In the process of crop growth and development, the requirements for nitrogen, phosphorus, potassium and other mineral nutrients often have two very important periods, which are also the key period of fertilization. If the demand for nutrients can be met in time during these two periods, it will improve crop yield and quality.

1. The critical period of crop nutrition: in the process of crop growth and development, there will be such a period when the requirement for a certain nutrient is not much in absolute terms, but the degree of need is very urgent. If this nutrient is lacking at this time, crop growth and development will be seriously hindered, and the resulting loss will be difficult to correct or make up for even if this nutrient is supplemented in the future. This period is often referred to as the critical period of crop nutrition.
Different crops, different nutrients of the critical period of nutrition is not the same; the same crop, different nutrients of the critical period of nutrition is not exactly the same. Studies have shown that the critical period for most crops to require phosphorus is at the seedling stage. Therefore, the production of phosphate fertilizer will be applied as a base fertilizer, in order to provide timely for the growth and development of the seedling stage. If the base fertilizer is not applied, a small amount of quick-acting phosphate fertilizer can also be used as seed fertilizer to meet the demand for phosphorus during the critical period of nutrition. The critical period of nitrogen nutrition is generally later than the critical period of phosphorus nutrition, and it is mostly when vegetative growth goes hand in hand with reproductive growth. The critical period of cotton nutrition for nitrogen is the initial stage of the current bacteria. At this time, if the nitrogen supply is insufficient, the current quantity will be significantly reduced, and it is easy to fall off, which will bring a lot of loss to the final yield. The critical period of crop nutrition for potassium is generally later than that of phosphorus and nitrogen. The critical period of potassium nutrition in rice is in the stage of grain distribution and young spike formation, which is earlier than the critical period of potassium demand for most crops. In this period, potassium supply is insufficient, the number of rice tillers increases slowly, and the number of grains per spike decreases significantly, resulting in yield reduction.

2. Crop nutrition efficiency period: In the process of crop growth and development, there is a period when the absolute amount of nutrients needed for crops is very large, and the absorption speed is also the fastest. During this period, if the corresponding fertilizer is applied, the yield increase efficiency and fertilizer utilization rate are also high. This period is called the crop nutrient efficiency period.

Most crop nutrition efficiency period is in the middle of crop growth and development. The characteristic of this period is that the crop is in the vigorous growth stage, the ability to absorb nutrients is the strongest, and the growth rate of the crop is fast. Research and practice show that the nitrogen efficiency of maize in the big bell mouth to the early stage of male, winter wheat in the jointing to heading stage, cotton in the flowering stage, peanut in the pod stage. It is very important to ensure the supply of required nutrients during the period of crop nutrition efficiency to increase crop yield and improve crop quality.

It should be pointed out that although crop nutrition has stages, it is very important to ensure adequate fertilizer supply during the critical period, but the continuity of crop nutrition should not be ignored. The growth of crops should be observed at any time. When it is found that there is a lack of nutrition, it must be adjusted through top dressing. Make crop nutrition meet the requirements of crop yield. Therefore, in the practice of crop cultivation, the combination of base fertilizer, seed fertilizer and top dressing (including foliar fertilization) is used, which can not only meet the continuous needs of crops for nutrients, but also seize the critical period of fertilization to achieve scientific fertilization and economic fertilization.