Chlorine is an essential element of plant nutrition. Based on twenty thorough and systematic studies on chlorine nutrition of plant and chlorine-containing fertilizers three new theoretical ideas are proposed.

(1) “Chlorine tolerance” and “critical value of chlorine tolerance of crop”. Plant chlorine tolerance can be divided into three types： (A)Crops of strong chlorine tolerance. It means under the condition of chlorine content of soil＞600 mg/kg,the crops can grow normally, (e. g. beet cotton, rice, cucumber, etc. ) (B) Crops of medium chlorine tolerance. The crtical chlorine tolerance values are 300～600 mg/kg. (e. g. wheat, maize, tomato, eggplant，soybean,etc. )(C)Crops of weak chlorine tolerance:The crops critical chlorine tolerance values are 150〜300 mg/kg. (e. g. potato, sweet potato, tobaccos, grape, apple, etc.)

(2) Chlorine allowance of soil. It is the differece between the critical value of chlorine tolerance of plant and chlorine-content of soil.This term cimplies that the higher the chlorine allowance of soil,the much chlorine-contained fartilizer can be applied to the soil.

(3) The residual chlorine content in soil is nearly a constant. Accumulation effect of residual chlorine content not exists in the soil.

These ideas will be beneficial to the application of Chlorine-contained fertilizers and the development of the “United-soda” Process.