LONG-TERM EFFECTS OF RATE AND FREQUENCY OF APPLIED-P ON CROP YIELDS, PLANT AVAILABLE-P, AND RECOVERY OF FERTILIZER-P IN A PEANUT-WHEAT ROTATION

Fertilizer P rates applied either to one crop (direct for current crop and residual for succeeding crop) or to both crops (cumulative) each year in peanut (Arachis hypogaea)-wheat (Triticum aestivum) rotation were evaluated for effects on crop yields, P-uptake, accumulation and depletion of Olsen's extractable P, and recovery of fertilizer P. The experiment was conducted for 8 years on a semiarid irrigated soil. Small and inconsistent responses by peanut to direct, residual, and cumulative P application on soil low in Olsen's extractable P were measured. Responses of wheat were substantial and consistent in all the years. Direct (fresh) application of fertilizer P to wheat was most efficient. Direct application of 13, 26, and 39 kg P ha-1 increased average wheat yield by 35, 50, and 54%, respectively, suggesting 26 kg P ha-1 as adequate. Removal of P by peanut from control and residual-P plots was comparable to that from direct and cumulative-P treated plots and equal to P-uptake by adequately fertilized wheat. These results suggest that P requirement of peanut can be met from soil-derived and residual fertilizer P. The initial low status of available P (11.8 kg ha-1) increased several fold after 8 years of fertilizer P applications (16.2 to 57.6 kg ha-1) with the increase proportional to rate and frequency of applied P. Wheat yield response to 13 kg P ha-1 treatment decreased in successive years due to declined available P in control soil and increased residual P in P treated soils. Consequently further response at high rates (26 to 39 kg P ha-1) continued to decrease over years. Recovery of fertilizer P after 8 years ranged from 14 to 39% in the plant and 34 to 93% in soil.