Use of the Nitrate Soil Test to Predict Sweet Corn Response to Nitrogen Fertilization

Abstract A precise procedure for routinely interpreting soil test NO 3 as a fertilizer guide should help achieve more efficient use of N on crops with a high N requirement. The present study was designed to calibrate soil test NO 3 for predicting relative (percent of maximum) yield and the N‐requirement index (NRI) of sweet corn ( Zea mays L.) using Mitscherlich‐type functions. The total N fertilizer requirement for the crop was equated to the potential yield increase multiplied by the NRI. The NRI was calculated from N rate studies as the amount of N required per unit increase in sweet corn yield. ‘Stylepak’ sweet corn was grown 2 years in a row at a Warden silt loam (Xerollic Camborthids) site with rates of NH 4 NO 3 the first year to give 84, 168, 252, 336, and 504 kg N/ha and a carry‐over of from 10 to 70 ppm N the second year. Functions showed a range from 10 to 40 ppm soil NO 3 ‐N was correlated with relative yields ranging from 62 to 98% of maximum and the NRI's range from 26 kg N/ton of yield increase up to 98 kg N/ton. It was concluded that with as low as 10 ppm NO 3 ‐N, the potential yield increase multiplied by the NRI equated to a requirement of 251 kg N/ha. A soil test of 40–50 ppm NO 3 ‐N correlated with 98–99% of maximum yield and indicated little or no fertilizer was needed. The NRI soil testing approach has definite advantages because it eliminates the controversial practice of calculating a fertilizer equivalent from soil NO 3 and fertilizer requirements can be calculated to fit different yield potentials of localized soil and cropping conditions.