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 (SH 3 BOH − 4 during drying of the soil. The aqueous B species were boric acid (H 3 BO 3 ) 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, saline‐sodic, and sodic soils. The effective K d (SH 3 BOH − 4 /total solution B) decreased with gravimetric water content (θ g ) for the range θ 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 θ 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.