Parent material

Underlying geological material in which soil horizons form

Parent material is the underlying geological material in which soil horizons form. Soils typically inherit a great deal of structure and minerals from their parent material, and, as such, are often classified based upon their contents of consolidated or unconsolidated mineral material that has undergone some degree of physical or chemical weathering and the mode by which the materials were most recently transported.

This keyword could refer to multiple things. Here are some suggestions: