How is the effect of topdressing with urea?

How is the effect of topdressing with urea? Please introduce in detail that urea is a variety with high nitrogen content (N46%) and soluble in water. 108kg urea can be dissolved in 100kg water at 20 ℃. In terms of chemical composition, urea is a kind of small molecular organic nitrogen fertilizer. After being applied into soil, molecular urea is soluble in soil solution. Because the mechanism of active absorption of nutrients by crop roots is mineral nutrition, organic molecular urea is soluble, but it can not be directly absorbed and utilized by crops. After urea is applied into soil, it should be hydrolyzed to form unstable ammonium carbonate under the action of urease secreted by microorganisms, and then further decomposed into ammonium and carbon dioxide, so that the mineral ammonium can be absorbed and utilized by crop roots and can also be absorbed and preserved by soil colloidal particles. After understanding these characteristics of urea, the following application suggestions can be put forward: the application period of urea as topdressing should be several days in advance. Because the fertilizer effect of urea as topdressing is actually slower than that of ammonium sulfate, ammonium bicarbonate, ammonium chloride and agricultural ammonia. In order to give full play to the effect of topdressing, urea should be applied earlier than several other nitrogen fertilizers, depending on the soil temperature a few days earlier. 10 ℃: 7 ℃ for 10 days, 20 ℃ for 4 min for 5 days, and 30 min for 2 mo for 3 days. Although urea is a chemically stable neutral molecular nitrogen fertilizer, it will not volatilize immediately when applied to the surface. However, the application of soil will eventually be converted into ammonium carbonate, otherwise it will be difficult to be absorbed and utilized by crops. If the urea of topdressing is concentrated on the surface, it will soon turn into ammonium carbonate in the current high temperature season, and make the root soil alkaline, which will lead to a large amount of ammonia volatilization, which will not only lose precious nitrogen, but also lead to the accident of burning stems and leaves. Therefore, when urea is used as topdressing, ditch application or hole application must be used, and the fertilization depth is 2mur. after application, the soil must be covered strictly in order to have the desired effect. When urea is used as topdressing in paddy field, shallow water is kept on the field surface, cultivated immediately after application, and then irrigated after urea conversion for 4 days, so as to avoid leakage loss. Click to get more nitrogen, phosphorus, potassium application techniques click to get more chemical fertilizer application techniques