Makezine_COTM_OpAmp-BadgeInstructables user diy_bloke built this op-amp controlled pump for his garden, using the classic 741 operational amplifier.

He wrote a very detailed how-to including showing how to etch a PCB, as well as how to make a humidity sensor out of gypsum:

The simplest form of a soil humidity sensor is made from two galvanized nails that are stuck in the soil. Though technically other metals or conducting materials can be used, stainless steel and galvanized iron seem to be the best resistent against corrosion in the soil. Stainless steel however is very difficult to solder, therefore galvanized material is a better choice.