The medium holds the roots, the water-nutrient solution, and air. selecting the right medium is very important to the plant, and the mixes are easy to prepare.
Here are some adaptaions of popular mixes:
4 parts topsoil, 1 part peatmoss, 1 part vermiculite, 1 part perlite. moist, contains medium high amounts of nutrients. best for hand watering. 1 part worm castings, 2 parts vermiculite, 1 part perlite. light weight, high in nutrients. 1 part worm castings, 1 part compost, 1 part topsoil, 2 parts vermiculite, 2 parts perlite, 2 parts styrofoam. holds high amounts of water and air. 1 part worm castings, 1 part peat moss, 1 part lava, 1 part vermiculite, 1 part perlite, 1 part styrofoam. good buffering capabilities.
Remember, its very important to have a good base/medium for the plants to live and grow in. So pick a good potting mix and spend some time getting the mix correct.