Curt Emanuel, Extension Educator, Purdue Extension – Boone County
The principle behind Nitrogen scavenging is pretty simple. Each growing season, some nutrients remain in the soil after the crop is harvested. In the case of most nutrients, we don’t worry much about them, so long as the soil stays in the field. They will be available for future growing seasons. Nitrogen (N) is different. It is not stable in soil so any excess N will usually be lost. This loss may occur in several ways. Some will be lost into the atmosphere through a process known as volatalization while some may be leached out of the soil or be carried off the soil surface as runoff by water.
Someone may look at the previous paragraph and say, “Shouldn’t farmers just use less nitrogen?” While there are certainly people who over-fertilize (but not many – fertilizer is expensive and farmers are no fonder than anyone else about spending money on something which does them no good), for the vast majority of farmers, the answer to that question is no. Farmers should plan to have a little nitrogen left over. It’s too fine a line to try to fertilize so the crop uses exactly the amount applied. All it takes is one rainfall event or some very hot, humid weather, and the crop runs out of N at the end of the growing season. The end of the growing season is when the plant sets its sights on reproducing – forming seed. By running out of N at this point, grain yields may be severely reduced. Corn ears may abort kernels, or they may add less starch to kernels, resulting in something known as low test weight, which will cause a dockage in the price farmers receive. Farmers should plan to have a little N left over; the financial risks of running short are too great.
When we plant cover crops, particularly those considered nitrogen scavengers – which all five fall, 2015 treatments are – those cover crops use the available N for their own growth. Instead of being lost, N remains in the field. Next spring as that cover crop is terminated, nitrogen remains available for the farmer to use in the next growing season.
In order to illustrate exactly how this works, we will need to do some math. The relevant information is that we applied 233.1 pounds of nitrogen per acre before planting the corn crop this May. That is enough N to grow 220 bushels of corn, which we were told is the yield potential of this field (our plot has very good, fertile soil). For those of you not in Central Indiana, this was a bad growing season. June and July had record amounts of rain. Waterlogged root systems are not efficient at transporting nutrients into the plant and producing high yielding corn. Our harvest for this plot was 150 bushels of corn per acre and we feel pretty good about that yield; many farm fields in this area did far worse.
In Indiana, the publication we use for making fertilizer calculations is Extension Bulletin E-2567, “Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat & Alfalfa.” If we go to page 9, we can see that 150 bushel corn should receive between 160 (for 140-bushel corn) and 190 (for 160-bushel corn) pounds of N per acre. We’ll split the difference and say that we should have applied 175 pounds of nitrogen per acre for 150-bushel corn.
When we subtract 175 from 233.1 we are left with 58.1 pounds of N remaining in the soil. The reality is that this year there probably isn’t quite that much. Due to the extreme wetness, some of this probably leached away. But much will remain. Instead of losing it either into the air or through future rainfall events this coming fall and winter, our remaining nitrogen will be taken up by our cover crops. This keeps it in the field where it will be available when we plant soybeans next spring. And while soybeans are a legume and produce their own nitrogen, they don’t start the nitrogen-fixation process until plants are several weeks old. Soybeans still use nitrogen to get started and what’s available will help in crop establishment.
There are many other cover crop benefits besides N-scavenging. However our crop situation this year, where yields are below expectations, provides a nice example of how cover crops can improve the environment by reducing the amount of nitrogen which finds its way into rivers and streams or the atmosphere. These cover crops may also help our financial situation by reducing the amount of fertilizer we need to use next spring. Double bonus!