Urea (carbamide) CO(NH₂)₂ is the most concentrated solid nitrogen fertilizer with a nitrogen content of 46%.
The main limitation is the presence of biuret: its excess inhibits plant growth, reduces seed germination and can cause leaf burns during foliar feeding. The permissible biuret content is up to 0.6% in brand A and up to 1.4% in brand B. Biuret decomposes in the soil within 10–15 days.
Urea is less suitable for centrifugal spreaders than ammonium nitrate due to its small granules and lower density, so the working width of the spreader is reduced by about a third.
In the soil, urea is rapidly hydrolyzed by urease to the ammonium form, and then to nitrates; the rate of the process depends on temperature, humidity, and pH. Surface application without incorporation can result in significant losses of nitrogen in the form of ammonia — up to 30–50%, especially on neutral and alkaline soils.
In terms of efficiency, urea is not inferior to ammonium nitrate, but the maximum effect is provided by applying it to the soil. It is best used for basic fertilization, row application under row crops and vegetable crops, and for foliar feeding. Plants are able to partially absorb urea without prior transformation, in particular through the leaves.
