For most plants , it is a crumbly , dark brown soil - you usually get this by adding compost or mulch ( eg lawn clippings ) to the soil.
If your soil is heavy clay , you can add gypsum and blood and bone to convert it to crumbly , rich soil.Clay soil , when treated , is richer than sandy soil.

If you have sandy soil , you usually need to add organic matter like compost or mulch , to help it hold water.However ,there are some plants that like it sandy.
Some alpine plants like soils that are very alkaline and rocky.I've read that you can add pieces of concrete to their planting hole or around them , to make the soil more alkaline.
Camellias and azaleas like acid soil , such as you often get in a forest.It is safe to use gypsum with them ,but not lime.

Skimmia prefers a slightly acidic soil, so if you're putting it in a pot, use ericaceous compost.