Because the soil is composed of mineral composition layers with different thicknesses. Soil can provide necessary "moisture, fertilizer, air and heat" for plants. Soil consists of solid, liquid and gas. Solid substances include soil minerals, organic matter and microorganisms. Liquid substances mainly refer to soil moisture. Gas is air existing in soil pores. These three substances in the soil constitute a contradictory unity. They are interrelated and mutually restricted, which provide necessary living conditions for crops and are the material basis of soil fertility. Therefore, all water, fertilizer (N, P, K fertilizer, etc. ), plants need air and temperature soil, which is naturally a natural culture medium.