The results obtained from pot and field test showed that the quick-acting K can be measured by NH₄ OAC method, which responsed very Favourably to the supply capability of K by soil, therefore this method used to measure of quick-acting K in soil is to be suitable for Jilin province. The critical value of quick-acting K in soil is 129ppm by this method. The yield of maize was increased about 10% by supplying 2-4Kg K₂O/Mu. When the value is lower than 129ppm in soil.