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Soil information plays an important role in guiding agricultural production. Soil electrical conductivity reflects soil information such as soil moisture, salinity, clay content and type, it is significant to obtain soil electrical conductivity accurately for realizing agricultural fine production. Among all kinds of soil conductivity measurement methods, the fourterminal method is widely used in practical measurement because of its low cost, high precision, fast measurement speed and simple operation, its performance directly determines the accuracy and measuring range of the measuring instrument. The effect of three constant current sources on the measuring performance of the measuring instrument was compared. It was found that the fourterminal method using Howland constant current source had the best measuring accuracy and the high measuring ability, but the low measuring range was small. The measuring range of low conductivity by using improved Howland constant current source was enlarged and the measuring precision was good. The measuring range of low conductivity by using constant current source based on differential amplifier was the largest, the measuring range of high conductivity was better and the measuring accuracy was good. 