Microwave sensors are used to detect changes in the soil dielectric constant (e.g. soil emissivity) or microwave attenuation. These properties are related to soil water content.
Typically used for remote soil moisture measurement (sensors measuring emissivity) however the technique has been used for real time proximal measurement of soil moisture [1].
Research.
1. Whalley, W. R. (1991) Development and evaluation of a microwave soil moisture sensor for incorporation in a narrow cultivator tine. Journal of Agricultural Engineering Research, 50(C), 25-33.