Integrating Soil Nitrate Level in Refining Nitrogen Fertilizer Management in Louisiana Corn Production Systems

In some corn (Zea mays L.) producing regions of the US, soil testing is still recognized as an effective decision tool for nitrogen (N) fertilizer recommendation. This study was conducted to investigate the early-season N requirement of corn based on early-season soil nitrate level and document the seasonal changes of inorganic N distribution throughout the soil profile. A Gigger silt loam soil and a Sharkey clay soil were selected to establish the trial located in northeastern Louisiana. Treatments of varying N rates were arranged in a randomized complete block design with four replications. Nitrogen fertilizer rates of 0, 67, 134, 201, 268, 335, and 403 kg N ha-1 were applied at early-season VE-V3 leaf stage. Four split application treatments of the Louisiana N recommendation (268 kg N ha-1) were applied at early-season and midseason V7-V9 leaf stages (0-268, 67-201, 134-134, 201-67 kg N ha-1). Grain yield and yield components were determined. Soil inorganic N content was determined by 1 M KCl extraction procedure followed by continuous flow injection analysis. Gigger silt loam reached a maximum yield of 13.2 Mg ha-1 when the N rate was 134-134 kg N ha-1 and soil inorganic N content was 80 kg N ha-1. Sharkey clay achieved a maximum yield of 13.1 Mg ha-1 when 268 kg N ha-1 was applied in early-season and soil inorganic N content was less than 60 kg N ha-1. Split N applications optimized yield for the Gigger silt loam, but experience a yield reduction for the Sharkey clay. The optimum N rate for the Gigger silt loam was 134-134 kg N ha-1 treatment and 201 kg N ha-1 for the Sharkey clay soil applied only at early-season (P\u3c0.05). Nitrogen rate had a significant effect on grain yield and 100-grain weight (P\u3c0.01) as well as N uptake and total N grain content (P\u3c0.05). Seasonal changes in inorganic N content occurred mostly in the 0-15 cm soil layer with decreasing variability with depth. These results show the potential for using inorganic N content to determine N application method in northeastern Louisiana corn production systems