New grain P and K concentration values for Illinois field crops

Most fertilizer P and K rate recommendations for the North-Central US are based on a combination of critical soil-test levels and the amount of fertilizer nutrient required to maintain that level. Our goal was to update P and K removal numbers by harvest of corn (Zea mays L.), soybean [Glycine max (L.) Merr.], and wheat (Triticum aestivum L.) in Illinois. With the collaboration of farmers, grain handlers, and stakeholders, we collected, over a 3-year period, a total of 2335, 2621, and 825 samples of corn, soybean, and wheat grain, respectively, with samples from most counties in Illinois. Yields were collected during the first year of sampling; grain nutrient concentrations were found to be independent of yield. We selected the 75th percentile as the new value for grain concentrations. For corn, these values were 0.37 lb P₂O₅ and 0.23 lb K ₂O per bushel; both are about 14% lower than the reference values. For soybean, these values were 0.75 and 1.18 lb P₂O₅ and K₂O, respectively, which are, respectively, 12 and 10% less than reference values. For wheat, new values are 0.46 and 0.28 lb P₂O₅ and K₂O,decreases of 22 and 8% for P and K, respectively, compared to reference values. While the exact origins of the reference values used in Illinois are not clear, our having found lower grain concentrations in this survey indicates that improved varieties and management practices over time have not led to increases in per-bushel nutrient removal, although increased yields will mean more removal of nutrients. But lowering, by about 12% relative to the old reference values, the per-harvested-bushel amounts of P and K fertilizer applied to replace nutrients removed should not lead to decreasing soil test values over time.