When you buy soil from a plant nursery or garden supplies store, you often buy what is called "topsoil". Topsoil is usually a premixed collection of decaying organic matter and microorganisms that are full of nutrients to support plants and retain moisture. It is very similar to mulch, which is an earlier stage in the process. Many organic gardeners and farmers will tell you there is no need to ever buy soil, but there are many circumstances where it might be preferable.