Corn nitrogen rates: Residual effects on soil fertility and soybean
Nitrogen (N) is one of the most important nutrients for crop production. Soybean is often rotated with corn in Midwestern states, so the large N demand of soybean and the fertilizer N applications to corn could intersect with crop rotation and N cycling. Our objectives were to determine the residual effects of corn N fertilization to (1) soybean growth and development, (2) N accumulation, (3) grain yield and N removal, and (4) soil fertility. Nitrogen response field trials of corn were established in 2006 across Indiana. These trials have been rotated with soybean every other year, and the residual effects of the six corn N rates were evaluated in 2013 and 2014 near Columbia City, Wanatah, and West Lafayette, Indiana. Plant biomass and crop canopy reflectance was collected at V4 (fourth trifoliate), R4 (full pod), and R6 (full seed). Grain yield and moisture was collected with field-scale combines. Plant and grain subsamples were analyzed for macro- and micro-nutrients determining concentrations and uptake. Deep soil samples (0 to 20, 20 to 40, 40 to 60 cm) were taken pre-plant and post-harvest of soybean. The soil samples were collected from plots that received the corn N rate treatments. Soil was analyzed for nitrate, ammonium, and general fertility macro- and micro-nutrients. Following the lowest N rate applied to the previous corn crop, early soybean growth was greatest only in Columbia City in 2014 and N concentration was greatest in the biomass sampled at R6 only in West Lafayette in 2014. The only grain yield response was in Columbia City in 2014. Yield was the highest (3.9 Mg ha-1) at the lowest previous corn N rate and decreased to 3.5 Mg ha-1 as the previous corn N rates increased. Canopy reflectance indices were related to previous corn N rates in a few scenarios, with the most direct relation to early soybean growth and N accumulation at Columbia City in 2014. The best correlation of reflectance indices to yield was at R4 and R6 in Wanatah in 2014 as it related to disease pressure. Pre-plant soil NO3-N content (20 to 40 and 40 to 60 cm depths) increased incrementally as previous corn N rate increased at Columbia City and West Lafayette in 2013. In 2014 pre-plant soil NO3-N (20 to 40 and 40 to 60 cm depths) increased curvilinearly at Columbia City and Wanatah. Residual NO3-N from the pre-plant samples were greatest at locations with low precipitation in the prior corn season and/or during the winter fallow period between growing seasons. Total N content generally followed NO 3-N trends except at West Lafayette in 2013 where higher NH4-N concentration at 0 to 20 cm contributed to differences in total N. Post-harvest N content was not significant in 4 out of 6 site-years. Available P was not limiting at any location and differences were attributed to differences in crop growth rates. No other nutrients were limiting due to routine soil sampling and proper fertilization practices during the corn years. Overall we conclude that previous corn N rate will not likely influence soybean growth, development, or yield under conventional tillage where all other fertility is adequate; however, potential influence exists on no-till fields where soybean growth, development, and yield followed similar trends with previous corn N rate in 1 of 2 years.
Casteel, Purdue University.
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