- Harvested grain removes 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.
- New studies conducted by University of Illinois measured grain removals of P and K from thousands of grain samples collected over the period 2014-2016.
- Results of the research have prompted the University of Illinois to reduce removal rates of P and K for corn and soybeans.
Corn and soybeans have relatively high total requirements for phosphorus (P) and potassium (K). A fairly large percentage of the P and K required by the plant is ultimately removed from the cropping system as harvested grain. In order to maintain soil test levels, the nutrients removed in grain must be replaced by fertilizer additions to the soil. Hence, these fertilizer recommendations are often referred to as 'maintenance' fertilizer. In the state of Illinois, maintenance fertilizer recommendations for P and K are specified in Table 8.6 of the Illinois Agronomy Handbook.
Maintenance recommendations in Illinois have been based on historic 'book values' of uncertain origin and have remained unchanged for many years. The grain removal numbers published by neighboring states have been considerably lower, so the Illinois Nutrient Research and Education Council (NREC) funded a project at the University of Illinois to measure the amount of removed nutrients in thousands of grain samples collected from all across the state of Illinois, during the period 2014-2016. A commercial lab analyzed 2,335 corn and 2,620 bean grain samples during that timeframe.
One key finding was that nutrient removals in grain were only slightly correlated to yield, to such a limited extent that adjusting grain nutrient removals for yield was not justified. The results also showed no consistent or significant effect by location or crop year. Nutrient concentrations in grain did vary across samples and showed a normal distribution around the mean. In other words, the data set was relatively simple to interpret and convert to maintenance fertilizer recommendations.
To establish the new recommendations, researchers adopted a 75th percentile approach used by other neighboring states, like Iowa. Said another way, the new recommendations were established at a level high enough to encompass 75% of the samples. This approach incorporates a conservative margin of safety to ensure that the new levels are high enough to represent most crop situations in the state, while simultaneously reducing the environmental risk of over-fertilizing. The new recommendations are now very close to those of neighboring states, pulling Illinois into regional alignment.
Figure 1 presents a summary comparison of the previous maintenance recommendation to the updated rates. Combined over two seasons of 200-bushel corn and 60-bushel beans, the new rates result in 13% less P2O5 and 12% less K2O maintenance fertilizer per acre.
Please see your FS Crop Specialist to discuss an agronomically sound soil fertility program for your farm.
Additional Reading:
New Grain Phosphorus and Potassium Numbers (University of Illinois)
Figure 1: Summary comparison of crop maintenance fertilizer recommendations (University of Illinois).