I am going to be using some very large pots. What should I fill them with? The answer, if I look at the bags in the stores, is some sort of special container mix. But why? Fill a huge cylindrical pot with garden soil. set it on the middle of my garden, and put a plant in the middle. It will not be affected by the fact that it is in a pot. To anthropomorphize a bit, it won't even know it's in a pot. It will grow exactly as if it were planted in the soil that now lies beneath the pot. So why wouldn't I use garden soil in my pots?