Unharvested or Hailed-Out Soybean Fields - Nutrient Supply to Corn?
John Sawyer, Professor, Department of Agronomy, Iowa State University
March 18, 2019
Flooded fields and wet soil conditions in the fall 2018 meant some soybean fields were not, or are not going to be harvested. Also, some fields occasionally experience significant shattering or a hailstorm in the fall where soybean seed is knocked from the plants and thus not harvested. Although the grain is not harvested, nitrogen (N), phosphorus (P), and potassium (K) nutrients returned to the soil in the soybean grain can be accounted for when planning nutrient applications for the next crop. Other nutrients will also be returned, but most not a fertilization need in most Iowa soils. An estimate of soybean bushels per acre lost is needed to approximate nutrient return to the soil.
For P and K, the calculation is straightforward. Either multiply the estimated yield lost times the bushel equivalent content for P2O5 and K2O (for soybean, 0.72 lb P2O5 /bu and 1.20 lb K2O/bu at 13.0% grain moisture content), or if the loss was significant or the crop not harvested, simply assume fertilizer or manure P and K applied before planting were never removed from the soil. With both approaches, P and K returned to the soil in soybean grain will be available for future crop use. Because the soybean residue was not affected, there would be no difference from a normal harvest.