Here's a Temp-Humidity example on http://arduino-info.info
http://arduino-info.wikispaces.com/PROJECT-Temp-Humidity-Display
Here's a soil moisture sensor that reverses polarity to help solve the electrolysis problem:
http://gardenbot.org/howTo/soilMoisture/#The_local_circuit_-_simple_voltage
(H-bridge chips/modules here: http://goo.gl/WcY5h )
DISCLAIMER: Mentioned stuff from my own shop...