If you're into greenhouse electronics or plantwatering systems you might know these popular low cost soil moisture sensors from China. http://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=soil+moisture+sensor&_sop=15 I found that I needed to make the connection between the wires and the actual fork that goes into the soil a little sturdier. This simple design requires only two additional M3 screws and bolts. As always my FreeCAD design is included for tweaking and reuse.