This sensor can be found on ebay for few dolars. Princip of measuring is similar with conductivity measuring. Potential among two electrodes is being measured. But in this case the electrodes are stuck in the ground. This modul measure with DC so the polarization effect occurs. It do not affect the measured value as much but it case degradation of electrodes. Modul has two outputs:
Sensitivity is set by trimr on modul. Electrodes are nickled for better durability. Inner circuit schema of modul is here.