In order to develop a continuous measuring method to detect both soil moisture and electric conductivity, experiments were conducted to increase the accuracy of soil moisture measurements by the thermal method.
It is the most important factor for this method that soil water movement induced by temperature gradient around the heat source should be neglected. Also, heating capacity and time in the heat source should be determined at designed certain values. Suitable heating capacity and time were 0.8 Watts and 15 minutes, respectively, in volcanic ash soil, sandy soil, and 0.2mm glass beads.
In the case of decreasing contact surface of heat source body with soil, especially sandy soil, the heat transfer coefficient tended to lower. Therefore, it was necessary to increase contacting surface of the heat source body and to improve its form.
Concentration of electrolytes: less than 0.01M KCl solution, in soil water did not affect to detect soil moisture and rising temperature in the center part of cylindrical heat source body.
As the rising temperature of the heat source decreased with rising soil temperature, we revealed that soil temperature compensation value (alpha) in the equation (θ0=θ/1-α(Θ-Θ0)) is 0.0051/°C in the volcanic ash soil.
The correlation coefficient between pF values and rising temperature of the heat source was 0.89 for non-compensation values; an application of this equation to the same data gave it 0.97.