Phosphorus sorption isotherm characteristics and availability parameters of Appalachian acidic soils

Phosphate (P) sorption isotherms have been widely used to characterize the P status and to establish fertilizer requirements of soils. Recently, mechanistic models have successfully described the nutrient uptake by plants under changing soil and plant parameters. Phosphorus sorption characteristics of eleven representative soils of the Appalachian Region of the United States were evaluated, and experimental P adsorption data were fitted to Temkin, Freundlich, and Langmuir equations to determine the relative importance of the soil parameters in P retention and supply to plant roots. The Barber and Cushman model was used to predict the effect of P fertilization on P uptake by plant. The soils of Appalachia differ considerably with respect to the retention of added P. All three isotherm equations gave good fit with the experimental data and were reliable to describe the P quantity/intensity relationships of these soils. The following sequence of P adsorption capacity in various soils was established: Tate (BA) much greater than Hayesville (Bt) > Westmoreland (Bt2) similar to Upshur (Btl) similar to Gilpin (A) > Lily (A) similar to Dandridge (E) similar to Watanga (Ap) > Ashe (A) similar to Berks (A) similar to Dekalb (A). In spite of the great differences among soil properties related to surface area and degree of weathering of these soils, the content of free iron (Fe) oxides was the only soil property that correlated with the constants of the isotherm equations. Amounts of P required to give 0.20 mu g P/mL varied with soils and were closely related to the constants of the isotherm equations. Soil P parameters from the P adsorption isotherms were used to determine a measure of Cli, b, and De instead of those used conventionally in the Barber and Cushman (1981) model to predict P uptake by corn. Predicted P uptake by corn varied with soils, even at the same P concentration in equilibrium solution (0.20 mu g P/mL) or when an equal amount (90 mu g P/mL) of P was added to all soils. Predictions of P uptake, however, were not correlated with the amount of P required to give 0.20 mu g P/mL in 0.01M CaCl2 solution or with the constants of the isotherm equations used. Diffusion coefficient and equilibrium P concentration in solution, and two intensity factors, were more important than buffer power in predicting P supplied to roots. These findings may help to explain the lack of significant correlations between the constants of the P isotherm equations and P uptake.