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A Quick Look at the Nitrogen Cycle and Nitrogen Fertilizer Sources

February 1, 2017

A look at the nitrogen cycle helps understand the advantages and disadvantages of particular nitrogen fertilizer sources. For example, if nitrogen fertilizer is to be top-dressed and not incorporated, understanding the risk of nitrogen loss through volatilization is important. If sulfur is needed, then selecting ammonium sulfate to provide nitrogen and sulfur may be a good choice.

The cost per pound of nitrogen applied is also an important factor. Selecting a fertilizer with high nitrogen percentage and managing it appropriately may be a top priority. Please follow the 4R stewardship practices (right source, right rate, right placement and right timing) whenever possible as a guide to fertilizer decisions. Assistance is available from your local Michigan State University Extension educators. A few key points on the synthetic nitrogen fertilizers below are adapted from the International Plant Nutrient Institute fact sheets on the various fertilizers:

Anhydrous ammonia (NH3):

  • Highest nitrogen content of any commercial fertilizer at 82 percent nitrogen.
  • Applied below soil surface through tractor-drawn knives or shanks as a pressurized liquid that immediately becomes a vapor after leaving the tank.
  • Potential safety hazard and requires careful safety practices.
  • Rapidly converts to NH4+, then converts to nitrate.
  • Seeds should not be placed close to a zone of recent ammonia application.

Urea-ammonium nitrate (UAN):

  • Liquid formulation.
  • Nitrogen concentration ranges from 28 percent nitrogen to 32 percent nitrogen (more dilute in regions with colder winter temperatures).
  • Fifty percent of the total nitrogen comes from urea, the other 50 percent from ammonium nitrate, resulting in 25 percent NO3-, 25 percent NH4+ and 50 percent urea.
  • Can be blended with other nutrients and many agricultural chemicals.
  • Versatile: Can be injected as a band application at planting, sprayed onto soil surface, dribbled as a band at planting or sidedress time, added to irrigation water or applied as foliar spray.
  • Subject to some volatilization (gas) loss or urea component if left on the surface. Inhibitors that slow nitrogen conversion and loss can be added.


  • Dry granular with 46 percent nitrogen.
  • Incorporation reduces nitrogen loss, but often surface applied as top-dress on perennial grass and other crops.
  • Most of the nitrogen in urea is not immediately available to plants and must be converted to more available forms.
  • Once applied, urea nitrogen is quickly (normally within two days) converted to NH3 and is vulnerable to volatilization for several days until the NH3 is converted to NH4+, and finally to NO2-.

Ammonium nitrate:

  • Dry granular with 33 to 34 percent nitrogen.
  • Fifty percent in ammonium form, 50 percent in nitrate form, so nitrate is immediately available to plants and ammonium provides delayed nitrogen supply.
  • Popular for pasture and top-dress application since very little nitrogen loss through ammonia volatilization occurs.
  • High density results in even spreading across wide distances.
  • Limited availability because of its potential use in illegal explosives

Ammonium sulfate:

  • Dry granular with 21 percent nitrogen and 24 percent sulfur.
  • Used primarily where there is a need for nitrogen and sulfur.
  • Not the most economical source of nitrogen since concentration is relatively low.
  • More acidifying effect on soils than ammonium nitrate due to the nitrification process, not because of the sulfur content.
  • If used on alkaline soils, it should be incorporated or watered in, if possible, to avoid nitrogen loss from volatilization.

Soil organic matter:

  • A major source of nitrogen used by crops.
  • Easily decomposed portions of organic matter break down quickly and release nutrients, leaving behind a much more stable residue referred to as humus, which builds up slowly over time. This is the more permanent component of soil organic matter.
  • About 2,000 pounds of nitrogen in organic forms is contained in each percent of soil organic matter, and releases approximately 20 pounds of nitrogen annually.
  • Soil testing labs generally calculate an estimate of nitrogen provided by previous crops.

For more information on the nitrogen fertilizers and the nitrogen cycle, see “A quick look at the nitrogen cycle and nitrogen fertilizer sources – Part 1.

Source: Jim Isleib, Michigan State University