Many farmers who fall-applied nitrogen have the same question this time of year: how much of the nitrogen (N) I applied can I count on being there now? The answer depends on the nitrogen cycle and the weather conditions at application time.
This past fall was warm and wet throughout much of Minnesota, with soils staying above freezing into December and above-average precipitation into November. At the University of Minnesota’s Research and Outreach Centers in Waseca and Morris, soil temperatures were not consistently below 50°F until the 9th of November, which is several weeks after the typical.
Why is this important? Because if nitrogen fertilizer is applied when soil temperatures are below 50°F, the conversion of ammonium to nitrate happens slowly- the microorganisms responsible for nitrification are not very active when soils are cold.
Spring rainfall is more likely to leach nitrogen than late fall rains, so if little fertilizer N converts to nitrate form, then little fertilizer N will be likely leached the following spring. Very little loss of nitrogen from fall fertilizer is likely to occur if soil temperatures are cold at application time. The challenge lies in getting fall fertilizer applied and incorporated in the time between when soils fall below 50°F and when they freeze.
In order for fertilizer N to be lost to leaching, large amounts of rain need to fall at certain times of year. Think about a situation where you had a soaking rain that dropped 2 inches of rainfall over 3 days. How much of that rainfall moves down through the soil, and how much nitrate it takes with it, depends heavily on the time of year and the crop growth stage.
As the soil warms up in the spring after corn planting, most of the fertilizer N has already converted to nitrate, and the corn crop is not removing much water or nitrate from the soil. Minnesota soils typically hold a lot of water left over from late fall rains and snowmelt during the spring, and cannot absorb much rain without it leaching. In a normal or wet spring, most of the 2 inch rainfall will move down through the soil below the root zone or to a tile drain, and there will be high concentrations of nitrate in the soil to move with it.
If the rainfall event comes during midsummer when corn is at tasselling, the crop is able to pull a lot of water out of the soil, and there will be much lower concentrations of nitrate in the soil. This is a situation with much lower potential for water movement to groundwater or tile drains, and much lower potential for nitrogen loss, than the spring rainfall event.
Lastly, if that rainfall comes in early November, after the corn has matured and been harvested, you have a different situation. If the soil is fairly dry, as is typically the case by harvest, it will be able to absorb most of the rainfall, without much of it leaching. In a wetter year, like the fall of 2016, most of this rainfall is likely to leach. However, as long as the corn crop was not over-fertilized, there are usually fairly low nitrate concentrations in the soil in the fall, so that leaching water might not take much nitrate with it. This is a situation where the timing of fall nitrogen fertilizer or manure is critical. If the nitrogen source was applied early in the fall, when soil temperatures were still warm, a lot of that nitrogen would be converted to the nitrate form and large losses of nitrate could result. However, if that nitrogen source was applied in late October or early November when soil temperatures were cooler, there would be very little conversion to nitrate, with low risk of loss from that fertilizer nitrogen. Potential for loss depends on the interaction between soil temperature and soil water movement.
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For questions about this post contact:
Greg Klinger, UMN Extension Regional Office Rochester, MN
Anne Struffert, UMN Extension Regional Office St. Cloud, MN