As crops are being harvested and grain is taken to the storage or elevator, it is time to estimate the amount of nutrients that are removed by this grain, particularly the three major nutrients: nitrogen (N), phosphorus (P) and potassium (K).
These amounts can be easily calculated by multiplying nutrient removal rate per unit of yield (shown in Table 1) by the actual yield.
For example, a 150 bu/A corn will remove 135 lbs./A N, 56 lbs./A P2O5 and 41 lbs./A K20, while a 40 bu/A soybean crop will remove 152 lbs./A N, 32 lbs./A P2O5 and 56 lb/A K20. As yields get higher, more nutrients are removed from the field.
Table 1. Nutrient removal rates per unit of yield by grain crops.
The nutrient removal rates are taken into consideration for making fertilizer recommendations.
If your soil test P and K values are in the maintenance range, you can maintain that level by simply adding the nutrient removal rates of the crop to be planted, without encountering any buildup or drawdown scenarios.
Please note that as a legume, soybeans can meet their own N needs by symbiotic fixation with bacteria. Soybeans also remove less P and more K compared to corn.
The MSU fertilizer recommendations for N and P in terms of rate, timing and placement are influenced by Michigan’s environmental constraints. Even though K is not an environmental risk, fall K fertilizer applications on sandy soils with low organic matter are discouraged because of potential leaching losses.