How effective is urea as a topdressing fertilizer? Getting the application method right is key
How effective is urea as a topdressing fertilizer? A detailed introduction: Urea is a variety with a very high nitrogen content (N46%) that dissolves easily in water. At 20°C, 100 kg of water can dissolve 108 kg of urea. Chemically, urea is a small-molecule organic nitrogen fertilizer; after being applied to the soil, molecular urea can dissolve in the soil solution...
How effective is urea as a topdressing fertilizer? A detailed introduction
Urea is a variety with a very high nitrogen content (N46%) that dissolves easily in water. At 20°C, 100 kg of water can dissolve 108 kg of urea. Chemically, urea is a small-molecule organic nitrogen fertilizer. After being applied to the soil, molecular urea can dissolve in the soil solution, but because the mechanism by which crop roots actively absorb nutrients is mineral nutrition, organic molecular urea, though easily soluble, cannot yet be directly absorbed and used by crops in large amounts. After urea is applied to the soil, it must be hydrolyzed under the action of urease secreted by microorganisms to form unstable ammonium carbonate, which then further decomposes into ammonium and carbon dioxide. Only the mineral-form ammonium can be absorbed and used in large amounts by crop roots, and it can also be adsorbed and retained by soil colloids. After understanding these characteristics of urea, the following application recommendations can be made:
The application time for urea as a topdressing should be appropriately advanced by a few days. This is because the fertilizer effect of urea as a topdressing is actually slower than that of ammonium sulfate, ammonium bicarbonate, ammonium chloride and agricultural ammonia water, etc. In order to exert the effect of the topdressing at the right time, urea should be applied earlier than these other nitrogen fertilizers, and how many days earlier depends on the soil temperature at the time. At 10°C, 7-10 days; at 20°C, 4-5 days; at 30°C, 2-3 days.
Although urea is a chemically stable neutral molecular nitrogen fertilizer and will not volatilize and be lost immediately when applied to the soil surface, once applied to the soil it will ultimately be converted into ammonium carbonate, otherwise it would also be difficult for crops to absorb and use it in large amounts. If the urea used as topdressing is concentrated on the soil surface, in the current high-temperature season it will quickly convert into ammonium carbonate and cause the soil around the roots to become alkaline, leading to a large amount of ammonia volatilization, which both loses valuable nitrogen and can cause accidents that scorch stems and leaves.
Therefore, when urea is used as a topdressing, it must be applied in furrows or holes, with an application depth of 2-3 cun, and after application the soil must be covered firmly to achieve the expected effect. When urea is used as a topdressing in rice fields, keep a shallow layer of water on the field surface, cultivate and weed immediately after application, and wait 3-4 days for the urea to convert before irrigating, so as to avoid seepage losses.
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