Perlite, common in all the popular soil for growing MJ on this side of the pond, sucks up excessive water and releases it again when needed (when possible actually, which is around the same time). And since over watering is something many growers do (unless using drippers or watering many times a day. Ie. many growers over water in the morning so the plant has enough to last for the entire day). If you're going to test make sure (sorry can't resist) you get a real
tensio meter.
I agree perlite is no miracle cure or anything, but it's cheap and convenient.