Effect of nitrogen- and potassium-based fertilizers on nitrogen fixation in the winged bean,Psophocarpus tetragonolobus

Abstract

Five rates of urea application (0 to 40 kg N/ha) and three rates of K+ (0 to 65 kg/ha) on the nitrogen-fixing activity of two varieties (‘SLS-40’ and ‘SLS-44’) of winged bean were examined in the field. Specific activity of nitrogenase was inhibited with increasing amounts of urea up to 75 days after sowing (d.a.s.), but this was not so apparent at 90 d.a.s. Nitrogenase activity increased in proportion to K+ fertilization especially during the early stages of growth (30 to 60 d.a.s.) and continued even with added N fertilizer. Nitrogenase activities correlated better with nodule dry weight than with nodule number. Nodule number and nodule dry weight were significantly decreased by added N. K+ had a significant effect on nodule number and nodule dry weight. Forty-five day-old plants gave high nodule number but the highest nodule dry weight was recorded on 75 day-old plants. K+ fertilizer stimulated early root growth (30 to 45 d.a.s.) but the overall root biomass at the end of the experiment was similar under all K+ treatments. The shoot weight increased with N but not with K+.