Harvesting crops removes valuable nutrients from the soil, making nutrient removal rates and soil testing important considerations when developing fertilizer plans for future crop rotations, according to Michigan State University Extension.

Phosphorus (P) and potassium (K) can be stored in the soil from year to year, but rebuilding soil nutrient concentrations can take several years once levels decline. Farmers must account for nutrients removed during harvest, along with existing soil test concentrations, when determining fertilizer application rates.

Nutrient removal rates vary by crop and have changed over time as grain nutrient concentrations have declined. According to the 2020 Tri-State Fertilizer Recommendations, crops are removing less phosphorus and potassium per bushel of yield, reflecting changes in grain yields, biomass production and root mass and density.

Nutrient removal can be calculated by multiplying the removal rate per bushel by the actual harvested yield. For example, a corn crop yielding 150 bushels per acre removes approximately 52 pounds of phosphate (P2O5) and 30 pounds of potash (K2O) per acre. A soybean crop yielding 49 bushels per acre removes approximately 39 pounds of phosphate and 56 pounds of potash per acre.

Although nutrient removal per bushel has declined, higher yields increase total nutrient removal. Without fertilizer applications, soil test phosphorus and potassium levels will gradually decrease over time.

However, nutrient removal through grain harvest and soil nutrient depletion are not directly proportional. Soil texture, physical properties and mineralogy influence how quickly nutrients are depleted or replenished.

Recommended nutrient removal rates vary among crops. Corn removes approximately 0.35 pounds of phosphate and 0.20 pounds of potash per bushel, while soybeans remove 0.80 pounds of phosphate and 1.15 pounds of potash per bushel. Wheat removes approximately 0.50 pounds of phosphate and 0.25 pounds of potash per bushel.

Other crops have different nutrient requirements. Canola removes approximately 0.80 pounds of phosphate and 0.40 pounds of potash per bushel, while oats remove 0.28 pounds of phosphate and 0.19 pounds of potash. Dry beans remove approximately 0.79 pounds of phosphate and 0.92 pounds of potash per bushel.

The corn, soybean and wheat removal rates are based on the 2020 Tri-State Fertilizer Recommendations. Rates for canola, oats and dry beans are based on 2016 data from the International Plant Nutrition Institute.

Michigan State University Extension moved away from its previous build-up, maintenance and drawdown fertilizer recommendation framework in 2020. The revised framework uses build and maintain categories, providing greater flexibility based on factors such as fertilizer prices and whether farmland is rented or owned.

Under the revised recommendations, fields with soil test phosphorus or potassium concentrations below critical levels require fertilizer applications sufficient to replace nutrients removed by crops and build soil nutrient concentrations. Fields below critical levels are considered deficient and have a greater probability of showing a yield response to fertilizer applications.

When soil test concentrations are above critical levels but below the maintenance limit, fertilizer applications based on crop nutrient removal rates are recommended. Although a yield response to additional fertilizer is less likely, maintaining nutrient concentrations within the recommended range remains important.

For fields with soil test concentrations above the maintenance limit, there is no agronomic reason to apply additional phosphorus or potassium fertilizer.

Building soil phosphorus and potassium concentrations can take multiple years. Farmers may need to apply fertilizer at rates exceeding crop nutrient removal for three to four years before soil test concentrations increase.

Michigan State University Extension recommends soil testing every three to four years to monitor changes in phosphorus and potassium concentrations and guide fertilizer decisions.

Accurate yield estimates are also important when calculating nutrient removal. Because total nutrient removal depends on harvested yield, overestimating yield goals can result in excessive fertilizer applications and higher production costs. Using actual yields and soil test results can help farmers develop nutrient management plans that maintain soil fertility while avoiding unnecessary fertilizer expenses.

Source: Michigan State University Extension, "Nutrient removal rates in harvested grain: corn, soybeans, wheat, oats, dry beans and canola"