You can visualize the soil to be a kind of resistor, similar to the LDR we used earlier. The lesser the water content, the more the resistance; and the more the water content, the lesser the resistance.
Now, before we try and figure out what kind of circuit to use for our project, we need to find a way in which we can connect our wires to the soil!
We know that metals are good conductors of electricity. We need to provide a voltage between two points in the soil. We can use a large metal object, like an iron nail that can be embedded into the soil such that it can be an interface between our circuit and the soil.
Usually, these objects that are used as an interface in order to make some measurement are referred to as probes. You might have seen doctors using probes used to measure muscle activity of an athlete:
Image source: http://i.ytimg.com/vi/yZ_nxY91yxI/hqdefault.jpg
Muscle activity of an athlete being tracked using muscle probes.
The following is an image...