Soil Temperature and Humidity Sensor SHT20
The sensor includes a temperature/humidity sensor module in a sinter metal mesh encasing. The casing is weatherproof and will keep water from seeping into the body of the sensor and damaging it, but allows air to pass through so that it can measure the humidity (moisture) of the soil. It is designed to be submersible in water, but it's always best to avoid long-term (over 1 hour at a time) submersion if you need something that can be submerged for over an hour you may want to find a different sensor. It can also be simply placed outside for exterior weather sensing.
The sensor is essentially just an SHT-20 with the 4 data/power wires brought out so any SHT-2X code for a microcontroller will work. The sensor works with 3.3V logic. The 1-meter long cable has four wires: Red = VCC (3-3.7VDC), Black = GND, Yellow = SCK, Green = SDA.