Field studies of the dynamics of transformation and availability of applied phosphorus in a Vertisol cropped with rainfed cotton showed that phosphorus application increased available phosphorous in the soil. However, 70–80% of the phosphorus was fixed when more than 40 kg P2O5 ha−1 was applied. Maximum uptake of phosphorus by cotton occurred during the active growth period 40 to 80 days after germination, as reflected in a steep decline in the labile pool of available phosphorus in the soil. Over 50% of the total inorganic phosphorus occurred as Ca-P, which was the major contributor to the labile pool available to the cotton crop. The maximum response of cotton in terms of both dry matter and seed cotton and the maximum uptake of nitrogen, phosphorus and potash occurred when phosphorus was applied at 40 kg P2O5 ha−1. Phosphorus application increased the length, spread, volume and weight of roots.