A plant moisture meter has a brass or steel probe attached to a hand-size box with a gauge that measures varying levels of moisture in the soil. Costing less than $20 (in 2009) and operating without a battery, a plant moisture meter can take the guesswork out of watering plants in the house or garden.
Confirm that the indicator arm on the face of the gauge is pointing to "Dry" (the default setting). Dry, Moist and Wet are the gauge's three levels, and each level is broken down using a numbering system: 1 to 3 (Dry), 4 to 7 (Moist) and 8 to 10 (Wet).
Insert the probe into the soil close to the base of the plant. The probe is designed to read moisture content at the root level. Most plants will have an extensive root system, so push the probe halfway into the pot to be assured of being in the middle of the roots. For very large pots, you may need to push the probe into the soil until the gauge touches the soil.
Read the face of the gauge, which should have immediately indicated the level of moisture. Some plants require a continuously moist soil or recommend that you water once the soil is completely dry. Base watering decisions off the gauge reading and plant needs. You can leave the probe in place while you water the plant.
Use the numbers on the gauge to determine the plant's water usage and the needed frequency of watering without constantly using the gauge. For instance, if the gauge reads Moist 7 on the first check and seven days later the reading is Moist 4, you know the plant can go up to seven days without being watered. Conversely, if the gauge reads Dry after seven days, you know you need to water more often given the dramatic decrease in soil moisture.
Rinse the probe and wipe dry after each use.