What a great idea Save soil, save money, grow better plants. Use those cheap plastic pots (the ones the plants are potted in when you get them at the nursery/garden center) in the bottom of a large planter to provide good drainage and better airflow.
When I was in the Master Gardener program we used the water crystals in planters and I knew it was the same thing in diapers. "The trick to amazing potted plants - diapers. I did this in my hanging baskets last year and they rocked all season.