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Carbamide (urea) CO(NH2)2 is the most concentrated of solid nitrogen fertilizers, containing 46% nitrogen.
A potential issue with urea is the harmful effects of biuret. The increased content of biuret in the fertilizer reduces seed germination, inhibits the growth and development of plants as well as may cause leaf burns after foliar top dressing. The critical limit of biuret content in urea is usually up to 2 %. Therefore, in the finished product, the content of biuret is allowed no more than 0.6% in Grade A and 1.4% in Grade B. Biuret decomposes in the soil in 10-15 days due to its transformation into ammonium compounds by the urobacterium enzyme urease.
If compared to ammonium nitrate, the use of urea has its peculiarities. The high degree of flowability, low backfill density, small granules and low density do not allow high-performance use of centrifugal spreaders. The width of their capture should be 1/3 less than when applying potassium nitrate.
The effectiveness of urea depends on numerous factors. Urea is an uncharged molecule, therefore it is able to overcome significant distances in the soil with the movement of water. Thus, urea nitrogen in molecular form is absorbed insignificantly through the root system, but it becomes available for absorbing after hydrolytic transformations of the amide form to the ammonium one. Under the action of the enzyme urease, which is produced by urobacteria and plant roots, urea undergoes hydrolysis and turns into ammonium carbonate. At a soil temperature of 2 °C, 75% of urea is converted to ammonium in 4 days, at a temperature of 10 °c it takes 2 days, at 20 °C – only 1 day. Ammonium turns into nitrates at a temperature of 5 °C in 6 weeks, and at a temperature of 8, 10 and 20 °C – in 4, 2 and 1 weeks, respectively. The moisture capacity of the soil should be at the level of 40 %. The nitrification process is almost suspended in soils with a pH < 5.5. It should be also noted that non-amonified urea can be washed out of the soil.
Ammonium carbonate is a low-resistance compound. Under the influence of air it quickly decomposes into ammonium bicarbonate and ammonia, as a result of which gas ammonia is lost, especially on low-buffer soils. Therefore, during surface application of urea without immediate embedding in the soil and in the absence of precipitation, nitrogen is lost in the form of ammonia, in particular on soils with a neutral or alkaline reaction, on crops of perennial grasses where urobacteria activity is high, as well as after fertilizing grain crops. Losses of gas ammonia can reach 30-50% of the mass of nitrogen applied, while in the case of ammonium nitrate application they are 1-3 %.
The main factors affecting the ammonia weathering into the atmosphere are: the cation exchange capacity (CEC), soil pH, СаСО3 content, and moisture content.
After urea is applied to the soil, the ammonium carbonate formed during its hydrolysis decomposes into bicarbonate and ammonium hydroxide. The resulting ammonium is absorbed by the soil, then gradually absorbed by plants and undergoes nitrification. Urea is able to be absorbed by the roots and leaves of plants without prior transformation.
In terms of efficiency, carbamide is not inferior to ammonium nitrate, however the much more higher efficiency is observed after applying it to the soil.
Carbamide is most effectively used for basic fertilizing, top dressing of row and vegetable crops, and for foliar top dressing of plants, since it is an organic compound. Surface top dressing of winter crops, meadows and pastures is less effective if compared to ammonium nitrate, which is explained by the loss of ammonia, the slower action of urea than ammonium nitrate. Unlike other nitrogen fertilizers, carbamide ensures plants to absorb a significant amount of nitrogen from urea without prior conversion. Urea is absorbed by leaf cells in the form of whole molecules.
- it does not lose its physical properties during storage;
- the statistical solidity of pellets is not less than 7 MPa;
- the mass fraction of pellets between 1-4 mm is more than 90 %;
- it is highly soluble in soil solution;
- it quickly compensates insufficient nitrogen quantity.
- Depending on the soil and climatic conditions, 100-400 kg/ha of carbamide is applied for winter tillage or for pre-sowing cultivation.
- 50-100 kg/ha of carbamide usually apply to the rows and during row top dressing.
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