Research Of Soil Moisture Measurement Based On Standing-wave Ratio Theory

Soil is a kind of peculiar dielectric which is composed of soil matrix , water and air. The permittivity of water (~77) is much greater than that of soil matrix (~4 ) ,and that of air (~ 1) . Thus the permittivity of soil is mainly determined by that of water. Therefore we can get the water content by measuring the permittivity of soil. This paper briefly summarized the research state of soil water measuring techniques at home and abroad, especially the techniques based on the standing-wave ratio theory. We develop a soil water sensor and it was used in laboratory trials so that we can study its working property. According to the standing-wave ratio theory we worked out the expressions of the sensor probe impedance , and of voltage on the sensor, and we designed the structure of the probes. Trials demonstrated that there is a distinct influence as to the probe structures. The sensor with four probes of unequal length has the advantage of that with equal length. Soil types and unit weight have limited impact on the measurement result, and if you want to minimize the errors you need calibrate the soils respectively. The salts dissolved in the soil moisture have a great impact on the measurement result. Especially when there is comparatively much salt, we need special calibration because of the big errors.