Green manuring in lowland rice – A review

Green manuring is an age old practice dating back to the beginning of the Christian era and is researched in India for long. Research on green manuring is ample and continued to be studied on potential green manures, their method of establishment, utility and release pattern of nitrogen, its role in Integrated Nutrient Management for rice, as an intercrop in rice, incorporation methods and similar other aspects. Review of literatures state that green manuring in no case substituted entirely the chemical N indicating the essentiality of integration of fertiliser N with green manures. Sesbania aculeata and S. rostrata were predominantly grown for organic manuring in rice. Among the green manures, S. aculeata and S. speciosa exhibited higher organic carbon content with wider C-N ratio than S. rostrata. Green manure contains two N fractions. One which decomposes immediately after incorporation is named as ‘Fast N’ and the other which decomposes slowly over several years as ‘Slow N’. With most of the green manures, ‘Fast N’ accounts for 50–80 per cent of total N. During the first crop period 65 per cent of the green manure decomposes and it is 14 per cent in the next crop. It was found that incorporation of young green manure ensured adequate mineralisation of N and the optimum age for tncorporation is 45 days. S. aculeata adds more biomass in shorter period than S. rostrata and it was also not necessary to allow any decomposition period to daincha before planting. S. cannabina and S. restrata are found to be more promising in soils with low P levels. Sesbania rostrata seemed to be sensitive to high pH whereas, S. aculeata can be grown in soils which are saline and also found promising in reclamation of the soil. From the review, it could be concluded that daincha is a most appropriate green manure crop for low land rice.