The Kinetics of Phosphorus Desorption from Soil

The kinetics of P desorption for several soils was investigated at different water/soil ratios, during a short period of time, so that the results could be related to P release from agricultural soil to rainfall and runoff water. For all soils the logarithm of P release ( P d ) was linearly related to the logarithm of contact time ( t ) at any given water/soil ratio ( W ) and P amendment and to logarithm of W at any given contact time and P amendment. The amount of P released was also directly proportional to the amount of desorbable P ( P o ) in the soil initially. The following simplified empirical model was developed to describe the desorption of soil P, P d = KP o t α W β , where K , α, and β are constants. The simplified model gave a reasonably good description of P desorption from five southwestern soils and provided similar values of the constants K, α, and β, for each soil, over a range of experimental conditions. Consequently, we suggest that average values of the constants for each soil can be used in the model to describe P release for general applications.