Phosphorus accumulation, leaching and residual effects on crop yields from long‐term applications in the subtropics

The effects of 25 years of annual applications of P fertilizer on the accumulation and migration of soil Olsen‐P, and the effects of soil residual P on crop yields by withholding P application for the following 5 years, were evaluated in a subtropical region. Annual application of P fertilizer for 25 years to crops in summer (groundnut), winter (wheat, mustard or rapeseed) or in both seasons raised the Olsen‐P status of the plough layer (0–15 cm) from initially very low (12 kg P ha −1 ) to medium (18 kg P ha −1 ) and very high levels (40–59 kg P ha −1 ), depending on the amount of P surplus (amount of fertilizer applied in excess of removal by crops) ( r  =   0.86, P  ≥   0.01). However, only 4–9% of the applied P fertilizer accumulated as Olsen‐P to a depth of 15 cm (an increase of 2 mg kg −1 per 100 kg ha −1 surplus P) in the sandy loam soil. In the following 5 years, the raising of 10 crops without P fertilizer applications decreased the accumulated Olsen‐P by only 20–30% depending upon the amount of accumulated P and crop requirements. After 29 years, 45–256 kg of residual P fertilizer had accumulated as Olsen‐P ha −1 in the uppermost 150 cm with 43–58% below 60 cm depth; this indicates enormous movement of applied P to deeper layers in this coarse textured soil with low P retention capacity for nutrients. Groundnut was more efficient in utilizing residual P than rapeseed; however, for both crops the yield advantage of residual P could be compensated for by fresh P applications. These results demonstrated little agronomic advantage above approximately 20 mg kg −1 Olsen‐P build‐up and suggested that further elevation of soil P status would only increase the risk of environmental problems associated with the loss of P from agricultural soils in this region.