RESIDUAL PHOSPHORUS AND LONG-TERM MANAGEMENT STRATEGIES FOR AN ULTISOL

Knowledge of residual benefits from previously applied P is crucial to maximize economic returns to current P inputs. We measured the residu al benefits of P fertilizer on three summer soybean crops grown on a H awaiian Ultisol over a three-year period. Four rates of P had been app lied to four crops (P build-up phase) prior to the residual phase of t his experiment. The P inputs during the build-up phase were (kg P ha(- 1) per crop): control (OP)=no P inputs; low P (LP)=50, 35, 35, 35; mod erate P (MP)=100, 70, 70, 70; and high P (HP)=300, 210, 210, 210. Dry matter yield (DMY) and P uptake in all P regimes declined with each su ccessive crop during the residual phase. The relative decline was grea test in the LP regime where DMY declined by 61% and P uptake by 71% du ring the residual phase. Even with cumulative P inputs of 930 kg P ha( -1) (HP), DMY and P uptake declined by 15% ad 36%, respectively, durin g the residual phase. The decline in P uptake with time was not relate d to Mehlich-1 extractable P (M1P), Olsen P, or P extracted by iron-ox ide impregnated filter paper (FeO-P). Initially, optimum DMY was achie ved with a MIP value of 2 mg P kg(-1), however, this value increased w ith time so that by the end of the experiment an MIP value greater tha n 5 mg P kg(-1) did not support maximum yields. The rapid decline of r esidual P benefits in this soil is in contrast to many reports of the lasting residual P benefits in highly weathered soils. Our results sug gest that to sustain crop productivity frequent, small applications of P to this soil may be more economical in the long-term than applying large amounts of P.