The best way to make sure you aren't inadvertently overwatering or underwatering a plant is to test the actual moisture content of the soil. Requiring no batteries, this meter has a bi-metal probe that gauges the conductivity of the soil to measure moisture content, indicated by readings from 0 to 10 on the display. By consulting a companion guide, which lists moisture requirements for 170 plant types, you can determine whether the plant needs water immediately, has good moisture, or has been over-watered.