YIELD AND NITROGEN RESPONSE OF LOWLAND RICE (ORYZA-SATIVA L) TO SESBANIA-ROSTRATA AND AESCHYNOMENE-AFRASPERA GREEN MANURE IN DIFFERENT MARGINALLY PRODUCTIVE SOILS IN THE PHILIPPINES

Organic-N fertilizers in the form of flood-tolerant, leguminous, stem- nodulating Sesbania rostrata and Aeschynomene afraspera may be useful alternatives to resource-poor rice farmers if applied as green manure. Therefore, the accumulation of N by these green manure species and th eir effect on the performance and yield of wetland rice (IR 64) was ex amined at four different sites in Luzon, Philippines. Soils deficient in N, P, and K were selected and compared with the fertile Maahas day of the International Rice Research Institute (IRRI) at Los Banos. The green manure plants were, grown under flooded ed conditions for 49 day s in the wet season of 1987, chopped, and then ploughed in before tran splanting rice seedlings. In a second experiment, the effect of S. ros trata green manure was studied under rainfed conditions. All green man ure treatments were compared to an urea treatment (60 kg N ha(-1)) and an untreated control. Both legumes developed well, even on the margin ally productive soils. S. rostrata accumulated up to 190 kg N ha(-1) a nd A. afraspera even accumulated 196 kg N ha(-1) in the shoots. In all treatments, green manure increased grain yield significantly (P = 0.0 5) over the untreated control, by 1.3 - 1.7 Mg ha(-1). The yields were comparable to those obtained with 60 kg N ha(-1) of urea fertilizer. S. rostrata caused the highest grain yield, of 6.5 Mg ha(-1) on the Ma ahas clay soil of IRRI. The apparent release of exchangeable NH4+-N in the soils after green manuring and the rice grain yield response Show ed that both green manure species may provide sufficient available N t hroughout the development of IR 64 in the wet season. In the rainfed m arginal soil site, green manure with S. rostrata produced even higher rice grain yields than urea. Green manure therefore seems particularly attractive for poor farmers on marginally productive soils, at least as a temporary strategy to improve yield and yield sustainability.