Soil science
"Soil" refers to anything between the top of the dirt and the top of the underlying rock (called "bedrock"). Soil is often broken into layers. The top layer is high in organic matter (roting plants and such). Then there is a layer of layer of soil where rain water has removed some metals (called "leaching"). Those metals get collected in another, lower layer. Finally, there is a layer made up mostly of broken bits of bedrock. A soil can have all of these layers, or only a few.