The first thing to understand is the purpose of crop topdressing and the requirements for fertilizer. Topdressing is a period of rapid growth of crops. It is generally a period of large-scale growth and development of vegetative and genital, which is the stage where crops require the most nutrients. In order to meet the stage requirements of high-yield crops, it is necessary to supplement a certain amount of quick-acting nitrogen fertilizer or nitrogen-potassium compound fertilizer. Since the root absorption efficiency is high at this time, as long as the amount and method of topdressing are used correctly, the utilization rate of nitrogen fertilizer will be very high, up to 50-70%. For the purpose and demand of top dressing, the fertilizer variety must have the characteristics of easy dissolution, easy absorption and fast fertilizer efficiency. Urea is often used by farmers as the preferred nitrogen fertilizer for topdressing. It is generally considered to have high nitrogen content and quick effect.
The actual fertilizer supply characteristics of urea: At present, most people directly engaged in agricultural production, the understanding of the "quickness" of urea is often not comprehensive enough. In fact, urea is an organic small molecular nitrogen fertilizer, although it appears on the surface after being applied to the soil. It dissolves quickly, but the molecular urea root system cannot absorb and utilize it directly. The urea must be converted into ammonium nitrogen by the decomposition of urease, which can be quickly and extensively absorbed and utilized by the crop roots. This shows that the fertilizer effect of using ammonium sulfate, ammonium chloride and ammonium bicarbonate as top dressing is definitely faster than that of urea. They can be directly absorbed and utilized without conversion, and they are more in line with the purpose and effect of top dressing.
How long does it take for urea to complete the conversion? What are the factors that affect conversion speed? Which determines that urea is topdressed a few days earlier than other ammonium nitrogen fertilizers? Scientific experiments have shown that the urea conversion process is affected by various factors such as soil temperature, moisture, organic matter content, pH and other factors. In general farmland, when soil moisture and pH are basically suitable for normal growth of crops, soil temperature is the rate of urea conversion. the main factor of influence. For example, in the early spring and winter wheat seasons in the north, the ground temperature is about 10 °C, and it takes about 7-10 days for the urea nitrogen to be converted into ammonium nitrogen. The local temperature rises to 20 °C, the spring corn trumpet stage top dressing, and the cotton bud stage top dressing When the wheat is topdressed at the booting stage, the conversion of urea to ammonium nitrogen takes about 5 days, while the local temperature rises to 25 °C and only takes 3 days. Therefore, using urea top dressing is better than other ammonium nitrogen fertilizers in a few days. As for the specific advance several days, it is necessary to adapt to local conditions.
When urea is used as top dressing in summer, it is necessary to pay attention to timely soil or irrigation. Because of the high temperature, urea conversion is fast, and the product after urea conversion is ammonium carbonate, which is less than the ammonium bicarbonate, which is commonly known as “qifeiâ€. Stable, will produce a lot of ammonia. If urea is not buried in the soil, or sprinkled on the surface without watering immediately, the consequences of ammonia volatilization will cause the leaves to burn. In severe cases, the leaves of the plants will boil like boiled water, even if the ammonia volatilization does not burn. The loss of nitrogen caused by the leaves can also pollute the environment and reduce the utilization rate. In short, the application technique of urea as a top dressing: one must be applied in advance, and the second is to cover the soil.
Professor of China Agricultural University Cao Yiping
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