Dr. Maria Villamil and Dr. Emerson Nafziger in the Crop Science Department at the University of Illinois collected some 2,300 corn and 2,600 soybean grain samples over the 2014 through 2016 seasons in Illinois in order to see if the crop removal numbers have changed since the currently-used numbers were generated some decades ago. As expected, they found a considerable amount of variability in grain P and K content, with the highest values as much as double the lowest values. In order to reduce the chances of having the number too low for a given field, they decided to use the 75th percentile – the number that is higher than 75 percent (and lower than 25 percent) of the values they found.
The new numbers for grain removal for corn are 0.37 lb P2O5 and 0.24 lb K2O per bushel. These compare to the old values of 0.43 and 0.28 for P2O5 and K2O, so the new numbers are about 15 percent lower than the older ones. For soybean, the new numbers are 0.75 lb P2O5 and 1.17 lb K2O per bushel. These are 10 and 12 percent lower than the old numbers of 0.85 and 1.30, respectively. They also gathered wheat grain samples and are collecting some more of those in 2017. Wheat samples collected so far show values that are considerably lower for P and a little lower for K than the older numbers. The new numbers for grain removal of P and K provide new guidance in calculating removal based on yield. They are only moderately lower than the older numbers, so calculated removal won’t change by a lot, but over time it will help us balance crop needs with the supply of these nutrients.