Gypsum and its role in trapping ammonia gas

Animal manures and composts inevitably release ammonia into the atmosphere. These waste products are rich in nutrients and contain organic forms of nitrogen. They are rich in biological life that feed on carbon and nutrients, and in the process, they mineralize or release nutrients, including nitrogen.

Table grapes in Chile
Table grapes in Chile

A very common problem during manure composting is the liberation to the atmosphere of certain amounts of nitrogen in the form of ammonia gas. Activity of microorganisms on the organic nitrogen compounds of the manure convert it to ammonia. Two consequences of this loss are the decrease of the amount of available nitrogen in the material and the increase of air pollution.

When microbes feed on organic material and they die, they form microbial biomass that is rich in amino nitrogen, a form of ammonia. If this decomposition occurred in the soil, the ammonia would combine with water and form ammonium, and that would stay intact in the soil until it eventually converts to nitrate. However, ammonia nitrogen released during decomposition of animal wastes and compost would be lost to the atmosphere. And that is very noticeable because you can smell the ammonia odor being emitted.

So ammonia is a by-product of decomposition – that is a biological fact of life. The key is figuring out how to trap this ammonia to prevent its loss. Perhaps there is another use for calcium sulfate or gypsum to trap ammonium. In a moist environment, gypsum disassociates (separates) into its individual salt components, calcium and sulfate. When gypsum is blended into a compost windrow or mixed in a manure pile, the sulfate salt naturally attaches to the ammonia molecules, forming ammonium sulfate – effectively trapping it in another stable form and preventing ammonia volatilization.

A key study was conducted that looked at trapping nitrogen with synthetic gypsum or superphosphate ( Calcium sulfate has 17-18% sulfate, while single super phosphate has 10% or less. Triple superphosphate has negligible amounts of sulfate

The authors of the study concluded:

  • “The amount of ammonia lost by volatilization from the manure decreased with the addition of both phosphogypsum and simple superphosphate, but for all rates applied, phosphogypsum was more efficient than simple superphosphate;
  • The reduction in volatilization increased with the rate of application of both additives;
  • There was an evidence that the reduction of ammonia loss was due to the presence of gypsum or sulfate in the studied additives.”

Gypsum has more sulfate available so it can trap more ammonium than superphosphate. The more product blended in the soil, the more ammonia was trapped.

The key is to predict the amount of nitrogen per ton of material that will volatilize and then apply enough gypsum to capture that ammonia into a stable ammonium sulfate form. Of course, the end product will contain calcium, ammonium sulfate and other nutrients and carbon.


Dr. Davidson posts articles on soil management and subjects to gypsum. If you have suggestions for topics or questions, feel free to contact him at or call 402-649-5919.