Why are different soils made of different chemical?

Soils are formed by a complex interaction between weathering and biological processes over millions of years. The chemical composition of soils is influenced in a large part by the parent rock from which they formed. For example a soil formed from a sandstone rock will comprise largely of quartzitic grains, and have a sand grain size, with little clay. There are large differences in chemical composition of the original rocks like, for example, between a granite, basalt or shale.

Besides the parent rock, other factors like the amount of rainfall, the type of organisms temperature and amount of organic matter all influence the chemical composition of the final soil.