Here are the instructions for the Luster Leaf Rapitest Moisture Meter. This will give you a good explanation on how to use the product and test for the correct number on which the product should be used. 1. Insert the probe, vertically if possible, into the pot half way between the edge of the container and the plant stem. The depth of penetration will be influenced by the size of the container. Probe deeper in a large pot, shallower in a small one. A good rule of thumb is to probe about 1/2 to 2/3 of the way down from the surface. For very large containers (over 12" in diameter) place the probe closer to the stem (about 1/3 of the way from the stem to the edge of the pot.) 2. While pushing the probe into the soil you may note that the meter reading varies as the probe moves downward. This is because the moisture conditions are not uniform. Certain types of soil have a tendency to create moisture pockets, which might give you a false high reading in one small area of the soil. We recommend you take at least two readings in order to confirm your findings. (Note: probing aerates the soil, which is good for the plant.) 3. Note your meter reading. 4. Remove the probe from the soil by pulling the probe handle, not the wire. Tugging the wire may result in a malfunction. 5. Wipe the probe completely clean with a soft cloth or tissue before taking another reading or when finished. Important: The meter is designed and constructed for test readings. The probe should not be left in contact with soil for extended periods of time. Here is a guide as well for information on the common plants and their numbers: http://www.domyownpestcontrol.com/msds/watering_guide-20141.pdf