A quick look at the nitrogen cycle and nitrogen fertilizer sources – Part 2

Various nitrogen fertilizers fit different needs.

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.”

Did you find this article useful?