Green manures for soil fertility and crop yield improvement in northeast Thailand

Four studies were undertaken to compare and examine the use of green manure crops, organic and chemical fertilizers, on crop yield in Northeast Thailand. In the first study undertaken over five years, the effects on corn yield of four treatments were compared (untreated control, farmyard manure, compost, and Crotalaria juncea as a green manure crop) in combination with N:P:K chemical fertilizer at rates ranging from zero to 625 kg/ha. In each year of the study the organic fertilizer treatments and green manure improved corn yields by between 24 to 74 percent; among these treatments the yield from the C. juncea incorporated plots was generally better than from the other two treatments. The response to the N:P:K levels varied with type of organic fertilizer treatment. A second study compared the effectiveness of four green manure crops over two years. The green manure treatments were C. juncea, Canavalia gladiata, Vigna sinensis and Sesbania rostrata. The relative response as measured in terms of improved maize yield was generally closely related to the biomass of the respective green manure crops when incorporated. A third study compared the effects of V. sinensis and C.juncea as green manure crops, when cassava was the each crop grown. The comparison also included a comparison of N:P:K at rates varying from zero to 625 kg/ha. Cassava tuber yields did not differ significantly between the use of the green manure crops and the use of inorganic fertilizer. Increasing levels of fertilizer application did not result in further increases in tuber yield. In the fourth and final study, the use of S. rostrata for improving rice yields was examined and compared with and without combinations of N:P:K fertilizer. Highest rice yields were obtained with the combination of green manure and chemical fertilizer. The use of S. rostrata alone resulted in an improvement in rice yield the equivalent of the application of 156 kg/ha of 16:16:8 compound fertilizer.