Soil water-content sensors based on the measurement of soil impedance are widely used for the detection of rain-fall induced slope failure and other agriculture applications. In principle, low-leakage sensors can measure soil water content down to 4%, but they require readout systems capable of generating soil excitation signals at frequencies ranging from 1KHz to 1MHz. However, existing commercial products do not cover this frequency range. To solve this problem, a new readout system is proposed in this thesis. By using sinusoidal excitation, implemented by a pulse-width- modulated driver, and synchronous detection, the readout system achieves less than 1% measurement error from 1KHz to 1MHz.