The frost on the night of Aug. 20 combined with the very cool growing season may force growers to make decisions that they have not faced before. These may involve nutrient removal by corn and soybeans and nutrient credits for succeeding crops.

Because of the severity of the frost in many areas of northern Minnesota, there are soybean and corn fields that will not be harvested. Crops in these fields will probably be plowed under, leaving serious questions about the nutrient value of the destroyed crop.

The phosphorus and potassium in the destroyed crop will become available to subsequent crops over time. Potassium will become available more rapidly than phosphorus. This availability, if substantial, will be reflected in changes in soil test values. So, it is more important to sample fields when the destroyed crops have been plowed under. It's reasonable to expect that most of the changes in soil test phosphorus and potassium will be measured in 2005 and 2006.

The nitrogen credits for the destroyed crop are more of a concern. Based on estimated dry matter production and knowledge of the nitrogen connection in soybean plants, we estimate an N credit for soybeans if destroyed at three states of development. These are:

  • R3 - 25 lb. N per acre
  • R4 - 45 lb. N per acre
  • R5 - 65 lb. N per acre

Pod development is just beginning at R3. There is full pod development at R4, and pods are three-fourth inches long at one of the four uppermost nodes. At R5 the pods are one-eighth inch long at one of the four uppermost nodes.

An additional credit of 40 lbs. N per acre should also be applied if a crop requiring nitrogen in a fertilizer program follows soybeans that are either harvested or destroyed. This credit is a consequence of factors other than the incorporation of soybean residue into the soil.

The amount of N contributed by a corn crop that is plowed under in the next two to three weeks is more difficult to estimate. This practice would be somewhat similar to fields where sweet corn residue is incorporated in August or September after harvest. A nitrogen credit of 30 lbs. N per acre appears to be reasonable for crops other than legumes that follow the destroyed corn in the rotation.

The N from the destroyed corn can be considered as a slow release source. This would be beneficial to wheat and other crops where quality is not a concern. Planting of barley after destroyed corn could be a problem. The late season release of N in 2005 could increase grain protein and disqualify the barley for the malting market. A sugarbeet crop would not be a good choice following a destroyed corn crop. Again, late season increase of N in 2005 would be a liability rather than a benefit because of the potential reduction in sugar content.

Soil sampled for nitrate-nitrogen is a recommended practice where damaged crops are plowed under. This sampling should be delayed as long as possible this fall to provide for a more accurate measure of mineralized N.

More detailed information on is available at http://www.extension.umn.edu/cropenews . Also, see the new Web site on frosted and immature crops at http://www.extension.umn.edu .