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How Urea Converts to NH4
Urea is a white crystalline granular material containing 46% N. These granules are larger, harder, and less affected by moisture (high humidity) than the prilled form which used to be the primary form of urea. Consequently, granular urea has become a suitable material for fertilizer blends. Urea weighs from 44 to 49 lbs per cubic foot, depending manufactured form.
Hydrolysis, the breakdown of urea in the presence of water, is a fundamental property of urea that greatly impacts the management of urea as fertilizer. Thus, much of the following discussion will relate to this property.
Urea hydrolyzes to ammonium carbonate very quickly when added to and incorporated into the soil according to the following reaction:
CO (NH2)2 + H2O + urease (NH4)2CO3 2NH3 + 2H2O + CO2
urea (an enzyme)
The ammonium carbonate is very unstable and decomposes to NH3 + carbon dioxide. The enzyme, urease, greatly stimulates this reaction. Urease is found in all soils in sufficient quantities to bring about rapid conversion of urea to ammonia (NH3) which then in the presence of water converts to ammonium (NH4+). The ammonium then attaches to the negatively charged soil particles and behaves like any other ammonium-based source of nitrogen fertilizer. This whole hydrolysis reaction normally occurs within a 2 to 4-day period. If the urea is not incorporated into the soil, hydrolysis can also occur and NH3 can then be lost to the atmosphere through volatilization.
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