555 circuit for moisture sensor
From basic capacitor physics, we know that capacitance, charge, and voltage are related by

$$V = \frac{Q}{C}$$

and given that parallel plate capicatance is described by $$C = \frac{\epsilon A}{d},$$

If the total charge stays constant, then the voltage across the capacitor will decrease with increasing permittivity.

$$ V \varpropto \frac{\alpha}{\epsilon}$$

(\$ \alpha \$ represents the constants of charge, area, and distance.)