U of M Extension Home : U of M Home

Minnesota Crop News > 2001-2008 Archives

The frost on the night of August 20 combined with the very cool growing season may force growers to make decisions that they have not faced before. In various situations, some of these decisions could be affected by some knowledge of crop removal.

There are a variety of values that can be used to calculate removal of major nutrients by corn and soybeans. Values that are frequently used are listed in the table that follows:

Removal of Major Nutrients by Corn and Soybeans

The values are for a harvested crop and are listed as lb. removal per bushel of grain. This listing assumes that yields are known or, at least, there is a good estimate of yield. This is not difficult when crops mature. Yields of immature crops that are not harvested can only be estimated unless the crop is used for silage.

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. If this action is taken, there are serious questions about the nutrient value of the destroyed crop.

Dr. Dave Franzen at North Dakota State University and I have discussed this situation and agree on recommendations that are discussed in the paragraphs that follow. It’s important to understand that there are no research projects that provide data specific for this situation. However, we can apply information taken from other sources and adapt it to the damaged 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 a 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 ¾ inches long at one of the four uppermost nodes. At R5 the pods are 1/8 inch long at one of the four uppermost nodes.

An additional credit of 40 lb. 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. In studies where the sugarbeet crop has followed sweet corn rather than field corn, spring sampling to a depth of 2 feet shows an additional 30 lb. of nitrate–nitrogen per acre where the sweet corn residue is incorporated. This additional nitrate–nitrogen is a result of mineralization of the residue of the immature corn. Therefore, a nitrogen credit of 30 lb. N per acre appeared 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. This delay will provide for a more accurate measure of mineralized N.