Integrating Soil Nitrate Testing to Refine Nitrogen Fertilizer Recommendation in Louisiana Corn Production Systems.

Continuous research is required to refine the current nitrogen (N) rate recommendation for corn (Zea Mays L.) in northern Louisiana.  Soil texture influences N dynamics within the soil profile and complicates the establishment of N fertilizer rate recommendation for corn.  This study was conducted to establish preplant N rate requirement of corn based on soil nitrate (NO₃) testing.  In 2013, 60 plots with a dimension of 9.1m-long x 4 rows were planted to corn on two different soil types:  Gigger silt loam and Sharkey clay.  Fifteen treatments consisting of preplant N rates ranging from 0 to 403 kg ha^-1 in 67 kg ha^-1 increments, different combinations of preplant and sidedress N rates (0-269, 67-202, 134-134, and 202-67 kg N ha^-1), and preplant N rates of 67 and 134 kg ha^-1 with sidedress application at rates based on yield goal + soil (NO₃) level and optical sensor readings were arranged in a randomized complete block design with four replications.  Soil samples collected at depths of 0-15 cm and 15-30 cm before planting and at harvest were analyzed for NO₃ content by 1 M KCl-extraction procedure followed by flow injection analysis method.  Grains collected at harvest using a plot combine harvester were used for yield determination. In previous years, corn planted on Sharkey clay soil required higher preplant N (at least 134 kg N ha^-1) and total N rates (302 kg N ha^-1) than on Gigger silt loam soil to maximize grain yield.  This was partly explained by soil NO₃ level; early season levels of soil NO₃ within the 0-30 cm on Sharkey clay (≈ 10 kg NO₃^- ha^-1) were found to be consistently lower than Gigger silt loam (≈ 35 kg NO₃ ha^-1).  In 2013, soil NO₃ at planting on Gigger silt loam averaged only 18 kg NO₃-N ha^-1 resulting in higher preplant N requirement than previous years (67 vs 134 kg N ha^-1); this when combined with 134 kg N ha^-1 application at V8 leaf stage had maximized grain yield (P<0.001). Our results demonstrated the value of integrating soil NO₃ testing in establishing preplant N rate recommendation for corn.