Submitted to: Vadose Zone Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/2/2002
Publication Date: 2/20/2003
Citation: Vaughan, P.J., Suarez, D.L. 2003. Constant capacitance model prediction of boron speciation for varying soil water content. Vadose Zone Journal. v. 2. p. 253-258. Interpretive Summary: Boron can cause toxic effects in plants at either low or high concentrations in the soil and it is known that there is a limited range of acceptable boron concentration. Soil boron may occur in solution in water that is present in the soil or it may be adsorbed on the surfaces of certain soil minerals such as clays. Only the soil solution boron is important to plants. We solved a set of mathematical equations known as the constant capacitance model to determine the fractions of boron that are adsorbed and in solution. These fractions change depending on conditions in the soil especially the soil pH, a measure of soil acidity. We calculated how the adsorbed and solution boron concentrations change as a soil loses its water due to drying. As the soil dried the solution became more acid and the soil solution boron concentration increased. Plants would be likely to take up less boron as a soil dried simply because there is less water available, but this study showed that the soil solution boron in dry conditions increased providing a counterbalancing effect. This work is important to the development of a comprehensive strategy to address the problems of boron deficiency and toxicity in plants.
Technical Abstract: This work considered the aqueous speciation of boron between a soil solution containing B and the tetrahedral surface B species (SH3BO4-) 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 boron 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 content ranging10-60% and three solution compositions representing saline, saline-sodic and sodic soils. The effective Kd (SH3BO4-/total solution B) decreased with gravimetric water content (theta g) over 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 7-9. Kd varied from 2.5 to 4.7 at theta g=1.5 due to 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 based on 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.