Time-course of N₂ fixation and its contribution towards n demand in ‘JS-335’ soybean (Glycine max)

The N yield ill soybean increased sharply after 45^th day of sowing and attained the maximun level at 85 DAS (days after sowing). The rate of N₂ fixation also increased sharply after 45^th day and 78% of the total fixed N (111.9 kg/ha) was accrued by the crop during 45–75 DAS. After 75^th day, N₂ fixation decreased and no significant improvement in the amount of symbiotically fixed N was observed. Soil N uptake, however, increased significantly even after 75^th day and its highest level was recorded on 85^th day. The contribution towards total N yield by symbiotical1y fixed N decreased from 61.9% at 75 DAS to 54.1% at 85 DAS. The same by soil N increased from 30.4% at 75 DAS to 40.2% at 85 DAS. Symbiotic N₂ fixation fulfilled the entire requirement on N only during 55–65 DAS. It was concluded that peak period of N₂ fixation was 45–75 DAS in ‘JS 335’ soybean.