Measuring direction of current to determine moisture is present
Let me suggest a slightly different circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

R1 is your moisture sensor. It's driven by AC, because the input and the output of the left inverter are L/H – H/L with a ratio of 1:1. C2 and R2 are a half-assed high pass (good enough). The more moisture, the lower the value of R1, the higher the frequency, the higher the voltage on the right inverter input.

You have to play with the values.