In a lot of areas of the world IRL soils aren't always brown. In Colorado for example, a lot of the soil here is a fairly deep shade of red. Why should it be added? More variety in terrain, it'd certainly liven things up a bit to come across red dirt instead of the same boring brown dirt that is everywhere else. IRL this sort of dirt has a lot of Iron in it (which is why it's red) and tends to form in drier areas.

We're not re-adding different dirt types