Just think of the splendid remains of mummies and the many other treasures from ancient Egypt. Artifacts such as human remains, papyrus, wood, and ivory found in Egypt have survived so well because of the hot, dry desert sands, which contain very little moisture and the kinds of tiny micro-organisms that live in damp soil that would eat these things. In North America, only a small part of the wonderful ancient remains have survived over time, because the climate is damp and the soils are full of living micro-organisms that feast on organic remains. Soil also varies in levels of acidity. Acids can destroy some substances yet help to preserve others.
Gardeners know about soil acidity because it affects the plants and flowers that they grow.
Gardeners who want their pink hydrangeas to turn blue will have to change the acidity of the soil. Even in a very small area, soil types can change dramatically. At one excavation, the broken remains of a plate with a pretty painted decoration was discovered. At one end of the trench, one fragment of the plate looked almost new and the decoration was mearly intact. At the other end of the trench, however, another fragment of the plate was found and the decoration was almost completely gone, eaten away by the chemical reactions that took place in the soil where it was buried.