Evaluation of the Pre-sidedress Soil Nitrate Test in Ohio

Growers need to identify optimum N application rates for corn grain production to maximize economic return and minimize N losses. The pre-sidedress soil nitrate test (PSNT) is used to improve N management decisions in several states, but critical values were established years ago with older corn hybrids. Our objectives were to determine the PSNT critical value for current Ohio corn production using three model types and identify the relationship between agronomic optimum N rate (AONR) and soil nitrate-N (NO3-N). From 2004 to 2008, 34 field experiments were established across Ohio. At each location, soil samples were collected at the V4 to V6 corn growth stage. Relative grain yield was correlated with soil NO3-N levels. Three types of models were evaluated to determine the PSNT critical value. The PSNT pre-sidedress soil nitrate test critical values were 17, 22, and 31 μg NO3-N/g soil for the linear-plateau, quadratic-plateau, and quadratic-plus-plateau model, respectively. The critical value of 31 μg NO3-N/g soil reduces the chance of yield loss due to N deficiency, but increases the chance of N over application; however, the lower critical value of 17 μg NO3-N/g soil increases chance of yield loss due to N deficiency, but decreases the chance of N over application. AONR was inversely related to soil NO3-N concentration; however, the rate varied at soil N levels <25 μg NO3-N/g soil.