SOIL-PHOSPHORUS DESORPTION-KINETICS AND ITS RELATIONSHIP WITH PLANT-GROWTH

The kinetics of P desorption in soils is important to consider from a plant nutrition perspective. This study was designed to compare the ab ility of several equations to describe P desorption kinetics from soil s and to relate the information derived from desorption kinetic curves to plant growth and P uptake. Samples of five soils (Psammentic Paleu dult, Typic Calciustoll, Udic Pellustert, Vertic Albaqualf, and Aridic Calciustoll) were treated with five P levels (0, 25, 50 75, and 100 m g kg-1) and incubated at field capacity and 24-degrees-C for 31 d. Pho sphorus desorption kinetic data for each treatment were generated by e quilibrating suspensions of the incubated soil samples with mesh bags containing HCO3--saturated anion-exchange resin for periods up to 96 h . Desorbed P was then determined by extraction of resin P with 0.5 M N aCl. Initial (after 0.25 h) and final (after 72 h) amounts of desorbed P and desorption rates were determined. The kinetic data were best de scribed by the expanded form of the Elovich equation. The initial and final amounts of desorbed P were closely related to relative plant gro wth and P uptake. The final desorbed soil P was the best soil P availa bility index. Soil P desorption rates generally had soil-specific rela tionships with growth of corn (Zea mays L.) and P uptake and did not s eem to limit the P nutrition of plants. The parameters of the expanded Elovich equation were not satisfactory indicators of the P-supplying capacity of the soil.