The nitrogen fertilizer can be urea [CO(NH2)2] and ammonia water (NH3. H2O) and ammonium salts, such as ammonium bicarbonate (NH4HCO3), ammonium chloride (NH4Cl) and ammonium nitrate (NH4NO3). Some compound fertilizers such as ammonium phosphate [a mixture of ammonium dihydrogen phosphate NH4H2PO4 and diammonium phosphate (NH4)2HPO4] and potassium nitrate (KNO3) can also be used as nitrogen fertilizer.
Urea is synthesized directly from liquid ammonia and carbon dioxide at high temperature and high pressure in industry. The chemical reaction is as follows:
2NH3+CO2 → NH2coonh4 → Co (NH2) 2+H2O Urea is easily soluble in water, and 105g can be dissolved in 100 ml water at 20℃, and the aqueous solution is neutral.
There are two kinds of urea products. Crystallized urea is a white needle-like or prismatic crystal with strong hygroscopicity. Granular urea is translucent, with a particle size of 1 ~ 2 mm, smooth in appearance and obviously improved in hygroscopicity. At 20℃, the critical moisture absorption point is 80% of the relative humidity, but at 30℃, it drops to 72.5%. Therefore, urea should be avoided to be stored in the open air in humid climate in midsummer. After adding hydrophobic substances such as paraffin wax in urea production, its hygroscopicity is greatly reduced.
Long-acting nitrogen fertilizer is suitable for various crops and soil conditions. There are two kinds of long-acting nitrogen fertilizers widely used in China: long-acting urea and long-acting ammonium bicarbonate, and their application methods are basically the same as urea and ammonium bicarbonate. Specific application points are as follows:
(1) Long-acting nitrogen fertilizer releases nitrogen slowly, and its peak release time is about 5 days later than that of urea, which should be earlier than the conventional application period of urea. Generally, 5-6 days in advance in early spring and 3-4 days in summer are appropriate.
(2) Long-acting nitrogen fertilizer has strong nitrogen retention ability and high utilization rate in soil. Therefore, its dosage is slightly less than that of ordinary nitrogen fertilizer, usually less than the constant 10%- 15%.
(3) Because of the different soil quality, the absorption and preservation ability of long-acting nitrogen fertilizer in soil are also obviously different. Clay has strong absorption and preservation ability and can be used for multiple purposes at one time; Sandy soil should be applied several times in small quantities.
(4) Scientific application of long-acting nitrogen fertilizer should be based on different nitrogen absorption characteristics of crops.