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Organic Phosphate Transport and Hydrolysis in Soil: Theoretical and Experimental Evaluation¹

ABSTRACT

An understanding of the movement, hydrolysis, and retention of organic phosphates in soils is necessary to evaluate possible advantages of the compounds as fertilizers, based upon their characteristic of increased vertical movement over that of inorganic phosphates when applied with irrigation water. Pulses of chloride and glycerophosphate were applied to 4-, 10-, and 20-cm long Panoche clay loam (Typic Torriorthents) soil columns maintained slightly water-unsaturated (soil-water pressure = –0.018 bar). The concentration of glycerophosphate in the pulse applications varied from 386 to 1,110 ppm P. An analytical solution assuming first-order kinetics and a reversible, linear isotherm for glycerophosphate hydrolysis and sorption, respectively, was compared with measured glycerophosphate concentrations in the column effluent. An analytical solution of the coupled equations for simultaneous transport of organic phosphate and orthophosphate assuming a linear, reversible isotherm for orthophosphate adsorption is given. This solution was compared with measured orthophosphate concentrations in the effluent from a continuous application of glycerophosphate to a 2-cm long column. Organic phosphate adsorption on soil colloids was effectively separated from organic phosphate hydrolysis using displacement techniques. The magnitude of the fitted, first-order constant for hydrolysis was dependent upon the initial concentration of the influent solution indicating that first-order kinetics may not be applicable for high organic phosphate concentrations. The magnitude of the fitted, adsorption-desorption constant for glycerophosphate was dependent upon concentration indicating that adsorption-desorption was nonlinear. The nonlinearity of glycerophosphate adsorption-desorption was verified from an equilibrium, adsorption isotherm. The magnitude of the fitted orthophosphate sorption constant for coupled glycerophosphate and orthophosphate transport was considerably larger than that determined from a 6-day equilibrium adsorption isotherm.

¹ Contribution from the Dep. of Land, Air, and Water Resour. Univ. of Calif., Davis, Calif.

² Research Assistant and Associate Professor of Soil Science, respectively, Dep. of Land, Air, and Water Resour., Univ. of Calif., Davis, CA 95616. Senior author's address is now Activo 20-30, numero 101, Ens. Ozama, Santo Domingo, Dominican Republic.

Received for publication September 27, 1976. Accepted for publication June 14, 1977.

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