DETERMINATION OF PHOSPHATE DESORPTION CHARACTERISTICS IN SOILS USING SUCCESSIVE RESIN EXTRACTIONS

An alternative method for examining phosphate desorption characteristi cs in soil was tested. Five tropical soils with very different phospha te sorption capacities were incubated with added phosphate under three different conditions: 2 days at 25 degrees C, 55 days at 25 degrees C , and 2 days at 50 degrees C. After incubation the phosphate was desor bed from the soil using successive cation-anion resin extractions. The data from these extractions were fitted to a first order rate equatio n describing desorption. From the equation, an asymptote (B) was found to represent the ultimate amount of phosphate desorbable from each so il after incubation. It was found that increasing the incubation time or increasing the temperature of the incubation lowered this parameter 'B' suggesting that the slow reaction of adsorbed phosphate had reduc ed the amount of readily desorbable phosphate. Differences between soi ls as reflected in this parameter may indicate differences in the resi dual value of added phosphate in the field.