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Peter C. Scharf
University of Missouri
Plant Science & Technology
(573) 882-0777

Spring 2017 Rain and Nitrogen

Peter C. Scharf
University of Missouri
(573) 882-0777

Published: May 18, 2017

illustration showing corn plus rain drops equals a question mark over laid on a flooded corn field

The spring was going great. Corn was nearly planted across Missouri and much of the corn belt. It was a little dry, but not too bad.

Then a couple of big weather systems dropped 5 to 15 inches of rain from Kansas to Indiana in about a week. Most of this rain fell slowly enough that it was absorbed into the soil, where it could (and in some cases did) do bad things to seeds and nitrogen.

I have heard a lot of worry about nitrogen loss, but my opinion is that not much was lost due to this rain. I do think that it left us much more vulnerable to future N loss than we were before.

Most fertilizer N was already in the nitrate form by the time the rain hit (more below), and nitrate is the form that can be lost. But for most people, the factors that control loss of nitrate do not yet add up to enough to hurt them.

Well-drained soils

Nitrogen Watch map for well-drained soils. Leaching is the main loss mechanism in these soils, and can start as soon as nitrate is formed. This will depend on N form and timing, but by April 1 nitrate was the dominant form in most fields. Cross-hatch shows areas that are on track to have 16" of April-June rain, which would create a high probability of N deficiency in corn.

For well-drained soils where leaching is the main N loss mechanism, the N has moved downward with the water—but not out of the root zone. I would guess that a lot of nitrate was moved down into the 2nd and 3rd foot of soil. Some may have moved back up with recent drying conditions pulling water back to the surface. It will take quite a bit more rain to flush nitrate out of the root zone for most folks. My rule of thumb is that 16 or more inches of rain April through June will do it.

Only south Missouri has hit that mark already. The only well-drained soils with corn in that region are in creek and river bottoms and probably went underwater. Even in these fields, I suspect that not much N was lost. As the creeks came up, water came up from below in the fields, so that there was not much empty space that could be filled by water coming from above. As the waters receded, they carried away some N, but it's hard to say how much. There wouldn't be much corn on well-drained uplands in this region, but there is quite a bit of grass that received N. Although N leaching from grass is rare, it could have happened with the weather we had. If you have grass that looks like it needs more N, and you need more grass, fertilize it again.

Poorly-drained soils

Nitrogen Watch map for poorly-drained soils. Denitrification is the main loss mechanism in these soils, and losses in April are usually temperature-limited. Cross-hatch shows areas that are on track to have 12" of May-June rain, which would create a high probability of N deficiency in corn.

For poorly-drained soils, denitrification is the main way that nitrate is lost. In converts to nitrogen gas and is lost to the air. It's very temperature-sensitive since bacteria are carrying out the conversion and they do it a lot faster when they're warm.

For many fields, I think that soil temperature protected them from major N loss. Brunswick, MO, for example, had a soil temperature of 65 on April 24 and 51 on April 28. The soil temperature dropped 14 degrees in 4 days because a lot of cold water was dumped on it. At 51 degrees, denitrification is slow. Only in southwestern Missouri and southeastern Kansas do I think soils were warm enough to result in N loss to worry about. Even in those places I would expect only moderate losses.

For the next big rain, I don't think that we can expect soil temperature to protect us. Poorly-drained soils will saturate more quickly than they did last time. As I write this on May 17, there is a low-pressure system slowly pushing its way across the middle of the country.

Track the situation

No matter what kind of soils you farm, or advise farmers on, there are large areas that are on track to have substantial N loss if it keeps raining at the same pace that it has so far. Cross-hatched areas in the attached Nitrogen Watch maps show where these areas are located. I call them the 'danger zone' for N loss. We update these maps weekly through the end of June. You can find them at https://plantsciencesweb.missouri.edu/nutrientmanagement/Nitrogen/Nitrogen_watch_2017/nitrogen_watch_2017.htm

Although rainfall totals and soil drainage alone are crude indicators of whether you actually have serious N loss in a particular field, they are easy to track and give a quick picture of whether you need to look deeper.

Anhydrous out early

Because of the open fall and winter, a lot of anhydrous ammonia was applied through fall, winter, and early spring. There were not many days when you couldn't apply in Missouri. I don't have statistics, but won't be surprised if I later find that it was a record year for anhydrous application in Missouri.

Timing of anhydrous application will have some impact on nitrate movement in well-drained soils, and potential for N deficiency. Any anhydrous applied before March 1 was probably 90% converted to nitrate by the time the big rains hit at the end of April, N-Serve or not. Only the luck of future weather will determine whether it's there in June and July when your crop needs it. For anhydrous applied in March, I would expect in the neighborhood of 60% converted by the time the rains hit, and therefore less downward movement of nitrate than in fields with earlier applications. Fields with anhydrous applied in April were probably more in the ballpark of 30% converted, meaning that even less N moved. For March and April applications, I see potential for N-Serve to have reduced downward movement of nitrate by slowing formation of nitrate.

Other N fertilizers

All N fertilizers other than anhydrous will have mostly converted to nitrate within 2 to 3 weeks of application in the spring. From what I know of when N was applied, most fields would have had mostly the nitrate form of N by the time the big rains hit. In well-drained soils this nitrate has moved down, but is probably still in the root zone. In poorly-drained soils, temperature has protected against large losses. In both, risk of future loss if it keeps raining is fairly high.

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REVISED: February 21, 2017