The application of potassium fertilizer depends not only on the potassium supply capacity of soil, but also on crop species, agricultural production level, climate and soil conditions. The content, form, transformation and supply capacity of potassium in soil is an important basis for rational distribution and application of potassium fertilizer. The content of total potassium in soil varies greatly, generally 0. 1% ~ 3%, and the average value is about 1%.

Potassium in soil includes three forms: ① mineral potassium. It mainly exists in coarse-grained soil, accounting for about 90% of total potassium, which is extremely difficult for plants to absorb. ② Slow acting potassium. It accounts for about 2% ~ 8% of total potassium and is the source of rapid release of potassium in soil. ③ Available potassium. Refers to the replaceable potassium adsorbed on the surface of soil colloid and potassium ions in soil solution. Plants mainly absorb potassium ions in soil solution. The potassium nutrition level of plants in season mainly depends on the content of available potassium in soil. Generally, the content of available potassium only accounts for 0. 1% ~ 2% of total potassium, and its content is not only affected by tillage and fertilization, but also controlled by soil slow-release potassium storage and transformation rate.