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Your Position: Home > Agriculture > Knowledge of Urea

Knowledge of Urea

Author: Hou

Nov. 30, 2022

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Tags: Agriculture

Urea, also known as urea or carbamide, is an end product of protein metabolism in humans and mammals, as well as a major organic nitrogen compound excreted by animals. The pure product is white granular or acicular or prismatic crystal, and it is light red or yellow when mixed with iron and other heavy metals. It is tasteless and odorless, soluble in water, ethanol and benzene, and insoluble in ether and chloroform.

At 20 ºC, 100 kg of water can dissolve 105 kg of urea, which absorbs heat. The aqueous solution is neutral. The nitrogen content of the pure product is 46.65%, the agricultural urea is 42%~46%, and it contains a small amount of biuret, generally less than 2%, which is generally harmless to crop growth; The specific gravity and unit weight of urea are small, and the weight of fertilizer per cubic meter is 0.65 tons. 


Urea is an important nitrogen fertilizer with high quality and efficiency. At the end of World War I, Germany began to use urea as fertilizer. Practice has proved that urea has a high nitrogen content and no side components, which shows a good yield increase effect on all crops. Since the 1950s, it has become a key nitrogen fertilizer variety developed by various countries. In the 15 years from 1960 to 1975, the global urea output has increased by 10.5 times, and it has gradually replaced ammonium nitrate since the 1970s, becoming the most widely used nitrogen fertilizer variety in the world. 

After urea fertilizer is applied to the soil, a small part of it is absorbed by plants in the form of amide, and most of it is ammoniated under the action of urease, converted into ammonia, and then absorbed by plants. The decomposition rate depends on the soil quality, temperature and water content, generally in 3 to 10 days. Therefore, the fertilizer efficiency of urea is longer than other nitrogen fertilizers, and the nitrogen loss is less. Urea is an ideal fertilizer for topdressing outside the root, because it does not contain harmful ingredients to crops, is neutral, does not burn the stems and leaves, has strong moisture absorption, and is easy to penetrate into the leaves with water and be quickly absorbed by crops. The concentration of topdressing outside the root is generally 0.5%~2%. In order to improve the utilization effect of urea, in addition to correctly mastering the properties and application conditions of urea, attention should be paid to rational application. Urea is a neutral fertilizer and contains no harmful components. Hydrolysate NH+4 and HCO-3 can be absorbed and utilized by crops. It is applied year after year, and generally has no adverse effect on soil properties. It is applicable to all kinds of crops and soils, and its fertilizer efficiency is similar to that of ammonium sulfate with equal nitrogen content; The effect is better than that of ammonium sulfate when it is used in the "aged" paddy field which lacks active iron and is prone to hydrogen sulfide poisoning. Too much urea or too high biuret content will easily cause ammonia and biuret toxicity to crops; The higher the urease activity of crops, the more sensitive to the toxicity of urea and biuret. Urea can be used as a supplementary feed for anti nestling animals. 


In industry, urea is the raw material for the synthesis of urea formaldehyde, melamine formaldehyde resin, etc. Medically, it is used to manufacture tetracycline, phenobarbital, caffeine and other drugs. The dye industry uses it to produce vat brown BR, phthalocyanine blue B, etc. 

Agricultultural Urea is packed with plastic film inside and woven bag outside. It shall be stored in a dry warehouse and shall not be stored and transported together with acids. The application methods of urea include base fertilizer, topdressing, seed fertilizer or topdressing outside the root. The application time of base and topdressing should be 4-5 days earlier than that of ammonium nitrogen fertilizer, and the application amount should be reduced according to the nitrogen content. When urea is used as the base fertilizer for paddy fields, it should be plowed immediately after 5-7 days of spraying before irrigation. Do not irrigate too early after application to reduce the direct loss of urea; When it is used as topdressing, the water shall be drained to the field surface to keep the shallow water layer. After fertilization, the field shall be tilled immediately and no irrigation shall be conducted for 2-3 days. The application method of urea as dry foundation fertilizer and topdressing fertilizer is basically similar to that of various ammonium nitrogen fertilizers. Generally, urea is not directly used as seed fertilizer. Due to the high concentration and pH value of urea or ammonia in the fertilization zone, protein denaturation will be caused, which will hinder seed germination and seedling growth, and even make the seed lose its germination capacity, especially for small seeds; However, if the urea is mixed with dry fine soil first, then it is applied to a certain depth under the seed, and the seed is sown after covering a thin soil layer; Or side application of urea 2.5 cm away from the seed has no adverse effect on seed germination. 

Generally, the leaves of crops absorb and utilize urea faster than the roots, and the effect of urea as a topdressing outside the roots is better than that of other nitrogen fertilizers. The reasons are as follows: Urea is a neutral organic nitrogen, free of side components, and the spraying solution of general concentration has little burning effect on the stems and leaves of crops. The degree of ionization is small, and the molecular volume is small, so it is easy to enter leaf cells through the cell membrane. Molecular urea has a high penetration and diffusion speed. When it penetrates into cells, it is not easy to have plasmolysis, and even if it does, it is easy to recover. It has strong moisture absorption. The solid urea left on the leaf surface due to the evaporation of water from the spraying liquid can still be re absorbed and dissolved, with a high absorption and utilization rate. 


The optimum concentration of urea as topdressing varies with crop types and their growth stages. Generally, the optimum concentration of spraying rice, wheat and gramineous grass is 2.0%, cucumber 1.0% ~ 1.5%, radish, cabbage, spinach and cabbage 1.0%, watermelon, eggplant, sweet potato, potato, peanut and citrus 0.4% ~ 0.8%, mulberry, tea, pear, apple and grape 0.5%, persimmon, tomato, strawberry, greenhouse cucumber and eggplant, flowers 0.2% ~ 0.3%. Generally, spray once every 7-10 days, 2-3 times in total. The dosage of the solution is 750~1125 kg per hectare each time, with the degree of spraying until the leaves are wet. Spraying time is better in the morning or evening. The content of biuret in urea as topdressing shall not be higher than 0.5% to avoid leaf injury. 


The United States began to add urea to ruminant feed in the early 1940s. Since the 1950s, urea has been widely used worldwide as a non protein nitrogen source for ruminants. In the late 1980s, the total amount of urea used in feed in the United States reached 1 million tons, which is about 5% of the total urea output in the United States. In the early 1990s, China's total urea output was about 6 million tons, and the amount used for feed accounted for about 2%~3%. Urea is mainly used for feeding ruminant livestock such as cattle and sheep. Up to now, there are few scientific evidences that can be used by monogastric animals.


In order to ensure rational and effective utilization and prevent the increase of pH value in rumen of livestock and ammonia poisoning, the following points should be paid attention to when using urea feed: Mix evenly, limit the intake, and do not drink immediately after taking urea containing feed. The diet should contain a certain amount of carbohydrate easy to ferment, among which molasses is the best, starch dextrin is the second, and fiber feed is the worst. The Chemicalbook should be supplied with proper and balanced minerals, especially the supply of cobalt, zinc and sulfur to meet the needs of rumen microbial reproduction. Diets should not be mixed with feed containing urease, such as raw beans, raw soybean meal, etc. The livestock and young animals fed with urea feed for the first time or only coarse fodder and hay shall be gradually fed with gradually increased urea feed to gradually adapt. Urea can be used in high energy and low protein diets or low energy and low protein diets, but the utilization rate of urea is poor in low energy and high protein diets or high energy and high protein diets. The appropriate amount of urea should be determined according to the potential value of urea fermentation. Urea poisoning occurs very quickly and the course of disease is short, usually within ten minutes to several hours after ingestion. The clinical symptoms are restlessness, trembling, salivation, shortness of breath, uncoordinated muscle movement, swelling, convulsions, etc. 


Generally, 6 liters of acetic acid solution containing 5% can combine acid with ammonia to prevent excessive ammonia from entering the blood. Solubility in water (g/100ml): grams dissolved per 100ml of water at different temperatures (ºC): 108g/20 ºC; 167g/40ºC; 251g/60ºC; 400g/80ºC; 733g/100 ºC identification test Solubility is easily soluble in water and ethanol (OT-42). Dissolve 0.1g of nitric acid precipitation test sample in water and add 1ml of concentrated nitric acid. There shall be white crystalline precipitation. Take 1g of sample for color reaction in the test tube, heat it for liquefaction, cool it until the solution is turbid, dissolve it in the mixture of 10ml water and lmL2mol/L NaOH, and add 0.05ml of copper sulfate test solution (TS-78). Red purple should appear. The melting range is 132~135 ºC. For content analysis, accurately weigh about 0.5g of the sample and dissolve it in 10% sulfuric acid solution to a constant volume of 100ml. Take 5.0ml of the solution and put it into a long necked combustion flask, add 10ml of sulfuric acid test solution (TS-240), and carefully heat it until no gas is evaporated. Boil carefully for 10min, and add 40ml of water carefully after cooling. Cool it again and put it into a steam distiller. Add 50ml of 10mol/L NaOH solution, and carefully distill it for 1h under the condition of sending steam through the mixed solution. Collect about 50ml of distillate into a collecting bottle containing 40mL of 4% (W/V) boric acid solution. Add 0.25ml of methyl red/methylene blue test solution (TS-150) and titrate with 0.1mol/L hydrochloric acid. At the same time, perform a blank titration. Each ml of 0.1mol/L hydrochloric acid is equivalent to 3.003mg of urea (CH4N2O). Toxicity GRAS (FDA § 184.19232000). LD5014300mg/kg (rat, oral). It can be used. No more than 3% of gum gum is not toxic (FAO/WHO, 2001). The use limit is unlimited (FDA § 184.19232000).

Maximum allowable use of food additive Maximum allowable residue standard Chinese name of additive Chinese name of food allowed to use this additive Maximum allowable use of additive function Maximum allowable residue (g/kg) Maximum allowable residue (g/kg) Urea processing aid for food industry/Generally, it shall be removed before the final product is made, except for colorless crystals with chemical properties with specified residue in food. Soluble in water, ethanol and benzene, almost insoluble in ether and chloroform. Use: It is used to separate nitrogen oxides, nitrite and chromatographic analysis, and also used as a biological culture agent. It is used as a raw material for fertilizer, animal feed, explosives, stabilizer, urea formaldehyde resin, etc. Urea is an important raw material for pesticide production. It can produce fungicides such as cyanurate, ethoxynol, pesticides such as triazophos, phoxim, pyrimidine oxyphosphorus, fenitril, fenvalerate, herbicide such as chlorsulfuron. Uses It is used as nitrogen fertilizer in agriculture, feed additive in industry, explosive, stabilizer, urea formaldehyde resin, etc. It is used as crop fertilizer to improve the utilization rate of nitrogen in urea It is used as yeast feed; Gum tissue improver. GB2760-90 is listed as processing aids. For alcoholic beverages, gelatin products, wine, yeast fermented baked products. Uses Urea is mainly used as chemical fertilizer. In industry, it is also used as raw material for manufacturing urea formaldehyde resin, polyurethane, melamine formaldehyde resin, and widely used in medicine, explosives, leather making, flotation agent, pigment and dewaxing of petroleum products. When urea is heated to 200 ºC, it generates solid melamine (i.e. cyanuric acid). The derivatives of cyanuric acid trichloroisocyanuric acid, sodium dichloroisocyanate, tri (2-hydroxyethyl) isocyanate, tri (allyl) isocyanate, tri (3,5-di-tert-butyl-4-hydroxybenzyl) isocyanate, triglycidyl ether of isocyanate, and melamine complex of cyanuric acid have many important applications. The first two are new high-grade disinfectants and bleachers. The total production capacity of trichloroisocyanuric acid in the world exceeds 80000 tons. Uses: Used as fertilizer, animal feed, explosive, stabilizer and raw materials for urea formaldehyde resin. It is used to test antimony and tin in application analysis. Determination of lead, calcium, copper, gallium, phosphorus, iodide and nitrate. When measuring blood urea nitrogen, prepare standard solution to measure serum bilirubin. Separation of hydrocarbons. Oxides and nitrous acid used to decompose nitrogen in analysis. Prepare the culture medium. The stabilizer for the determination of uric acid by Folin method is homogeneous precipitation. Production method Urea is the final product of protein metabolism in mammals. In 1922, the industrial production of urea from ammonia and carbon dioxide was realized in Germany. Ammonia reacts with carbon dioxide to form carbamate, and then dehydrates to form urea. The production method uses carbon dioxide and ammonia to synthesize ammonium carbamate under high temperature and high pressure, which is crystallized, separated and dried after decomposition, absorption and conversion.