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Rice Response to Waterlog-Tolerant Green Manures

Multiple Cropping Dep., Rice Res. Inst. (IRRI), P.O. Box 933, Manila, Philippines
Dep. of Agric., Kathmandu, Nepal
Agronomy Dep., Int. Rice Res. Inst (IRRI), P.O. Box 933, Manila, Philippines
Multiple Cropping Dep., Int. Rice Res. Inst (IRRI), P.O. Box 933, Manila, Philippines

^* Corresponding author.

Chemical N fertilizers are usually applied to modern rice (Oryza sativa L.) cultivars, but yield responses are obtained from green manuring as well. To be considered as a viable alternative to chemical N fertilizer in tropical Asia, however, green manures must tolerate the ephemeral waterlogging that precedes rice cultivation. Experiments on an Aeric Tropaqualf soil during three early wet seasons determined green manure N accumulation in 60 d or less, and rice grain yield response to it. In the first experiment (1983), cowpea [Vignu Unguiculata (L.) Walp.] and Sesbania cannabina [(Retz.) Poir.] were grown 24, 36, and 48 d before incorporation. Nitrogen accumulation was greatest (79 kg N ha^–1) in 48-d S. cannabina. Accumulation in cowpea was apparently depressed by waterlogging. Rice grain yield response to green manure N was unaffected by location of green manure cultivation (in situ vs. transported from a nearby field). In the second (1984) and third (1985) experiments, Sesbania sp. were flooded during the last 25 d of growth to assure early wet-season waterlogging. Mean N accumulation in flooded S. rostrata (Brem. and Obrem.), which nodulates on its stem, was 90 kg ha^–1 in 48 d and 164 kg ha^–1 in 60 d. Nitrogen accumulations in nonflooded S. rostrata were 128 kg ha^–1 in 48 d and 198 kg ha^–1 in 60 d. Accumulation in a Nepalese S. cannabina cultivar averaged 84% of that in S. rostrutu. Rice grain yield response was a function of green manure N regardless of species or age. Both S. rostrata and S. cannabina tolerate waterlogging and accumulate sufficient N in less than 60 d to increase rice yields more than 2 Mg ha^–1.

Received for publication February 16, 1988.