Increased P application to lateritic soil in 1976 increased Colwell soil test P for P applied in 2000

In a field experiment, 6 amounts of superphosphate [0 - 800 kg phosphorus (P)/ha] were applied in July 2000 to an acidic lateritic ironstone gravel sandy soil treated 24 years previously ( May 1976) with 6 amounts of superphosphate ( 0 - 599 kg P/ha). In October 2001, samples of the top 10 cm of soil were collected to measure the capacity of the soil to sorb P by the phosphate retention index (PRI) and P buffer capacity (PBC) methods, and to measure soil test P by the Colwell ( sodium bicarbonate) procedure. The capacity of the soil to sorb P, as measured by both PRI and PBC, decreased with increased P application in 1976. For all amounts of P applied in 2000, Colwell soil test P increased with increased P application in 1976. We conclude that increasing amounts of P applied in 1976 decreased the capacity of the soil to sorb the subsequent application of P, thereby increasing soil test P values of soil treated with the subsequent P.