An Assessment of Grain NPK Content in Illinois: Have Removal Rates Changed?.

Since most fertilizer P and K rate recommendations for the North Central US are based on a combination of critical soil-test levels and mass-balance calculations, a high level of accuracy in estimating grain removal rates is essential. The objective of this study was to modernize P and K removal rates by analyzing P and K concentrations in corn, soybean, and wheat grain samples collected from producers across Illinois. Grain samples were collected from producers in the nine Illinois crop reporting districts in 2014, 2015, and 2016 and sent to a commercial lab for N, P, and K analysis. With more than 1500 samples analyzed for each corn and soybean crops, and more than 500 wheat grain samples analyzed so far, our results consistently showed that 99% of the grain P and K levels sampled were below the recommended ‘book’ removal values in IL for all field crops studied. There was no effect of crop yield on the content of N, P, or K in grain. While there is little evidence to suggest that modern hybrids and varieties need higher soil test levels in order to assure enough P and K, it is certain that nutrient removal numbers are important for both calculating replacement amounts of nutrients and in calculating mass balances of nutrients and fertilizer needs over large areas.