In the process of plant growth, there is often a period when the demand for a certain nutrient is not much in absolute quantity, but it is very sensitive and urgent. At this time, if there is no such nutrient, the impact on plant growth will be extremely obvious, and the losses caused by it will be difficult to correct and supplement even if the nutrients are supplemented later. This period is called the critical period of plant nutrition.

The key period of phosphorus nutrition in most plants is at seedling stage, with cotton at 10- 20 days after emergence and corn at about one week after emergence (three-leaf stage). The critical period of crop nitrogen nutrition is often slightly later than that of phosphorus. Usually, in the period when vegetative growth turns to reproductive growth, winter wheat is in the tillering and young panicle differentiation period, cotton is in the budding period, and corn is in the young panicle differentiation period.

In the process of plant growth and development, in another period, the absolute amount of nutrients needed by plants is the largest and the absorption speed is the fastest. The absorbed nutrients can maximize the production potential and improve the production efficiency, which is the maximum efficiency period of plant nutrition. This period is often in the middle of crop growth, when crops grow vigorously, and the response of crops to fertilization is the most obvious from the external form. The maximum nitrogen efficiency period of maize is from the big bell mouth stage to the early heading stage, and wheat is at the jointing stage.

The critical period of crop nutrition and the maximum efficiency period are two key periods of crop nutrition and fertilization. In these two stages, it is necessary to meet the nutritional requirements of crops according to their own nutritional characteristics, and at the same time pay attention to the continuity of nutrient absorption of crops in order to meet the nutritional needs of crops reasonably.