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The origin of coated urea
Coated urea is a modified urea. Why do you want to modify urea? There are two reasons: First, urea is the highest nitrogen fertilizer variety (N ≥ 46%). In China's chemical fertilizers, the current industrial production of urea and agricultural consumption are ranked first. At the same time, the production and consumption of urea in China is also ranked first in the world. A survey of the actual effects of fertilization in recent years shows that urea is a fertilizer with low utilization rate. Due to the large total consumption and low utilization rate, the nitrogen loss caused by urea and the negative impact on the atmosphere and water environment are also large. Second, the reason for the low utilization rate of agriculture is determined by the weakness of the agrochemical nature of urea. The agrochemical traits of urea after entering the soil are similar to those of ammonium bicarbonate. They cannot be preserved by the soil before ammoniation, and the ammoniation process is accompanied by the alkalization of the soil in the micro-region, which leads to the aggravation of ammonia (NH3) volatilization. As early as more than 20 years ago, a large number of experiments have proved that the application of urea, ammonium bicarbonate and ammonium sulfate on the surface of calcareous fluvo-aquic soil in North China, the cumulative volatilization loss rate after 12 days is 12%, 15% and 9.0% respectively. It can be seen that the loss rate of ammonium sulfate is the least, and the loss rate of urea is relatively high, second only to ammonium bicarbonate. Moreover, the nitrogen content of urea is higher than that of ammonium bicarbonate, so the total amount of volatilization actually exceeds that of ammonium bicarbonate. For example, in the cultivation of millet in the black soil of northeast China, the comparative effect test of urea and ammonium bicarbonate showed that ammonium bicarbonate is better than urea regardless of crop absorption rate or apparent loss rate.
In view of the weakness of the agrochemical nature of urea after entering the soil, as early as the end of the 1980s, chemical fertilizer enterprises and scientific research units were committed to the modification test of urea. It is hoped that new varieties will be developed through modification to improve the utilization rate of urea.
Material composition and processing technology of urea coating liquid
The coating solution is a gel composed of organic and inorganic substances. In addition to organic substances such as humic acid, a small amount of potassium (K), magnesium (Mg), manganese (Mn), and zinc (Zn) are added. Colloidal substances formed after nutrients such as iron (Fe) and boron (B). The coating solution is yellow-green, with a specific gravity of 1.17 to 1.18 and a pH between 3 and 4.
The coating process uses a certain affinity between the coating solution and the surface of the urea particles, and uniformly sprays a small amount of the solution on the surface of the urea, and has a small amount of penetration into the urea particles. The amount of the coating solution per ton of urea is only 6-10. Kg; dried and solidified by the heat of urea in the granulating tower, the coating solution can form a very thin coating film on the surface of the urea granule after dry solid oxidation, which can be processed at one time.
With the rapid increase of urea consumption in China, in 1987, Guangzhou Nitrogen Fertilizer Plant was the first to develop coated urea, and there were Shijiazhuang Agricultural Modernization Research Institute of Chinese Academy of Sciences, Hebei Province, Guangdong Province, Zhejiang Province and other agricultural science and technology institutes. The multi-year test of the unit has a better understanding of the agrochemical properties and fertilizer efficiency of the coated urea.
The evaluation of the agrochemical properties of coated urea and the difference between the compositional urea and the ordinary urea: the coating film of the gelatinous coating is very thin, the weight is only 0.2% to 0.3%; the nitrogen content of the coated urea The change is small and contains a little trace element. The coated urea is physically improved due to the barrier effect of its outer coating. Even if the long-released block is easily broken, the particles can remain as they are after breaking, so the coated urea has more than ordinary urea. Better storage performance. In the release rate, the international common water dissolution rate is used as an indicator. The time required for 1 gram of common urea to completely dissolve in 100 ml of water is 9 minutes, and 1 gram of coated urea is completely dissolved until 13 to 20 minutes. It shows that its solubility in water is slightly slower than ordinary urea. Similarly, the coated urea diffuses in water more slowly than ordinary urea. In addition, the diffusion rate of coated urea in the soil also has a slight decrease and lag.
The application of coated urea, through field fertilizer test in many provinces, the application of coated urea has a tendency to increase production compared with the same amount of nitrogen. In addition to the abnormal results of individual results, the general yield increase is 2% to 10%.
Coated urea has been around for more than 20 years, and the cumulative area of ​​promotion has exceeded millions of hectares. But it has not become a mainstream product. The reasons for the analysis may be generally recognized by more users in terms of performance improvement, coating quality, price and fertilizer stability.
Although the agrochemical performance of coated urea has improved, it is not classified as a slow release fertilizer. Because China's slow release fertilizer national standard stipulates that the sustained release performance index is stricter and is in line with international standards. The sustained release performance of coated urea does not meet the standard requirements. At present, some advertisements have described the coated urea as a "controlled release fertilizer" category, which is obviously misleading to farmers' consumers and is not conducive to the promotion and application of coated urea.
(Senior Consultant of Sinofert, Professor of China Agricultural University, Cao Yiping)
Source: Farmers Daily 
In the past year, farmers and grassroots marketers have repeatedly inquired about the properties and properties of coated urea. They usually ask two questions: Is the property of coated urea a slow release fertilizer? Can 40 kg of coated urea be used on top of 50 kg of ordinary urea? In this regard, the experts of this issue answered the questions about the agrochemical traits and attributes of coated urea.
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