Macy put forward the concept of "critical percentage of nutrient elements" based on dry matter of leaves. He believes that every nutrient element in crops has a fixed "critical percentage". If the concentration of this element in plants exceeds this percentage, it means luxury absorption; if it is lower than this percentage, it means nutrient deficiency. This critical percentage is a conceptual value, which can be said to be a range value, and will change due to other factors. But for each crop, it is possible to reflect the approximate range of its characteristics. Accordingly, he divided the correlation curve between plant response and nutrient concentration into several regions or parts.

1. Minimum percentage area

The concentration of a nutrient element in plants does not change much, but the growth, yield and quality of plants will increase or decrease greatly.

2. Areas with poor supervision

When the concentration of an element in plants increases, the reaction of plants will also increase.

3. Luxury absorption zone

The concentration of an element in plants increases, while the reaction of plants remains unchanged.

According to the above analysis, the task of leaf analysis is to find out all kinds of critical concentrations (nutritional diagnostic indicators) and find out what kind of nutrient element concentrations of the tested fruit trees belong to. In particular, in many orchards, the lack of potential elements is often overlooked. Therefore, in nutritional diagnosis, special attention should be paid to distinguishing the potential deficiency of various elements so as to correct it through appropriate fertilization measures.