The composition of plants is very complicated. Generally speaking, fresh plants contain 75%~95% water and 5%~25% dry matter. If dry matter is burned, elements such as carbon (C), hydrogen (H), oxygen (O) and nitrogen (N) escape in the form of carbon dioxide, water, molecular nitrogen and nitrogen oxides, leaving a residue called ash. Therefore, in addition to carbon, hydrogen and oxygen, the essential nutrient elements of plants can also be divided into two categories: nitrogen and ash. So far, it has been found that there are about 70 chemical elements in plants, but the contents of these chemical elements in plants are different, and these elements are not necessarily necessary for plant growth. Some elements may be accidentally absorbed by plants or even accumulated in large quantities; On the contrary, some elements are indispensable nutrients for plant growth, although they have little demand for plants.

On the research of plant essential nutrients, Arnon and Stout put forward three standards of higher plant essential nutrients in 1939:

1. Without some nutrients, plants cannot complete their life cycle;

2. The role of essential nutrients is irreplaceable by other nutrients; When it is deficient, plants will have specific and special deficiencies, and only after supplementing this element can they return to normal.

3. Essential nutrients directly participate in plant metabolism, such as enzyme components or participate in enzymatic reactions.

According to the above three principles, the following 16 kinds of essential nutrients for higher plants were determined.

Carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, zinc and copper.

Although all higher plants are sure to need the above 16 nutrient elements, the demand varies greatly, and they are generally divided into macro elements and trace elements.