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Rice Responses to a Short-Duration Green Manure. I. Grain Yield¹

For many Asian farmers, inorganic N fertilizer for rice (Oryza sativa L.) is costly. Biologically fixed sources of N are being reexamined as alternatives to inorganic N fertilizer. Land that is fallow during the first 5 to 7 weeks of the wet season, before monsoon rains intensify, can be used to accumulate N in a green manure. This experiment determined quantities of N that well-adapted tropical legumes accumulated in 20 to 45 days and rice yield responses to green manuring with legumes grown for 45 or fewer days. The experiment, repeated in 1981, 1982, 1983, and 1984, was conducted on a fine, mixed, isohyperthermic Aeric Tropaqualf at IRRI, Los Baños, Philippines. Rice was transplanted in factorial arrangements of inorganic N rates (0 and 80 kg N ha ¹) and green manure growth durations. In 1981, 1982, and 1983, mungbean [Vigna rudiata (L.) Wilczek] was the green manure, and growth durations were 0 (clean fallow), 20, 30, and 40 days. In 1984, cowpea [Vigna Unguiculata (L.) Walp.] was the green manure, and durations were 0,25,35, and 45 days. Average N accumulation in the oldest green manure was 83 kg N ha ¹. Mean rice yield response to 80 kg inorganic N ha ¹ was 1.0 Mg ha¹. Mean response to the oldest green manure was 2.0 Mg ha^–1. The study demonstrated that a fast-growing tropical legume can accumulate more than 80 kg N ha^–1 in 45 days and that rice yield responses exceeding 2 Mg ha^–1 are possible from green manure incorporation.

Key Words: Mungbean • Vigna radiata L. • Wilczek • Cowpea • Vigna unguiculata L. Walp. • Inorganic N fertilizer • Organic N fertilizer • Philippines

² Head (agronomist), research assistant and research aide, Multiple Cropping Dep., The International Rice Research Institute, Los Baños, Philippines.

Received for publication January 21, 1985.