The makeup of the growing medium depends upon the plants you choose to grow, as the media should have last to match the life expectancy of the green wall plant.
For Annuals, Vegetables, Small Fruits, and Herbs
Plants are expected to be used for one full year need a good quality peat-based or coconut fiber-based potting soil. These are available at local garden centers.
For Perennials, Vines and Tropicals
Plants that are expected to last for 3 to 5 years before dividing, use a good quality potting soil, with perlite or pumice and/or composted pine bark. Such potting soils will tend to decompose less.
For Succulent Plants
Plants that can last decades without repotting, succulents need a good quality mix, typical of one marketed for rock gardening. The unique quality of such mixes is that they have a high mineral content (e.g. perlite, pumice, or expanded shale) and less organic matter, and therefore do not decompose over time.