Urea is currently the highest concentration of solid nitrogen fertilizer produced in China, and its aqueous solution is neutral. In addition to the base fertilizer used in crops, urea is mostly used for crop topdressing. So, how can we improve the fattening effect of urea? The main points should be as follows:
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　　 1. Determine the amount of topdressing based on soil fertility. Practice has proved that increasing the amount of fertilizer applied to the middle and low-yield fields can significantly increase the yield. Maintain general fertilization for high-yield fields
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Level can be. Generally, on the basis of applying the fertilizer and fertilizer as the base fertilizer, taking the two crops of corn and sorghum as an example, on the high-fertility soil, 20 kg per mu is applied ; in the medium-fertile soil, 23 kg per mu is applied ; On low-yield soils, 27 kg should be applied .
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　　 2. Topdressing at the peak of crop nutrition. The main role of top dressing is to use fertilizer to regulate the growth of crops, to meet the nutritional needs of crops in the middle and late stages of growth, and to achieve the purpose of increasing production. The key to improving the utilization rate of fertilizers is to grasp the critical period and maximum efficiency period of nutrient absorption of various crops, and try to improve the utilization rate of fertilizers in the nutrient period of crops. Corn should be in the period from jointing to big bell; sorghum should be from jointing to booting; rice from greening to tillering; sunflower in the bud stage; wheat in the three-leaf stage; soybean in the early flowering stage.
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　　 3. Do deep application of soil. Practice has proved that after applying urea to the surface of the ground for 4~5 days at room temperature , most of the nitrogen will be ammonia volatilized, and its utilization rate is only about 30% , especially on the surface of calcareous and alkaline soils. The loss of volatilization is more serious. Therefore, when using urea to topdress the dry crops, it is best to apply the pits or trenches to a depth of 10 cm, so that the urea can be placed in the moist soil, which is beneficial to the conversion of urea, and is also beneficial to the ammonia fertilizer. Adsorption reduces the loss of volatilization. When using urea to topdress rice, the deepest water layer should not exceed 3 to 5 cm.
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　　 4. Keep a certain distance from the crop. Because of its high nitrogen content, urea has a large nutrient concentration and is highly hygroscopic. Therefore, in topdressing, it is necessary to prevent urea from being applied to the vicinity of the crop line, and it is not possible to drop urea into the heart of the crop to avoid burning the seedlings and affecting the growth. It must be kept at a certain distance from the crop.
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　　 5. Apply earlier than other nitrogen fertilizers. Because urea is a low-molecular organic compound, it must be ammoniated after being applied to the soil and converted into a highly volatile ammonium carbonate before it can be absorbed by the roots of the crop. Therefore, the application of urea to crop topdressing should be applied about 7 days earlier than other nitrogen fertilizers .
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　　 6. Do not mix with alkaline fertilizer. Urea is a neutral fertilizer. When topdressing, avoid mixing with alkaline fertilizer to prevent reducing fertilizer efficiency. If mixed with a non-alkaline fertilizer is not administered, but also shifted the date of fertilization, generally separated 3--5 days to. However, when urea is mixed with fertilizers such as potassium chloride, phosphate rock and superphosphate, the yield increase effect is remarkable.         7. It is not advisable to fill the water immediately after the application. After urea is applied to the soil, it cannot be adsorbed by the soil before it is decomposed and converted. If you fill the water immediately after chasing, it will cause a large loss of urea. Soil water shortage is serious, non-irrigation is not time, but also to do small water and dip irrigation, avoid flooding. When topdressing rice, the characteristics of urea with greater fluidity before conversion should be taken into account. After the application, it is not possible to immediately irrigate the water. Generally, it is better to pour a small water 3 to 5 days after the chasing .
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　　 8. Do root dressing. Urea has less damage to crop leaves, is soluble in water, has strong diffusibility, and is easily absorbed by leaves. It is not easy to cause separation of the walls after entering the leaves, so it is very suitable for top dressing. However, urea with a biuret content of not more than 2% should be used to prevent loss of the blade. The concentration of urea top dressing varies with crop types. The concentration of top dressing of gramineous crops is 1.5%~2% ; the concentration of top dressing for double-leaf crops is 1% , and the concentration of crops at flowering stage It should be smaller. Generally, the amount of top dressing per urea is 0.5~1.5 kg. The spraying time is preferably carried out after 4 pm. At this time, the transpiration amount is small, the leaf surface pores are gradually opened, and the diluted urea solution can be basically absorbed overnight, thereby greatly improving the utilization rate of urea.
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