> Anyone can explain the difference between potting soil , garden soil, > soilless mix and whatever else there might be? > Garden soil comes out of someone's garden - and if you put it in a pot, it usually compacts to a heavy consistency, not allowing good "porosity" (soil pore spaces). Especially if it has a lot of clay, and little organic material. So... plant roots can't penetrate easily, and water and air flow is impeded. Those roots need both water AND air. So, potting soil has amendments to keep it loose and open. Like perlite, pumice, or shredded, decomposed wood or bark. If you get a bag of potting soil, you can see the compaction-resistant pieces of whatever they thought to use. Compaction and loss of porosity can also be a problem out in the garden.