Geeetech Soil Moisture Sensor Self-watering Automatic Irrigation Garden Tool for 328 2560 The sensor uses the two probes to pass current through the soil, and then it reads that resistance to get the moisture level. More water makes the soil conduct electricity more easily (less resistance), while dry soil conducts electricity poorly (more resistance). This moisture sensor can read the amount of moisture present in the soil surrounding it. It's a low tech sensor, but ideal for monitoring an urban garden, or your pet plant's water level. It will be helpful to remind you to water your indoor plants or to monitor the soil moisture in your garden.