300pF capacitor is the soil probe, with two parallel metal plates right next to one another, with the soil between then, forming a H structure.
The diode is a schottky type, with the lowest voltege drop possible.
The circuit operates between 1 and 2MHz
High soil humidity = higher voltage in the output, and vice versa.
The output of the circuit is the output of the voltage follower, will varry with slightly with soil nutrient and mineral content, and small deviations from every component.
