The main forms of nitrogen (N) used in modern farming systems include urea, nitrate, ammonium and organic N (contained in living, or formerly living, plant and animal materials). Applying and using N by plants on farms in Michigan and elsewhere involves a dynamic and complex system referred to as the N cycle.

The N cycle involves the fixation of atmospheric N by lightning, microbes (alone or in combination with leguminous plants) and the addition of synthetic fertilizers. The resulting nitrate, ammonia or ammonium, when included in a plant-growing system in the soil, then undergoes a variety of changes. 

Some of these changes are desired and encouraged by farming practices such as incorporating fertilizers to discourage volatilization of N-containing ammonia as gas. Some parts of the N cycle are undesirable to farmers, including denitrification by soil microbes under saturated soil conditions with resulting loss of plant-available N to the atmosphere as N gas.

Plants can take up two forms of N including nitrate (NO3-) and ammonium (NH4+). The nitrate form has a negative electric charge and doesn’t “stick” to clay and organic matter particles in the soil as well as the positively charged (cation) ammonium form. For this reason, nitrate N moves through soil with soil moisture and can be more easily lost to leaching. Also, nitrification in the soil — a soil microbial processes in aerobic and above-freezing conditions with favorable pH of 5.5-7.5 — converts ammonium to nitrate.

In short, there is a lot to consider regarding N sources for crop production. Crop nutrient requirements, soil conditions, forms of N available, cost and available equipment all come into play. 

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

The cost per pound of N applied is also an important factor. Selecting a fertilizer with high N 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 N fertilizers below are adapted from the International Plant Nutrient Institute fact sheets on the various fertilizers:

Anhydrous ammonia (NH3):

  • Highest N content of any commercial fertilizer at 82% N.
  • 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.
  • N concentration ranges from 28% N to 32% N (more dilute in regions with colder winter temperatures).
  • Fifty percent of the total N comes from urea, the other 50% from ammonium nitrate, resulting in 25 percent NO3-, 25 percent NH4+ and 50% 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 N conversion and loss can be added.


  • Dry granular with 46% N.
  • Incorporation reduces N loss, but often surface applied as topdress on perennial grass and other crops.
  • Most of the N in urea is not immediately available to plants and must be converted to more available forms.
  • Once applied, urea N is quickly (normally within 2 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 33-34% N.
  • Fifty percent in ammonium form, 50% in nitrate form, so nitrate is immediately available to plants and ammonium provides delayed N supply.
  • Popular for pasture and topdress application since very little N 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% N and 24% sulfur.
  • Used primarily where there is a need for N and sulfur.
  • Not the most economical source of N 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 N 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 N in organic forms is contained in each percent of soil organic matter, and releases approximately 20 pounds of N annually.
  • Soil testing labs generally calculate an estimate of N provided by previous crops.