Input / output analysis of the cumulative soybean response to phosphorus on an Ultisol

23 K.G. Cassman, P.W. Singleton b and B.A. Linquist International Rice Research Institute, Manila, Philippines NiFTAL Project, 1000 Holomua Ave, Paia, Hawaii, USA (Accepted 14 December 1992 Although biological N2 fixation (BNF) by legumes can provide significant N inputs to crop systems on highly weathered tropical soils, potential inputs from BNF largely depend on soil P supply. We compared the cumulative effects of P input regimes on yield, N and P budgets, and soil P availability in four consecutive soybean crops in a 2year period on a Humoxic tropohumult. In each crop cycle, nodulating (nod) and nonnodulating (nonnod) isolines were subplots in P-regime mainplots (kg P ha by crop cycle): Po= control without P inputs; LP= 50, 35, 35, 35; MP= 100, 70, 70, 70; and HP= 300, 210, 210, 210. Seed yields of the nod isoline in the HP regime were 3700 kg ha in the two summer seasons and 2400 to 2500 kg ha in the fall seasons, with a mean increase of 85% compared to yields of the nod Po control. Nonnod seed yields and N accumulation were unaffected by the P regime, averaging 870 and 48 kg ha, respectively. The contribution of BNF to nod soybean N assimilation was linearly related to P uptake, and mean P uptake by nod plants was 60% greater than by nonnod soybean, despite 35% greater root length of nonnod plants at 0-50 cm depth. For the four crop cycles, total BNF input to the system ranged from 330 kg N ha (PO), or 65% of total aboveground N, to 710 kg N ha (HP), which was 78% of total aboveground N. After accounting for P removal at harvest, a net P input of just 99 kg P ha after four crops increased cumulative seed yield by 3600 kg ha and BNF by 227 kg N ha in the LP treatment. A positive net P balance also resulted in (1) an increase in extractable P in the 0 to 25 cm topsoil, (2) a reduction in the proportion of P that would be fixed from subsequent additions as indicated by a shifted P sorption isotherm, and (3) greater apparent P uptake efficiency from applied fertilizer in subsequent crop cycles. As a result, the yield response of nod soybean per unit P input increased from 13 kg seed kg P applied in the first crop to 44 kg seed kg P applied in the fourth crop of the LP treatment. Cumulative effects of this magnitude emphasize the need to consider the longer-term nutrient balance of the crop system in developing costeffective P management strategies on highly weathered soils. The potential for greater P input use efficiency with time when inputs exceed outputs means that farmers' average and marginal return from investment in P fertilizer will also increase with time. Correspondence to: Dr. K.G. Cassman, International Rice Research Institute, P.O. Box 933, 1099 Manila, Philippines. 0378-4290/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved. Input/output analysis of the cumulative soybean response to phosphorus on an Ultisol