As crops are being harvested and grain is taken to the storage or elevator, it is time to recognize the amount of nutrients that are removed from your field by this grain, particularly the three major macronutrients: nitrogen (N), phosphorus (P) and potassium (K). Nutrient removal amounts can be easily calculated by multiplying the nutrient removal rate per bushel (shown in table) by actual yield.
For example, a 150-bushel-per-acre corn will remove 135 pounds per acre N, 56 pounds per acre P2O5 and 41 pounds per acre K20. At current fertilizer prices, total N, P and K nutrient value in the grain would be about $112 per acre. Likewise, a 40-bushel-per-acre soybean crop will remove 152 pounds per acre N, 32 pounds per acre P2O5 and 56 pounds per acre K20.
Nutrient removal rates indicate the variations in N, P and K needs among different crops. As crops yields increase, more and more nutrients are removed from the soil. As a result of this removal, the soil test P and K levels will gradually decrease over the years if no fertilizers are added.
Information on nutrient removal alone is not adequate for making fertility recommendations because it does not take into account the ability of the soils to retain and supply nutrients. Another important aspect is the current soil test level.
According to Michigan State University Extension’s build-up, maintenance and drawdown approach, the soil test P and K should ideally be in the maintenance range. When this level is attained, farmers can avoid any buildup or drawdown scenarios simply by adding the nutrient removal rates of the crop to be grown. When the soil test level is low and in the build-up range, there will be a response to P and K, and the recommendations will provide for both build-up and maintenance. However, if the soil test level is in the high drawdown range (greater than adequate), there may not be a yield response for adding P and K at crop removal rates. Several years of cropping without the addition of fertillizer will help to reduce nutrients and bring the soil test down to a desired level.
Soil testing every three years would enable farmers to monitor the changes in the P and K levels. Please refer to MSU Extension publication E2904, “Nutrient Recommendations for Field Crops in Michigan,” for guidelines on MSU fertilizer recommendations and nutrient removal rates of Michigan field crops.
Soybeans as a legume crop can meet its own N needs under favorable growing conditions by symbiotic fixation with bacteria. So no N fertilizer is currently recommended on soybeans. Soybeans also remove less P and more K in a year compared to corn.
Whether you are planting wheat or contemplating fall P and K applications for corn and soybeans, nutrient removal rate is an important consideration in fertilizer rate decisions for sustainable crop production. As the nutrient removal rate amount is a function of yield, an over-estimated yield goal will lead to over application of fertilizer and higher production costs.
Source: George Silva, Michigan State University Extension