Do the Math on New P and K Rates



As crops are being harvested and grain is taken into storage, it is time to estimate the number of nutrients that are removed by this grain. Harvested grains remove large quantities of phosphorus (P) and potassium (K) from the cropping system that must be replaced with adequate rates of fertilizer in order to maintain soil test levels. These amounts can be easily calculated by multiplying nutrient removal rate (see below) per unit of yield by the actual yield. Maintenance/Removal Rate recommendations in Illinois have been based on historic 'book values' of uncertain origin and have remained unchanged for many years. Recently the University of Illinois conducted a study (2014-2016) funded by the Illinois Nutrient Research and Education Council (NREC) to measure the number of removed nutrients in thousands of grain samples collected from across the state of Illinois. Following the analyses of 5,000+ samples of grain, the new recommendations are now very close to those of neighboring states such as Iowa, drawing Illinois into regional alignment.



Corn- the new grain removal numbers of 0.37 lb. P2O5 and 0.24 lb. K2O per bushel are both about 15 percent lower than the book values currently in the Illinois Agronomy Handbook. Soybeans- the new numbers of 0.75 lb. P2Oand 1.17 lb. K2O per bushel are 12 and 10 percent lower than the book values, respectively.


How does this impact your fertility program?

In a two-season application cycle- YR1: 200-bushel corn; YR2: 60-bushel soybeans

P removal using the old book values- 137 lb. P2O5 per acre

P removal using the new values- 119 lb. P2O5 per acre (13% reduction)

K removal using the old book values- 134 lb. K2O per acre

K removal using the new values- 118 lb. K2O per acre (12% reduction)


What’s your strategy for reducing nutrient loss?

Effectively, re-considering your fall fertility program may make sense as part of your strategy for reducing nutrient loss. This new research indicates less P & K is needed by the crop, meaning you can reduce nutrient loss in a way that makes both economic and environmental sense. 


Read the official University of Illinois bulletin here.