By Emerson Nafziger
Early this week, a brief respite from heavy rains allowed for some corn and soybean planting (or replanting) to resume in many parts of Illinois. But, given the amount of recent precipitation, many farmers are concerned about nitrogen loss and wondering if they need to apply more. Emerson Nafziger, professor in the Department of Crop Sciences at the University of Illinois, provides some insight.
“The return of cooler weather along with the rainfall slowed nitrification - the conversion of ammonium to nitrate - slightly, and also slowed the denitrification process,” Nafziger explains. “Both nitrification and denitrification are biological processes, so they happen faster at higher temperatures. We know from finding nitrate in the soil that there has been a lot of nitrification. Denitrification requires both saturated soils and warm soils, and there has been less of it.”
Soils with standing water are slow to warm up, limiting the rate of denitrification. But it is happening in some areas where water is still standing. In those locations, it will be some time before a crop can be planted, and Nafziger says adjustments to fertilizer nitrogen may be in order as the crop gets established.
Ammonium moves little in the soil, but when it is converted to nitrate, it can move. “We know from our research that nitrogen applied last fall was about 70 percent nitrate by early May, and ammonia applied in March or April was more than half nitrate when the weather turned wet,” Nafziger says.
Somewhat surprisingly, Nafziger found little change in soil nitrogen levels from the unusually heavy rainfall, “We sampled six trial sites both before and after the heavy rainfall of late April and early May, and found virtually no change in soil nitrogen content. We expect that mineralization of soil organic matter added some nitrogen between samples, and that is no longer around, so some nitrogen moved out. The good news is that most of the nitrogen added as fertilizer is still in the soil. That may not be the case in every part of every field, but we don’t see any reason to imagine that most of the nitrogen we applied has been lost.”
Soil drainage is an important factor in movement of water and nitrate. Soil texture is a critical component of drainage, but field tiles change the relationship between texture and water movement.
“As an example, a typical Drummer silty clay loam soil in eastern Illinois allows hardly any water to move through it unless the soil is tile-drained. Tile becomes the exit route for soil nitrogen into surface waters, replacing denitrification as the main way nitrogen is lost in such soils. So, tile drainage changes the assumption that heavy-textured soils will lose nitrogen to denitrification while lighter-textured soils lose more to leaching,” Nafziger explains.
While it’s possible that some nitrogen may be lost before crop uptake begins in a few weeks, Nafziger says that a decision to apply more nitrogen than planned is premature. As soils dry, rainfall returns to normal, and plants grow, roots will begin to draw water and dissolved nitrogen towards the surface, and mineralization will kick into high gear. “Last year,” Nafziger recalls, “under good temperatures and without unusually heavy rainfall, we saw mineralization provide as much as 150 pounds of nitrogen per acre to the crop.”
One indication that the topsoil has not been stripped clean of nitrogen is the recovery of green leaf color that has been happening as the soil dries out. “Most fields are not as dark green as we saw at this point in 2016, but as the root system starts to expand and as soils continue to warm, this will change,” Nafziger says. “The corn crop at this point looks the way it does not because of lack of nitrogen, but due to the effects of temperature and rainfall on crop growth and early development.”
While it is premature to revise nitrogen management based on what has happened so far, farmers shouldn’t rule out the possibility that the crop may need more nitrogen. The good news is that farmers still have time to make such decisions. As long as soil conditions continue to improve, a crop provided with normal amounts of fertilizer nitrogen rarely runs out during vegetative development. According to Nafziger, this year will be no exception.
Nafziger plans to continue soil sampling to learn more about the status of soil nitrogen over the next two months. But, he says, because similar weather patterns have not happened this early in the season in recent years, he cannot easily predict what will happen later in the season.
“Nitrogen deficiency develops over time, and is almost always more related to current soil moisture than to the amount of soil nitrogen. So, if soils do not get extra wet or extra dry over the next month, this season could turn out to be much more typical than we expect.”