University of Missouri Extension

G9218, New January 2006

Managing Nitrogen to Protect Water Quality

John Lory and Steve Cromley
Division of Plant Sciences

How is nitrogen lost from the soil?

Nitrogen is primarily lost by three pathways:

  • Nitrate leaching happens when excess rainfall moves water through the soil. As the rainwater moves downward in the soil, it carries nitrate nitrogen below the root zone. Nitrate leaching is most likely to occur during wet periods of the year when the crop is not actively growing, such as late fall through early spring. All nitrogen fertilizers convert to nitrate-nitrogen in warm soil.
  • Ammonia volatilization happens when urea fertilizers and liquid manures are surface-applied. The nitrogen is lost as ammonia to the atmosphere. Volatilization is enhanced when the applied nitrogen coats plant and plant residues without contacting the soil. Nitrogen is also lost as ammonia when anhydrous ammonia injection slots fail to close during application.
  • Denitrification happens when warm soils are waterlogged for more than a day or two when there is nitrate in the soil. These conditions are most likely to occur during wet spells in May or June. During denitrification, nitrogen is lost as a gas to the atmosphere.

Nitrogen is a highly mobile nutrient that can be lost to the air, in runoff and through the soil. The high mobility of nitrogen creates unique nutrient management requirements to ensure fertilizer remains in the soil long enough to benefit your crop.

A significant proportion of a poorly timed nitrogen fertilizer application can be lost before the target crop has a chance to use it. For example, if nitrogen for a corn crop is applied in early fall losses from winter and spring rains can deplete much of the nitrogen fertilizer from the soil before the plant can use it during the growing season.

There are three water quality concerns associated with loss of nitrogen from agricultural fields:

Improving nitrogen management improves both water quality and the effectiveness of fertilizer nitrogen for meeting agronomic goals.

How to prevent nitrogen losses

Avoid overapplication of fertilizer nitrogen
Research has shown that nearly all nitrogen applied in excess of crop needs can be lost from the root zone in humid regions of the United States, including Missouri. To avoid overapplication of nitrogen

Soil sampling strategy for an 80-acre field. Figure 1
Active growing season of common Missouri crops.
 

Apply nitrogen during periods of active uptake

Prolong the time nitrogen is held by the soil
There are a number of strategies for extending the window of opportunity for applying nitrogen fertilizer.

More on ammonia volatilization

Specific chemical conditions are needed for ammonia volatilization to occur. High pH conditions (greater than 7) promote ammonia volatilization.

  • Almost all Missouri soils have a pH below 7 so most ammonium-containing fertilizers do not promote volatilization. Volatilization is not a problem with ammonium sulfate and ammonium nitrate.
  • Surface-applied urea is prone to ammonia volatilization because the fertilizer temporarily raises soil pH near the urea particles as they convert to a form of nitrogen usable by the plant.
  • Many manure types have a pH above 7 that promotes ammonia volatilization until the manure equilibrates to the soil pH.
  • Missouri soils naturally adsorb ammonia. We take advantage of this characteristic when we inject manure and anhydrous ammonia under the soil surface.

Minimizing nitrate leaching

Minimizing ammonia volatilization

G9218 Managing Nitrogen to Protect Water Quality | University of Missouri Extension

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