Pre-sidedress soil nitrate test (PSNT) provide a method of estimating soil nitrogen supply, and can aid producers in fine-tuning their nitrogen application rates for corn. One drawback of current soil nitrate tests is that they are only calibrated for situations where all of the nitrogen being measured has come from soil mineralization: that is cores need to be taken in soil that has not received any commercial nitrogen fertilizer. Soil nitrate samples are taken 12” (30 cm) deep. Past research indicates that the soil nitrate in the top 12” of soil, under most conditions, represents approximately 60% of the total plant available N in the rooting profile.
Predicted soil nitrogen supply will be overstated under circumstances where additional nitrate has been added through fertilization (i.e. broadcast pre-plant or pre-emerge N applications), and as a result nitrogen requirements will be underestimated. To date, no soil nitrate calibration is available which can aid in evaluating the relative soil nitrogen supply for the large portion of Ontario’s corn acreage that receives pre-plant nitrogen.
Recent nitrogen research in Ontario has identified a pre-sidedress soil nitrate threshold of 36 ppm which has been effective in identifying whether additional nitrogen is probable to provide an economic yield response when a modest (i.e. 100 lb-N/ac) amount of nitrogen has been applied around planting. Soil tests less than 36 ppm indicate there will be response to additional application of nitrogen, while those over 36 ppm typically do not. This threshold could allow a grower to apply pre-plant nitrogen, and identify at side-dress time if there is a requirement to add additional N. The purpose of this research is to verify the validity of this threshold concept.