Davis Soil Moisture Sensor 
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Soil Moisture Sensor, Vantage Pro® and Vantage Pro2
Watermark® soil moisture sensor uses electrical resistance to measure the moisture level of the soil. Does not require periodic maintenance during the growing season. Simply bury at the desired depth, then monitor moisture levels throughout the season. Add up to 4 sensors to a single Leaf & Soil Moisture/Temperature Station. Includes 15' (4.6M) cable.
Compatible with Vantage Pro and Vantage Pro2. The Soil Moisture Sensor must be connected via a Leaf & Soil Moisture/Temperature Station (#6345).
Quick Guide (pdf)