Phosphorus release from coated monoammonium phosphate: Effect of coating thickness, temperature, elution medium, soil moisture and placement method

Abstract

Coating fertilizer granules with a polymer slows the release of nutrients over a long period of time, so that the fertilizer uptake by plants may be enhanced. Two laboratory experiments were conducted to determine the effects of coating thickness, temperature, soil moisture, and placement method on the release rate of P from coated monoammonium phosphate (MAP) in soil; to compare the release rate of P from coated MAP in soil and in water; and to evaluate different kinetic equations for simulating the release of P from coated MAP. In exp. 1, release of P from 10 pre-weighed granules of a non-coated MAP and three polymer-coated MAP fertilizers (Coated I, 4%; Coated II, 4.5%; and Coated III, 5%) was determined in water at 10 and 23°C. In exp. 2, release of P from the four MAP fertilizers in soil was determined at two temperatures (10 and 23°C), two moisture levels (90 and 60% field capacity) and two placement methods (incorporation and banding). Four kinetic equations, i.e., simple Elovich, Parabolic diffusion, Power function, and First order kinetics, were evaluated to simulate the release of P from coated MAP. Activation energy of P release was calculated using the P release versus ln time plots of the simple Elovich equation. The release rate of P from coated MAP was strongly affected by coating thickness and temperature but was not affected by method of P placement and soil water content. The release of P from coated MAP was slower in soil than in water, and it decreased markedly with increasing thickness of polymer coating on MAP, whereas it increased with increasing temperature. The simple Elovich equation best described the release of P from coated MAP in both water and soil. The activation energy was not affected by elution medium and coating thickness but it was lower at the higher temperature. In conclusion, the test of P release rate from coated MAP in water may not represent the true release rate of P from coated MAP in soil, as it is influenced by elution medium, temperature and coating thickness. The simple Elovich equation best described the P release kinetics from coated MAP in soil and in water.