not OP, but one of the tricks is to activate the sensors only when measuring, so there's no constant DC applied to the sensor wires/pads. once you have that, reduce measurement frequency, so to mainimise the time when voltage is applied to the sensors. for example once every hour for moisture is sufficient, and 1/sec isn't really going to help much.
That's what they do when performing catheter ablation (a medical procedure for curing cardiac fibrilation by destroying minute parts of muscle with electric current).
DC would work just as fine on this procedure, but due to electrolysis of water, oxygen and hydrogen bubbles would form, which could get stuck somewhere. Using a square wave AC quickly reverses the reaction every period, like you suggested for the moisture meter.
