Phosphorus Sorption and Movement In FINE Sand Spodosols and Entisols.

Phosphorus is a critical nutrient in citrus production whose deficiency or excess can affect crop yield and quality. Sound stewardship and management of P would help growers maintain high yields, conserve environmental quality and realize high economic returns. Appropriate estimation of sorption coefficients and subsequent characterization of P movement would provide necessary information for sustainable environmental management, reduce problems of eutrophication, and prevent groundwater contamination. Laboratory experiments and computer simulations were conducted to 1) determine the effect of supporting electrolyte on P sorption coefficient (K_D), 2) evaluate the effect of sorption coefficients on prediction of P movement at 30- and 60 cm soil depths using HYDRUS-1D, and 3) compare the P concentrations and fluxes as a function of irrigation frequency and soil type. Results indicate P sorption was of the order: 0.005 M CaCl₂ > 0.01 M KCl ≈ fertilizer mixture [NH₄NO₃ + KH₂PO₄] suggesting an influence of the cation on P adsorption. The sorption coefficients (K_D) determined using 0.005 M CaCl₂ as the supporting electrolyte were about threefold greater than the K_D determined in the fertilizer mixture suggesting that Ca⁺² in solution might result in overestimation of P sorption on sandy Spodosols and Entisols. Model simulations confirmed the laboratory results with slower P mobility using K_D estimated with 0.005 M CaCl₂ than the other two supporting electrolytes. Irrigation frequency did not affect P movement in the two soils.