Constant Capacitance Model Computation of Boron Speciation for Varying Soil Water Content

content (Keren et al., 1985). The range of tolerance for both sensitive and tolerant plants to soil solution This work considered the aqueous speciation of B between a soil B concentration is 0.028 to 1.39 mmol L 1. Plants are solution containing B and the tetrahedral surface B species (SH3 BOH 4 during drying of the soil. The aqueous B species were boric not sensitive to adsorbed B. Adsorption of B on clay acid (H3BO3) and the borate anion B(OH) 4 . A computer program minerals, calcite, and organic matter in soil reduces B was written to calculate solution speciation of major ions using a in soil solution and, therefore, its availability to plants. matrix-type numerical solution including cation exchange and dissoluAs pH is increased, the adsorption of B by soils intion–precipitation of calcite. The B speciation was calculated sepacreases. Conversely, the lowering of soil pH in the range rately but utilized the H concentration as determined in the major 7 to 9 increases the availability of B, potentially inducing ion speciation. Numerical simulations of soil drying were performed B toxicity effects in plants (Goldberg, 1997; Keren et for 20 hypothetical soil textures with clay contents ranging from 10 to 60% and three solution compositions representing saline, salineal., 1985; Goldberg and Glaubig, 1986; Bingham et al., sodic, and sodic soils. The effective Kd (SH3BOH 4 /total solution B) 1971). Adsorption–desorption reactions are primarily decreased with gravimetric water content ( g) for the range g 1.5 responsible for controlling soil B concentrations, as to 0.05. A decrease in H concentration caused decreasing Kd consiscompared with other types of chemical reactions involvtent with earlier experimental work showing decreasing fractional ing B (Goldberg, 1997). adsorbed B with decreasing pH in the range 7 to 9. Kd varied from Boron adsorption and desorption reactions have been 2.5 to 4.7 at g 1.5 because of variation of the equilibrium constants modeled by several tools, including the Langmuir and in the constant capacitance model (K and K ) with varying soil texFreundlich isotherms (Rhoades et al., 1970; Fleet, 1965), ture. Kd increased with increasing sodicity of the soil water. An application of this program would be prediction of adsorbed and solution B the Keren equation (Keren and Mezuman, 1981), and concentrations at field water content on the basis of experimental the constant capacitance model (Goldberg, 1997). Nondeterminations of adsorbed and solution B concentration for saturated equilibrium models describing hysteresis in the adsorppaste extracts. Such predictions would be useful to generate initial tion–desorption reactions include characterization of conditions for solute transport modeling and for determining whether the kinetics of B desorption (Griffin and Burau, 1974) solution B concentrations at field water contents would be beneficial and evaluation of adsorption–desorption rate constants or harmful to plants. (Keren and Sparks, 1994). Boron adsorption–desorption reactions in hysteretic soils were modeled by separate sets of Freundlich parameters for each type of reaction T occurrence and transport of B in the vadose (Elrashidi and O’Connor, 1982). Drying of montmorilzone has long been of interest due to environmental lonite suspensions substantially increased adsorption of concerns and its agroeconomic importance. Considering B at a fixed pH compared with nondried systems (Keren transport, the relevant forms of B are soil solution speand Gast, 1981). Keren and Gast also found decreases cies and sorbed B on surfaces of various minerals and in adsorption of B on montmorillonite with decreasing organic matter (Goldberg, 1997). The objective of our pH in the range 7 to 9. work was to develop a method for calculating the speciaVariation in soil water B concentration with varying tion of B for varying soil water content. This is relevant water content was calculated using a nonreacting model to further understanding of the phytotoxicity of B and to developing a method for extrapolating the determinaand an adsorption model (Mezuman and Keren, 1981). tion of soil B concentration in saturation paste extracts The nonreacting model merely displayed the concento lower water contents characteristic of field conditions. trating effect of water removal, whereas the adsorption Such extrapolations could be applied to specification of model indicated substantial adsorption of B and only a initial conditions for modeling transport of B in soils. minor increase in solution B concentration. These calcuIn the interest of generality, the program we have devellations relied on the assumptions of equilibrium and oped for B speciation can also be used to calculate the constant solution pH. The equilibrium assumption can water content dependence of the concentration of other be considered reasonable for nonhysteretic soils, but the chemical species that are commonly present in soils. constant pH assumption may not be valid during drying The toxic effect of soil B on plants operates through of most soils. We examine a soil drying simulation in which the soil solution, as determined by experiments with pH is calculated by a generalized speciation model to varying soil solution B content at “field capacity” water evaluate the combined effects of varying pH and dilution on prediction of B speciation by the constant caP.J. Vaughan and D.L. Suarez, George E. Brown, Jr. Salinity Laborapacitance model. tory, USDA-ARS, 450 W. Big Springs Road, Riverside, CA 92507. Received 16 Sept. 2002. Original Research Paper. *Corresponding author ([email protected]). Abbreviations: CEC, cation exchange capacity; EC, electrical conductivity; ESP, exchangeable sodium percentage. Published in Vadose Zone Journal 2:253–258 (2003). 253 Published May, 2003