A reliable estimation of the amount of residual available P in soil is needed for a relatively accurate fertilizer recommendation. In an attempt to predict the amount of residual P by means of a model, the kinetics of recovery of applied P were studied in different soils of Iran. Thirty composite soil samples covering wide range in relevant properties were collected from different areas of the country. The soils were treated with 0, 50 and 500 mg P kg–1 and incubated at 85 % field capacity under greenhouse conditions for 365 days. Samples of the soils were taken at certain intervals and were extracted at the same moisture for Olsen-P. Results indicated that decrease in recovery was sharp at initial periods of incubation followed by a slow decrement at longer times, in all soils. The results were well described by the following equation: R =100/ (1+ ktb), where R is the percent of recovery at time t, and k and b are empirical constants. In 25 alkaline soils of this experiment, the amounts of k and b were in the ranges of 0.19 to 0.73 and 0.11 to 0.55, respectively, while in four acid soils (pH< 6.8) the same parameters were in the ranges of 0.86 to 1.83 and 0.085 to 0.211, respectively. These constants were correlated to some soil properties. The constant k in alkaline soils was correlated with carbonate-free clay content, free iron oxide and active calcium carbonate equivalent, while the same constant was correlated with pH in acid soils. The constant b in alkaline soils was correlated with organic carbon and pH while a good correlation was obtained between b and free iron oxide in acid soils.
)