hi
the cheap way is to just build a "voltage divider".. five cent solution. Put two probes in the soil.. the probes form one of the resistors in the divider. Do a little googling and you will see how it's done.
For something more accurate, try one of the many schematics around. I found this moisture sensor in ten seconds... do a google image search for moisture & schematic. Here's another. A decent moisture sensor will use an op-amp to conditon the input, and to provide a high-impedance input.
D