Soil Liquid Phase Composition
The work sums up the vast experience of the authors' research into soil liquid phase composition in various ecosystems of Central and Eastern Europe (Russia, Ukraine, Hungary, Czech Republic). It describes the methodological basics of soil liquid phase research: methods of soil solution extraction, the main problems of application of ionselective electrodes for immediate in situ assessment of ionic activity in soil liquid phase and redox potential, and ways to overcome those problems. Data are presented on soil liquid phase composition in natural and agricultural ecosystems, their redox, pH, carbonate and other regimes as well as the relations between the composition of the soil liquid phase and different ecological properties. The work is intended for soil scientists, agricultural chemists and ecologists.
Translated from Russian by A.O. Korsunsky and N.A. Moskalenko
Soil Liquid Phase Composition V.V. Snakin, A.A. Prisyazhnaya Institute of Basic Biological Problems, Russian Academy of Sciences Pushchino, Moscow Region, Russia
and E. Kov~cs-Lfing Institute of Ecology and Botany, Hungary Academy of Sciences Vacratot, Hungary
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CONTENTS CONTENTS
INTRODUCTION I N T R O D U C T I O N .........................................................................................................
5
ACKNOWLEDGMENTS A C K N O W L E D G M E N T S .............................................................................................
7
CHAPTER C H A P T E R 1. SOIL SOIL LIQUID LIQUID PHASE PHASE AS A A STRUCTURAL S T R U C T U R A L ELEMENT E L E M E N T OF OF AN AN ECOSYSTEM E C O S Y S T E M ................................................................................................................ 1.1. Types Types of of soil water water .................................................................................................. 1.1.1. Pellicular Pellicular water water .................................................................................................... 1.1.2. Capillary Capillary water water ..................................................................................................... 1.1.3. Gravitational Gravitational water water ................................................................................................ 1.2. Soil liquid liquid phase phase in environmental research research ...........................................................
9 9 10 14 16 17
CHAPTER C H A P T E R 2. SOIL SOIL LIQUID LIQUID PHASE PHASE INVESTIGATION I N V E S T I G A T I O N ........................................... 2.1. Methods Methods of of soil solutions extraction extraction ....................................................................... 2.2. Ionometric Ionometric analysis of of soil samples samples ........................................................................ 2.2.1. Activity Activity and and concentration concentration of of ions ....................................................................... 2.2.2. ISE selectivity coefficients coefficients .................................................................................. 2.2.3. The The influence influence of of solid phase phase on ionometric ionometric measurements measurements in soil ...................... 2.2.4. Influence of of soil moisture on the ionometric measurements measurements ............................... 2.2.5. Influence of of the gas phase phase on the ionometric ionometric measurements in soil (incomplete (incomplete contact contact between between the electrode electrode and and soil) ......................................................................... 2.3. In situ measurements measurements of of ionic activity in soil ......................................................... 2.3.1. Compensation Compensation of of temperature dependence dependence in ion-selective systems ................... 2.3.2. The The selection selection of of electrodes .................................................................................. 2.3.3. Getting the electrodes electrodes set for work work ...................................................................... 2.3.4. The The process process of of measurements in soil ................................................................... 2.4. Measurement Measurement of of the soil redox potential ................................................................ 2.5. Comparison Comparison of of different different methods methods of of soil liquid phase phase investigation investigation ...................... 2.5.1. Laboratory Laboratory methods methods ............................................................................................. 2.5.2. In situ measurements measurements and displaced displaced soil solutions .............................................. 2.6. Soil solution, soil and plant plant analytical methods methods ..................................................... 2.7. Data Data base base ................................................................................................................. Conclusions ............................................................................................................. 2.8. Conclusions
21 22 24 25 28 29 36
C H A P T E R 3. STUDY S T U D Y AREAS AREAS ..................................................................................... CHAPTER environment ..................................................................................................... 3.1. The environment Climate .................................................................................................................... 3.2. Climate Vegetation ............................................................................................................... 3.3. Vegetation 3.4. Soils ........................................................................................................................
69 71 73 74 76
37 39 40 46 50 52 54 58 58 58 65 66 67
2
CHAPTER CHAPTER 4. ENVIRONMENTAL ENVIRONMENTAL IMPACT IMPACT ON THE THE SOIL LIQUID LIQUID PHASE PHASE ....... phase ....................................................................................................... 4.1. Soil solid phase Atmosphere and soil air .......................................................................................... 4.2. Atmosphere Hydrological regime ............................................................................................... 4.3. Hydrological Temperature ............................................................................................................ 4.4. Temperature Vegetation ............................................................................................................... 4.5. Vegetation 4.5.1. Atmospheric Atmospheric precipitation precipitation and forest vegetation ................................................. Ecosystems and soil types ...................................................................................... 4.6. Ecosystems Anthropogenic factors ............................................................................................ 4.7. Anthropogenic management and soil liquid phase phase composition composition ......................................... 4.7.1. Field management 4.7.2. Soil resistance resistance to acid rain ................................................................................... 4.7.3. Soil liquid phase phase under recultivation recultivation ................................................................... Conclusions ............................................................................................................. 4.8. Conclusions
84 84 87 88 92 95 98 103 109 109 118 118 121
CHAPTER CHAPTER 5. SPATIAL SPATIAL AND AND TEMPORAL TEMPORAL PROPERTIES PROPERTIES OF SOIL LIQUID LIQUID PHASE ........................................................................................................................... PHASE composition of of soil liquid phase ...................................................................... 5.1. The composition potential (Eh) ...................................................................................... 5.1.1. Soil redox potential 5.1.2. pH pH properties properties ....................................................................................................... Calcium activity ................................................................................................... 5.1.3. Calcium 5.1.4. Potassium Potassium activity ................................................................................................ 5.1.5. Nitrate Nitrate activity ..................................................................................................... 5.2. Spatial Spatial heterogeneity .............................................................................................. 5.3. Temporal Temporal variability variability ............................................................................................... 5.4. The estimation estimation of of the necessary necessary number number of of collected collected data for the reliable determination determination of of soil characteristics characteristics .............................................................................. 5.5. Dynamics Dynamics of of the soil liquid phase phase ........................................................................... 5.5.1. Sandy semi-desert semi-desert steppe .............. ..................................................................... 5.5.2. The Middle Middle Danube Danube steppe .................................................................................. 5.5.3. The Priazov Priazov steppe ............................................................................................... 5.5.4. The Colchid Colchid forest ............................................................................................... 5.6. Conclusions Conclusions ...............................................................................................................
147 149 149 155 158 164 170
CHAPTER CHAPTER 6. MATERIAL MATERIAL AND AND ENERGY ENERGY EXCHANGE EXCHANGE IN ECOSYSTEMS ECOSYSTEMS ......... carbonate equilibrium equilibrium ...................................................................................... 6.1. Soil carbonate 6.1.1 Assessment Assessment of of carbon carbon equilibrium equilibrium status ............................................................. 6.1.2. Atmospheric Atmospheric CO CO22 and soil liquid phase phase ............................................................... 6.2. Soil liquid phase phase oxidation oxidation and pH pH as indices of of ecosystem ecosystem functioning............... functioning ............... Oxidation-reduction in soils .................................................................................. 6.2.1. Oxidation-reduction Eh-pH graph ................................................................................................. 6.2.2. The Eh-pH 6.2.3. Soil redox regime and grassland grassland productivity ..................................................... 6.2.4. Thermodynamic Thermodynamic interpretation interpretation ofEh of Eh changes ..................................................... 6.2.5. In vivo Eh measurements in animals ................................................................... 6.3. Potassium Potassium dynamics in the soil liquid phase phase .......................................................... 6.4. Potassium Potassium and nitrate in the liquid phase of of agricultural agricultural soils ............................... 6.5. Silicon in soil solutions.. solutions ........................................................................................... Organic matter matter in soil solutions .............................................................................. 6.6. Organic 6.7. Heavy metals .......................................................................................................... 6.8. Correlation Correlation between between soil solid and liquid phases phases composition composition ............................... Conclusions ............................................................................................................. 6.9. Conclusions
175 175 175 178 184 185 191 196 200 200 203 204 204 210 210 212 218 220 220 232 240
122 122 122 127 133 135 136 138 145
3
CHAPTER C H A P T E R 7. ENVIRONMENTAL E N V I R O N M E N T A L PROCESSES P R O C E S S E S AND A N D SOIL SOIL LIQUID L I Q U I D PHASE P H A S E ...... Photosynthetic intensity intensity .......................................................................................... 7.1. Photosynthetic Transpiration and and evaporation evaporation ................................................................................ 7.2. Transpiration 7.3. Plant Plant matter matter dynamic dynamic .............................................................................................. 7.4. Ecological Ecological assessment assessment of o f the the degree of o f anthropogenic anthropogenic changes changes in soil .................. 7.5. Soil liquid liquid phase phase and and ecosystem ecosystem contamination contamination .......................................... ........... 7.6. Conclusions.............. Conclusions .............................................................................................................
244 244 244 244 248 248 250 250 251 256 256 259 259
SUMMARY S U M M A R Y ...................................................................................................................
261
GLOSSARY G L O S S A R Y ..................................................................................................................
263 263
REFERENCES R E F E R E N C E S ..............................................................................................................
268 268
C O R R E L A T I O N BETWEEN B E T W E E N SOIL SOIL N A M E S .............................................................. CORRELATION NAMES
305 305
SUBJECT S U B J E C T INDEX I N D E X .........................................................................................................
306 306
A U T H O R INDEX I N D E X ......................................................................................................... AUTHOR
311
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5
INTRODUCTION INTRODUCTION soil (soil (soil solution) solution) is a very thin, penetrating and all-embracing water layer. layer. The liquid phase of soil
It has the most extensive surface among the biosphere components and interacts with all these components. Investigation of the soil liquid phase can of great significance in environmental research. Soil water is one of the most important natural water category in the biosphere (Vernadsky, (Vemadsky, 1960). 1960). V.1.Vernadsky V.I.Vemadsky considered it "the basic element of the biospheric mechanism" and "the basic life substratum". According to K.K.Hedroitz (1975a), ''to "to move on in solving some theoretical as well as practical issues of agronomy we have to find another approach to solve the problem of soil solution; we extemal should study the composition of the solution and its temporal changeability as depending on external
conditions. It will not be an exaggeration to say that further achievements of agronomy depend on the of this problem". solving ofthis
Soil liquid phase investigations have not become an efficient instrument in ecology or applied soil science, despite extensive soil solution data. This is due to the difficulties in studying soil solutions in unchanged state, spatial heterogeneity of soil properties (including soil liquid phase) and dynamic composition of soil solutions responding to environmental changes. The more difficult the problem, the more interesting it is to fathom its depths. Soil liquid phase investigation dates back to the start of of experimental environmental research. Two trends have emerged from the very beginning: (i) attempts to separate soil solution from soil in order to analyze its composition (Schloesing, 1866; Ishcherekov, 1910), and (ii) experiments on soil liquid phase caning carrying out immediate investigation in soil, without preliminary extraction, through electrometric methods (Whitney &, Means, 1897; Briggs, 1899). The first trend was used for a long time, though it was noted that "all the attempts at extracting soil solution from soil at a low moisture content are bound to fail"
1975a). Development of of the second trend was drawn back by the imperfection imperfection of of (Hedroits 1975a). electrometric electrometric techniques. It was not until the ion-selective electrodes technology (ISE) was introduced
made and the first ISE (glass (glass H+-electrode) It-electrode) was used used in soil investigations that progress was made (Nikol'skii, (Nikol'skii, 1930).
Development of of different different ISE technology and and field ionometers allowed to expand expand the the circle circle of of Development determinable determinable ions ions in in water water (liquid) (liquid) phase phase of of different different soils, soils, and and to investigate investigate natural natural soil liquid liquid phase phase
6
in sire situ under field conditions without breaking their internal physico-chemical balances (the so-called in
of data is the case, which enables us to assess parameters parameters of of measurements). A brand-new class of of physico-chemical and biological processes in soil under natural conditions. It is often that analysis of of investigations. Soil sample soil samples results in unreliable data, especially at the preliminary stage of of selection and preservation, and its redox, gas-exchange and properties reflect the stages of
microbiological processes are different from soils in the field. Investigation of of soil as a component of of natural and cultivated ecosystems should be dynamic and should reveal its nature and the links within the solid, liquid and gas phases. We agree with Ruellan (1983), that to study recent soil processes the newest technical means should be used in order of soil components in situ. to find out the structure and composition of of new approaches to soil liquid phase analysis and This study is devoted to search and back-up of fmd out, the role of of soil liquid phase in the functioning of of natural and agricultural ecosystems in aims to find of recent soil-formation, formation of primary biological production, and in bio-geochemical turnover of of soil liquid phase is the determination of of the concentration (activity) of of elements. Direct investigation of ions or redox potential in situ; while the analysis of of soil solution implies that the soil solution is extracted from soil. The authors have aspired to give insight into the development of ideas and theories as well as certain results of Russian schools of soil science and ecology on problem of studying of soil liquid phase. The references therefore contain mainly articles in Russian. As compared with earlier publications on soil liquid phase investigation (Bystritskaya, Volkova, Snakin, 1981; Snakin, 1989; 1989; Snakin, Kovacs-Lang, Kov~ics-L~g, Bystritskaya Bystdtskaya et al., 1991; Snakin, Prisyazhnaya, Rukhovich, 1997) this work is substantially expanded. It includes new field investigation data as well as all data generalization carried out by the means of a special complex database «Demetra»
0.6
'~ 0.4 rv 0.2 |
I
I
I
i
0.2
0.4
0.6
0.8
1.0
Co,,/T--.,C,
Fig. 6. The relationship between Ca concentration and total ion concentration on growth of cotton roots (Howard & Adams, 1965)
The material showed that the composition of the soil solution is a satisfactory indicator of plant nutrition conditions. Nevertheless, the problem of soil liquid phase in the functioning of ecosystem has received little attention. This is explained by the relatively poor development of study methodologies.
21 CHAPTER 2. SOIL LIQUID PHASE INVESTIGATION
In order to study the role of soil liquid phase (SLP) in the functioning of a ecosystem, it is necessary to find more advanced methodologies. In our opinion, it should consist of developing methods of in situ analysis of the composition of soil liquid phase without violating the interactions within the ecosystem. This approach does, however, not exclude the use of traditional methods. The investigation of SLP has a history of more than 100 years. During this period, numerous analytical techniques were proposed (Fig. 7). There are two different methodologies: (i) application of various types extractions of soil liquid phase such as soil solution in order to further determination of their composition; (ii) attempts to analyze soil liquid phase composition in situ.
Soil liquid phase
I
I
Field measurements in situ (ionometry, conductometry)
Laboratory analyses of soil samples
Direct analysis with the help ISE
Non-solving volume measurement
Lysimetric water investigation
Preparation of water extract
Extraction of soil solution (by liquid displacement, pressurization, centrifuging, combined methods)
Fig. 7. ,Scheme of investigations on the soil #quid phase
The first approach allowed wide range of analytical techniques for determining the numerous components of soil solutions. Concerns have however been expressed that the
22 composition of extracted soil solutions differs from that of the SLP (Shilova, 1964; Kriukov, 1971; Kovda, 1973; Hedroitz, 1975a, etc.). The second approach could start only with the development of potentiometric (ionometry, determination of soil redox potential) and conductometric (determination of the overall salt concentration) techniques. It is characterized by a limited range of determined parameters due to the lack of ion-selective electrodes (ISE) suitable for soil-chemical investigations. This enabled us to begin the in situ measurements in SLP investigations. The method of water extracts that have been named as soil solutions before, should be considered a method of SLP analysis. Water extracts are widely used in chemical analysis for determination of the amount of readily soluble salts in soils and their temporal changes. They provide only indirect information about the SLP composition, and in our investigations, they were only used for comparison. Below are brief descriptions of the methods of soil solutions extraction. More detailed information can be found in reviews by Komarova (1968); Kriukov (1971); Skrynnikova (1977).
2.1. METHODS OF SOIL SOLUTIONS EXTRACTION
The displacing liquid method was first introduced by T.Schloesing and water was used as the displacer (Schloesing, 1866). The insufficient degree of displacement, the impossibility of precise determination of the borders of out-flowing soil solution in this variant prompted the idea to search for some new displacers. Ischerekov (1910) chose ethanol and Komarova (1956) brought improvements into the technique (Table 2). The most suitable for this process was to use 100-150 cm long plastic or glass tubes with the inside diameter of 4 cm filled with a mixture of the investigated soil with purified quartz sand. On the top 10-20 ml of ethanol was poured every 2 hours. The degree of extraction is indicated by the appearance of ethanol in the outflowing solution. The testing of the solution for alcohol is made organoleptically.
Table 2 The percentage of soil solutions displacement by various displacers (Komarova, 1956) Displacing liquid
Displaced soil solution (%)
1,4-dioxane
61
Ethanol
57
Methanol
45
Acetone
58
23 The Ishcherekov-Komarova method proved to be convenient, which accounts for its wide usage. The possibility to automatize the feeding of displacer (5-10 ml/hr) by the use of peristaltic pump or the appropriate capillaries (Fig. 8) enabled to increase the degree of soil solution displacement and to standardize the displacement conditions. In the course of the investigations we did not face the need to mix soil samples with sand for better displacement. Automatic inflow of alcohol made it impossible for ethanol to go stagnant except for soil samples of heavy mechanical composition with high moisture content. This method allows to obtain soil solution even at low moisture condition of the soil samples.
t
....
.:.....
r~.-; .......
J
Fig. 8. The unit for ethanol displacement of soil solutions operating with capillaries for maintaining automatic inflow of ethanol Suction or press out soil solutions became widely used after publication of works by Richards (1941) and Kriukov (1947). The high pressure equipment (up to 10,000 kg/cm2) was used. This allowed to obtain soil solution and to extract part of the soil bound water. The method of pressing, despite the complicated equipment it requires, proved to be efficient to investigate the soil liquid phase. As a rule, a pressure of 50-200 atm. is used. Instead of excessive pressure, suction is often used, but soil solution can only be extracted in case the soil is moist.
24 Centrifugation can also be used to extract soil solutions, but it lacks the benefits of the two previous methods. Usually, a speed of 5000-9000 rpm is used, and the remaining moisture corresponds approximately to one third of the hygroscopic moisture (Komarova & Knyazeva, 1967). The willingness to obtain soil solutions under minimal change as compared to their natural state has resulted in a number of combined methods. Doyarenko (1924) noted, that displacement by ethanol does not extract a true solution in terms of its physical properties as ethanol changes the osmotic pressure as well as the conductivity. The use of high pressures leads to additional dissolving of components. Depression causes an increase in evaporation which leads to a higher concentration. That is why Doyarenko suggested to extract soil solution using oils that do not mix with water like purified linseed or vaseline oil. Fresh soil should be mixed with oil during which soil solution turns into an emulsion and can be further extracted at a low pressure and the oil is extracted after centrifuging. Shmuk (1923) used a similar method. It is also possible to combine the methods of displacement by ethanol and pressing (Kriukov & Komarova, 1956), centrifuging and displacement by a liquid that does not mix with water, e.g.,
CC14 (Mubarak & Olsen, 1976; Adams et al., 1980), extraction of soil solution by pressure of compressed gas with the use of Richards' press (Fedorovsky, 1964).
2.2. IONOMETRIC ANALYSIS OF SOIL SAMPLES
Analysis of soil liquid phase without extraction of soil solution was carried out for the first time in the 19th century (Whitney & Means, 1897). However, the electrometric method for determination of the presence of readily soluble salts in soil on the basis of impedance measurements never became popular mainly because of the necessity to correlate the obtained data with the type of soil and moisture content. Later the conductomentric method turned out to be rather useful, for example, in studies of salt affected soils (Gorbunova, 1977). The method of direct determination of the concentrations of salts in soil solution by the freezing temperature of the soil also never came to be widely used (Bonyoncos, McCool, 1915). Potentiometric technique allowed direct measurement of the soil liquid phase composition. Long before the first ion-selective electrode (glass hydrogenous) was designed, works on pH determination in soil and redox potential in soil suspensions and pastes had been published (Remezov, 1929; Trofimov, 1931). As various new ion-selective electrodes were designed, the number of researchers applying ionometry in soil investigations grew 1. The new method eliminated
See: Nikol'skii (1930); Nikol'skii, Evstropiev (1930); Marshall (1942); Avakyan(1953); Eisenman et al. (1957); Kerzmn, Gorbunova (1973); Materova (1969).
25 what was seemingly impossible to cope with - the problem of extraction of soil solution in an unchanged form. At the same, time ionometry arouse a number of problems connected primarily with the notions of separate ions' activity and the possibility of distortion induced by the charged suspensed particles and the gas phase while carrying out measurements in suspensions and soils ("the suspension effect"). Excessive exaggeration of these problems o~en makes some scientists reject the possibility to interpret the data obtained by ISE in colloidal systems. Let us consider these troublesome issues without detailing the general problems of ISE usage which have been discussed by other 2.
2.2.1. ACTIVITY AND CONCENTRATION OF IONS
The potential (E, mV) of a system of electrodes (ISE - reference electrode) registered by a measuring unit (pH-meter, millivoltmeter, ionometer) in general correlates with Nernst's equation: 2,3. R T E-
E°+
zF
lga x
where E ° - the system's standard potential, mV; R - the universal gas constant; T -
(1) the absolute
temperature, °K; z - the measured x ion's charge; F - Faraday constant; az- the measured ion's activity. The symbol "+" is valid when measuring the cation activity; the symbol "-" - when measuring the activity of anions. Under room temperature (+25°C), if the ionic activity changes 10 fold, the system's potential should change by 59 mV for monovalent ion, or by 29.5 mV for bivalent ion. Thus, the electrode system's potential is the function of the activity, not the concentration of ions in the solution. The concentration of ions in the solution signifies its quality in a volume unit (or mass) of the solvent. A common way to express the concentration is molarity, i.e., the amount of matter expressed in gram-moles per 1 litre of the solvent, mol/L. In soil investigations, due to small concentrations of soil solutions, the mmol/L values are o~en used 3. To perform precise thermodynamic calculations it is necessary to express the concentration in molar parts (the number of moles of the matter per 100 moles of the solvent) or molality values (for water solutions - mol/kg H20), which are very close to molarity for low concentration solutions. Most of the thermodynamic relationships expressed through concentration are applicable only to ideal systems, i.e., such systems where the ionic and molecular interactions are absent. To characterize the real ecosystems' behavior, G. Lewis introduced in 1907 the notion of ion activity
2 See: Durst (1969); Bates (1973); Nikol'skii, Materova (1980); Cammann(1973); Morf (1981); Handbook of Electrode Technology (1982). 3 The data quoted below uses millimole-equivalentper liter, meq/Lto express ion concentration (activity).
26 instead of concentration so that its use should allow applying the laws of ideal systems to real systems. Thus, activity is a function of concentration, differing from the latter by a certain factor, which was called by Lewis the activity coefficient: a, - y °i "Ci where a; - activity, Ci - concentration and ~° _ ionic activity coefficient. "Different methods to express the concentration correspond to different chemical potential values in standard hypothetical solutions of unit concentration, and, therefore, to different values of ai in one solution" (Chemical Encyclopedic Dictionary, 1983). Therefore, activity has the same dimensionality through which the concentration is expressed. It is said that the activity is a non-dimensional value (Sposito, 1984), because in a number of thermodynamic equations, activity is expressed through a ratio of fugitivity. The activity coefficient is described by the dimensionality opposite to the concentration. Naturally, activity is an artificial notion and therefore its dimensionality (or nondimensionality) can be made conditional. However, we think that the mentioned approach disagrees with the basic idea of introduction of activity instead of concentration. Special mention should be made of the fact that in thermodynamic calculations the concentration is immeasurable, since the molal part- moles ratio; molality - the ratio of the substance mass to the solvent's mass are nondimentional themselves. We assigned to activity values the same mode of expression as to concentration. An important obstacle in using ionometry is the thermodynamic uncertainty of the notions of activity and the activity coefficient of a certain ion. This is a topical problem in physical chemistry and has been discussed for a long time (Rabinovich, 1985). The contemporary techniques to determine the activity can only be used for electroneutral component, and the activity coefficient determined this way is applied for an average ionic one. The real coefficients of the anion and cation activity of a salt may vary. To proceed with an individual activity coefficient from the average ionic ones, various suppositions are used, which give close results with weak ionic strength in the investigated solution. One of these suppositions holds that the anion and cation activity coefficients are equal in water solutions of KCl: Y K + = Y cr
= 7 +_KCl.
An practical issue is standardization of the activity scales of individual ions. At present, a standardization has been performed only in the field ionometry. Due to the absence of accepted standard solutions with a known value of activity for various ions, there inevitably arises the
27 problem of ionic activity calculation for standard calibration solutions in the working area of the electrodes. 4 Table 3 shows the activity coefficients of some ions in water solutions with various degrees of ionic strength. For diluted solutions the activity coefficient can be calculated through the DebyeHuckel equation: Az 2 lgy, = - 1 + B.b~fi
"
where z - the ion's charge, I -
1
the ionic strength in the solution: I = ~ . ~]
ciz 2
(ci
-
the ion's
concentration); b - the ion's size parameter, with the same exponent as the diameter of the hydrated ion (Table 4); A and B are the constants depending on the temperature and the solvent's dielectric properties (Table 5).
Table 3 Some ion activity coefficients (Butler, 1964) Ions
Ionic strength of the solution (/) 0.0005
0.001
0.0025
0.005
0.01
0.025
0.05
0.1
K ÷, I-, NO3-, CI-, NH4+, Ag+ 0.975
0.964
0.945
0.924
0.899
0.850
0.800
0.750
OH-, F-
0.975
0.964
0.946
0.926
0.900
0.855
0.810
0.760
Na+, HCO3-
0.975
0.964
0.947
0.928
0.902
0.860
0.820
0.775
Pb2+, CO32-
0.903
0.868
0.805
0.742
0.665
0.550
0.455
0.370
Ca2+, Fe2+
0.905
0.870
0.809
0.749
0.675
0.570
0.485
0.405
Mg2+
0.906
0.872
0.813
0.755
0.690
0.595
0.520
0.450
Table 4 The meaning o f ' b ' index (Butler, 1964) Ions
b
IT Na+, HCO3-, HzPO4OH-, F-, K+, CI-, Br-, F, HS-, NO3-, NH4+, Ag+ Mg2+ Ca2+, Cu2+, Zn 2+, Fe2÷, Mn 2+ Ba2+, S2-, Pb2+, CO32-
9 4 3 8 6 5
SO42-, HPO42A12+,Fe3+
4 9
PO4 3-
4
4 These calculations are not necessary if the task is to directly determine the ion's concentration in the investigated substrate. However, in this case the standard solutions should be prepared so that ionic strength of the standard solution and that under investigation be the same. The latter cannot be carried out to a full extent due to the unknown composition of the soil liquid phase under investigation.
28 Table 5 Parameters for the Debye-Huckel equation (Bates, 1973)
Parameters
Temperature(°C) 0
5
A
0.4918
B
0.3248
10
15
20
25
30
0.4952 0.4989
0.5028
0.5070
0.5115
0.5161
0.3256 0.3264
0.3273
0.3282 0.3291
0.3301
For ionic strength over 0.1 the hypothesis of different degree of preciseness is used. Such is the hypothesis of ionic couples in solution which concentration is subtracted from the total concentration of the given ion determined by conventional analytical methods.
2.2.2. ISE SELECTIVITY COEFFICIENTS
Since there are no absolutely selective electrodes, the potential at the ISE surface depends on the presence of various ions in addition to the measured ones. If the electrode's potential is influenced by both the A z' and the B~2ions, the system's potential cannot be calculated by the equation (1). Here, the following equation is applicable:
2,3RT lg~a A + KA/8 .a~/Z=]
e = e ° + zl : F
(2)
where aa and aa are the activity of ions A and B, Kn/~ is the ratio of an A-selective electrode selectivity coefficient to ion B. If/(Am = 1, then the electrode is not selective to any of the two ions and can be only used to measure the sum of their activity; if KA/~ 20%) - in suspension of 10:1. The difference in the value of potential in suspension and filtrate varies depending on the type and the texture of the soil. In a suspension of 10:1 of sandy gray-brown soil (silt 1%) it is 13%, while in clay meadow soil (silt 20-40%) and in meadow-steppe solonetze it is up to 60%, increasing with the depth (Table 7).
Table 6 Average ion activity deviation values for 8 soil types (see Table 7) in suspensions and filtrates for some ISE (%) The Ion Determined
soil : w a t e r
Field Moisture
1:10
1:5
1:1
Ca 2+
23.3
9.0
1.2
5.2
Ca 2+ + M g 2+
20.0
7.0
0.1
2.9
K÷
-
18.2
4.2
2.0
Na +
22.0
14.5
6.2
-
NO3-
22.2
8.3
2.1
4.0
C1-
17.5
21.7
1.7
1.7
Kriukov and Komarova (1956), comparing pNa values measured in pastes and in centrifuged or pressed solutions in a number of soils, silts and clays, observed significant differences only for hydrophilic askangel. In most cases the pNa value in pastes and solutions were equal (Table 8).
32
Table 7 A v e r a g e i o n a c t i v i t y d e v i a t i o n v a l u e s for 6 I S E (see T a b l e 6) in s u s p e n s i o n s a n d f i l t r a t e s for v a r i o u s soils ( % ) Soil*
Depth, cm
Silt
Soil • water
fraction, %
1 • 10
1 "5
Field moisture 1•1
Grey-brown
0 - 10
1.0
13.6
8.5
2.7
Grey forest
0 - 10
5.5
23.1
11.1
2.4
2.1
Sod-podzolic
0 -20
6.3
25.5
11.8
2.4
Ordinary chernozem
0 - 10
9.0
20.7
8.8
1.5
3.8
Dark chestnut
0-25
9.5
23.8
10.8
1.8
-
Ttmdra soil
55 - 65
-
27.3
10.0
2.8
-
Meadow solonchakous
30 - 80
24.1
42.0
55.4
57.7
-
80 - 190
37.3
41.0
43.7
56.2
-
Meadow steppe solonetze
0 - 30
15.6
36.4
26.3
5.1
-
30 -80
39.6
68.4
32.9
29.9
-
8 0 - 160
41.1
42.9
61.0
26.5
-
* The soil type by FA 0 U N E S C O - see Section "'Correlation between soil names"
Table 8 C o m p a r i s o n o f p N a v a l u e s in p a s t e s and s o l u t i o n s ( K r i u k o v & K o m a r o v a , 1 9 5 6 ) Object
pNa in paste
pNa in solution
Na - kaolin in 0.0 In NaCl
1.91
1.91
Na - kaolin in 0.05n NaC1
1.35
1.35
Na - kaolin in 0.10n NaCI
1.07
1.07
Na - kaolin in 0.50n NaCI
0.37
0.37
Na - askangel in 0.0 In NaCI
1.65
1.87
Na - askangel in 0.05n NaC1
1.29
1.29
Na - askangel in 0.10n NaCI
1.07
1.07
Na - askangel in 0.50n NaCI
0.49
0.49
0 -12 cM
2.47
2.47
12 -26 cra
1.11
1.11
26 - 32 cra
0.99
0.99
210 - 240 cM
Solonetz, medium-columnar:
0.94
0.94
Silt, Karachailag
0.94
0.94
Silt, Pacific
0.31
0.31
Bentonite, Oglanlinsk
1.16
1.16
Askangel
1.18
1.68
It m i g h t b e i n t e r e s t i n g to e v a l u a t e t h e o b t a i n e d d a t a in t h e light o f " n o n - s o l v i n g v o l u m e " c o n c e p t (see S e c t i o n 1.1.1). In the m e n t i o n e d case w i t h askangel, a m a x i m u m v a l u e o f the n o n - s o l v i n g
33 volume was achieved, and it is possible that in the given case and otherwise the deviations of pX values in the paste and the extracted solutions were caused rather by emission of some pellicular water than by suspension effect. As a result, the solution obtained was less concentrated than in the soil liquid phase measured directly in the paste. Let us have a look at another possible aspect. The data for soils of natural moisture condition prove that the suspension effect is within the error of ionometric analysis or can be absent (see Tables 6 and 7). It seems that in undisturbed soils of natural humidity the colloidal particles are coagulated (~ - potential close to zero, the size of the aggregates is larger), which also suggests an insignificant suspension effect. The following experiment provided important information. If the displacement tube was filled with ordinary chernozem soil and, instead of a filter, a net with cell diameter lmm to mechanically support the soil was inserted at the bottom, the ethanol displaced soil solution was almost always transparent. Except when soils were collected immediately after rain or in winter: in this case soil solutions were turbid even if filters were inserted into the tubes. In order to assess the contribution of various phenomena to the SE, we have carried out a number of experiments within the soil-solution system to measure the ion-selective system's potential in the supernatant and in the pellet after centrifugation (20 min, 2000 rpm) in accordance with Fig. 9. The difference in the potential E1 - E2 or E2- E3 reflects the actual differences between the properties of the intermicellar liquid and the extracted equilibrium solution. This difference is maximal for the given suspension since the difference between the supernatant and the sediment, and not the initial suspension is the subject of research. The difference between the potential E3-E4 or E2-E1 reflects the change in the liquid junction potential's value, and the maximum value for the given suspension. To analyze the third cause, let us consider the differences in the results of the in
situ measurements in soil and in the ethanol-displaced soil.
E1 F-I.
F E2 7
E~
7
F E,II_
Fig. 9. Position of the ion-selective (1) and reference electrode (2) while measuring the suspension effect (3 - supernatant; 4 - sedimenO
34 The ionic activity in the macrophases of the heterogeneous system (suspension, sediment, supernatant) is different, therefore the ionic activity in the supernatant cannot be characteristic of the system on the whole (Tschapek et al., 1966). In this sense, the in situ ionometry is the only method to determine the real activity of the ion in the soil liquid phase, while all the methods of extraction of the natural solution from the colloidal substrate give distorted information. Table 9 shows the possible value of such distortion.
Table 9 The difference between potential (mV) of ion-selective electrodes in an equilibrium solution and sediment (El - E4 and E2 -E3; see Fig. 9) of grey forest soil suspension, for different ions S:L
I-1+
Na+
NO3-
CI-
(ESL-43-07)*
(ESL-51-07)
(EM-NO3-01)
oP-Cl
1 :1
17 + 2
1 +3
1 +2
-4,0+
1 :2
14+4
2+2
-2+4
-7,0+1,2
1 :5
10+5
2+2
-2+4
-11+3
1,3
*The type of sensing electrode, reference electrode is EVL-1M (silver~silver chloride) - see Table 15
The influence of the solid phase on the activity of various ions differs and is higher for H + ions with 10-17 mV, which is about 0.3 pH unit. For cations (H +, Na+), there was an increase in the potential when the electrode was moved from the supernatant to the sediment, while for anions (CI, NO3"), a decrease was observed. Since for cations and anions the calibration curves are opposite
directed, the presence of the solid phase the ionic activity decreases. A possible reason for this might be the presence of the non-solving volume, i. e. the soil water around the solid particles with no dissolved salts. This also explains the so-called negative absorption phenomenon described by A.V. Trofimov (1925)). While centrifuging, the volume of the film liquid decreases, and the supernatant comes out more diluted than the intermicellar liquid. The issue of the liquid junction potential is interesting in terms of the ionometry method, since it brings controversy into measurement results. Let us try to assess the value of this uncertainty. Table 10 shows that the difference in the potential value on the boundary between the reference electrode and the medium in case of sediment and the supernatant is about 8 mV. As expected, this difference does not depend on the type of the indicating electrode and the ratio of S:L. Such liquid junction potential value can cause uncertainty in estimation of the activity value of about 0.15 pX for monovalent ions and 0.30 pX for bivalent ions. When the potential of ion-selective system is measured separately in the supernatant and the sediment only the first and the second reasons for SE are relevant. Both the addition (cations) and
35 the partial compensation (anions) of the SE potentials are possible. Our investigations of various soil suspensions also showed an insignificant change in the liquid junction potential (Table 11).
Table 10 The difference in the potential (mV) of an ion-selective pair when the electrode is placed into the sediment and supernatant (E2 -El and E3-E4; S :L
see
Fig. 9) of grey forest soil suspension
Ion-selective pair E S L - 4 3 - 0 7 / E V L - 1M*
ESL-51-07/EVL-1M
EM-NO3-01/EVL-1M
1•1
7 +2
4 +2
7 +6
1 2,5
6_+3
4_+2
8_+4
15
-1 _+5
4_+2
4_+4
*The type o f electrode - see Table 15
Table 11 The value of the suspension effect (mV) for various soils (average for suspensions 1:2.5; 1 5 1'10 by 36 measurements) Type of soil*
Type of the electrode pNO3
pK
pCa
Grey forest
-1.5 + 0.8
-3.9 + 4.9
4.8 + 1.7
Typical chernozem
-1.2 + 2.0
-11 + 6.2
5.0 + 2.2
Sod-calcareous
-2.8 + 1.5
0.9 + 4.0
5.0 + 1.7
Chestnut
-1.7 + 1.6
-
8.8 + 5.4
Sod-podzolic
-3.2 + 1.8
-
4.5 + 1.4
Sierozem
-3.1 + 3.0
-0.4 + 2.7
2.5 + 2.5
Solonetz
-2.2 + 2.9
2.4 + 3.2
-3.7 + 6.3
* The soil type by FA 0 U N E S C O - see Section "Correlation between soil names "
It is worth mentioning that measuring such insignificant differences in potentials is difficult in terms of fixing their values on the existing equipment, especially on the field ionometers in the presence of the electrode potential drift. The scale point value for field ionometers, made in Russia, is 1-5 mV, for field ionometer 407A made by "Orion" - 10 mV. There is information that as the soil suspension is diluted, the SE can increase (see Table 7). This does not contradict the data of the increase in the suspension effect with the increase in the solid phase concentration (Chernoberezhsky, 1978). The latter was observed under the constant concentration of soil liquid phase, while if the soil suspension is diluted with water, the electrolyte concentration decreases, as a result the process of peptisation of colloids can occur, which can make SE grow. Our investigation on the SE linked ~-potential showed, that ~-potential of soil colloids is
36 lower when soil interacts with the equilibrium soil solutions than when the solution is diluted with distilled water (Snakin et al., 1989). Therefore we could assume that under natural moisture condition in soils, SE is not be larger than in the pastes and suspensions. The third reason for SE exists irrespective of ionometry. In the process of extraction of the solution from its original colloidal system, various changes can occur in the composition of this solution. Most of the changes take place while collecting soil samples, transporting them and in the course of their further processing while extracting the soil solution. As a multiphase, the soil undergoes critical changes in its regime during this procedure. This affects the soil liquid phase composition, e.g., on its pH value (Table 12).
Table 12 The value of pH in grey forest soil under corn measured in situ (1) and in ethanol displaced soil solution (2) Fertilizer
Date of measurement 29.04.1989 1
08.06.1989 2
24.08.1989
1
2
1
2
Reference
6.6 + 0.7
6.6
6.4 + 0.2
6.7
-
-
N,~oP6oK60
6.9 _+0.4
7.1
6.6 + 0.3
7.1
6.3 + 0.1
7.5
NgoP6oK9o
7.4 _+0.2
7.1
6.4 + 0.2
7.2
-
-
NgoP6oK9o+ manure
7.0 + 0.3
7.4
6.1 + 0.2
7.1
-
We have shown that the difference in the pH value measured in situ and in the soil solution, is closely linked to the activity of the soil living components (Snakin, 1989). This causes changes in concentration to a maximum of pH 1.5 units. The considered material allows us to conclude that the methodological error of ionometry while analyzing the soil colloidal systems is insignificant, the uncertainty in the obtained results does not exceed 0.15 pX for monovalent and 0.3 pX for bivalent ions and remains within the range of the field method error.
2.2.4. INFLUENCE OF SOIL MOISTURE ON THE IONOMETRIC MEASUREMENTS
Here we shall discuss the reliability of the electrodes' data in soils of various moisture content. The very first ionometric investigations faced the problem of the moisture level dependence of measurement results reliability. The natural threshold of soil moisture, above which the measurements could be possible, is determined by the measurement chain resistance including soil. Taking into consideration the high
37 Ohm impedance of the pH-meters such a threshold is low. Using a quinhydrone electrode to measure pH in sand is not complicated even when its moisture is 1%, though the accuracy of such measurements is lower under lower moisture levels (Trofimov, 1931). The experiments with the non-salty sierozems with various moisture contents led to the conclusion that there are no reasons to deny the reliability of the meter's data within the moisture range of 1-10% (Kerzum et al., 1970). At low soil moistures, another problem can arise: the influence of the suction force of the dry soil on the speed of the electrolyte flowing out of the reference electrode. Apart form that, on the boundary between the reference electrode and the investigated substrate, there can arise an additional out-flow potential which adds uncertainty to the results. The value of this potential (EoF) depends on the speed of the outflow of the solution from reference electrode (Bates, 1973). One could assume that the value EOF is important when the soil moisture is low and the electrode's electrolytic bridge contacts with small capillaries, high suction pressure of which is characteristic under low moistures. If the moisture exceeds the wilting point of plants, the role of this factor should decrease but it is hard to perform calculations and assessment for this factor separately. It was determined that the overall systematic error of pH measurements in soil solution under low moisture levels in the soil samples under investigation amounted to 0.2-0.3 pH units (Meleshko & Pachepsky, 1981). The insignificant impact of out-flow potential on measurement error under sufficient moistures seems to have made Kerzum et al. (1970) confident of the fact that the glass pH electrodes' data are quite reliable when soil moisture exceeds 10%. Such confidence is proved by an earlier work by Trofimov (1931) in which soil samples were used buffered by the saturated solution of magnesium biphthalate and the phthalic acid. For soil moisture of 15% or higher the deviations between the parallel determinations under similar experimental conditions did not normally exceed 0.05 pH for both the buffered and the untreated soils. We should note here that the liquid junction potential's value can be excluded by using a solid reference electrode (Bound & Fleet, 1977) which has no leak of the solution. When measuring in chains without transfer, that is, in the absence of the liquid junction potential, the ion-selective electrodes sensitive to H ÷, Na ÷ and CI ions give correct data even under moisture close to that of air-dry soil (Goncharov & Kiselev, 1987).
2.2.5. INFLUENCE OF THE GAS PHASE ON THE IONOMETRIC MEASUREMENTS 1N SOIL (INCOMPLETE CONTACT BETWEEN THE ELECTRODE AND SOIL)
In addition to the influence of the solid phase on ionometric measurements in soil with natural moisture, the gas phase can also have its own impact. This is of special importance while
38 determining pH because of the components soil gas phase, primarily CO2, can influence parts of the electrode that do not touch the soil particles and as a result the electrode potential can change. When using the quinhydrone electrode to measure pH by an electrode partially immersed in liquid Trofimov (1931), who was the first to investigate this phenomenon, found that the fact of partial immersion had nothing to do with the deviation in electrode potential. The possibility of distortion of the results of pH measurement in soils by a glass electrode because of the incomplete contact with soil liquid phase has been considered in detail by (Meleshko & Pachepsky, 1981). The possibility of such distortion has been shown when the ball of the glass electrode was only partially plunged into solution. The presence of isopotential point can be observed here, where the potential is independent of the degree of submergence the electrode; and the coordinates of this point are close to the pH value of distilled water, which is in equilibrium with CO2 of the air. It is assumed that on the part of the electrode that is not in contact with soil, there is an absorbed liquid or condensate film. Though interaction with atmospheric CO2 this contributes to the change in electrode potential. The experiments with sand mixed with buffer solutions showed that if the sand moisture is low (2%), the electrode's calibration is close to the scale of the half-submerged one, and under high moisture (20%) to that of an electrode fully submerged into the buffer solution. The error is calculated by the following equation: ApH
= pHmeasurement-
pHtrue = (or- 1)" (pHtrue- pHi),
(3)
where pH~ is the coordinate of the isopotenial point, ct is a coefficient reflecting the degree of the electrode ball's contact with soil; its value for the soils under investigation when the moisture exceeds 5% makes up 0.90 - 0.97. If the soil solution and the film on the electrode's surface are influenced by the same soil air (pHtrue and pHi are very close), then the error of pH determination due to the soil air influence becomes insignificant (
