Urea is a highly soluble nitrogen-containing species (N46%). It can dissolve 108 kg urea in 100 kg water at 20°C. In terms of chemical composition, urea is a small-molecule organic nitrogen fertilizer. After application into the soil, molecular urea can be dissolved in the soil solution. Since the active root nutrient absorption mechanism of crop roots is mineral nutrients, organic molecular urea is easily dissolved. However, it is not yet directly absorbed by crops. After urea is applied to soil, under the action of urease secreted by microorganisms, it is hydrolyzed to form unstable ammonium carbonate, which is further decomposed into ammonium and carbon dioxide. Only mineral ammonium can be absorbed and utilized by crop roots, and can also be used as soil rubber particles. Adsorption preservation. After understanding these characteristics of urea, the following application suggestions can be made:
The application period of urea as top dressing should be appropriately several days in advance. Because urea is used as a fertilizer, it is actually slower than ammonium sulfate, ammonium bicarbonate, ammonium chloride, and agricultural ammonia. In order to give full play to the utility of topdressing, urea should be applied earlier than other nitrogen fertilizers, and the soil temperature at the time should be considered several days in advance. 7-10 days at 10°C, 4-5 days at 20°C, and 2-3 days at 30°C.
Although urea is a chemically stable neutral molecular nitrogen fertilizer, it will not volatilize and lose instantly when applied to the surface. However, the applied soil is eventually converted to ammonium carbonate, which is otherwise difficult to absorb and utilize in large quantities. If the topdressing urea is applied to the ground surface, the current high temperature season will soon turn into ammonium carbonate, and make the rhizosphere soil alkaline, which will lead to a large amount of ammonia volatilization, loss of valuable nitrogen and burning into the leaves and leaves Accident.
Therefore, urea used as topdressing, must use the ditch or hole application, fertilization depth of 2-3 inches, after the cover should be covered with soil, in order to have the desired effect. When urea is used as a top-dressing fertilizer in paddy fields, shallow surface water is kept on the field surface, and tillage is applied immediately after planting. After urea conversion for 3-4 days, water is poured to prevent leakage loss.
Professor of China Agricultural University Cao Yiping
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