Critical period of plant nutrition: In the process of plant growth and development, there is often a period when the demand for a certain nutrient is not much in absolute quantity, but it is very urgent. At this time, if this nutrient is lacking, the growth of plants will be seriously damaged, and even if it is supplemented later, it is difficult to make up for the damage. This time is called the critical period of plant nutrition.

Generally speaking, the critical period of plant nutrition is often in the early stage of its growth. For example, the critical period of phosphorus in Gramineae plants is around the three-leaf stage of seedlings. The critical period of nitrogen is often in the period when vegetative growth turns to reproductive growth. It is of great significance to master the critical period of plant nutrition for guiding fertilization.