Yes I purchased and used some of those capacitive soil moisture sensors as well. I had trouble getting them dialed in. I build an automatic watering rig for an office plant; parts included the capacitive sensor, a small water pump, an ATTiny, an 8x8 LED matrix (to show a smiley face when the plant was well watered and a sad face when it was not), and a relay. Programmed it in ATMel Studio. I think I ended up over watering the plant and killing it, or maybe the other way around. In any event I couldn't get that sensor dialed in. The rig is stuck in my office at work and we haven't had access for months now; I'll have to dig in again once I get back in there. Thanks for sharing your experience. I think trying the original sensor (the resistance based one) and turning it on and off 4 times an hour might be the next step...

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Thanks for the tip Paul. How long do you suppose it would last if you do 6 measurements a day using the technique you describe? Do you have experience with this?

I agree with Mr. Delibašić, more description would be great. How well do the servos work at dispensing the soap? Great idea, thanks for sharing and I look forward to more from you.