I've been cautioned not to repot my indoor plants to often, o.k. but after a couple of months the soil in the pots gets so hard that water just sits on top for a long time. I use Miracle Grow potting soil and the plants are healthy looking, its just the soil itself gets hard. Calif. water is hard and full of "stuff", could this just be a build up of minerals and if so, can it be remedied?