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

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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 their effect on the performance and yield of wetland rice (IR 64) was examined at four different sites in Luzon, Philippines. Soils deficient in N, P, and K were selected and compared with the fertile Maahas clay of the International Rice Research Institute (IRRI) at Los Banos. The green manure plants were grown under flooded conditions for 49 days in the wet season of 1987, chopped, and then ploughed in before transplanting rice seedlings. In a second experiment, the effect of S. rostrata green manure was studied under rainfed conditions. All green manure treatments were compared to an urea treatment (60 kg N ha−1) and an untreated control. Both legumes developed well, even on the marginally productive soils. S. rostrata accumulated up to 190 kg N ha−1 and A. afraspera even accumulated 196 kg N ha−1 in the shoots. In all treatments, green manure increased grain yield significantly (P=0.05) 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 Maahas clay soil of IRRI. The apparent release of exchangeable NH 4 + -N in the soils after green manuring and the rice grain yield response showed that both green manure species may provide sufficient available N throughout the development of IR 64 in the wet season. In the rainfed marginal 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.