Hello Richard, I have used i-buttons by Dallas Semiconductor (http://www.ibutton.com) for monitoring soil temperatures with success. I used their most basic datalogger (DS1921L) that only records temperature, and they go for $15-$25 CAD depending on the number you order. They have one that records humidity as well as temperature (DS1923) but is substantially more expensive ($80-$95 CAD for the number you are looking for). They are small and rugged and can withstand harsh environments (I buried mine in the soil over the winter with minimal protection and they were fine).
Good luck, Andy Teucher
