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With a view to assessing the influence of in situ growing and incorporation of Sesbania rostrata on the growth and productivity of rice and to study the economy of nitrogen use by rice, a field experiment was conducted at College of Agriculture, Vellayani during the period from July 1991 to December 1991. The experiment was laid out in Randomised Block Design with three replications. Three levels of Rhizobium inoculation for Sesbania rostrata (i1-seed inoculation, i2 - stem inoculation, i3 – seed + stem inoculation), five levels of nitrogen to succeeding crop of rice raised after incorporation of Sesbania rostrata (n0 – no nitrogen, n1 – 25 percent of the recommended dose, n2 – 50 percent of the recommended dose, n3 – 75 percent recommended dose, n4 – 100 percent recommended dose) and one control plot of rice (treated as per KAU package of practices recommendations) were fixed as treatments. The rice variety used was Jyothi. The soil of the experimental site was sandy clay loam, medium in available nitrogen and phosphorus and low in available potassium. An abstract of the results is given below.
Among the three different levels of inoculation, seed + stem inoculation was found to influence, the green matter and dry matter yields of Sesbania rostrata positively, though not significantly. The number and dry weight of root nodules and shoot nodules, nitrogen uptake, phosphorus uptake, potassium uptake, calcium uptake and magnesium uptake were not found to be influenced significantly by the different inoculation levels. From these results, it can be concluded that, the easiest method of inoculation viz. seed inoculation is sufficient for satisfactory growth and nitrogen fixation by Sesbania rostrata. The present study also revealed that the growth and nodulation of Sesbania rostrata is not satisfactory under conditions of continuous submergence, as that prevailed during the cropping period of the present experiment. Rhizobium being an aerobic bacteria might have got inhibited under the anaerobic conditions created by continuous water logging. Furthermore, the acidic condition of the soil of the experimental site might have also inhibited the activity of Rhizobium. Thus it shows that liming is absolutely necessary for the satisfactory growth of Sesbania rostrata in acid soils.
Among the different treatments tried, though the control (KAU package of practices recommendations) gave the maximum yield, it was on par with the treatment comprising 50 percent of the recommended dose of nitrogen along with Sesbania rostrata incorporation. This shows that there is considerable scope for economising nitrogen by the substitution of chemical nitrogen applied to rice by Sesbania rostrata.