The effect of topdressing urea is small.

At present, most of China is in a hot and rainy season, and it is also a key period for the vigorous growth and yield formation of various crops. It is necessary to absorb a large amount of nutrients from the soil, which is the maximum efficiency period of nutrition indicated by the theory of plant nutrition stage; The supply capacity of the soil may not be able to keep up with the rapid growth of the crop, which requires the timely addition of fertilizer; at this time, the yield of the top dress will be the highest. It can also be said that in agricultural production, it is now the golden season for topdressing crops.
"Top dressing" refers to fertilization during crop growth. The longer the growing period of crops, the greater the need for top dressing, and the number of times may be more than once. The specific top dressing time is determined by the growth period of various crops, such as the jointing stage and the booting stage of rice, wheat and spring corn, the flowering period of cotton, tomato and other crops, and the fruiting (peach) stage. Critical period. Therefore, the name of topdressing is often named after the growth period of the crop, such as rice tiller and panicle fertilizer, and wheat jointing fertilizer.
For the three elements of NPK fertilizer, the nitrogen fertilizer is too active after being applied to the soil, and it is easy to lose waste. It is not suitable for all the basic fertilizers to be applied at one time, but it needs to take a certain proportion of top dressing to achieve high yield and high efficiency. The goal. In the commonly used nitrogen fertilizer varieties, the current market mainly provides urea, and other varieties such as ammonium sulfate and ammonium nitrate are rare. In fact, urea has become the first choice for farmers in China. However, the choice of urea for topdressing has both high efficiency and low efficiency. The key is how to grasp the application technique. From the survey and statistics of the agricultural sector and the hotline for the service of farmers, the accidents of smoked leaf burning in agricultural production have occurred frequently in recent years. Why is there a constant accident and inefficiency in urea top dressing? How to improve the utilization rate of urea for topdressing? Here, the role of topdressing is compared with the characteristics of urea, so that the majority of farmers can understand and master the law, and use urea in the current season to obtain high yield and high efficiency of this autumn harvest crop.
In view of the basic purpose of top dressing, the nitrogen fertilizer varieties used should preferably meet the following two requirements: First, the water solubility is strong and the effect is quick. Second, the chemical properties are stable. Because of the field fertilization during the period of vigorous crop growth, it is difficult to use large-scale mechanical overturning and ditching, and farmers are used to the method of topdressing such as rain or irrigation on the surface. If the nitrogen fertilizer is unstable and the volatilization loss is too large, such top dressing It is not cost effective.
From the above two requirements, we can analyze the characteristics of urea: urea is a kind of nitrogen-rich (N46%) water-soluble variety. At 20 ° C, 108 kg of urea can be dissolved in 100 kg of water. In chemical composition, urea is a small molecular organic nitrogen fertilizer. After being applied to the soil, molecular urea is soluble in soil solution. Because the mechanism of active nutrient absorption by crop roots is mineral nutrition, organic molecular urea is soluble. However, it cannot be directly absorbed and utilized by crops. After urea is applied to the soil, it is hydrolyzed to form unstable ammonium carbonate under the action of urease secreted by microorganisms, and then further decomposed into ammonium and carbon dioxide. The mineral ammonium can be absorbed and utilized by the crop roots, and can also be used by soil colloidal particles. Adsorption preservation. After understanding these characteristics of urea, the following application recommendations can be made:
The application period of urea as top dressing should be appropriate several days in advance. Because urea is used as a fertilizer for topdressing, it is actually slower than ammonium sulfate, ammonium bicarbonate, ammonium chloride and agricultural ammonia. In order to play the role of top dressing in a timely manner, urea should be applied earlier than several other nitrogen fertilizers, and the soil temperature at that time should be seen a few days in advance. At 10 °C, 7 to 10 days in advance, 4 to 5 days in advance at 20 °C, and 2 to 3 days in advance at 30 °C.
Although urea is a chemically stable neutral molecular nitrogen fertilizer, it will not be immediately volatile when applied to the surface. However, the application of the soil will eventually be converted into ammonium carbonate, otherwise it will be difficult to be absorbed and utilized by the crop. If the topdressing urea is concentrated on the surface, the current high temperature season will quickly turn into ammonium carbonate, and the root surrounding soil will be alkaline, which will cause a large amount of ammonia to volatilize, which will lose valuable nitrogen and will form smoldering stems and leaves. The accident.
Therefore, urea should be used for top dressing. It must be applied by ditch or acupoints. The depth of fertilization is 2~3 inches. After application, the soil should be covered tightly to have the expected effect. When urea is used as top dressing in paddy field, the surface of the field is kept shallow, and immediately after application, the ploughing is carried out, and the water is re-irrigated after 3 to 4 days of urea conversion to avoid leakage loss. (Author: Cao Yiping senior adviser Sinochem fertilizer, China Agricultural University)
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