At present study, a capacitively-coupled contactless conductivity detection (C4D) device integrated with capillary electrophoresis was expected to fulfill rapid detection of potassium ion concentration in soil. The device contained a C4D cell and a C4D detector. Firstly, a C4D cell matched with the 360 μm fused quartz capillary was designed. In the C4D cell, a receiving electrode, a shielding electrode and an exciting electrode were constructed by metal cylinder electrode with conductive silver paste. The lengths of the exciting electrode and the receiving electrode were both 10mm, and the distance between the two electrodes was 2mm. Secondly, the C4D detector included exciting signal generator circuit, pre-amplifier receiving circuit and detection amplifier circuit. After the potassium ion electrophoresis tests, the optimal excitation signal for soil solution analysis was determined as 136kHz, 80V sine wave. Finally, the C4D device was used in preliminary test of the potassium ion contained in six kinds of soil samples. The six national standard soil samples were digested by pure water, and then the solutions were filtered as test samples. The potassium ion test results of the six soil samples tended to correlate with the standard values. The results indicated that the C4D device had potential to complete preliminary tests of actual soil samples. The proposed capacitively-coupled contactless conductivity detection method modulated the soil samples for the first time. The optimized detection method can be used for rapid on-site soil nutrient detection in the future.