Soil Moisture Measurement by the Dielectric Method

Abstract

Measuring soil moisture by the dielectric method is considered. It was revealed that the complex dielectric permeability (CDP) affects the readings of sensors using different physical principles in different ways. The results of CDP measurements for six samples are given: river sand, bentonite, three samples of Haplic Chernozem (Loamic) of the Omsk region and a mixture of chernozemic material and river sand. Content of particles less than 0.01 mm in size in the samples varied from 1.7 to 72%; the content of organic carbon ranged from 0 to 4.12%. Measurements of CDP were performed at different values of moisture at the frequencies from 10 kHz to 4–8 GHz in the same cell using the original technique developed by authors. It was noted that at frequencies below 20–30 MHz, the dielectric permittivity is an ambiguous function of moisture owing to the strong influence of the conductivity of the soil solution and of interlayer dielectric relaxation on it. Moisture measurements are best to perform in the frequency range 0.5–3 GHz, where the effect of interlayer relaxation and soil solution conductivity on the CDP is negligible. Measurements of soil moisture and conductivity were performed using a Decagon 5TE capacitive sensor. The calibration dependency that is valid for different soils, considering the conductivity of the soil measured by this sensor was found. The error of moisture measurements using this dependency is 0.035 m³/m³. The use of individual calibration dependencies for each of soils reduces the error to 0.014 m³/m³. Results of the work can be used to improve the accuracy of soil moisture measurement by the dielectric method.