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.