I’ve been waning to try a soil sensor for a while now but never done anything about it. You can always put two nails in the ground and apply small amounts of electricity and take a reading. This will work but not very well and the nails (or other metal) will eventually corrode.
Well, Adafruit has a soil sensor that uses capacitive measurements as opposes to resistive measurements that looks interesting. Currently only $7.50. It will give you a reading between 200 – 2000 with 2000 being very wet. The only other thing you need is a jst cable and a microcontroller. This looks very interesting as Spring is almost upon us.
There is also a different soil sensor available from Sparkfun currently priced at $7.95. This sensor uses two prongs that are Gold Finished to help prevent corrosion. This sensor also seems easy to use with a microcontroller.