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

This work considered the aqueous speciation of B between a soil solution containing B and the tetrahedral surface B species (SH3 BOH4- during drying of the soil. The aqueous B species were boric acid (H3BO3) and the borate anion B(OH)(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, salinesodic, and sodic soils. The effective K-d (SH3BOH4-/total solution B) decreased with gravimetric water content (theta(g)) for the range theta(g) = 1.5 to 0.05. A decrease in H+ concentration caused decreasing Kd 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 theta(g) = 1.5 because of variation of the equilibrium constants in the constant capacitance model (K+ and K+) with varying soil texture. Kd 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.