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Published online 25 January 2008
Published in Soil Sci Soc Am J 72:355-361 (2008)
DOI: 10.2136/sssaj2007.0124
© 2008 Soil Science Society of America
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SOIL CHEMISTRY

Influence of Soil Moisture Content on Soil Solution Composition

Carmen L. Dyera, Peter M. Kopittkeb,*, Anna R. Sheldona and Neal W. Menziesb

a School of Land, Crop and Food Sciences, Univ. of Queensland, St. Lucia, QLD 4072, Australia
b School of Land, Crop and Food Sciences, Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC-CARE), Univ. of Queensland, St. Lucia, QLD 4072, Australia

* Corresponding author (p.kopittke{at}uq.edu.au).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Despite the importance of the soil solution to plant growth, few studies have considered the influence of moisture content on the composition of the soil solution. Indeed, soil solution has seldom been extracted from soils below field capacity, despite the relevance of such conditions to plants grown in the field. Soil solution was extracted from a variable-charge soil (Oxisol) and from several permanent-charge soils (Vertisols) at various moisture contents (potentials ranging from –5 to –230 kPa) using polyacrylonitrile hollow-fiber filter elements and pressure chamber apparatus. For the Vertisols, a decrease in moisture content resulted in a proportionate increase in the soil solution ionic strength, a behavior similar to that expected from a solution without any solid-phase interaction. In contrast, for the Oxisol, the ionic strength remained constant as the soil moisture content decreased due to "salt adsorption." For soils containing excess gypsum (CaSO4·2H2O) or lime (CaCO3), the dissolution (with increasing moisture) and precipitation (with decreasing moisture) of these minerals controlled the solution concentrations of their constituent elements. The soil solution composition appeared to conform to that expected from the ratio law (valency effect). The results of this study provide evidence that the method used can extract representative soil solution from soils at moisture contents lower than field capacity. They also demonstrate that changes in soil solution ionic strength and composition, while obeying simple control mechanisms, could not be readily predicted without a detailed understanding of the soil's surface charge and exchange behavior.

Abbreviations: PZNC, point of zero net charge


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The soil solution is of particular importance in soil science and plant nutrition, as it is from this medium that plants take up the majority of the nutrients required for growth. Similarly, it is the presence of elements in the soil solution at phytotoxic concentrations that causes a reduction in plant growth. Changes in the soil moisture content are known to influence the soil solution composition due to changes in ionic concentrations, ionic speciation, and ionic activities (Wolt, 1994). Yet, despite the importance of the soil solution, there are almost no data available describing changes in the soil solution composition for moisture contents representative of those observed in the field (Wolt, 1994). Rather, due to the difficulty of extracting solution from soils that are drier than field capacity (nominally designated as –10 kPa), almost all previous studies of the effect of moisture content on soil solution composition have been conducted in soils at field capacity and wetter. For example, Karlen et al. (1980) worked from "just below field capacity" to 150% of field capacity, Reitemeier (1945) from field capacity to 500% moisture content, and Fotovat et al. (1997) from 150 to 1000% moisture content. While some studies have had treatments drier than field capacity, these studies still tended to focus on soils wetter than field capacity. For example, although Csillag and Redley (1989) examined soils at potentials as low as –250 kPa (pF 3.4), the majority of the soils analyzed by these researchers had moisture contents greater than saturation (–0.1 kPa, pF 0) with little consideration given to the soils below saturation. Similarly, Khasawneh and Adams (1967) worked from approximately –30 kPa (15% moisture) to a moisture content of 1000%, with the data set dominated by the wet soils.

Although the soil solution composition is recognized as a useful predictor of nutrient and contaminant availability to plants, our understanding of the processes that control soil solution composition is limited by the difficulty and tediousness of extracting representative soil solution. Numerous laboratory methods have been proposed for the extraction of the soil solution, including centrifuge drainage (Gillman, 1976), immiscible liquid displacement (Menzies and Bell, 1988; Whelan and Barrow, 1980), vacuum displacement (Wolt and Graveel, 1986), polyacrylonitrile hollow-fiber filter elements (Menzies and Guppy, 2000), and pressure-membrane displacement (Csillag and Redley, 1989; Karlen et al., 1980; Richards, 1941). For field studies, the use of drainage (zero-tension) lysimetry (Litaor, 1988) or suction extraction (tension) using porous cups or plates (Wagner, 1962), and more recently hollow-fiber elements (Jones and Edwards, 1993), are common. Irrespective of the method chosen, consideration must be given to the influence that each particular method will have on the soil solution composition during the extraction process (Gillman and Bell, 1978; Wolt, 1994).

In this study, we examined the effect of changing soil moisture content on the soil solution composition across a range of moisture contents applicable to the field situation. Soil solution was extracted from two soil types, an Oxisol and several Vertisols, at potentials ranging from –5 to –230 kPa. The soil solution was extracted using polyacrylonitrile hollow-fiber filter elements in combination with the pressure-membrane displacement technique. We aimed to establish that the extraction method used produced solution that accurately represented the solution within the soil matrix. In addition, we aimed to determine the main mechanisms controlling soil solution composition as soils dried.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soils Preparation
To examine the effect of soil moisture content and soil type on the soil solution composition, the surface soil of an Oxisol was collected from Redland Bay, Queensland, Australia (27°37'34'' S, 153°17'52'' E). In addition, Vertisols were collected from several locations throughout Queensland: a nonsaline gypsiferous subsoil (0.6–1.0 m) and a saline gypsiferous subsoil from Roma (26°34'43'' S, 148°47'5'' E); a saline subsoil from Goondiwindi (28°32'54'' S, 150°19'19'' E); and a nonsaline surface soil from Gatton (27°33'29'' S, 152°19'58'' E). Following collection, all soils were air dried and ground to pass through a 2-mm sieve. Soil-water characteristic curves were determined for each of these four soils using the pressure plate method (Klute, 1986).

Nine soil treatments were prepared, consisting of four for the Oxisol and five for the Vertisols. For the Oxisol, the treatments consisted of: control (no amendment); gypsiferous (control soil plus 10 g CaSO4•2H2O kg–1); low salinity (control soil plus 1 g NaCl kg–1); and strongly saline (control soil plus 2 g NaCl kg–1). For the Vertisols, the treatments were: nonsaline (Gatton soil, no amendment); nonsaline calcareous (Gatton soil plus 100 g CaCO3 kg–1); saline (Goondiwindi soil, no amendment); nonsaline gypsiferous (Roma nonsaline gypsiferous soil, no amendment); and saline gypsiferous (Roma saline gypsiferous soil, no amendment). For brevity, all treatments will be referred to according to the following classification: O = Oxisol, V = Vertisol, S = saline, G = gypsiferous, C = calcareous, S(1) = NaCl added at 1 g kg–1, and S(2) = NaCl added at 2 g kg–1. All soils that received an amendment were wet to field capacity (–10 kPa; 37% for the Oxisol and 40% for the nonsaline Vertisol) with deionized water. Soils to which NaCl was added were allowed to equilibrate for 1 d, while those to which CaSO4•2H2O or CaCO3 was added were allowed to equilibrate for 5 d at 25°C. Following the equilibration period, these soils were air dried (3–5 d drying time) and ground to pass a 2-mm sieve.

Soil solution pH and electrical conductivity (EC) were determined for all treatments at field capacity (Table 1 ). Exchangeable cation concentrations were determined by inductively coupled plasma–optical emission spectroscopy (ICP–OES) following extraction with 0.1 mol L–1 BaCl2 and 0.1 mol L–1 NH4Cl at a 1:10 soil/extractant ratio (Gillman et al., 1982). The exchangeable cation concentration was corrected for the soil solution cations measured at field capacity (So et al., 2006).


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  		Table 1. The pH and electrical conductivity (EC) of soil solution extracted at field capacity, soil organic C and exchangeable cation concentrations, and the effective cation exchange capacity (ECEC) calculated as the difference between extractable and soil solution cations.

 
Soil Solution Extraction and Analysis
Soil solution samplers were assembled from hollow-fiber polyacrylonitrile filter elements (50,000 Da molecular weight cut-off) according to Menzies and Guppy (2000), with each filter element arranged in a loop using a Tygon T-connector.

The nine soil treatments were then placed in a series of polyethylene bags (0.025-mm wall thickness), with each wet to four moisture contents with triple deionized water (Table 2 ) and allowed to equilibrate for 3 d at 25°C (Menzies et al., 1991). This sample preparation approach aimed to ensure that a uniform soil solution composition would exist throughout the sample. Following the 3-d equilibration period, the 36 treatments (nine soil treatments x four moisture contents [Table 2]) were placed into a plastic petri dish (90-mm diameter) with a soil solution sampler. Each of the 36 treatments was replicated four times, thus giving a total of 144 petri dishes. Soil was packed around the filter element to ensure good contact between the soil and the element. Each petri dish was placed inside a pressure chamber and connected using a small-bore polytetrafluoroethylene tube to a Venoject blood-collecting vial (Terumo Corp., Tokyo), which was external to the chamber and at atmospheric pressure. A pressure of up to 900 kPa was applied within the chamber, forcing the soil solution through the sampler and into the collection vial. While the majority of soil solution was extracted within 1 or 2 d, for soils at lower matric potentials (more negative than –40 kPa), up to 7 d was required. Using this method, no soil solution was extracted from soils with matric potentials exceeding –300 kPa.


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  		Table 2. The gravimetric moisture content and corresponding matric potential for the various soil treatments.

 
Soil solution pH and EC were measured immediately following extraction. Samples were then stored at 4°C before analysis by ICP–OES for Ca, Mg, K, Na, Fe, Al, and Si, and by ion chromatography (Dionex ICS-2000, Dionex Corp., Sunnyvale, CA) for Cl–, SO42–, NO3–, and PO43–. The ionic strength of the solutions was calculated from the EC according to Menzies and Bell (1988). Soil solution chemical speciation calculations were performed using PhreeqcI 2.12 with the Minteq database (Parkhurst, 2005).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
The soil solution composition was influenced by the soil moisture content, although the effect was dependent on the soil type (Vertisol or Oxisol) and the soil treatment (Fig. 1 ). For the Vertisols, as moisture content decreased, soil solution pH decreased in the V, V-C, and V-G treatments, while it remained relatively constant in the V-SG and V-S treatments (Fig. 1). A similar pH decrease with decreasing moisture content has been reported for a calcareous soil by Misra and Tyler (1999). In the Oxisol, soil solution pH decreased slightly (an average of 0.5 pH units) across the measured moisture content range (Fig. 1).


Figure 1
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  		Fig. 1. Effect of soil moisture content on the soil solution ionic strength (top) and pH (bottom) for several Vertisols (V, left) and an Oxisol (O, right); S = saline, G = gypsiferous, C = calcareous. Ionic strength estimated from the electrical conductivity. The solid gray lines without data represent the expected ionic strength that would result from a decrease in moisture content without any interaction with the soil solid phase. The vertical bars represent the standard deviations from the mean of four replicates.

 
As observed for pH, the soil solution ionic strength (and thus the concentrations of cations and anions) was also influenced by the soil moisture content. For all Vertisols other than the gypsiferous Vertisol, soil solution ionic strength increased as the moisture content decreased (Fig. 1). The magnitude of this ionic strength increase was generally similar to that predicted for a solution without any solid-phase interaction (i.e., without any adsorption, desorption, or precipitation reactions). Indeed, this increase in Vertisol ionic strength was associated with an increase in soil solution ion concentrations for all treatments other than the V-G treatment (Fig. 2 ). For the V-G treatment, the concentrations of the major ions were relatively constant with changing moisture content (Fig. 2). In contrast to the Vertisols, the soil solution ionic strength of the four Oxisol treatments remained relatively constant across all measured moisture contents (Fig. 1) and the change in concentration for the major anions and cations was generally much less than that expected if there was no interaction with the soil solid phase (Fig. 3 ).


Figure 2
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  		Fig. 2. Effect of soil moisture content on the composition of the soil solution of a saline Vertisol (V-S, left), a saline gypsiferous Vertisol (V-SG, middle), and a gypsiferous Vertisol (V-G, right). The dotted lines without data represent the expected ionic strength that would result from a decrease in moisture content without any interaction with the soil solid phase. The vertical bars represent the standard deviations from the mean of four replicates.

 

Figure 3
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  		Fig. 3. Effect of soil moisture content on the composition of the soil solution of an Oxisol with no amendment (left), an Oxisol with NaCl added at 1 g kg–1 (middle), and an Oxisol with CaSO4·2H2O added at 10 g kg–1 (right). The dotted lines without data represent the expected ionic strength that would result from a decrease in moisture content without any interaction with the soil solid phase. The vertical bars represent the standard deviations from the mean of four replicates.

 
The changes observed in the soil solution composition, which were dependent on the soil type, can be explained through the consideration of mechanisms outlined by Fotovat et al. (1997) and Khasawneh and Adams (1967): exchange, changes in particle surface chemistry, dissolution or precipitation, and the "valency effect."

Particle Surface Chemistry and the Diffuse Double Layer
For the majority of soils, particularly those that did not contain free (reactive) minerals such as CaSO4•2H2O and CaCO3, the most important factor controlling the influence of the soil moisture content on the soil solution composition is the soil mineralogy. For soils (such as the Vertisols) that contain predominantly permanent-charge minerals, the surface charge density of the soil is largely independent of the soil solution properties. Hence, for such soils, as the moisture content changes, the soil solution will behave almost entirely as a simple one-phase system (i.e., there will be minimal liquid–solid interaction). The modest changes in cation composition (Fig. 2) driven by exchange reactions are discussed below.

As expected, a decrease in the soil moisture content increased the soil solution ionic strength for all of the Vertisols (other than V-G) at a rate similar to that predicted for a system without any solid-phase interaction (Fig. 1). For the V-G treatment (saturated with respect to CaSO4•2H2O), ionic strength remained relatively constant across all moisture contents, due primarily to the dissolution and precipitation of CaSO4•2H2O (see below). Similarly, given that there is little interaction between the solution and solid phases in the Vertisols, it was anticipated that soil solution pH would not be influenced by the soil moisture content. Indeed, for the V-SG and V-S treatments, soil solution pH tended to remain relatively constant across the moisture contents examined (Fig. 1). As moisture content decreased in the V, V-G, and V-C treatments, however, soil solution pH decreased (Fig. 1). For the V-C treatment, this pH decrease was an experimental artifact associated with the pressurization with N2 gas, a decrease in the partial pressure of CO2, and a loss of HCO3– and CO32– from the soil solution (see below). It is unclear why pH decreased for the V and V-G treatments.

In contrast to the permanent-charge system, for soils (such as the Oxisol) dominated by variable-charge minerals, the equilibrium between the solid phase and the soil solution is influenced by the properties of the solution, such as ionic strength and pH (Uehara and Gillman, 1981). In variable-charge soils, an increase in the soil solution ionic strength increases the surface charge density and hence increases the net uptake (adsorption) of ions from solution (e.g., Donn and Menzies, 2005). Similarly, depending on the soil solution pH in relation to the point of zero net charge (PZNC), a change in ionic strength will increase or decrease the soil solution pH. The PZNC for Oxisols typically ranges between approximately 3.5 and 5.0 (Appel and Ma, 2002; Appel et al., 2003; Fox, 1982), although a range of 2.7 to 6.5 has been reported for a range of Oxisols (Theng, 1980). The Oxisol surface soil used in this study has a pH above the PNZC, as demonstrated by a {Delta}pH (pH1:5 1 mol L–1 KCl – pH1:5 water) of –0.8. The negative {Delta}pH indicates greater release of protons through cation exchange capacity increase than adsorption of protons through anion exchange capacity increase (Uehara and Gillman, 1981). Nevertheless, this {Delta}pH value also indicates that the soil carries some anion exchange capacity; Black and Waring (1976) reported that this soil adsorbed NO3– when the {Delta}pH was more positive than –1.0.

For the Oxisol, which is dominated by variable-charge minerals, it was anticipated that, as the soil moisture content decreased, the increase in soil solution ionic strength would be partially offset by an increase in surface charge density. It was noted that the ionic strength did not increase at all, however, but rather was constant across all moisture contents (Fig. 1). It is considered that for the Oxisol, the expected partial increase in ionic strength of the bulk soil solution was offset by "salt adsorption" (see below). While the ionic strength of the Oxisol soil solution remained relatively constant, the pH decreased slightly (an average of 0.5 pH units) as moisture content decreased (Fig. 1). Considering that the Oxisol used in this study is above the PZNC, an increase in ionic strength would result in a decrease in pH due to a net movement of H+ off the soil surface (Uehara and Gillman, 1981; van Olphen, 1977). Although the ionic strength of the bulk soil solution did not increase as the soils became drier (Fig. 1), the ionic strength of the solution at the liquid–solid interface would have increased due to salt adsorption (see below). Thus, it is considered that the slight decrease in soil solution pH resulted from an increase in ionic strength at the liquid–solid interface (salt adsorption) and the release of H+ from the soil surface.

Salt Adsorption
Although it was anticipated that the ionic strength of the Oxisol would increase as moisture content decreased (and that this increase would be partially offset by an increase in surface charge density), ionic strength remained relatively constant across all treatments (Fig. 1). For the O-S(1) and O-S(2) treatments, it was noted that at any given moisture content, the solution cation and anion concentrations were less than expected (calculated according to the Na and Cl added, the solution cation and anion concentrations measured after NaCl addition, the solution cation and anion concentrations of the control, and the moisture content of the soil) (Fig. 4 ). Indeed, even though the bulk solution ionic strength was constant across the various moisture contents (Fig. 1), the difference between the expected cation and anion concentrations and measured cation and anion concentrations (i.e., the cations and anions adsorbed) increased as the soil moisture content decreased. At each of the moisture contents, the magnitude of the reduction in the cation concentration was similar to the reduction in the anion concentration (Fig. 4). It is considered that this apparent lack of change in ionic strength was due to the movement of cations and anions from the soil solution onto the solid phase at the lower moisture contents (Fig. 3 and 5 ) by a process known as salt adsorption.


Figure 4
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  		Fig. 4. Effect of soil moisture content on the difference between the expected cation and anion concentrations and the measured cation and anion concentrations in the soil solution of an Oxisol with NaCl added at 1 g kg–1 [O-S(1)] and an Oxisol with NaCl added at 2 g kg–1 [O-S(2)]. The expected cation and anion concentrations were calculated from the Na and Cl added, the moisture content of the soil, and the soil solution cation and anion concentrations of the Oxisol with no NaCl added.

 

Figure 5
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  		Fig. 5. Effect of soil moisture content on the soil solution cation composition per unit mass (total cations = Ca + Mg + Na + K, divalent cations = Ca + Mg, monovalent cations = Na + K) of soil for several Vertisols (V, left) and an Oxisol (O, right); S = saline, G = gypsiferous, C = calcareous, S(1) = NaCl added at 1 g kg–1, S(2) = NaCl added at 2 g kg–1.

 
The pH of the Oxisol (pH 6) is conducive to salt adsorption, as at this pH the soil will possess both negatively charged sites (such as kaolinite, PZNC 2.8–2.9, and organic matter) and positively charged sites (such as Al and Fe oxides, PZNC 8–9) (Qafoku and Sumner, 2002). Thus, for the Oxisol, it is considered that the increase in ionic strength associated with a decrease in moisture content resulted in salt adsorption, described by Qafoku and Sumner (2002) as: (i) the compression of the oppositely charged diffuse layers; (ii) a decrease in diffuse layer overlap and mutual charge neutralization, (iii) the adsorption of ions in oppositely charged diffuse layers to counteract the decrease in mutual charge neutralization, and (iv) a corresponding reduction in the ionic strength of the bulk solution to the original level (Fig. 3, 4, and 5).

Salt adsorption is defined as the simultaneous adsorption, in equivalent amounts, of the cation and anion of an electrolyte with no net release of other ions into the soil solution (Qafoku and Sumner, 2002) and occurs only in variable-charge soils when both negatively charged and positively charged particles are present. At low ionic strengths, the diffuse layers are comparatively large, and some mutual charge neutralization occurs between the positive and negative surfaces. As ionic strength increases (due to the addition of salts or a decrease in moisture content) and the diffuse layers are compressed, however, mutual charge neutralization decreases and indifferent ions are adsorbed in the oppositely charged diffuse layers (salt adsorption) to offset the reduction in mutual charge neutralization (Qafoku and Sumner, 2002). Thus, while the process of salt adsorption results in an increase in the ionic strength of the diffuse double layer (and thus causes an increase in surface charge density), the ionic strength of the bulk soil solution does not change.

Mineral Dissolution and Precipitation
For soils such as the Vertisols that are dominated by permanent-charge minerals, salt adsorption does not occur, and changes in moisture content (and the ensuing change in ionic strength) have minimal influence on the surface charge density. Hence, soil solution ionic concentrations are controlled mainly by the dilution or concentration resulting from moisture content changes and by mineral dissolution (with increasing moisture) and precipitation (with decreasing moisture).

For the V treatment (data not presented) and V-S treatments (Fig. 2), concentrations of measured cations and anions increased at a rate approximately equal to that expected for a simple decrease in moisture content (i.e., no interaction with the solid phase). In contrast, for the two gypsiferous treatments (V-G and V-SG), concentrations of Ca and SO42– increased less than expected for the decrease in moisture content, as these elements were controlled by the dissolution or precipitation of CaSO4•2H2O (Fig. 2). Indeed, modeling with PhreeqcI indicated that for both treatments, the soil solutions were saturated with respect to CaSO4•2H2O at all moisture contents (saturation index of 0.26 to 0.28 for V-G, and 0.32 to 0.34 for V-SG [a saturation index of zero indicates saturation]). Precipitation of gypsum would produce a stoichiometric removal of Ca and SO42–, thus an equimolar reduction of Ca and SO42– in solution would be expected; however, the difference between the expected concentration (assuming no interaction) and measured concentration was greater for SO42– than for Ca for both V-G and V-SG (Fig. 2). This difference was due to the buffering effect of the cation exchange complex, and to cation selectivity or the valency effect (see below), with the proportion of Ca in the soil solution increasing (compared with Na) as the moisture content decreased (Fig. 2).

For the calcareous Vertisol (V-C), it was expected that Ca concentrations would remain constant as the moisture content decreased due to the precipitation of CaCO3; however, Ca concentrations were observed to increase as moisture content decreased (data not presented). It was also noted that, although the soil solution pH for the highest moisture content (pH 8.19) was similar to that expected for a solution in equilibrium with atmospheric CO2 and CaCO3 (pH 8.34) (Lindsay, 1979), soil solution pH tended to decrease as the soil moisture content decreased (Fig. 1). It is considered that for the V-C treatment, the increase in Ca concentration and decrease in pH are experimental artifacts resulting from the use of a chamber pressurized with N2 gas to extract the soil solution. While the majority of soil solution was extracted within 1 h from the wettest treatments, up to 7 d of extraction were required for the lower moisture contents. Disruption of the soil through sieving and wetting and drying is known to alter the solution composition (Cabrera and Kissel, 1988; Chapman et al., 1997), and to accelerate mineralization (Seneviratne and Wild, 1985). For the drier soils, the extended period of moist incubation will have resulted in additional mineralization occurring, increasing the NO3– concentration in solution. The similar behavior of the two monovalent anions, Cl– and NO3– (Fig. 2 and 3), however, indicates that in these soils the effect was small. Mineralization will also have resulted in a lowering of the soil pH, with this further accentuated by the effect of changing CO2 partial pressure during and after the extraction. During the extraction period, gas leakage from the pressure chamber necessitated repressurization with additional N2, thus reducing the CO2 partial pressure in the chamber and increasing the solution pH. Once the soil solution passed across the filter element membrane and was discharged to the collection vial, however, the CO2 partial pressure to which it was exposed rose to that of the atmosphere, lowering the pH of the extracted solution. The extracted solution was no longer buffered by the presence of CaCO3, and speciation calculations showed that the solution was undersaturated with respect to calcite and atmospheric CO2.

The dissolution and precipitation of CaSO4•2H2O was also an important mechanism controlling the concentration of Ca and SO42– in the gypsiferous Oxisol (O-G) treatment (Fig. 3), although the soil solution concentrations for this treatment are also confounded by the concurrent change in surface charge density and salt adsorption (see above). Nevertheless, speciation calculations showed that the soil solution was saturated with CaSO4•2H2O across all moisture contents (saturation indices of 0.19–0.24).

Valency Effect
According to the ratio law proposed by Schofield (1947), as the moisture content of a soil increases, divalent (and trivalent) cations will tend to preferentially move from the solution onto the soil's exchange sites. Thus, an increase in soil moisture content is expected to decrease the proportion of divalent cations in the soil solution compared with the monovalent cations. This trend has been reported in soil solutions by Ulrich and Khanna (1972), Khasawneh and Adams (1967), Csillag and Redley (1989), and Fotovat et al. (1997). If the soil–water–electrolyte system conforms to this ratio law of Schofield, then a plot of Na/(Ca0.5) against v0.5 (where Na and Ca are the number of millimoles of the ion present in the soil solution of volume v [expressed in liters per kilogram of soil]) should give a linear plot passing through the origin (Khasawneh and Adams, 1967). Indeed, in the current study, Na/(Ca0.5) generally decreased with decreasing moisture content (Fig. 6 ), with linear regressions of most passing approximately through the origin (data not presented). Thus, despite the narrow range of moisture contents used in the current study (when compared with previous studies such as Fotovat et al. [1997] and Khasawneh and Adams [1967]) (Table 2), the soil solution composition generally followed that expected from the ratio law of Schofield (1947).


Figure 6
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  		Fig. 6. Effect of soil moisture content (presented as the square root of the volume of water in the soil [v, L/kg]) on the Na and Ca ratio (where Na and Ca are the number of millimoles of the ion present in the soil solution of volume v [see Khasawneh and Adams (1967) for details]) for several Vertisols (V, left) and an Oxisol (O, right); S = saline, G = gypsiferous, C = calcareous, S(1) = NaCl added at 1 g kg–1, S(2) = NaCl added at 2 g kg–1.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Changes in soil moisture content were observed to impact on the composition of the soil solution, although the effect was dependent on the soil treatment and soil type. For the Vertisols (predominately permanent-charge soils), a decrease in moisture content increased the soil solution ionic strength at a rate similar to that expected from a solution without any solid-phase interaction (i.e., a simple dilution–concentration effect). In contrast, for the Oxisol, which is dominated by variable-charge minerals, the ionic strength remained relatively constant as the soil moisture content decreased and was associated with a decrease in the total sum of cations in the soil solution at lower moisture contents. This observed behavior of the Oxisol is attributed to salt adsorption. For soils containing excess CaSO4•2H2O or CaCO3, the dissolution (with increasing moisture) and precipitation (with decreasing moisture) of these minerals controlled the solution concentrations of the corresponding elements. The soil solution composition appeared to conform to that expected from the ratio law (valency effect).


   ACKNOWLEDGMENTS
 
The technical assistance of Douglas Crim is appreciated. This research was funded through the Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC-CARE) Project 3-3-01-05/6 and the Grains Research and Development Corporation (GRDC) Project SIP08.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication April 3, 2007.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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Right arrow Articles by Menzies, N. W.
Agricola
Right arrow Articles by Dyer, C. L.
Right arrow Articles by Menzies, N. W.
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Right arrow Soil Surface Chemistry
Right arrow Soil Chemistry

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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome