Testing soil moisture of container plants by probing with a finger is tricky for most people to learn. The results are often inaccurate. Do your plants and fingers a favor and use a soil probe. The red one in the photo is called a Soil Sleuth.
This soil probe is in widespread use in the interior plantscaping business. It is a fine product that should be available in retail stores across the country but isn't. You need to order one online. Do yourself a favor and order one for yourself and as gifts for friends.
The 12” durable plastic Soil Sleuth will work well for pot sizes up to 10”. Use the 16” aluminum probe for larger planters such as sub-irrigated grow boxes. .
The most important zone of a container plant soil system is down at the bottom rather than the top. This is where the highest concentration of roots is located. It is beyond the range of fingers.
Push the probe down to the bottom of the planter. Twist the probe and pull it back out of the soil. There will be small plugs of soil in the notches. Pinch the bottom plug. If it feels moist and the soil particles stick together, do not water.
It’s time to water when the soil particles do not stick together. If the soil was allowed to dry too much it will not form plugs and will be light in color. If the soil is very dry, the probe will come out empty of soil particles.
The probe will not harm the root system. If you inadvertently hook the probe on a root, merely twist the probe to unhook before removing it.
Probing also has the beneficial effect of aerating the soil each time you use it. A novice can learn to use a soil probe and judge soil moisture with great accuracy. A green thumb is not required. It takes longer to explain it than use it.