The transformation of potassium （K） forms in soil plays an important role in the improvement and evaluation of potassium fertility in soil，efficient fertilization techniques of potassium，and regulation of potassium nutrition in plants.In recent years，the team of studying potassium in soil at Nanjing Institute of Soil Science，Chinese Academy of Sciences，has carried out a series of studies on the differentiation method and content range of different forms of potassium in soil，the mechanism of efficient utilization of potassium in soil by plants，the release mechanism of non-exchangeable potassium （NEK） in soil and its influencing factors，especially the method of evaluating the bioavailability of non-exchangeable potassium in soil.The content of non-exchangeable potassium in soil had a maximum value in different types of soil based on the results of soil K extraction with a strong sodium tetraphenylboron （NaTPB） method.The non-exchangeable potassium extracted through exchange and diffusion can account for 20% to 60% of the total potassium in the soil.The release of non-exchangeable potassium from minerals mainly depends on the content of other ions and the concentration of potassium in environment，and the efficient absorption of non-exchangeable potassium by plants depends on the ability of root systems to absorb potassium at low concentrations as well.It has been confirmed that the formation and release of non-exchangeable potassium in soil play an important role in the soil holding capacity and bioavailability of potassium in soil.The new methods for measuring the total amount of non-exchangeable potassium in soil，determining changes of potassium in soil，and grading plant-available potassium in soil have been established.It will provide powerful methods and means for accurately evaluating and predicting the potassium fertility in soil，and studies related to soil potassium，which will facilitate the transition of studying potassium in soil with the qualitative and semi-quantitative methods to the quantitative methods.