Strange! Does the soil mix contract when it dries out?
I think you said "miracle grow soil" Wouldn't it be better to use the "potting mix" instead of potting "soil" or just garden soil?
It always seemed misleading for stores to call something "potting soil" if it's too heavy to use for pots and containers!
Anyway, I know that either "miracle grow soil" or "miracle grow [/u]potting[/i]soil" would kind of prone to compacting in pots and being too heavy and dense to start with. But I would not have guessed they would get as hard and water-repellant as you describe.
At first I thought the "water-repellent" effect might mean there was a lot of peat moss in the mix. Peat moss can be hard to re-moisten once it gets dry.
But I still think you have something special going on that I never heard of. Can excessive hard minerals in water make soil like concrete when it dries? If you keep pouring them in the top and the water keeps evaporating out through the clay pots.
If that's what's going on, maybe a mineral crust COULD form overnight on some clay pot in a hot, dry draft.
What would be a solution? Water them with bottled water? De-desalinized water? Lots of supermarkets have a "water machine" where you can buy a gallon of reverse-osmosis-ed water for 25 or 50 cents. I guess "rain water" would be good if you had enough rain for that to be reliable.
You probably already know to avoid watering with "softened" water. That kind of water purifier usually gets rid of Ca, Mg and iron ions by replacing them with salt (sodium chloride salt). That's as bad or worse for potted plants than most minerals.
