The scientific application of fertilizer should first be based on the above four fertilization theories and aim at high yield, high quality, high efficiency, no pollution and improving the fertile soil. It should not only be based on the nature of fertilizer and the characteristics of crop nutrition, but also consider the factors such as soil fertility, cultivation system (scientific fertilization under plastic film mulching, scientific fertilization under different rotation systems and intercropping), and the combination of irrigation and fertilization.According to the availability to crops, soil nutrients can be divided into quick acting nutrients, slow acting nutrients and ineffective nutrients; According to the existing forms in soil, soil nutrients can be divided into water-soluble state, exchange state, mineral state and organic state.