SparkFun gator:soil Hookup Guide
Englandsaurus 
Other Troubleshooting Tips
Here are a few tips for troubleshooting this device.
Sensor Corrosion
A commonly known issue with soil moisture senors is their short lifespan due to corrosion of the electrodes from electrolysis. To combat this, we've had the PCB coated in Gold Finishing (Electroless Nickel Immersion Gold), which has a high decomposition potential. However, the life span of the sensor is limited. Applying a higher input voltage, taking frequent readings, and/or leaving the sensor on for extended periods of time will decrease the lifespan.
A simple way to extend the lifespan of your sensor is to only power it for the short period of a reading.
Waterproofing
A commonly known issue with soil moisture senor is that it may be exposed to water when you water your plants. Unfortunately, water and electronics do not mix. To prevent shorting out your board and sensor, you should always disconnect your cables when watering your plant. (*You can remove the sensor, but then you will need to recalibrate the sensor every time and this will affect the consistency of your readings.)
Calibration
To get any sort of useful data out of your sensor, it is advised that you calibrate it to whatever soil you plan to monitor. Different types of soil have various affects on the sensor; you may get different readings from one soil sample to the next. Before you start storing moisture data or triggering events based on that value, you should see what values you are actually getting from your sensor.