The TMP36 is the sensor.
Look at a datasheet here:http://www.analog.com/static/imported-files/data_sheets/TMP35_36_37.pdf
I'll assume you have the T0-92 3 wire package.
I think you want Pin one to power. Pin three to ground. Pin two to analog in on the Arduino. You are reading the change in the voltage on Pin 2 to get the temp.
You must have or there must be a conversion table. This volt reading equals that temp. Maybe you calibrate and make your own. Don't know.
If you are shoving it in the soil, you may need to pay attention to mounting it and protecting it. Don't know your application so can't speak to that.