The present article deals with the determination of potassium content in soil of rhizosphere in situ by the potassium selective micro-electrode. It was found that the equilibrium potential was established within 3 minutes in the soils with higher moisture content. While under lower mossture content, it was delayed up to 5 minutes. Similarly, the equilibrium in the soil with higher K⁺ concentration was established more quickly than that in soil with lower K⁺ concentration. Forty samples collected from yellow cultivated loessial soil, white bleaching paddy soil and sandy soil with different K⁺ content had been studied. The results showed that Brood correlation was found between the K⁺ concentration determined by the electrode method and the concentration of water-soluble K⁺ determined by the flame photometric method (r= 0.931).Generally speaking, the electrode method may be a useful method for estimation of K⁺ concentration in situ.