Green Manure for Paddy Crops of Karnataka: Investigation of Effectiveness of Stem Nodule Legume Sesbania Rostrata

Paddy is grown in over 1 million hectares in Karnataka. This crop needs a nitrogen dose of 100-120 kg/ha. Many marginal and small farmers cannot afford to supply this through chemical fertilizers.

Green manuring is effective in meeting this demand and is also environmentally sustainable. Leguminous crops can be used as green manures, capable of fixing the atmospheric nitrogen to the average extent of 100 kgN/ha. Sesbania rostrata, a stem nodulating legume in symbiotic association with nodule bacteria is reported to have fixed upto 267 kgN/ha in a 52 day crop. It holds a lot of promist for nitrogen fixation under water-logged conditions rendering it ideal for rice fields. With the small and marginal farmer in mind, this project was initiated for steadying the rhizobial ecology of Sesbania rostrate, quantifying nitrogen fixation and testing it under field conditions. Dr. K. Shivappa Shetty of the University of Agricultural Sciences is the convener.

Initially nodulation induced by inoculating different species of Sesbania and Siratro with bacterial isolates from set and root nodules of sesbania was compared and found to nodulate sesbania rostrata and S. aculeata. A test for utilization of carbon sources showed that sodium succinate, sodium lactate, manitol and sucrose supported maximum azorhizobial growth. Plant growth, nodulation and quantity when observed indicated that S. rostrata showed higher nitrogen yield but lower fresh weight than S. aculeata. It responded to plant harmones by increasing dry matter as well as nitrogen yield.

Field studies on the influence of mineral nitrogen revealed that incorporation of 8.76 t/ha green biomass (with 25 percent of nitrogen fertilization recommended ) could produce the maximum yield of paddy grains as well as straw. In the field S. rostrata was found superior to other conventional green manuring legumes (viz, Dhaianch, Sunhemp, blackgram etc. ) in nitrogen accumulation. It also significantly increased the grain and straw yield. A green house experiment indicated that growing S. rostrata along with Azolla is highly beneficial.