What are the technical key points for efficient fertilization with commonly used chemical fertilizers?

Urea

Urea is a high-concentration amide nitrogen fertilizer. It is currently the fertilizer with the highest nitrogen content among solid nitrogen fertilizers in China. It is a neutral quick-acting fertilizer. After being applied to the soil, it must be acted on by soil microorganisms for 3~4 days and converted into ammonium carbonate or ammonium bicarbonate before it can be absorbed and used by crops. Therefore, when used as a base fertilizer, it should be applied deeply, and it should be applied 4~5 days earlier than ammonium bicarbonate and ammonium sulfate. Before conversion, urea is in molecular form and cannot be adsorbed by the soil, so it should be prevented from being lost with water. The ammonia formed after conversion is also volatile, so urea should also be applied deeply and covered with soil, and it should not be irrigated immediately after application. If irrigation is done immediately after topdressing, urea is likely to be lost with water, so irrigation should be carried out after an interval of 4~6 days. Urea is not suitable for direct use as a seed fertilizer, because direct contact of high-concentration urea with seeds often affects seed germination and causes uneven emergence. When urea is used as a foliar fertilizer, the concentration must be strictly controlled. Generally, the concentration for grass crops is 1.5%~2%, for fruit trees about 0.5%, for open-field vegetables 0.5%~1.5%, and for greenhouse vegetables between 0.2%~0.3%. For crops in the vigorous growth stage, or for mature fruit trees, the concentration of urea applied can be appropriately increased.

Ammonium bicarbonate

Abbreviated as ammonium bicarbonate, with a nitrogen content of about 17%, it is the variety with the lowest nitrogen content among solid nitrogen fertilizers. Ammonium bicarbonate generally has a water content of 5%~6%, is prone to deliquescence, and is prone to caking. It is still relatively stable at temperatures below 20°C. If the temperature is slightly higher or the moisture in the product exceeds a certain standard, ammonium bicarbonate will decompose into ammonia gas and carbon dioxide, and the gas will escape into the air, causing loss of nitrogen fertilizer efficiency. It is itself a physiologically neutral quick-acting nitrogen fertilizer, suitable for various crops and various soils, and can be used as both a base fertilizer and a topdressing. When ammonium bicarbonate is used as a base fertilizer, it can be applied in furrows or holes. If it can be deeply applied in combination with ploughing, its effect will be better. However, note that the application depth must be greater than 6 cm (sandy soil can be deeper), and after application it must be immediately covered with soil. Only in this way can nitrogen loss be reduced. When ammonium bicarbonate is used as a topdressing, in dry fields it can be combined with intertillage, applied deeply below 2 cun, immediately covered with soil, and watered in time. In paddy fields, a shallow water layer of about 1 cun should be maintained, but it should not be too shallow, otherwise roots are easily damaged. After application, timely harrowing should be carried out to promote good absorption of the fertilizer by the soil. When ammonium bicarbonate is used as a topdressing, be sure not to broadcast it just after rain or before the dew has dried. Whether ammonium bicarbonate is used as a base fertilizer or a topdressing, it must never be broadcast on the soil surface, to prevent nitrogen volatilization, causing nitrogen loss or burning crops. At the same time, avoid application in high-temperature seasons and high-temperature periods, and apply as much as possible in seasons when the temperature is below 20°C.

Diammonium phosphate

Diammonium phosphate is a high-concentration quick-acting fertilizer, suitable for various crops and soils, especially for crops that prefer ammonium and need phosphorus. It is suitable for use as a base fertilizer. If used as a topdressing, it should be applied early and deeply at 10 cm and then covered with soil. It must not be too close to crops, so as not to burn the crops. When used as a seed fertilizer, it must not come into direct contact with seeds. Do not broadcast diammonium phosphate with water, otherwise most of the nitrogen in it will remain on the soil surface. Do not mix it with alkaline fertilizers such as plant ash and lime, to prevent nitrogen volatilization and reduce the effectiveness of phosphorus.