Relating Soil-Profile Inorganic Nitrogen Distribution and Grain Yield in Corn Production in Northeastern Louisiana.

The use of soil inorganic N content to derive corn N fertilizer requirement has not been fully adopted in Louisiana. This study was conducted to document and relate the seasonal changes on soil profile inorganic N distribution to grain yield of corn grown on a Gigger silt loam (sl) and Sharkey clay soil. Plots applied with preplant N rates of 0, 202, and 403 kg N ha^-1 were selected from on-going N response trials in Northeast Louisiana for sequential deep core soil sampling within a two-year period: months of March, May (V8 leaf stage), August, October, and December.  Two ~76-cm deep core soil samples were obtained within a 300 cm center area of the two middle rows of each plot and cut into 15-cm sections.  Soil samples were analyzed for NH₄-N and NO₃-N using 1 M KCl extraction procedure and continuous flow injection analysis system. A plot combine harvester collected grains for yield determination. The application of 202 kg N ha^-1 raised inorganic N content of the upper 30 cm of Gigger sl from 20 to 59 kg N ha^-1 in May, whereas Sharkey clay soil’s inorganic N was raised from 30 to 41 kg N ha^-1. Similarly, a higher yield response to 202 kg N ha^-1 was recorded on Gigger sl with an increase in yield of 5.9 times the control plot than Sharkey clay (5.4 times). With 403 kg N ha^-1, Gigger sl’s inorganic N averaged 98 kg N ha^-1 compared with the 57 kg N ha^-1 of Sharkey clay which explained why the maximum yield was attained on Sharkey clay at lower amount of N fertilizer than on Gigger sl.   The changes of in-season inorganic N content of the upper 30 cm of soil profile due to N fertilization were associated to corn yield response to N.