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

George E. Brown, Jr. Salinity Laboratory, USDA-ARS, 450 W. Big Springs Road, Riverside, CA 92507
^* Corresponding author (pvaughan{at}ussl.ars.usda.gov)

Received 16 September 2002.

This work considered the aqueous speciation of B between a soil solution containing B and the tetrahedral surface B species (SH₃BOH^-₄ during drying of the soil. The aqueous B species were boric acid (H₃BO₃) and the borate anion B^-₄. A computer program was written to calculate solution speciation of major ions using a matrix-type numerical solution including cation exchange and dissolution–precipitation of calcite. The B speciation was calculated separately but utilized the H⁺ concentration as determined in the major ion speciation. Numerical simulations of soil drying were performed for 20 hypothetical soil textures with clay contents ranging from 10 to 60% and three solution compositions representing saline, saline-sodic, and sodic soils. The effective K_d (SH₃BOH^-₄/total solution B) decreased with gravimetric water content (_g) for the range _g = 1.5 to 0.05. A decrease in H⁺ concentration caused decreasing K_d consistent with earlier experimental work showing decreasing fractional adsorbed B with decreasing pH in the range 7 to 9. K_d varied from 2.5 to 4.7 at _g = 1.5 because of variation of the equilibrium constants in the constant capacitance model (K^- and K⁺) with varying soil texture. K_d increased with increasing sodicity of the soil water. An application of this program would be prediction of adsorbed and solution B concentrations at field water content on the basis of experimental determinations of adsorbed and solution B concentration for saturated paste extracts. Such predictions would be useful to generate initial conditions for solute transport modeling and for determining whether solution B concentrations at field water contents would be beneficial or harmful to plants.

Abbreviations: CEC, cation exchange capacity • EC, electrical conductivity • ESP, exchangeable sodium percentage