Rice Response to Waterlog-Tolerant Green Manures

AbstractChemical 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 (Vigna 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