Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot
Instead of measuring soil moisture try:    - measuring air moisture of small upside down cup on top of the soil 
    - measuring weight of the whole pot

Or don't measure at all and instead use precise amount of water on precise times (RTC module, medical grade piezo-electric pump). My similar project is on hold at 20 or so sketches of various types of water pumps.