Urea and the ammonification process

Urea or urea (NH₂)₂CO is a white crystalline solid containing 46% nitrogen. In agriculture, urea is used as a mineral nitrogen fertiliser. Urea contains a high concentration of nitrogen, which promotes plant growth and increases yields. It can also be used as a feed additive for animals.

Urea is applied to the soil as a solid (granules) or as a solution, depending on the needs of the crop and the application method. Once applied to the soil, urea undergoes chemical decomposition by bacteria, which results in the formation of ammonium ion (NH₄⁺). This process is called ammonification. Thanks to the enzyme urease, and in the presence of a small amount of soil moisture, urea is hydrolysed and converted into ammonium ion (NH₄⁺) and carbon dioxide (CO₂):

(NH₂)₂CO + 2H₂O + urease → 2NH₄+ + CO₂↑.

The rate of ammonification depends on soil moisture, temperature, acidity and the presence of bacteria that can decompose urea. In moist soil, bacteria are more active, so ammonification is faster.

However, if the soil becomes too wet, it can lead to nitrogen loss. Therefore, it is important to maintain optimum soil moisture to ensure efficient use of urea and prevent nitrogen losses.

When urea granules are dissolved, a zone of high ammonia concentration and elevated pH is created. This zone can be toxic to the seeds and sprouts of young plants and should be avoided. Usually, this toxic zone is neutralised within a few days. Once this zone is neutralised, the plants can effectively use the nitrogen that has been applied as urea.