If you are on a low, low, (very low) budget, this simple diy one will work.
All you will need is two nails (stailess steel will be best)
An analog sensor cable or just 2 single wire cables connecting to the 5v and sensor value ports on the analog input of the PhidgetsSBS or 8/8/8 board
Connect the analog sensor cable to your board, cut the connector at the other end and connect the white and red cable to the nails. Put the nails in the ground. Experiment with the distance the nails should be and the depth the nails should be.
if you the soil is getting wet, resistance will reduce and sensor values will increase
You will have to manually calibrate the sensor (build a formula to put meaning in the sensor value)
You can isolate the nails if you want to measure moist on a very specific depth
Not only moist will influence the resistance (or sensor value) but also soil type (i.e. metals in the soil)