They measure electrical conductivity, and not actually moisture. They basically measure across the shaft of the probe, and the tip, with a small voltage. With that said, the reading depends on the electical conductivity of whatever it is you are sticking it into, which isn't always accurate. For example, distilled water is pure water, which is not conductive, hence its use in batteries. If you stick a "moisture" meter in it, you will read "dry", even though you know it's wet. So, in the case of a free draining mix like 5-1-1 or gritty mix, you will also read dry due to the air space, thus not completing the electrical circuit. If you're using miracle grow, it'll work ok, but don't bet anything on its accuracy. Persoanally, I don't use them; had one before, but tossed it because it gave wacky readings. I'd stick it in a freshly watered pot, with miracle grow for soil, and slowly poke it deeper. The needle would bounce all over from dry to wet, so I gave up on it. I've never used a digital one, but I suspect it's the same thing, with a digital screen, and a higher price tag.

Joe

I'm inclined to agree with Joe. There have been times where they worked properly, but whatever information they provided was always tainted by the worry that they were "wacky readings." A bamboo skewer is simple, cheap, and much more reliable.