Corn Nitrogen Rates: Residual Effects on Soil Fertility and Soybean.

Nitrogen (N) is one of the most important nutrients for crop production and health. Soybean requires more N for seed production than any other commercially grown grain crop. Depending on mineralization rates and soybean demand, soil N availability can account for 18 to 60% of soybean N demand. Bradyrhizobium japonicum form a symbiotic relationship with soybean, which allows for biological nitrogen fixation (BNF) to occur by taking atmospheric N and converting it to ammonium. This process can fix 50 to 75% of the N required by soybean annually. A potential soybean N limitation could occur where soil organic matter is low and BNF is limited. Our objectives were to determine the residual effects of corn N fertilization to (1) soil fertility and (2) the growth and yield of rotated soybean. Nitrogen response trials of corn were established in 2006 across Indiana. These field-scale trials have been rotated with soybean every other year, and the residual effects of the six corn N rates were evaluated in 2013 in fields near Columbia City, Wanatah, and West Lafayette, Indiana. Deep soil samples (0 to 20, 20 to 40, 40 to 60 cm) were taken prior to planting and after harvest of soybean. Plant biomass was collected at V4, R4 (full pod), and R6 (full seed). Grain yield and moisture was collected with field-scale combines. Soil was analyzed for nitrate, ammonium, and general fertility. Plant and grain subsamples were analyzed for macro- and micro-nutrients to determine concentrations and uptake. Pre-plant soil nitrate concentrations (20 to 40 and 40 to 60 cm depths) increased incrementally as previous corn N rate increased at Columbia City and West Lafayette. Grain yield ranged from 3434 to 3590 kg ha^-1 in Columbia City, 3811 to 4056 kg ha^-1 in Wanatah, and 3832 to 4015 kg ha^-1 in West Lafayette. Post-harvest soil N and grain N concentration did not differ among any of the treatments and locations. Rotated fields are being evaluated in 2014.