(1) comes from the air.
That is, oxygen and carbon, which account for more than 85% of plant weight, are the most needed for vegetable growth, and are mainly supplied by air. Oxygen and carbon in the air can be directly utilized, and enter plants through the stomata of plant leaves in the form of oxygen molecules (O2) and carbon dioxide (CO2). Generally, oxygen accounts for 20.946% and carbon dioxide accounts for 0.033%. Oxygen and carbon contained in the air above the ground are inexhaustible. Although the concentration of carbon dioxide in the air is low, it will reduce the concentration of carbon dioxide in the nearby air when the photosynthesis of plants is intense. But because the air is flowing, it can be exchanged and replenished constantly, and there will be no shortage of carbon dioxide supply. Therefore, although a lot of oxygen and carbon are needed in vegetable cultivation, it is generally not necessary to consider supplementary supply. However, when vegetables are cultivated in greenhouses, the circulation and exchange of air are affected due to the tightness of buildings, and sometimes the supply of carbon dioxide is insufficient. Therefore, it is necessary to apply carbon dioxide as fertilizer (see Question 40 for specific application techniques), and it is precisely because of the construction facilities in the reserve that the conditions for applying carbon dioxide are provided that the vegetable yield can be effectively improved.
(2) from water.
Water consists of hydrogen and oxygen (H2O). When the roots of vegetables absorb water, they get hydrogen from water. Although there is hydrogen in the air, it cannot be directly used by plants like oxygen and carbon. As long as the cultivated vegetables have sufficient water supply, they will not be short of hydrogen, which accounts for more than 5% of the plants. Water is the source of hydrogen supply and the carrier of other nutrients. When it is dissolved in water, various nutrients can be absorbed by the roots and grown and utilized by plants.
There is such a saying in agricultural proverbs that "it will be windy in three days, it will rain in five days, the weather will be favorable, and the grain will be abundant", which is a profound understanding and summary of the nutrient sources needed to cultivate crops through long-term agricultural production practice.
(3) from soil.
Among the 16 nutrient elements necessary for vegetable growth, except oxygen, carbon and hydrogen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, boron, manganese, molybdenum, zinc, copper and chlorine all come from soil. The contents of these nutrients in soil vary greatly, some of them can account for more than 1% or even more than 10% of the soil weight, while others are very few, only about 0.000 1%. Vegetable growth needs more nutrients such as nitrogen, phosphorus, potassium, calcium and magnesium. Whether the amount contained in the soil can meet the needs of vegetable growth is the key to obtain vegetable yield. Among them, nitrogen, phosphorus and potassium are particularly important, commonly known as "three elements", not because the total content in the soil is too small, but often because the available nutrients are not enough to meet the needs of vegetable growth in the season, so they should be supplemented by fertilization. Although nitrogen is the most in the air, accounting for 78.084%, it is a pity that ordinary plants can't directly use molecular nitrogen in the air, and only leguminous vegetables such as kidney beans and soybeans can be used by nitrogen-fixing bacteria in nodules, so the amount of nitrogen fertilizer can be reduced by planting leguminous vegetables generally, and other vegetables can only come from soil, which is insufficient and needs to be supplemented by nitrogen fertilizer. Some nutrient elements, such as boron, copper, molybdenum, etc., have little content in soil, and the demand for vegetable growth is also small, but their influence on vegetable growth is as important as other nutrient elements. Soil contains a variety of nutrients, which is a nutritional bank for vegetable growth.