2BCb 2Cr

166-188 188-224 224-248 248-255

N 310 l0YR 511 2.5Y 612 lOYR 811, 712

A1 A2 2Bsslb

0-25 25-46 46-70

IOYR 312 lOYR 411

2Bss2b

70-89

2.5Y 512

0-9 9-39 39-72 72- 148 148-160 160-173

2.5Y 210 2.5Y 210 2.5Y 210 2.5Y 210 lOYR 612 2.5Y 210, 510

Horizon

w

P

A1 A2 BA Bss 2c 3Bss

5Y 511

Mottling

Texture

None C None C None C None sil Soil Unit P2 (S78GU5)C C None None gc flf C 2.5Y 516 d P C 5YR 518 Soil UnitP 17 (S78GU17)d C None None C None C None C None gsl None C

Structure

Slickensides

Bound

Strong Strong None None

cw cw cw

3mgr 2msbk lcsbk

None None Strong devel.

cs as gs

lcsbk

Strong devel.

-

None None None

cs cs cw 2s 2s

Om Om

Om Om

2-3f~ 2-3 mgr 2msbk Om 0% Om

Mod.devel. None Strong devel.

-

a Sampled near Bullard Causeway; gypsum 2% of Matrix at 82-103 cm, 5% at 103-185 cm; there appear to be two sets of slickensides, with moderate to weakly developed slickensides 40-103 cm and strong development 137-185 cm. Sampled on Gifford Causeway; soil developed on calcareous fill materials over limestone (marl); the buried 2Ab ranges in thickness from 12 to 74 cm; appears to be a wave cycle of 140 cm; small amounts of gypsum (1-2%) are present in the 2B; calcium carbonate equivalents in upper fill material ranged from 7.6% in A1 to 39.7% in C1; the Cr horizon had 58.6% calcium carbonate equivalent. Sampled on Bullard Causeway; soil developed on calcareous fill materials over limestone (marl); 40% coarse limestone fragments occur in A2; slight effervescence in A1 and strong effervescencefrom 25 to 89 cm; calcium carbonate equivalents in A1 and A2 are 3.1 and 21.6%, respectively. Sampled in Aquada Limon; slight effervescence in upper 39 cm, mod. 39-72 cm, and strong 72-173 cm; lense of limestone pebbles at 148-160 cm.

Depth (cm)

EC (dS/m)

PH

2

1

Figure 9 Guatemala.

EC (dS/m)

PH

-

6

A1

7

8

0

1

2

1

I

3

4

5

\

50

i

Soil morphology, pH, and conductivity of soil unit PI at El Mirador bajo, Peten,

Depth (cm) 0

3

Bw

c1

100

150

c2

c3

....

I

........

2Ab

200

2Bsst PBCb

250

ZUL

Figure 10 Soil morphology, pH, and conductivity of soil unit P2 at El Mirador bajo, Peten, Guatemala. 35

36


The preserved buried soils under the causeways provide an indicator of the conditions prior to construction of these features. For example, Fig. 10 shows the pH and electrical conductivity measurements of Profile 2, in which the original buried surface began at 166 cm (Table 111). The high salt content in the buried Ab (greater than 4 ds/m) would be deleterious to plant growth; the extractable Na content in the buried Ab and Bss horizons (ranging from 2112 to 4080 ppm) would also be a problem for many biological systems.

C. CHEMICAL PROPERTIES OF SOILSAT HAD RIA"^ VILLA, ITALY 1. Introduction

The use of chemical properties of soils in archaeological interpretations has been increasing in the last decade or more in the United States. The introduction of the inductively coupled argon plasma-atomic emission spectrometer (ICAP) to laboratories has greatly facilitated the elemental analyses of soils from archaeological sites (Lewis et al., 1993). The Villa, near Rome, Italy, was built by Emperor Hadrian from A.D. 118 to 133 and occupies approximately 121 ha. Some initial work on heavy metal distribution, in conjunction with soil characterization of gardens at the Villa, showed increased Pb concentrations in several gardens and adjacent areas (Foss el al., 1994). This resulted in more detailed sampling of soils from gardens, agricultural fields, and areas outside the Villa to determine the locations of soils with high levels of Pb and other elements. 2. Methods

Some 49 sites were sampled at Hadrian’s Villa; samples were obtained from surface and subsoils to represent the major soils and landscapes of the Villa. The elements analyzed included Al, As, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, P, Pb, S, Sr, Ti, and Zn. The elemental distribution was determined using an extracting solution of HCI-HNO, acid of 0.61 and 0.16 M , respectively, and analyzing the extract using an ICAP (Lewis el al., 1993).

3. Results Figure 11 shows the distribution of Pb at Hadrian’s Villa. The Pb content in the Villa ranged from 47 to 953 mg/kg; off-site the range was 19 to 29 mg/kg. The areas with the highest Pb contents were in the older portion of the Villa and near some of the major buildings. In the agricultural fields some distance from the buildings, the Pb levels dropped to near background values. The source of the

Figure 11 Areal distribution of Pb at Hadrian’s Villa in Italy.

Figure 12 Areal distribution of Zn at Hadrian’s Villa in Italy.


37

increased Pb at the Villa is still unknown, but the data on Pb distribution may aid in developing the answer to this vexing question. In addition to Pb, other elements show individualized distribution patterns. Figure 12 gives the distribution of Zn at the Villa. In general the Zn levels are higher near the buildings than in outlying areas, but the distribution pattern is different from that of Pb. Other elements such as As, Cu, Mn, Ni, and P showed increased levels in the gardens and areas surrounding buildings in comparison to off-site locations or to the agricultural fields in the Villa. These data, in conjunction with analyses of heavy metals at other Roman sites in Italy and Tunisia, have shown that Roman cultural levels can actually be identified by the increased concentrations of elements such as Pb, Cu, Zn, Ni, and P (Foss et al., 1994). Information such as this is vital to the interpretation and development of site history.

D. PALEOSOLS NEAR MT. VESUVIUS 1. Introduction

The large catastrophic volcanic eruption in A.D. 79 covered Pompeii with 5 to 6 m of volcanic material. However, significant eruptions preceded the eruption of A.D. 79, and the paleosols developed during intervals of volcanic activity have been preserved. The objective of this investigation was to study paleosols developed in volcanic deposits of Mt. Vesuvius dating from 1871 B.P. (A.D. 79) to 17,000 B.P. The interpretation of these paleosols can provide information on the number of eruptions and length of weathering time between eruptions.

2. Methods Soil profiles were described and sampled at the following locations around Mt. Vesuvius: Pompeii, Villa Oplontis, Herculaneum, Boscoreale, Ottaviano quarry, and Pozzelle quarry. Description and sampling were in accordance with procedures outlined in the Soil Survey Manual (Soil Survey Division Staff, 1993). The elemental distribution was determined using an extracting solution of HClHN03 of 0.61 and 0.16 M, respectively, and analyzing the extract using an ICAP (Lewis et al. 1993).

3. Results Figure 13 shows the general morphological characteristics of soil profiles sampled below the A.D. 79 level at Pompeii (Polybius and Pompeii garden sites), Boscoreale, and Oplontis. All profiles exhibited numerous pedologic (de-

Polybius

(ssntl)

Pompeii Garden

Boscoreale

Oplontis

(S8.W (S87It2) (ssnt4) Hgure 13 Morphological characteristics of soil profiles described at Pompeii, Boscoreale, and Oplontis near Mt. Vesuvius, Italy.


39

velopmental) and lithologic (textural) discontinuities and buried surfaces (Ab horizons). The Ab horizons indicate previous surfaces on which organic matter from plants and animals decomposed into humus and developed into an A horizon. Some of the profiles had cambic (Bw) horizons that indicated longer periods of weathering. The major paleosols were the Pompeii (1871 B.P.), Avellino (3800 B.P.), and the Mercato (7900 B.P.). Correlation of paleosols was possible using the chemical properties of the soils. Figure 14 shows the Ba/Pb contents of the Avellino and Mercato paleosols at the Ottaviano quarry. Figure 15 presents the Ba/Pb ratios of soils located near Pompeii. Note the high Ba/Pb ratios of the Mercado paleosol at each of the four sites. The chemical properties of the Mercado paleosol were also closely associated with a 14Cdate of 8200 2 230 years B.P. at Boscoreale. Thus, the combination of soil chemical properties, 14C dates, and soil morphology is an effective tool in mapping and interpreting paleosols in the Mt. Vesuvius area. Figure 16 shows the detailed morphology of a 15-m section at the Ottaviano quarry. Both the Avellino and Mercato paleosols were complex profiles in that discontinuities were present within each; thus, the soil-weathering sequence of these paleosols was interrupted by additional deposition of volcanic sediment. For example, the Mercato paleosol had two additional Ab horizons below the upper Ab. Using the numerous I4C dates, cultural artifacts, and soil morphology, a solum thickness based on age (i.e., duration of soil weathering) was proposed (Fig. 17). These data permit an approximation of soil age based on morphologic charac-

Wgure 14 Content of extractable Ba and Pb in paleosols at the Ottaviano quarry near Mt. Vesuvius, Italy (Lewis et al., 1993).

40 P

Boscoreale

Oplontis

Pompeii Garden

Figure 15 Ba/Pb ratios of soils at Pompeii, Boscoreale, and Oplontis near Mt. Vesuvius, Italy.

Polybius

figure 16 Soil morphological characteristics of column at Ottaviano quany near Mt. Vesuvius.

42


Age of Sol1 (yrs.x103)

Figure 17 Solum thickness versus age for soils developed on volcanic materials near MI.Ves-

uvius.

teristics. For example, the Pompeii paleosol (soil at the A.D. 79 surface) was less than 30 cm in most locations. Using soil morphology (solum thickness) to determine its approximate age, it can be seen that Pompeii had little volcanic activity for perhaps 600 to 700 years prior to the massive eruption in A.D. 79. The P content of soils has been used as an indicator of human activity in a number of archaeological sites. Phosphorous is of special interest because of the amount generated by human populations, and because of its use in fertilizer; it is also relatively immobile in many soils. Figure 18 shows the acid-extractable P in soil profiles at Boscoreale, Pompeii (Garden and Polybius), and Oplontis. The P contents in the Pompeii paleosol (upper 0.5 m) and the Avellino are generally between 500 and 2600 Fg/g; these values are greater than the background values for P found in the Bw and C horizons and at sites outside populated areas. The Mercato paleosol has P values less than 200 pg/g, except at Polybius where the Mercato paleosol is close to the surface and the Avellino paleosol is absent. The P values provide evidence of human activity throughout the time period of the Pompeii paleosol (1871 B.P.) and the development of the Avellino paleosol (3360 B.P.).

E. GEOMORPHOLOGY AND SITE SELECTION AT THE SEMINOLE REST SITE, VOLUSIA COUNTY, FLORIDA 1. Introduction

The Seminole Rest archaeological site is located on the shore of Mosquito Lagoon in Volusia County on Florida’s east-central coast. The site consists of a

Ext. P(ug/g)

Ext. P(ug/g) 1000 2000

0

1

2

I

8

b

Y

9

8

3

4

Boscoreale

5

3000

I

Pompeii Garden

Ext. P(ug/g) 1000 2000

Ext. P(ug/g) 3000

1000 2000

- 0

P

I

A

7 \ 4 \ \

1

\ Pompeii Polybius

1 '

I

Oplontis

Figure 18 Extractable P in soils sampled at Pompeii, Boscoreale, and Oplontis near Mt. Vesuvius, Italy.

3000

1

44


large clam-shell mound (Snyder’s Mound) and several smaller interlayered shell/sand mounds bordered by salt marsh. Several aboriginal cultural periods are represented (Horvath er al., 1995) with inclusive calibrated radiocarbon dates ranging from 210 to 1460 A.D. East of the mound is Mosquito Lagoon which is separated from the Atlantic Ocean by a barrier island and mangrove keys. West of the mound is a small water-control canal, then coastal marsh and low Pleistocene terraces now covered by oak woodland (Schmalzer and Hinkle, 1990; White, 1970). The mound today is 227 m long and 155 m wide, and extends from 2 m below to 4.5 m above sea level (Horvath et a l ., 1995). In 1993 archaeological investigations were conducted at Seminole Rest by the National Park Service. Field archaeologists collaborated with faunal analysts, paleobotanists, and soil scientists to reconstruct the cultural and ecological history of Snyder’s mound, one of the smaller ancillary mounds (Fiddle Crab Mound), and the adjacent lagoon and upland habitats (Horvath er al., 1995). One of the questions posed by archaeologists of the Seminole Rest site was its relationship to sea level: was the mound constructed on dry land, or was moundbuilding begun by throwing clam shells into shallow water adjacent to a living area or shellfish-processing site? The 1993 excavations of the site encountered midden material below present sea level. Cores, placed at intervals on Snyder’s Mound and penetrating to the culturally “sterile” sediments below, also indicated below-sea-level accumulations of shells. To address this question, particle-size distribution analysis of the mound substrate and of adjacent marsh surface samples from approximately contiguous topographic positions was used. Two adjuncts to particle-size analysis that were also employed in understanding the geomorphology of the area and site selection by past human inhabitants were clay mineralogy and an examination of the infauna of the submound sediments.

2. Methods Mechanically extracted core samples were taken by geochemical engineers using a split-spoon, 1.5-in. core which extracted 46-cm core intervals. Placement of the cores at various points on the top of Snyder’s Mound was directed by archaeologists. Cores penetrated the shell layers and terminated in the submound sandy sediments. Thirteen auger sites were also chosen, including the east and west margins of Snyder’s Mound, areas below and adjacent to the smaller Fiddle Crab Mound, and marsh areas south and southwest of the excavation units. Samples were taken at 10-cm intervals to depths ranging from 75 to 140 cm. Sampling terminated at the upper boundary of the water table. Particle-size distribution was determined using the pipette method (Day, 1965). Samples with an estimated 1% or more organic carbon (judged by a dark


45

brown or black color) were pretreated with hydrogen peroxide and heat to remove organic matter. Samples for clay-size mineral identification were plated onto ceramic tiles, saturated with either magnesium chloride and glycerol or potassium chloride, and X-rayed at room temperature and at 1 10°C using a Nicolet diffractometer and Cu K-alpha radiation. Faunal material from each core and auger sample was identified using comparative specimens housed in the zooarchaeology laboratory at the Florida Museum of Natural History. Particular attention was paid to small invertebrate infauna that actually resided in the soils and sediments.

3. Results Grain-size distributions in all areas sampled were dominated by fine sand: from 52 to 77% by weight of the sand-to-clay size fraction. Subrnound (Snyder’s Mound) core samples and samples taken beneath and immediately adjacent to Fiddle Crab Mound contained more fine sand (mean of 70%)than soils from the area between those two features, adjacent to the modem canal. In those areas, mean fine sand content was 58%. Extremely high silt contents were encountered in all levels of the tests taken on the western margin of Snyders’s Mound (mean of 25%) and in the surface levels of the southern margin. In contrast, samples from beneath Fiddle Crab Mound contained an average of 4.8% silt. The submound core samples contained only 5.6 to 12.5%silt, with an average of 7.5%. Clay content was less than 1% in all samples analyzed. Clay-size mineral species from selected Seminole Rest soil samples appeared as two somewhat overlapping assemblages: (i) high smectite with kaolinite and quartz, and (ii) low or minimal smectite with hydroxy-interlayer vermiculite (HIV), kaolinite, gibbsite, and quartz. The latter grouping was found in the lower levels of auger samples in the vicinity of Fiddle Crab Mound and in 75% of the submound core samples tested. Broad smectite peaks, indicating the presence of a poorly crystalline, minimally weathered smectite, were seen in all upper-level marsh samples, in upper Fiddle Crab Mound zones, and throughout the samples from the western margin of Snyder’s Mound. Soil faunal analysis revealed that submound core samples and auger samples on the eastern margin of the mound bordering the lagoon contained high numbers (>25% by volume) of both articulated and disarticulated shells of the genus Parastarte, the Brown Gem Clam. Shell numbers decreased with depth in the auger samples. Samples from the western margin of the mound and the Fiddle Crab Mound zone samples contained no Parastarte. The auger test to the south of the mound had a few Parastarte at 90-100 cmbs.

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4. Discussion Although particle size was dominated by fine sand in all samples, relationships between the coarser (medium and coarse sands) and finer (especially silt) fractions helped outline subsurface landscape features and aided in understanding the original rationale for mound placement. In the basal core sediments beneath Snyder’s Mound, relatively high coarse and medium sand contents and low silt contents suggest a mixing of finer aeolian sands with coarser marine sediments, resulting in a texture characteristic of a beach ridge or sand bar or bank. The finer textural class on the western margin of the mound and in the upper 50 cm of the test to the south defines an accumulation of silt over sand. The Volusia County soil survey (Baldwin et al., 1980) offers a basis for these differences. The soil map unit directly south of Snyder’s Mound is the Canaveral fine sand series, which forms on dune flanks and in interdune areas. Prior to modem dredging and canalization, this unit probably extended northward and provided either a low, subaerially exposed ridge or a shallowly submerged sand bar, which represented a suitable substrate for mound building. North and west of the mound is the Turnbull muck soil series. This is a mucky- or clayey-over-sandy series, the development of which has been augmented by the gradual closing of the inlet and subsequent silting of the lagoon (Mehta and Brooks, 1973). The silty areas west of the mound may have been a low swale behind a bar, or a tidal creek, which gradually infilled with fine sediments. During the period of occupation of Snyder’s Mound, the combination of a low bar and swale or tidal creek would have presented the local residents with a comfortable “landing” or haven for canoes filled with shellfish gathered from the lagoon. Extracting the meat from the heavy-shelled clams would have made good sense energetically if the clams had to be carried inland for any distance. The shells would have simply been tossed out at the shore. Two other lines of evidence support the premise that the mound is underlain by a sand feature such as a low ridge or bar. These are clay mineralogy and invertebrate soil fauna. Clay-size mineral distributions in the submound core samples reflect a relatively low smectite content and the presence of HIV and gibbsite, both highly weathered forms compared to smectite. Similar mineral assemblages were found only in the deepest levels of Fiddle Crab Mound. Mound-peripheral and marsh auger samples contained an abundance of smectite. Highly weathered minerals such as HIV and gibbsite are commonly found in old, leached soils, and in reworked marine sediments such as dune or beach deposits which have been transported by wind and water and repeatedly chemically and physically altered. Those soils and sediments are a product of Pleistocene and early Holocene climatic conditions; they form Florida’s inland sand ridges and underlie or mantle its coastal areas. The clay assemblages in the submound core samples and some deep auger samples suggest that the sediments


47

there are composed of such materials. The cation-enriched smectites mantling the more weathered forms are common in quiet-water lagoonal sediments which are high in soluble salts and have a neutral to high pH. The composition of the invertebrate infauna of the submound and peripheral samples is the second line of evidence supporting a sand feature beneath Snyder’s Mound. A high concentration of the Brown Gem Clam, Purasturte triquetru, was found in 6 of 10 of the submound core samples and in four of the auger samples, particularly on the eastern margin of the mound. Virtually no Purusturte were found in samples to the west of the mound. Parustarte is a small bivalve which is common in sand bars (Abbott, 1974). The combination of high numbers of Parustarre, a low smectite and high HIV/gibbsite mineralogy, and coarser grained sediments in the submound samples suggests a sand bar or submerged ridge. In contrast, the finer grained, high smectite environment lacking Purustarte west of the mound indicates a low swale area landward of such a bar or ridge. It is evident that “modern” constructions and excavations (e.g., mounds and canals) have been superimposed over older landforms (sand bars and tidal creeks). The question of whether mound construction began on dry land or whether debris was initially thrown into shallow water can be answered directly only by the clam evidence. Grain-size distributions and clay mineral types might be similar in both subaerially exposed and shallowly submerged environments. But the presence of articulated clam valves, particularly of immature individuals of such a diminutive species, is strong evidence that they were in situ and alive when covered by midden debris. It is difficult to construct a scenario in which they could colonize sediments already covered by coarse midden debris. Considering their aquatic lifestyle, then, it would seem that at least a minimal layer of water covered the site at the onset of midden accumulation.

5. Summary and Conclusions The archaeopedologic study at Seminole Rest focused on broad questions of geomorphology and site/substrate relationships. In particular, the nature of the soil or sediment on which Snyder’s and Fiddle Crab Mounds rested and the character of the sediments which separated the two mounds were addressed. The accumulated evidence suggests that both of these mounds were built on a sandy substrate much different than the marsh sediments found in the area today. Clay mineralogy, particle-size distribution analysis, and invertebrate infauna distributions contributed to this interpretation. The Seminole Rest site offers the opportunity to gain understanding of human adaptation to a dynamic environment. The links between the natural coastal environments, resources, and processes, and early humans’ response to them, are the mounds and middens themselves. The botanical and faunal remains are clear

48


documentation of resource availability and choice. Carbon dates, clam seasonality studies, and the stratigraphy of the mound are records of the temporal relationships between humans and their resources (Horvath et al., 1995). The sediments, soils, and geomorphology of the area are tangible clues to the rationale behind site selection for habitation (or deposition), and to the conditions present at the onset of such activities.

F. SOILSAND LANDSCAPES: ARCHAEOPEDOLOGY AT THE PINELAND SITE

1. Introduction The Pineland archaeological site, once home to Florida’s Calusa Indians (Marquardt, 1992; Walker, 1992) is a study in complexity, incorporating monumental earthworks and canals, peopled by accomplished and highly socially stratified inhabitants, and dependent on a rich, natural mosaic of terrestrial and marine resources. Huge shell and sand mounds flank-or are encircled by-hand-dug canals and lakes (Cushing, 1897; Luer, 1989; Marquardt, 1992b, 1995) covering an area of approximately 25 ha (Luer, 1991). Generations of archaeologists have focused attention on interpretations of the site’s cultural features (Cushing, 1897; Douglas, 1885; Gilliland, 1975; Goggin and Sturtevant, 1964; Luer, 1989). More recently, advances in techniques of bioarchaeology (incorporating studies of plants, animals, soils, paleonutrition, and human osteology), and a redefinition of research questions and goals (Marquardt, 1992a), have highlighted environmental issues: how do humans exploit and manipulate their surroundings? What environmental effects of human habitation are in evidence and, conversely, what indications of environmental change are found in abandoned human settlements? One question about the Pineland site posed by archaeologists is: What was the initial relationship between settlement elements and the natural landscape? Did the original inhabitants begin mound construction on the flat, marshy soils adjacent to the shore, or did they use relict sand dunes, beach ridges, or other remnant geomorphic features as the bases for their architectural creations? This study focuses on the soils and sediments that underlie and surround two of the architectural features of Pineland. It characterizes physical and chemical attributes of natural soils and compares them with anthropogenic landscape features, giving evidence of human creation and manipulation of landforms and clues to the original rationale for placement of those features. One of the archaeological features is Smith Mound, the sand burial mound whose imposing size and shape are thought to be a central element in the display


49

of social and political power evoked by the site as a whole (Luer, 1989). The entire mound, some 70 m in diameter and 7 m high, was completely encircled by an artificial lake. Was this mound completely human-constructed, or is it an augmentation of a relict dune? The second feature is known as the Citrus Grove, a low northwest- to southeasttrending feature which today supports an orange grove. One small midden was uncovered beneath the surface horizon, as well as scattered, eroded shells, and intermittent shell lenses at various depths. Calusa canals and modem pasture ponds flank the long sides of this small rise. Is it simply a spoil bank thrown up during canal construction, or is its core an old beach ridge, much attenuated by human use and/or continued sedimentation through time? This study addresses the question of the original forms of Smith Mound and the Citrus Grove.

2. Methods Smith Mound soil samples were taken from three excavated squares and two points along a GPR transect. Four sets of Citrus Grove samples were augered from ground surface to approximately 2 m depth. Complete morphologic descriptions were made for each trench profile sampled, including horizon presence, thickness, and arrangement; color and mottling; boundary thickness and topography; approximate texture; structure; and presence and size of roots and other inclusions such as shells, plant material, and potsherds. Soil content of extractable phosphorus, calcium, magnesium, zinc, and copper was determined by ICAP spectroscopy using an extracting solution of 0.05 N HCl in 0.025 N H,SO,. Total phosphorus content of soils from two of the three squares excavated in Smith Mound was determined by the alkaline oxidation method of Dick and Tabatabai (1977).Organic carbon content was determined using the Walkley-Black potassium dichromate/ferrous ammonium sulfate digestion method (Soil Survey Laboratory Staff, 1992).A 2:l water-soil solution was used to measure pH. Iron and aluminum contents of selected samples were determined using citrate-dithionate extraction (Soil Survey Laboratory Staff, 1992) and atomic absorption. Particle-size distribution was analyzed using the pipette method of Day (1965).Samples with an estimated 1% or more organic carbon (judged by a dark brown or black color) were pretreated with hydrogen peroxide and heat to remove organic matter.

3. Results Particle-size distribution in all of the Pineland soil samples was dominated by the fine sand fraction, from 79.6to 89.5% in the Citrus Grove and from 82 to

50


91% in Smith Mound. The second most common particle-size class, very fine sand, averaged approximately 5.5% in the Citrus Grove and 5.2% in Smith Mound. Clay content was less that 1% in all samples. A ratio of “fine” particles (very fine sand, silt, and clay) to “coarse” particles (very coarse, coarse, and medium sands) was calculated for all samples. This ratio excluded the dominant fine sand fraction in order to highlight the more subtle shifts that may occur among grain size classes. In all four Citrus Grove tests, the ratio of fine to coarse particles decreased with depth until the water table was reached, at which point it increased. The ratio fluctuated in the Smith Mound samples, depending on which soil was under consideration (see discussion below). Soil profile descriptions are simplified here to include only features necessary to discuss the genesis of native soils and the creation and manipulation of anthropogenic ones. The Citrus Grove soil profile is described by one auger transect and is therefore a limited approximation of the subsurface morphology of that landscape feature. Figure 19 depicts the subsurface morphology derived from plotting field horizon descriptions. The Ap horizon, subdivided by slight color changes, was dark to light gray, single grain (structureless), dry, fine sand with individual scattered and worn shells. A few lamellae were recorded where the Ap was thickest and formed the flat ridge or “spine” of the Citrus Grove area. The topography of the boundary between the Ap and subadjacent horizons differed from the surface contours of the Ap horizon. Figure 19 shows that boundary, and all subsequent ones, to be flattened in the central portion of the profile, draping downward on the western edge and rising at an angle of 30 to 45” on the eastern edge. The Smith Mound soil profiles recorded a sequence of buried soils (Fig. 20) capped in some areas by a young soil forming in disturbed sediments thrown up by looters’ activities. The square in the topographically lowest position, B-1, exhibited a profile typical of a Spodic Quartzipsamment: salt and pepper A horizon, white E horizon with albic leaching into the light brown Bw below. Auger samples below the floor of the square retrieved a possible E’ horizon above a reddish Bw that articulated with the water table. The intermediately situated square B-2 had a similar horizon sequence, with the Bw horizon occurring at the same depth as in B- 1, and a thicker E‘ horizon. Square B-3 was uppermost on the flank of the mound and excavated deeper due to the discovery of a human burial at 2.18 m below the surface. This square revealed traces of at least two buried soils below the upper 24 cm, which was comprised of an Inceptisol forming in disturbed soil. Below this, a strong A’ horizon was underlain by a Bw’ that gradually faded to a thick C horizon. Below this sequence, the second buried soil exhibited an intact A horizon (A”) overlying thin E’ and Bw” horizons. A second sequum followed, in this case an E” horizon and subadjacent


51

TEST NUMBER

Citrus Grove Figure 19 Profile of Citrus Grove feature, Pineland Archaeological Site, Lee County, FL (Permission for use of figure granted by W. Marquardt, I.A.P.S. Books).

Bw”, which contained the human burial. The topography of the buried horizon boundaries, except for the intrusive interruption, were smooth, parallel, and angled toward the top of the mound, as if following a former hillslope surface. The final auger sample, at 390 cmbs, recovered a portion of the reddish Bw (in this case the Bw’””!) horizon. Soil chemical data are published in Scudder (1995) and are summarized briefly here. Organic carbon (OC) content of Citrus Grove soils was less than 1% by weight in all but the Ap horizon. There, OC content averaged 1.4%. Organic carbon generally decreased with depth to the Bw (or Bhs) horizon which articulated with the water table, at which point it increased. The A horizon of Smith Mound squares contained from 1.1 to 1.7% OC. Organic carbon content of all subsurface horizons there was less than 1%, with slight increases in some A‘, Bw, and Bw‘ horizons.

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Smith Mound Figure 20 Profile of Smith Mound, Pineland Archaeological Site, Lee County, FL (Permission for use of figure granted by W. Marquardt, I.A.P.S. Books).

Extractable calcium (Ca) content of the Citrus Grove horizons varied considerably, from 249 mg-kg-' in the Ap horizon of Test 1 to 4080 mg-kg- in the AE horizons of Test 2. The surface horizon of Smith Mound contained abundant Ca when compared with native soil values-from 450 to 970 mgmkg-1 in the three A horizon samples versus 52 to 72 mg-kg-I in native soils. Contents of all other major and


53

trace element, with the exception of extractable P, were also high. The contents of most of these elements decreased abruptly in the subadjacent E horizon, then increased again in the lowest Bw horizon of each square. Extractable phosphorus (P) content of Citrus Grove Tests 1 and 3 ranged from 31.7 to 180 mgekg-’ and showed no pattern of accumulation with depth. P content of Tests 2 and 4 showed a “mirror image” pattern of accumulation as did Ca, with higher amounts in the upper horizons of Test 2 and in the lower horizons of Test 4. Extractable P content of the surface horizon of Smith Mound was comparable to that of the native soil samples. In square B-3, which contained the repeating sequences of horizons, extractable P was generally lower in the A or A‘ horizons, and increased with depth to the Bw or Bw’. Citrate-dithionite (CDT) extractable aluminum content of the Citrus Grove samples increased with depth in Tests 1, 3, and 4, diminishing only in the E horizon. Aluminum content decreased with depth from surface through the E horizon of Test 2, then increased in the Bw’ horizon. Aluminum content of Smith Mound samples was determined using the Mehlich-1 extraction method, not the citrate-dithionate method used for the Citrus Grove, so only relative increases and decreases in aluminum content are discussed. In all squares, surface horizon samples contained more A1 than the subadjacent E horizon but less than the Bw horizon. In the multiple soils of square B-3, A1 content followed the same trends.

4. Discussion a. The Citrus Grove The existence of Pleistocene dunes and Holocene beach ridges and barrier islands in Pine Island Sound, and of coast-parallel soil series on the island itself (Gagliano, 1977; Widmer, 1988), offers insight into the possible origin of the Citrus Grove feature at Pineland. Figure 21 is a tracing of the Pineland area soil survey sheet (Henderson, 1984) superimposed over a 1940 aerial photo of the site and vicinity. The two NW- to SE-trending hatched black lines on the tracing follow vegetation changes evident on the photo. Although the lower of the two lines bisects the Matlacha soil series, this series is of human construction and does not reflect original soil configurations. The lower line marks the boundary between the marshy Peckish soil (a Typic Sulfaquent) and the sandier Myakka (an Aeric Alaquod). It also passes through the Citrus Grove, placing that feature on the transition between the two soils. This suggests that the Grove-or at least its core-is a product of natural coastal processes at work on a former shoreline. The profile of the Citrus Grove (Fig. 19) shows two distinct horizon boundary topographies. The first is the ground surface itself, following a gently mounded convex line. The second is seen at the boundaries between all subsequent sub-

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Figure 21 Pineland site soils and vegetation from 1940 aerial photo (Permission for use of figure granted by W. Marquardt, I.A.P.S. Books).

horizons. There, the horizons are level and subparallel in the central portion of the profile, but rise toward the east and slope down toward the west. The profile resembles a beach and berm or terrace feature buried under loose sediments. The sediments composing the Ap horizon are not characteristic of unmodified A horizons of native soils in the Pineland area. They are loose, dry, and uniformly gray to a depth of almost a meter, and contain scattered shells and occasional lamellae, which indicate incipient pedogenesis in recently deposited sediments. They also differ from native soils in some chemical and particle-size characteristics. The Citrus Grove area is composed primarily of fine sand, as is Smith Mound and the native soils examined. This indicates an aeolian origin for the sediments from which the soils formed. The resemblance of the subsurface horizons’ topography to a terrace or beach may mean that those features were cut into a preexisting dune, which may be identified in future work by looking for sedimentary


55

structures characteristic of dune deposition. The difference between the texture of the Citrus Grove and the native soils is apparent in the fine-to-coarse-grain ratios. Native soils generally contain more fine particles deeper in the profile: the ratio increases with depth. This is a manifestation of the mechanical translocation of small particles downward through a matrix of larger ones-the normal pedogenic process of eluviation through time. Citrus Grove soils contain more fine particles in the surface horizon than in the subsurface E horizon, suggesting that relatively fresh sediments with a high proportion of “fines” have been piled onto the original soil surface. The chemistry of the Citrus Grove soils also departed from that of the local native soils in several ways. Aluminum content, as measured by CDT extraction, was approximately 10 times higher in the Ap horizon of the Citrus Grove than in the A horizon of the nonmidden soils. It decreased with depth in Test 2, in a reversal of the normal pattern of accumulation of aluminum with depth due to weathering (Birkeland, 1984). This also suggests the piling of fresh-or at least Al-enriched-sediments on the original surface of the Citrus Grove, especially on the highest point, which was sampled at Test 2. A plausible hypothesis for the unusual physical and chemical character of the Citrus Grove A horizon is that it is derived from the poorly drained soils flanking the preexisting, now buried, sand ridge. Spoil from excavation of the Calusa canals would have contained fine particles and aluminum, particularly the subsurface B horizons transected during excavation. Spoil piled on the low sand ridge would form a dark, aluminum-enriched horizon. Calcium content of the Citrus Grove soils was higher in all cases than the native soils. In particular, the Ap horizon of Test 2 and the AE through EBw horizons of Test 4 contained 3500 to 4000 mg.kg-’, compared with 20 to 70 mgekg-1 in the A horizon of the native soils. Extractable phosphorus contents in the same provenances showed similar proportional relationships. The Ca and P contents of the Citrus Grove soils indicate an augmentation of these elements over normal levels. More interesting is the pattern of accumulation in Tests 2 and 4. High contents of these elements occur in Test 4 below th Ap horizon, in what is indicated by soil morphology to be a former surface horizon. Similar high contents of the same elements are found in the upper (Ap) horizon on the highest part of the Citrus Grove. These proveniences correspond to shell and bone accumulations encountered in an independent auger survey of the Citrus Grove. The findings here corroborate those results, although no actual faunal or cultural remains were recovered from the soil samples. Based on the evidence it appears that the Citrus Grove was a natural feature such as a beach and berm, wave-cut into well-sorted fine sands, with loose sands heaped upon it. The accumulation of fine particles and aluminum in the mounded Ap horizon suggests that the source of that horizon was the poorly drained soils that surround the Grove, and particularly the subsurface horizons, which would

56


have been transected during excavation of canals and other waterways in the vicinity of the Citrus Grove. b. Smith Mound This sand burial mound is located to the north of the Citrus Grove in a lobe of Matlacha soil (Henderson, 1984). The human-made Matlacha soil was probably based on either the Myakka or the Immokalee soil (an Arenic Alaquod), according to the projected soil map unit boundaries in Fig. 21. Although there are no round remnant Holocene sand features in the vicinity of Pineland today, the upper boundary of the deepest Bw(') horizon from the three squares forms a convex surface that rises below square B-2 (Fig. 20). All of the horizon boundaries are subparallel and angled upward, as if following the rise traced by the underlying Bw' horizon. Three soils are depicted in the figure: (1) an Inceptisol forming in the disturbed surface spoil, (2) a Spodic Quartzipsamment with weak Bw development capped by a dark gray buried A horizon, and (3) a Spodosol with an intact A horizon underlain by a human burial intruding into what appears to be an E'-Bw" and Err-Bw'" bisequum. The continuous gray former surface horizon of that soil is evident in field photographs, as is a clear break in the E'/Bw" boundary leading to the burial at 2.18 m below the ground surface. Four additional sets of samples taken on the opposite side of the mound corroborate a buried A or darker horizon at approximately the same depth as the former surface of the burial-containing soil. Ceramic analysis (Cordell, 1995) showed an increase in two ceramic styles, and the first appearance of Weeden Island Style ceramics, in the excavation levels corresponding to the burial-containing soil. The burial has been 14C dated to A.D. 1020-1170 (Beta-72995) (approximately 800 years B.P.), so the A" horizon of the Spodosol had a maximum of 800 years to reform over the intrusion. That is of course an overestimation of the time that soil was left subaeirally exposed, since the Spodic Quartzipsamment that developed in sediments mounded over it would have taken some part of the 800 years to form. The very thin and weak appearance of the A" horizon, generated in a region which could have supplied abundant organic plant remains to the soil surface, suggests that minimal time was allowed for the development of that horizon. The chemical characteristics of the two buried soils differed considerably. Organic carbon, calcium, magnesium, potassium, phosphorus, and aluminum contents decrease by about 50% between the Bw' of the upper Spodic Quartzipsamment and the A" horizon of the lower one. The relative enrichment of the overlying soil may indicate that the soils heaped upon the burial-containing soil were derived from the surrounding low-lying areas, as was hypothesized for the Citrus Grove. Spoil from lake or canal construction, particularly as it cut into the deepest Bw horizon at the water, was a likely source of these sediments. Additional augering revealed the presence of yet another set of soil horizons below the burial soil, beginning at about 297 cmbs. The difficulty of interpreting


57

augered soil samples near the zone of saturation makes it unclear whether those lower horizons are another entire soil, beginning with a buried surface horizon (A”‘-E”’-Bw””), or whether they are a third sequum of the burial soil, separated from the upper two sets of horizons by a diffuse E horizon. Whatever the origin and genesis of this fourth set of horizons, the chemical and physical data, along with photographic documentation, clearly indicate that Spodosol was an intact soil when disturbed by an intrusive human burial. It remained subaerially exposed long enough for a thin A horizon to reform and was then covered by sediments that developed into a Spodic Quartzipsamment. The chemical nature of that overlying Spodic Quartzipsamment, its topographic position, and the absence of sediments of like thickness and horizonation anywhere else on the site, lead to the conclusion that the sediments were probably derived from the immediate vicinity and that they were human-deposited.

5. Summary Soil science techniques and analyses were used to distinguish natural soils and landscape features from anthropogenic ones at Pineland. Those techniques, applied to the Citrus Grove area, revealed a body of parallel subsurface horizons whose boundaries differed from the surface topography. Chemical and grain-size analyses indicated a core of reworked dune sands in the form of a wave-cut beach or terrace mantled by a surface (Ap) horizon unlike local natural A horizons. This study suggests that the Ap horizon was derived from the surrounding landscape, based on aluminum content, grain-size ratios, and patterns of calcium accumulation. Smith Mound was underlain by a series of convex, subparallel horizons and was composed of at least three soils of varying maturity. A minimally developed Inceptisol forming within surficial spoil mantled a Spodic Quartzipsamment with weak Bw horizonation. Below this was a third, more strongly developed soil incorporating a human burial. The A horizon of this soil was complete, having reformed subsequent to the intrusion of the burial into the E’ and Bw“ horizons. A fourth set of horizons below the burial-containing soil was found, but was neither correlated with that soil nor identified as the beginning of a fourth complete soil due to the saturated condition of the samples.

G. PEDOARCHAEOLOGICAL ANALYSIS OF A PREHISTORIC SHELL-BEAFUNG ISLAND,FLORIDA 1. Introduction

A.B.’s Midden (8-Lv-65) is located on North Key; one of a series of small islands comprising the Cedar Keys National Wildlife Refuge (Fig. 22). North

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Figure 22 Location of the Cedar Keys region.

Key has remained in near pristine condition throughout historic times and has been virtually insulated from human influence. A.B.'s Midden, a prehistoric shell midden, fringes the southeast shore of the island (Fig. 23) There is no evidence to suggest that the soils in A.B.'s Midden have ever been plowed or chemically altered through fertilization or liming. A multidisciplinary research team (Seahorse Key Maritime Adaptations Program) was initiated to investigate the archaeological resources in the Cedar Keys

LOCATION MAP

SEABREEZE I.

Kilometers

RAULESNAKE I.

NOR7 ATSENA OTlE KEY

Gardiner's Midden

~

~~~

~~~

Figure 23 Cedar Keys region location map.

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region. The investigation included the excavation of two sites in addition to A.B.’s Midden. The objectives of this study were to determine the type and extent of human influence on soil formation by characterizing and comparing the site’s archaeological soils to the island’s natural soils. Using the pedologic information, it was anticipated that site perimeters, intense occupation areas, and a site development would be identified.

2. Methods a. Field Methods Six conventional 1 x 1-m excavation units were placed across the site, following the crest of the midden along the natural coastline (Fig. 24). The unexpected thickness of the cultural deposits required the enlargement of three of the test units (1, 3, and 6) to 1 X 2 m. Zones of stratigraphically uniform material within the units were noted and excavated in 5-cm levels. If a soil horizon could be differentiated within a zone, each horizon was excavated separately. Every unit was excavated to a minimum of 25 cm below the last cultural material recovered, except when intrusion of the water table made this impossible. Soil samples for chemical and particle-size distribution analysis were taken from both upper and lower areas of thicker zones. Two pedons of the naturally occurring soil Zolfo sand (sandy, siliceous, hyperthermic Grossarenic Entic Haplohumod) were sampled as controls to examine the midden’s influence on soil development. b. Laboratory Methods Organic carbon content was determined for those samples which were estimated to have more than 1 % organic carbon. A modified version of the WalkleyBlack method was used in the determinations (Nelson and Sommers, 1982). The pH was measured in a 1:1 suspension of water and soil. Total phosphorus (TP) determinations were made using the alkaline-oxidation method (Dick and Tabatabai, 1977). A variation of Chang and Jackson’s (1957) method was used to fractionate phosphorus. Particle size was determinated using the pipette method (Day, 1965).

3. Results and Discussion a. Archaeological Interpretations Analysis of cultural and zooarchaeological samples from A.B .’s Midden indicated that the site was occupied year-round, but intermittently. There was no evidence to support the presence of a permanent occupation. Stratigraphy indicates the site was used less intensively in its initial formation stages. Discrete

Figure 24 Location of excavation units at A.B.’s Midden.

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depositional episodes were distinguished in the zones beneath the dense shell midden. The dense midden component of the site was separated from the previous component by a zone containing very little shell which was interpreted as being a period of very low activity, possibly of site abandonment. Following this period, the site became heavily used as a refuse area, resulting in the shell overlay. The dense accumulation of shell at A.B.’s Midden can be viewed as a discontinuity in the soil, essentially functioning as new parent material for the formation of soil above it. It is also suggested that the presence of the midden has altered the development of the island’s geomorphology through differential erosion patterns. Accelerating shoreline erosion over the last half century, coupled with the effects of major humcanes (i.e., Elena in 1982) severely impacted North Key. The shell midden sites may have helped stabilize the shoreline by retarding the effects of marine erosion. b. Morphological and Chemical Features of Archaeological Soils Morphological features in the excavation units revealed characteristics typical of human-altered soils. Horizons within site boundaries had deeper and darker matrix colors than those in the naturally occurring soils. These colors ranged from very dark gray (lOYR 3/ 1) in the upper 10-cm horizon of some units to yellowish brown (IOYR 5/8) below 2 m. Fine and medium sands made up the largest percentages of the sand fraction in A.B.’s Midden soils. Samples from the dense shell midden components of the site revealed larger percentages of very coarse and very fine sands than from the below the shell deposit (Table IV). These differences reflect the discontinuity between the shell midden component and the occupation levels. Units closer to the edge of the site exhibited particle-size distributions closer to those of the natural soils (Table V). Clay and silt contents in the archaeological soils were higher than in North Key’s natural soils and were highest in the upper shell zones. The large percentages of silt and clay at such shallow depths would be unusual for the island’s natural sandy soils and, thus, are clearly a result of human activity. Organic carbon content of the dune sand parent material of North Key is naturally very low (< 1%); therefore, organic carbon levels were predicted to be very low across the site. However, the highest amount of organic carbon was 3.77% at a depth of 75-80 cm in Unit 1 (Table VI). This reflects the intensity of occupation during these depositional episodes, and is substantiated by the large amounts of phosphorus (Table VII). Organic carbon levels dropped in zones beneath the shell midden, but remain relatively high. The pH levels across the site were significantly higher than those of the natural soils (6.3 to 7.4) and varied. The average site pH was 8.1. In midden soils the pH

Table IV Particle-Size Distribution (%) in Selected Archaeological Soils at A. B.’s Midden

Sand

VCa

C

M

F

VF

Silt

Clay

20-25 45-50 75-80 100-105 130-135 205-210

87.3 80.2 77.6 94.4 94.8 95.9

5.5 4.6 3.0 1.o 0.4 0.4

12.2 11.5 9.6 7.7 5.9 8.4

44.6 23.5 29.6 44.9 43.2 44.7

32.2 23.7 43.4 43.5 49.0 45.3

3.4 25.8 11.9 3.2 1.5 1.2

6.3 9. I 12.6 2.9 5.3 3.3

6.4 10.7 9.8 2.8 0.0 0.9

50-55 70-75 95-100 125-130

89.9 98.3 98.4 98.2

0.9 0.3 0.3 0.3

9.0 6.7 7.2 7.1

48.1 43.2 44.5 45.9

38.6 48.1 46.4 45.5

2.7 1.6 1.5 1.5

6.9 0.9 0.0 0.7

3.2 0.8 1.6

10-15 25-30 85-90

89.0 82.8 86.2 97.1 99.7 99.8 98.4 97.8

2.3 2.4 3.1 0.4 0.3 0.4 0.4 0.4

6.5 5.2 8.5 5.8 6.0 6.1 6.4 5.4

40.6 21.7 25.9 41.3 40.7 39.4 45.1 42.0

45.8 50.9 52.3 49.6 50.0 50.8 46.3 50.2

3.5 20.0 9.9 2.6 2.1 3.2 I .7 2.0

5.8 6.9 3.0 3.5 0.5 0.3 0.3 0.5

5.2 10.3 10.9 0.0 1.8 0.0 1.5

85-90

94.1 97.2 98.4

0.5 0.2 0.3

7.4 8.0 7.5

48.4 50.6 48.0

41.8 39.8 42.4

1.6 1.4 1.8

3.4 0.0 0.9

2.5 2.8 0.7

40-45 80-85 95-100

98.2 98.2 95.9

0.3 0.2 0.2

1.4 6.8 6.5

49.7 49.0 41.4

40.9 42.0 44.0

1.8 1.7 I .9

0.0 1.8 4.1

1.8 0.0 0.0

5-10 20-25 40-45 75-80 100-105 125-130 140-145 150-155 185-190

90.7 94.7 84.8 95.1 95.8 93.2 91.6 97.9 97.6

0.8 1.2 3.2

5.0 5.3 8.2 10.2 4.6 5.4 5.3 4.7 6.2

43.4 44.2 18.9 46.8 35.6 35.5 38.5 36.1 43.3

46.1

1.3 3.8 20.6 2.4 5.5 6.5 3.5 3.5 1.8

3.9 0.0 7.6 2.4 3.0 5.8 2.4 2.1 2.4

5.4 5.3 1.7 2.5 1.2

Depth (cm) Unit 1

B C D F G I Unit D E F G Unit B B E F F G J J Unit B C C Unit B B B Unit A B C E F H I J K

2

I .o

3

105-110

110-115 135-140 180-185 195-200

1.9

4 15-20 50-55

5

6

1.o

0.4 0.2 0.3 0.3 0.3

a Very coarse (VC), coarse (C), medium

44.2 48.2 39.5 53.8 52.3 52.4 53.7 41.1

(M), fine (F), and very fine (VF) sand fractions.

I .o 0.0 0.0 0.0


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Table V Particle-Size (%) and Total Phosphorus (TP) Distribution in Natural Soils at A.B.’s Midden

Horizon

A1 A2 E2 E2 E2 E3 E3 E4 Bh BC Al

A2 E2 E2 E3 E3 E3 E4 Bh Bh

TP

Depth (cm)

Sand

VCa

C

M

F

VF

Silt

Clay

(pg/g)

0-10 40-50 80-90 100-105 105-115 120- 130 135-145 155-166 180- 190 225-235 25-35 45-50 70-80 90- 100 100- I 10 110-120 135-145 150- 160 200-210 2 15-220

92.8 96.9 99.1 97.2 97.7 97.5 97.4 97.7 99.1 98.5 97.9 98.2 98.4 98.8 98.3 97. I 97.4 98.5 99.1 99.2

0.8 0.4 0.4 0.6 0.4 0.5 0.7 1.o 1.7 0.8 0.4 0.3 0.2 0.4 0.4 0.7 0.6 0.9 0.5 0.3

7.0 6.4 7.3 8.4 9.1 8.2 8.4 9.2 9.0 5.2 8.3 8.6 8.4 8.7 8.5 9.2 9.6 12.5 10.8 11.6

49.0 45.9 48.9 50.5 49.6 46.2 46.6 48.5 44.8 42.2 50.0 51.4 51.7 48.2 48.6 46.4 46.2 47.5 48.0 52.4

41.3 46.0 42.2 39.3 40.0 43.9 43.3 43.4 43.7 50.8 39.2 37.8 38.0 40.9 41.5 40.2 41.9 37.2 39.2 39.6

1.3 1.3 1.2 0.9

3.1 0.4 0.0 0.8 0.0 0.2

4.0 2.8 0.9 2.0 2.3 2.3 1.2 1.2 0.9 1.5 0.0 2.8 0.0 0.9 1.5 2.5 2.3 0.0 0.0 0.8

145 160 100 133 139 166 180 124 180 99 78 108 64 67 112 215 316 254 364 304

a Very coarse

1.0 1.1

1.0 1.0 0.8 0.7 1.8 1.7 1.5

1.8 1.4 1.6 1.8 1.8 1.2 1.1

1.5

1.2 0.0 0.0 2.1 0.4 1.6 0.0 0.3 0.4 0.3 1.6 0.9 0.0

(VC), coarse (C), medium (M), fine (F), and very fine (VF) sand fractions.

tended to increase with depth, which is the result of weathering and leaching of Ca from the thick overlying shell zones. Total phosphorus levels were extremely high across the extent of A.B.’s Midden, but varied with depth, reflecting changes in depositional episodes and pedogenesis (Table VII). As expected, TP levels in the dense shell midden component of the site were the highest due to the presence of large quantities of food refuse, primarily bone. The soft parts of mollusks are particularly high in P; however, their shells contain almost no P, and do not contribute to the high P levels found in the shell midden (Cook and Heizer, 1965; Carr, 1982). Phosphorus levels dropped dramatically as the midden thinned out. It should be noted that P levels in natural soils vary widely. Therefore, concentrations from archaeological sites from differing regions should ideally be correlated with local, natural concentrations before being compared. For example, phosphorus levels in the natural soils ranged from 64 mg/g in the eluvial (E) horizons to 364 mg/g in the spodic horizon (Table V). In the natural soils, the P increased gradually with depth to the spodic horizon, then decreased.


65

Table VI Organic Carbon Content in Selected Archaeological Soils at A.B.’s Midden

Depth Zone Unit 1 B C

D F Unit 2 D Unit 3 B B E F

F

(cm)

% Organic carbon

20-25 45-50 75-80 100-105

3.23 3.06 3.77 1.20

50-55

1.76

10-15 25-30 85-90 105-1 10 110-1 15

3.42 3.37 2.71 2.39 1.43

5-10 20-25 40-45 75-80 125-130

3.17 2.95 2.77 3.62 0.76

Unit 6 A

B C

E H

4. Summary

The pedological analysis of A.B.’s Midden contributed to the overall understanding of site use and formation processes, yielding data on the impact of prehistoric human activities on pedogenesis. Morphologically, site zones were evidenced by deeper, darker colors developed from the accumulation of organic material during occupation. Stratigraphic breaks indicated discrete occupation sequences. Textures from the site suggest the A.B.’s Midden was used irregularly during its early formation, allowing natural accumulation of aeolian or storm-driven sands to separate occupations. Later, more intense occupations resulted in the rapid accumulation of shell zones, deposited sequentially with little accumulation of soil. Although sand dominated both natural and site soil textures, archaeological zones were higher in fine and very fine sand, while the natural soils had more medium sand. Some archaeological zones were high in silt- and clay-sized fractions.

Table VIl Total and FractionatedPhosphorus Contents in Selected ArchaeologicalSoils at A.B.'s Midden Zone Unit 1 B C F G H I Unit 2 D E F G Unit 3 B B E F F G J J Unit 4 B C C Unit 5 B B B Unit 6 A B C E F G

H I

J K

Depth (cm)

HzOu

Al'

Fe"

Caa

20-25 45-50 100-105 130-135 170-175 205-210

54 48 75 6 8 9

906 2000 1190 14 38 23

76 186 65 11 23 22

50-55

93 40 30 31

1443 32 25 36

55 138 I28 71 56 36 19 18

P subtotal

TP

3,264 21,208 1,326 12 40 22

4,300 23,541 2,656 43 100 76

4,778 26,157 2,951 48 110 84

269 6 14 37

7,296 89 18 18

9,101 167 86 122

10,112 185 96 135

372 1155 2162 118 52 57 54 60

85 128 150 68 70 74 51 89

1,977 7,992 9,422 350 91 57 51 43

2,489 9,413 1 1,863 607 269 224 175 210

2,766 10,459 13,118 674 299 249 195 322

85-90

92 65 19

198 52 44

59 29 28

266 59 22

614 204 113

682 227 125

40-45 80-85 95-100

2 4 5

3 15 11

2 3 9

8 0 10

14 22 35

16 24 39

172 94 115 38 57 80 88 15 39 34

948 1259 I340 475 111 67 626 100 118 22

128 75 122 23 37 41 60 61 26 23

1,635 3,420 11,529 647 417 200 1,126 54 1 222

2,885 4,847 13,106 1,183 62 1 388 1,901 716 405 130

3,206 5,386 14,562 1,314 690 43 1 2,112 796 449 144

70-75 95-100 125-130 10-15

25-30 85-90 105-1 10 110-1 15 135-140 180-185 195-200 15-20 50-55

5-10 20-25 40-45 75-80 100-105 110-1 15 125- 130 140-145 150-155 185-190

50 ~~

(I

Renodes water-soluble (HzO), aluminum (Al), iron (Fe), and calcium (Ca) phosphates.


67

The higher organic carbon content of the site produced morphological and chemical changes in the diagnostic spodic horizon compared to the spodic horizon occurring in the natural soils. Within site boundaries, particularly under the most densely occupied areas, the spodic horizon exhibited darker colors, firmer consistence, more developed structure, and a higher P content than samples of the spodic horizon in the natural soils. The natural acidity of North Key’s soils was raised by 1 to 3 units within the site perimeter, caused by large concentrations of calcium carbonate in shelldense horizons of the site. Total P values indicate relative occupation intensity of the site through time, and suggest that dense shell zones were associated with habitation areas. This interpretation is supported by zooarchaeological and sclerochronological assessments of A.B.’s Midden. The presence of shell-bearing sites on North Key, including A.B.’s Midden, has had a dramatic effect on the erosional history of the island (Borremans, 1990), while the addition of human-introduced nutrients to the soil environment has altered specific aspects of its character, including the expression of diagnostic subsurface horizons in the soil.

VI. SUMMARY Fundamental changes in the focus of modem archaeological research have resulted in the integration of the earth sciences with archaeology. This new focus emphasizes the synthesis of cultural and environmental information into a cohesive interpretation of human ecology, including settlement patterns, land use practices, and evidence of human impact on soils and landscapes. This chapter defines the role of the emerging disciplines of geoarchaeology, pedoarchaeology, and archaeometry in the archaeological context. It reviews recent works in pedoarchaeology at a wide range of sites, some with a broad, regional landscape approach and some with a more narrowly focused intrasite view. The foundation of modem earth science disciplines includes traditional soil science techniques: soil morphological descriptions, particle-size distribution analysis, chemical element distributions, clay mineralogy, landscape analysis, and micromorphology. These techniques are outlined, as are uses of soil maps and modem geophysical tools such as resistivity, electromagnetic survey, and ground-penetrating radar. Paleosols are defined and their use in the interpretation of archeological sites is illustrated. The use of soil morphology as a stratigraphic marker in both terrestrial and underwater sites is also discussed. The synthesis of pedology and archaeology is illustrated in a series of case studies selected from the authors’ works. Topics as diverse as the content of toxic

68


metals in garden soils at Hadrian’s Villa, the role of soil resources in the collapse of Mayan culture in Guatemala, relationships of alluvial soil sequences to early archaeological sites in the southeastern United States, the origin of monumental sand earthworks in southwest Florida, and the effects of middens on the soils and geomorphology of a coastal Florida island are addressed. Though the analytic techniques used in these studies are basic, the information they yield is as varied as the terrains and cultures they describe. The first two objectives of this chapter-to stress the mutual advantages of new interdisciplinary efforts in earth sciences and to summarize unique contributions of pedology to archaeology-are met in the examples of the work itself. The third objective-to encourage other pedologists to become involved in archaeological studies-can only be evaluated by future chapters such as this.

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PHOSPHATE ROCKSFOR DIRECT APPLICATION TO SOILS S. S. S. Rajan', J. H. Watkinsonl, and A. G. Sinclair2 IAgResearch, Ruakura Agricultural Research Center, 3 123, Hamilton, New Zealand LInvermay Agricultural Center, 50034, Mosgiel, New Zealand

I. Introduction 11. Reactivity of Phosphate Rocks

A. Definition of Reactiviry B. Measurement of Reactivity C. Mineralogy and Reactivity 111. Measurement of Phosphate Rock Dissolution in Soil A. Measurement in Acid Soils B. Measurement in Calcareous Soils IV. Factors Affecting Phosphate Rock Dissolution in Soil and Availability to Plants A. Factors Affecting Rate of P Release from Phosphate Rock Applied to Soil B. Factors Affecting Plant Availability of P from Dissolved Phosphate Rock V. Modeling the Rate of Phosphate Rock Dissolution in Field Soil A. Kirk and Nye Model B. Watkinson Model VI. Agronomic Effectiveness of Phosphate Rock A. Determining Agronomic Effectiveness B. Quantifying Comparative Performance of Phosphate Rocks C. Residual Effectiveness of Phosphate Rocks VII. Economics of Using Phosphate Rock Fertilizers VIII. Soil Testing Where Phosphate Rocks Are Used A. Current Research B. Future Research Needs IX. Amendments to Phosphate Rocks A. Composting with Organic Manures B. Phosphate Rock-Sulfur Assemblages C. Partially Acidulated Phosphate Rocks X. Concluding Remarks References

77 Adilnces in Agrnnmny, Vobine Y7

Copyright 0 1996 by Academic Press, Inc. All rights of reproduction in any form reserved

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S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

I. INTRODUCTION Interest in phosphate rocks (PRs) as direct application fertilizer stems from the facts that (i) per kilogram of P, PR is usually the cheapest fertilizer; (ii) direct application, with or without amendments, enables utilization of PRs which are unsuitable for manufacturing phosphoric acid and other soluble fertilizers such as triple (TSP) or single superphosphate (SSP); (iii) because PRs are natural minerals requiring minimum processing they are environmentally benign (Schultz, 1992); and (iv) PRs could be more efficient than soluble fertilizers in terms of recovery of phosphate by plants, even for short term crops in soils where soluble P is readily leached, as in sandy soils (Yeates, 1993) and possibly for long-term crops also in other soils (Rajan et al., 1994).

In spite of this PRs are not widely used as direct application fertilizers. The reasons are: (i) not all soils and cropping situations are suitable for use of PRs from different sources; (ii) the large number of factors controlling their dissolution in soil and availability to plants coupled with the inability to predict their agronomic effectiveness in a given soil climatic and crop situation; and (iii) their lower P content compared with high-analysis fertilizers which make PRs more expensive at the point of application if long-distance transportation is required. It has been more than 15 years since the last comprehensive review on PR for direct application was published by Khasawneh and Doll (1978). More recently Hammond er al. (1986b) reviewed the use of PRs and amended PRs in tropical soils. Since then considerable progress has been made in several areas of PR research. This includes a better understanding of the factors that affect PR dissolution, critical evaluations of the methods used to measure PR dissolution in soils, and its availability to plants and development of mechanistic models to predict the dissolution and availability to plants of PR-P. We found the literature on PR research rather overwhelming. In this chapter, instead of reviewing the numerous published reports, we will concentrate on the advances made on the fundamentals of PRs dissolution and their agronomic use, with specific examples. We have mostly quoted references published since 1978, although for the sake of continuity and comprehensiveness we have also cited some earlier publications. The philosophy behind this review will be that, paraphrasing Nye (1992), if we really understand the fate of PRs applied to soil and

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develop mechanistic models describing their reactions we should be able to predict their effectiveness for any combination of soil, climate, crop, and management. This should enable decisions regarding PR use to be made without time-consuming and expensive field trials in every location.

11. REACTIVITY OF PHOSPHATE ROCKS

A. DEFINITION OF REACTIVITY There are two occasions when some measure of the agronomic performance of PRs would be desirable. One is the performance of a given PR when added to a given soil/plant system, and the other is the relative performance of a number of PRs when added to a soil suitable for using PRs. It seems preferable to restrict the usage of reactivity to the second occasion because it will then depend on only PR properties. The first would also require a knowledge of whether the soil/plant system would be suitable for even the most reactive PR. Although Khasawneh and Doll (1978) indicated that the reactivity of a PR is related to agronomic effectiveness, they did not define the term explicitly. Rather it was discussed in relation to measured PR properties, such as the relative amount dissolved in a particular organic acid solution. More recently Sinclair et al. (1992) described reactivity as the ability of a PR to release P, or the rate of release of P to soil and plant; Rajan et al. (1 992) defined it as the magnitude and rate of dissolution of a PR. Building on these ideas, it is proposed that Reactivity is the combination of PR properties that determines the rate of dissolution of the PR in a given soil under given field conditions. Reactivity is defined as a property of the PR, and deliberately excludes properties of soils and plants. It is a direct measure of the amount of PR that dissolves in a given time, and hence is related to agronomic effectiveness. Sometimes there is apparently no relation because it is masked by other factors (see Section 1V.B). Also, the relation is discontinuous since the relative agronomic effectiveness (RAE) is not increased by a decrease in size below about 0.15 mm (Khasawneh and Doll, 1978), even though the amount dissolved will still continue to increase with decreasing size (Kanabo and Gilkes, 1988a,b,c,d). Reactivity is also defined in terms of the dissolution in a soil in the field, not in the laboratory or glasshouse, i.e., under the most appropriate conditions for providing information on the use of PR as a fertilizer. Reactivity as defined is a kinetic property, which is also consistent with the use of PR as a fertilizer in the open system of a field soil. Any method of measurement must therefore avoid the establishment of concentrations approaching a

80


saturated solution, or conditions approaching a quasi-equilibrium. Such equilibrium values would exclude purely kinetic properties, particularly surface area, which is an important factor in the dissolution rate modeling of RPRs (Kirk and Nye, 1986a,b; Watkinson, 1994a). The definition of reactivity should not be based on methods, as at present (Khasawneh and Doll, 1978), such as the amount extracted by a solution of an organic acid. The emphasis should be on dissolution in field soil, and any method can only give an estimate of this. Relative reactivities could be measured from the amount of PR remaining in a field soil at a set time after application. The relative amounts dissolved would give an indication of their relative effectiveness as P fertilizers. However, such a method would require appreciable time and resources, and so a laboratory method that provides an acceptable estimate would be desirable. As will be shown later, the PR reactivity is the combination of several properties measureable in the laboratory, including particle size. As soils become more acidic (or plants more acidifying), differences in PR reactivity become less important; conversely, as soils become more neutral, amounts dissolved are less and differences in reactivity become more important (Khasawneh and Doll, 1978). Consequently it is important to choose a soil for measurement of dissolution that gives a good range of amounts dissolved from a range of PRs. The above definition of reactivity is useful in that it is precise and the property can be measured specifically, i.e., by measuring residual PR in a soil (Section Ill), or calculated from relevant PR properties using mechanistic dissolution rate models (Section V).

B. MEASUREMENT OF REACTMTY Several types of measurement have been proposed, more particularly: dissolution in soil; dissolution in acid or salt solutions; measurement of crystal unit cell dimensions; and calculation of dissolution in soil using parameters from mechanistic dissolution models. Since rapid results for reactivity measurements are required, only aqueous extractions in the laboratory have been used routinely (Chien, 1978, 1993; Rajan er al., 1992; Sinclair et al., 1992), except for a recent laboratory test based on fundamental PR properties from dissolution rate model parameters (Watkinson, 1994~). 1. Principles

Most existing methods are purely empirical (although dilute organic acids have been used to simulate the action of root exudates), largely because the concept of


8I

reactivity has not been defined explicitly. Without an exact definition, methods based on precise principles are not possible. Generally, reactivity is equated with the solubility of the PR (Khasawneh and Doll, 1978; Chien, 1993), but modified by other factors, notably free carbonate content, crystal and particle size and porosity, and intermixture with silica (Chien, 1993). However, the term solubility itself is used in two senses: in the usual thermodynamic sense of the equilibrium concentration (Chien, 1993), but mostly as the rate of solution into one of several extractants (Chien, 1993). Furthermore, different extractants give different results. For example, Watkinson ( 1 9 9 4 ~ pointed ) out that citric acid apparently dissolves more fluorapatite, but less francolite than formic acid (Rajan et al., 1992; Chien, 1993) (Fig. 1). Largely because of these difficulties Sinclair et al. (1992) put in a plea for an approach to reactivity measurement based on fundamental properties of the PR. Watkinson (1994~)attempted such an approach incorporating the above definition of reactivity, a dissolution rate model (Watkinson, 1994a), and fundamental properties of the PR measureable in the laboratory.

2. Methods a. Empirical Since the review by Khasawneh and Doll (1978), no new chemical methods seem to have been proposed, only improvements to existing methods (Chien, 1993). Those most commonly used are heated neutral ammonium citrate, 2% citric acid, and 2% formic acid (Chien, 1993). Less common methods include absolute citrate solubility and acid ammonium citrate (Chien, 1993). To overcome the problem of appreciable impurity in the PR, adding a constant mass of P Formic-P Citric-P

a

-

4:

CAs FAs

~

0

A

0

A

10 20 30 40 50 PR dissolved in one year, DRF (“YO)

0

Figure 1 Relationship between citric-P and formic-P for carbonate- (CAS) and fluor- (FAS) apatites as shown plotted against dissolution rate function (DRF) (Rajan ef a / . . 1992; Watkinson, 1995).

82


or apatite to the extracting solution has been proposed (Axelrod and Gredinger, 1979). The preferential reaction of free carbonate with the extractant was overcome by measuring the amount of PR dissolved in the second rather than the first extract (Chien and Hammond, 1978; Mackay et al., 1984b; Mishra ef al., 1985). Rajan et al. (1992) recognized the importance of the geometric surface area in rate of solution methods, and adjusted the shaking time to include an effect of initial area or particle size. In contrast, Khasawneh and Doll (1978) treated geometric area (i.e., particle size) as being of lesser importance than the much larger specific area, which includes internal surfaces, but without supporting evidence. (Later discussion, Section V, will show that on the diffusion-controlled model, internal surface area is of small importance compared with geometric area.) b. Theoretical Olsen (1975) found that the dissolution kinetics of PRs into EDTA solution, the rate of which was increased through the chelation of the dissolved Ca2+, was consistent with a second-order reaction rate. However, he did not further test the model or use fundamental PR properties as variables. Watkinson (1995) proposed a Dissolution Rate Function (DRF), using fundamental properties of RPRs measurable in the laboratory, for estimating the reactivity of PRs. The DRF represents the amount of PR dissolved in a standard soil in a given time. It was derived from a simple rate equation in a dissolution rate model (Watkinson, 1994a,b), and the standard soil represented the properties of an average New Zealand pastoral soil used for direct application of PRs. In this soil 30% of Sechura (Peruvian) PR (also referred to as Bayovar PR) of particle size 0.075-0.15 mm dissolved in 1 year. For a PR with particles of the same diameter, do, the amount dissolved in time, t , the DRF was given by (Watkinson, 1994c) DRF

=

1 - [ I - 8D,t(C,/F)/(pdo2)]”2,

(1)

Where D, is the mean diffusion coefficient for phosphate in soil (the value for the standard soil mentioned above is 0.5 cm2 year-’), C , is the phosphate concentration at the PR surface (strictly C , - C s , where Cs is the phosphate concentration in the bulk soil, but C , Cs), p is the PR particle density, approximately 3.2 g cmP3, and F is the fractional P content of the PR. For convenience, the time, t , was 1 year, while values for do and F were measured using standard methods. The value for C , was measured as the equilibrium phosphate concentration of the PR in a simulated soil solution of constant pH (held at pH 5.5 using an automatic titrator) and set initial values of calcium (0.5 mM) and ionic strength ( 5 mM) (Watkinson, 1994~).This solution took into account the calcium from the soil and that dissolved from the PR and the calcite impurity in the PR at pH 5.5. Congruent dissolution was assumed because of the very low solubility of PRs (Kirk and Nye, 1986a), and the large ratio of solution to solid of 500: 1 used (Watkinson, 1994~).


83

The DRF can therefore compare a PR of high solubility and large size with a contrasting one of low solubility and small size. A large pH buffering capacity in the soil was assumed in that all the alkali generated by the dissolving lattice phosphate and carbonate was neutralized by the automatic titrator in holding the pH constant. This is generally true for New Zealand pastoral soils (Edmeades er al. 1985). The DRF for a fertilizer mixture with sieve analysis of particle sizes resulting in n successive sieve fractions, each of size range bi+, to bi and of weight n

wi = I , is given by,

fraction wi, where i= I

n

DRF = 1 -

wJ(2

+ at/b?)(b: -

- (2

+ at/b:+,)(b?-?_,- at)”2

i= I

+ ( 3 ~ Z ) ( s i n - l ( f i / b ~-) s i n - 1 ( f i t / b ~ - ~ ) ) 1 / 2 (-b ~bi-,),

(2)

where a = 8 D, ( C , / F ) / p , and after 1 year ( t = 1) the smallest particle, b,, has not dissolved. If the smallest particles have dissolved, additional sets of similar equations are required (Watkinson, 1994c). The DRF for 1 1 PRs, ground and unground, correlated with published values of relative response of ryegrass as the test plant in three soils (Rajan et al., 1992) (Fig. 2) at least as well as acid-extractable P using citric and formic acids (Watkinson, 1994c, 1995). A comparison of the ground and unground PRs in a plot of RAE against the solubility function, C,/F, i.e., only size excluded, showed the effect of grinding (particle size) on RAE (Fig. 3). The difference between ground and unground PRs increased with increasing solubility (Watkinson, 1994~).Conversely there was a neglible difference at very low solubility, which is consistent with evidence cited earlier that it was not possible to convert an unreactive PR into a reactive PR by grinding it to a very small size. These data 1201

0

I

I

1

I

10 20 30 40 Lab test, DRF (“7)

1

50

Figure 2 Relative response of unground (UG)and ground (G) PRs (ground Sechura PR = 100) (Rajan ct a / . , 1992). in relation to the Dissolution Rate Function (DRF) (Watkinson, 1995).

84


? = 0.87 Ground

1201

:

n

0.91 Unground

r I

I

I

I

I

1

10 20 30 40 50 60 RPR solubility, CR/F (mg L-l)

figure 3 The effect of grinding on the relative response values of PRs (0,unground, and 0, ground) (Rajan et a/.. 1992) having different levels of solubility, CRIF, at pH 5.5 (Ca = 0.5 mmol liter- I ) (Watkinson, 1995).

indicate that DRF was applicable to fluorapatites with and without carbonate substitution and of different size distributions. The effect on dissolution rate of grindinig, in three stages, a PR with size distribution typical of North African PRs has been calculated from the model (Watkinson, 1994b), and is shown in Fig. 4. The soil properties are such that 30% of Sechura PR would be dissolved in the first year. The economics of grinding could be estimated from such data (Sinclair et al., 1990a,b).

C. MINERALOGY AND REACTIVITY 1. Fluorapatite Fluorapatite (FA) is much less soluble than hydroxyapatite (HA) or even the carbonate substituted fluorapatites (CAs) (Khasawneh and Doll, 1978). This very 1001

e l m m (unground) \

8.25

---I

0

2 4 6 8 1 Dissolution time (years)

0

Figure 4 Predicted effect of grinding a PR to different sizes on its dissolution rate in a soil dissolving 30% of Sechura PR in the first year (Watkinson, 1994b).


85

low solubility precludes its use as a direct application fertilizer, and therefore it is classed as unreactive. It was not possible to convert an unreactive to a reactive PR, even by ultrafine grinding to a size c 0 . 0 2 mm (Khasawneh and Doll, 1978).

2. Carbonate Apatites (Sedimentary) Sedimentary fluorapatites in which carbonate substitutes for phosphate in the apatite lattice form two distinct series of phosphate rocks on the basis of their physical and chemical properties (McClellan and Van Kauwenbergh, 1992). The most common are those with an excess of fluorine over that in FAP. In this case carbonate plus fluoride together substitute for phosphate to preserve the charge balance within the lattice. The second series comprise those PRs in which phosphate is substituted by carbonate plus hydroxide and/or fluoride leading to a deficit of fluorine over that in FAP. a. Excess Fluorine The properties of those PRs with excess fluorine are controlled by the extent of carbonate substitution (McClellan and Van Kauwenbergh, 1992). The planar carbonate substitution for tetrahedral phosphate makes the lattice less stable, resulting in increasing solubility with increasing carbonate content (Khasawneh and Doll, 1978). Increasing carbonate also decreases the unit cell a-dimension and increases the solubility, and therefore the reactivity compared to FA, all other things being equal (Section 1V.A). b. Deficit Fluorine Where there is a deficit of fluorine, the combinations of substitutions are more complex (McClellan and Van Kauwenbergh, 1992), and the correlations with other properties more diffuse. The unit cell a-dimension is controlled more by the fluorine than the carbonate content, and decreases with increasing fluorine. In contrast to the excess fluorine series, the solubility decreases with decreasing unit cell size and increasing fluorine, the latter in line with the decreasing hydroxyl. The fluorine content ranges from zero (equivalent to hydroxyapatite) to that in FAP, so as a class they are generally more soluble and akin to hydroxyapatite, and more reactive than the excess fluorine class. c. Carbonate Impurity Calcite and, less commonly, dolomite impurities are often present as discrete minerals (McClellan and Van Kauwenbergh, 1992). Both dissolve to completion because the product is evolved as carbon dioxide increasing the local soil pH and calcium. Calcite dissolves much more rapidly (Sverdrup and Bjerle, 1982; Watkinson and Kear, unpublished data). These effects lower the amount of PR dissolved, and therefore the apparent reactivity. For PRs with excess fluorine, lattice carbonate is usually inversely related to calcite impurity (Watkinson and

86


Kear, unpublished data), so that these two effects also accentuate differences in reactivity.

111. MEASUREMENT OF PHOSPHATE ROCK DISSOLUTION IN SOIL PR for direct application has been advocated almost exclusively for noncalcareous acid soils. Consequently the techniques developed to measure dissolved PR have notably been for use in acid soils, although the inorganic fractionation scheme of Baifan and Yichu (1989) may pave the way for developing methods for calcareous soils. The extent of PR dissolution, and therefore the release of PR-P, can be measured (i) directly by determining the PR remaining in soil and (ii) indirectly by determining the reaction products released, P and Ca. A third category of methods measure apparently a constant fraction of PR dissolved. Under this category fall NaHCO, (pH 8.5) extractable P, anion resin (with or without cation resin) extractable P and isotopically exchangeable P which is measured either at a given interval after PR application to soil or at intervals. The third category is for estimating plant available P in soils and not for measuring PR dissolution per se. For that reason these methods are discussed in Section VIII. The direct measurement of residual PR is applicable in all circumstances, including field, greenhouse, and incubation studies. The indirect methods are suitable only for closed incubation systems, where the reaction products are not removed from the soil. They are not suitable for field or greenhouse studies unless the amounts of P or Ca removed from the soil by plant and microbial uptake and/or leaching are also measured. The prerequisites for any method used for estimating PR dissolution are that PR should not dissolve during preextraction and, in the case of indirect methods, the extractant should remove all of the reaction product(s) (Bolan and Hedley, 1989).

A. MEA~URFMENT IN ACIDSOILS 1. Measurement of Phosphate Rock Remaining by Inorganic P Fractionation Methods used for measuring the amount of PR remaining in soil are based on the inorganic P fractionation procedure of Chang and Jackson (1957) and later modifications (Petersen and Corey, 1966; Williams el al., 1967; Syers et al., 1972). The fractionation procedure was originally developed to characterize the


87

distribution of soil phosphate in various chemical forms, but has been subsequently applied to measure the dissolution products of P fertilizers and the residual PR to calculate the extent of PR dissolution (Chu et al., 1962; Shinde et al., 1978; Chaudhary and Mishra, 1980; Grigg, 1980a,b; Rajan, 1983, 1987a,b; Chien et al., 1987b; Bolan and Hedley, 1989; Perrott, 1992; Perrott et al., 1992; Rajan and Watkinson, 1992; Tambunan et al., 1993; Perrott and Kerr, 1994). Briefly, the method generally adopted to measure PR dissolution consists of prewashing soil (30 min) with NaCl or BaCI, solution buffered to more than pH 7.8 to remove soil-exchangeable Ca, followed by extraction (17 h) with NaOH (0.5- 1 M ) to extract nonoccluded Fe-P and AI-P, and then with an acid (HCI or H,SO,, 0.5-1 M )solution (4 or 17 h for HISO,) to extract Ca-P. Although some modified methods included Chang and Jackson’s technique of citrate dithionate extraction prior to acid extraction (Peterson and Corey, 1966; Williams et a l . , 1967; Syers et al., 1972), omission of this step has not been found to affect the amount of acid extractable P (Rajan, 1983). The amount of PR remaining is calculated from the increase in the Ca-P fraction (acid extractable P) of the PRtreated soil over that of the untreated control. In applying the procedure it is considered that (i) the P in PR is present as calcium apatite, (ii) the apatite P is not soluble in NaOH but is dissolved by HCI and HISO, (Williams, 1937), and (iii) the P dissolved in soil is transformed into AI-P and Fe-P. In the fractionation procedure a prewash with NaCl or similar electrolyte is necessary to remove soil exchangeable Ca which, if present, could result in precipitation of calcium phosphate during NaOH extraction (Syers et al., 1972). Dissolution of the calcium phosphate in the subsequent acid extract overestimates the Ca-P fraction and therefore the PR present (Perrott , 1992). Hughes and Gilkes (1984) reported that a prewash with unbuffered solutions of NaCI, KCI, or NH,CI of soil, incubated with PRs for a week resulted in the release of exchangeable acidity from soil and therefore dissolution of apatite. They concluded that the extracting solution pH should remain above 7.3 to prevent PR dissolution. These authors recommended prewashing with Bascomb solution (Bascomb, 1964) which consists of 2 M BaCI, solution buffered with triethanolamine (TEA) at pH 8.1. Perrott and Ken (1994), using both soil/PR mixtures and field soils collected 8 months after surface application of PRs, found that prewashing mineral soils with 1 M NaCl resulted in significantly less recovery of PR (70-95%) than when using NaCl solution buffered with 0.1 M H,BO, and NaOH to pH 7.8. The loss was significantly related to soil pH(H,O) (Fig. 5A) but not to the exchangeable acidity (Fig. 5B). H,BO,/NaOH buffers are preferred over TEA because they do not interfere with the molybdenum blue/ascorbic acid method used for P analysis and also are easier to prepare. To improve Ca extraction, Perrott and Kerr (1994) added EDTA to the buffered solution in amounts equivalent to the concentration of exchangeable Ca. Rajan (personal communication) determined the residual PR in a volcanic ash soil

88


0

"V

-

0

10

20

Titratable acidity (Cmol kg-l) figure 5 Effect of (A) soil pH(H20), and ( B ) titratable acidity on recovery of Sechura PR-P from soil-PR mixture using NaCl prewash. Recovery values are expressed as percentages of those using buffered NaCl prewash (Perrott and Kerr, 1994).

(Typic vitrandept) collected 3 months after surface application of North Carolina PR. He found that soil prewash with unbuffered NaCl solution gave only 8% less acid extractable P than a prewash with EDTA buffer (Fig. 6). It is possible that a higher soil pH of 5.8 resulted in a smaller loss in acid extractable P in the unbuffered solution. Most authors have used H2S04 to extract Ca-P although Williams et al., (1967) and Syers er al. (1972) have used HCI. Tambunan et al. (1993) reported that, for reasons they could not explain, the recovery of PR residues using up to 4 M HCI solution was less than that when 0.5 or I M H2S04 was used and thus H,S04 is a preferred option. While the sequential extraction procedure to determine the extent of PR dissolution has generally been satisfactory, it is too lengthy for routine testing. They also require a sample of unfertilized soil to enable the calculation of the remaining PR. Perrott and Wise ( 1 995) proposed a simpler procedure to determine PR residues remaining in soil. A mild acid extraction (acetate buffer) was used to differentiate between native soil fluorapatite-P and PR-P. The procedure consists of extracting P from two subsamples as follows: the first subsample is shaken with an acetate solution of pH 4 for a specified time after which a NaOH/citrate

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS .c0

89

Y=0.08 + 1.08 X, R2=0.963

$ , -ool

$

300 500 700 Acid-P, NaCl prewash (rng kg-l soil)

100

Figure 6 Acid-extractable P of soil samples collected from Sechura PR treated plots prewashed with either NaCl or buffered EDTA solution (soil pH 5.75) (S.S.S. Rajan, unpublished data, 1992).

solution is added. The suspension is shaken for a further specified time and centrifuged, and P is determined in the supernatant solution. A second subsample is similarly treated except that the intial shaking is with a borate solution of pH 8. The PR-P is calculated by subtracting the amount of P extracted in the second subsample from that of the first subsample. Their results averaged over 1 1 different soils from New Zealand showed a very significant (0.1% level) variation of PR-P recovery with rock reactivity (Table I). The recovery of PR-P from Sechura and North Carolina PRs was complete, which illustrates the usefulness of this method with highly reactive PRs. The recovery with medium reactive (and also the unreactive PR) PR was less than that present in the soil. Soil types had no significant influence on the recovery of

Table I Percentage of PR-P Recovered from PR-Soil Mixtures (Values Averaged across 11 Soils) Phosphate rock

Recovery (96 of added PR)

Sechura North Carolina Arad Egyptian Florida SED

93.7 101.0 80.9 77.3 37.5 4.8***

90


the two most reactive PRs, although when the recovery values were averaged across the five PRs, soils did have a significant effect (1 % level).

2. Measurement of Phosphate Rock Dissolved from ANaOH-P Measurement of PR dissolution in soil from the increase in the amount of NaOH-extractable P determined after prewashing soil to remove exchangeable Ca was proposed by Mackay et al. (1986). The underlying principle behind this method is that P released from PR largely forms complexes with soil Fe and A1 which are extracted by NaOH solution (usually 0.5 M . Since apatite P does not dissolve to any significant extent in this reagent (Williams, 1937) the method provides a direct estimate of the PR dissolved in soil. It has the advantage of needing only two instead of three extractions as in the fractionation procedure. Bolan and Hedley (1989) concluded that this method is suitable for use in incubation studies where the reaction products are not removed and where there is no significant active net mineralization or immobilization of P. This method has been used extensively in short-term incubation studies. The usefulness of this method has not been investigated for long term incubation studies where the sorbed P may be converted from NaOH extractable to occluded forms of P (Hagin et al., 1990).

3. Measurement of Phosphate Rock Dissolved from ACa PR dissolution has also been estimated from the increase in the exchangeable Ca (ACa) content of the PR treated soil over that of the control. In the ACa method it is assumed that the Ca released on dissolution of PR accumulates in the soil as exchangeable Ca which is extracted with appropriate electrolyte solutions. Khasawneh (quoted in Khasawneh and Doll, 1978) used neutral 1 M NH,OAC to determine the increase in exchangeable Ca in the PR-treated soil over the control soil. Smyth and Sanchez (1982) measured PR dissolution using a 1 M KCl solution. As pointed out in a previous section, use of unbuffered solutions could dissolve PR during the extraction because of the release of exchangeable acidity from the soil (Hughes and Gilkes, 1984). These authors therefore advocated the use of BaCI, solution which has been buffered to an alkaline pH. Bolan and Hedley (1989b) reported poorer recovery of Ca by NH,OAC and the recovery decreased as the pH increased. The ACa method is the simplest of the three techniques. However, it is not suitable for use in greenhouse, field, or open incubation studies where the Ca released is removed by plants or by leaching. In closed incubation studies also, overestimation of PR dissolution will result where the PR contains appreciable amounts of free CaCO, because of its preferential dissolution and hence increase in the measured exchangeable Ca.

PHOSPHATE ROCKS FOR DIRECT APPLICATION T O SOILS

91

B. MEASUREMENT IN CALCAREOUS SOILS Although not commonly found, there are situations where PR application to calcareous soils could be plant effective (Edwards, 1956; Singaram et al., 1995). Measurement of PR dissolution in calcareous soils is made complicated by the dissolved P forming not only Fe-P and Al-P but also Ca-P (Holford et al., 1975; Hooker et al., 1980). Unlike methods for noncalcareous soils where Ca-P is treated as one component, methods aimed at measuring PR remaining in calcareous soils should distinguish between the calcium apatite applied as PR and the Ca-P formed after reaction with soil components. Reports indicate that at relatively low solution P concentrations (< 10 mg liter-'), which is far higher than would exist at the PR/soil interface, the Ca-P is probably present as adsorbed P on CaCO, and as Ca-P complexes formed with the exchangeable soil Ca (see Sample et al., 1980). Baifan and Yichu (1989) proposed an inorganic P fractionation scheme which included separation of dicalcium, octacalcium and apatite P. They used sequential extractions with NaHCO, (pH 8.5), NH,Ac (pH 7.0), NaOH plus Na,CO,, and H,SO,. Fractionation schemes similar to the above may be appropriate for PR measurement in calcareous soils but are yet to be investigated.

rV. FACTORS AFFECTING PHOSPHATE ROCK DISSOLUTION IN SOIL AND AVAILABILITY TO PLANTS Several factors affect the rate of PR dissolution in soil and its availability to plants. The availability of PR-P to plants largely depends on its rate of dissolution. However, this is not always so because of the influence of soil characteristics, the plant and fertilizer management factors. This section reviews various factors influencing the rate of PR-P release and its availability to plants.

A. FACTORS AFFECTING RUE OF P RELEASEFROM PHOSPHATE ROCKAPPLIED TO SOIL 1. PR Properties

Two important properties determining the rate of PR dissolution in a given soil are chemical composition, which includes apatite lattice composition and the type of accessory materials, and particle size (Section 11).

92


PR deposits fall into three broad classes based upon their mineral assemblages in order of their increasing economic importance. These are Fe-A1 phosphates, Ca-Fe-Al phosphates, and Ca phosphates (McClellan and Gremillion, 1980). Commercial mineral Ca phosphates belong to the group of apatite minerals which are similar in crystal structure to fluorapatites but vary significantly in chemical composition. The Ca apatites of sedimentary origin have generally been found to be suitable for direct application as phosphate fertilizers. It has been well established that increasing substitution of C032- for PO,3- in the lattice structure increases the solubility of carbonate apatites. This occurs through decreased unit cell a-dimension and crystal instability on increased incorporation of planar CO,2- and F- for PO,,- tetrahedra (Lehr and McClellan, 1972; Chien, 1977). Unit cell a-dimensions in turn have been found to be closely correlated with the chemical extractability of P from PRs (Figs. 7A-7C) (Dash et al., 1988; McClellan and van Kauwenberg, 1992). For details on the chemistry of isomorphic substitution and its effect on crystallite properties the readers are referred to the review by Khasawneh and Doll (1978). There is a scarcity of studies relating directly measured dissolution of PRs in soil with the chemical composition of the PRs, under conditions where the reactant products were removed from the PR-soil interface. However, numerous reports have been published measuring PR dissolution indirectly as plant P uptake. Chien et a/. ( 1987b) determined residual PR remaining in a Columbian Oxisol 5 years after application. Six PRs (carbonate apatites) of varying citrate solubility were applied and the inorganic P fractionation of Chang and Jackson (1957) was employed to determine the PR remaining. The apatite dissolved, calculated as a difference between the PR applied and that remaining, ranged from 79 to 98% of that applied and there was a positive correlation between the PR dissolved and the citrate-soluble P of the PRs (Fig. 8). Rajan (1987b) reported that in 1 year after surface application to permanent pastures 27% of Florida PR (low carbonate substitution) dissolved compared with 42% for a North Carolina PR (highly carbonate substituted). Several publications provide evidence of a close positive correlation between increasing carbonate substitution in the lattice structure, determined by direct physical and chemical measurements or as indicated by chemical extractable P, and PR dissolution as measured by crop P uptake (Mackay et al., 1984a,b; Anderson et al., 1985; Leon et al., 1986; Dash et al., 1988; Rajan et al., 1992). Calcium carbonate is the most abundant accessory mineral in PRs. Because CaCO, is more soluble than the most chemically reactive apatites (Silverman et al., 1951) and since its dissolution increases the Ca concentration and pH at the apatite surface it is not surprising that accessory CaCO, can reduce the rate of PR dissolution in some soils (Anderson et al., 1985; Robinson et al., 1992a). How-


93

0 1

141

2{ 0

9.32

B

,

,

9.34

,

,

,o,

9.36

9.38

Unit-ceIP Figure 7 Relationship between unit-cell a dimension of apatite sample and solubility of P in (A) neutral ammonium citrate. (B) citric acid, and (C)formic acid (McClellan and Van Kauwenbergh, 1992).

ever, under field conditions where Ca may be removed by plant uptake and or leaching this effect will be minimized. Because PRs are relatively insoluble materials their geometric surface area

94


0

0,5 1.0 1.5 2.0 2.5 3.0 Citrate-soluble P (“Aof PR)

Hgure 8 Percentage of PR-P dissolved 5 years after application as related to their solubility in ammonium citrate (Chien er a / ., 1987).

will have an important bearing on their rate of dissolution in soil (Section 11). Thus the finer the particle size, the greater the degree of contact between PR and soil and therefore the greater the rate of PR dissolution, provided the PR application rate is such that the zones of PR dissolution between the particles do not overlap. The results of Kanabo and Gilkes ( 1 988c) support the above reasoning. These authors conducted a laboratory incubation study in a lateritic podzolic soil to estimate the dissolution of North Carolina PR ground to four size fractions ranging from 0.15-0.25 mm to

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