Yield Response to Applied Nutrients and Estimates of N₂fixation in 33-Year-Old Soybean–wheat Experiment on a Vertisol

SUMMARY Soybean–wheat systems are the major grain production systems on vertisols in Madhya Pradesh, India. A study on yield response to nutrients (N, P, K, S and Zn) and estimation of N 2 fixation by soybean under different nutrient combinations was studied in a 33-year-old, long-term experiment on soybean–wheat–maize system. For estimation of N 2 fixation, annual input–output N balance technique was used. The experiment was initiated in June 1972, comprising eight treatments, viz. control (no fertiliser and no manure), 100% N, 100% NP, 100% NPK, 150% NPK, 100% NPK + 15 t farmyard manure (FYM), 100% NPK + Zn and 100% NPK – S with four replications arranged in a randomised block design. The amount of N applied (100%) to each crop of soybean, wheat and maize was 20, 120 and 80 kg ha −1 , P (100%) 35, 35 and 26 kg ha −1 and K (100%) 16, 32 and 16 kg ha −1 , respectively. FYM was applied one week before the onset of monsoons. Both soybean and wheat yields responded to applied N and P during all these years. The yield response to K was observed after 10 years. The estimated amount of N 2 fixed by soybean annually varied from 62.8 to 161.1 kg ha −1 ; however, the net gain of N in soil after offsetting the N derived by soybean from soil varied from 24.2 to 66.5 kg ha −1 annually. Maximum N gain was recorded on application of P. There was a linear relationship between the amount of harvestable biomass N and residual biomass N, whereas quantity of N added to soil has a curvilinear relationship with the harvestable biomass N. The highest percentage of N derived from the atmosphere (% Ndfa) was recorded in the control treatment, but the highest amount of N 2 fixed was found in the 100% NPK treatment. Balanced use of nutrient is the best option to harness the N 2 fixation potential of soybean.