The more I compare the topsoil of the Earth with human skin the more parallels I see. I may even be struggling to see distinguishing factors.
The soilsphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes.
It exists at the interface of the lith(stone)osphere(the rigid outer part of the earth, consisting of the crust and upper mantle.), atmosphere(the envelope of gases surrounding the earth), hydrosphere(all the waters on the earth’s surface; including the clouds) and biosphere(the regions of the surface, atmosphere, and hydrosphere of the earth occupied by living organisms). The soilsphere is the skin of the Earth and only develops when there is a dynamic interaction between the atmosphere (air in and above the soil), biosphere (living organisms), lithosphere (unconsolidated regolith{a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials} and consolidated bedrock) and the hydrosphere (water in, on and below the soil). The soilsphere is the foundation of terrestrial life on Earth.
The soilsphere acts as the mediator of chemical and biogeochemical(relating to or denoting the cycle in which chemical elements and simple substances are transferred between living systems and the environment) flux into and out of these respective systems and is made up of gaseous, mineralic, fluid and biologic components. The soilsphere lies within the Critical Zone, a broader interface that includes vegetation, soilsphere, groundwater aquifer systems, regolith and finally ends at some depth in the bedrock where the biosphere and hydrosphere cease to make significant changes to the chemistry at depth. As part of the larger global system, any particular environment in which soil forms is influenced solely by its geographic position on the globe as climatic, geologic, biologic and anthropogenic changes occur with changes in longitude and latitude.
The soilsphere lies below the vegetative cover of the biosphere and above the hydrosphere and lithosphere. The soil forming process (soilgenesis) can begin without the aid of biology but is significantly quickened in the presence of biologic reactions. Soil formation begins with the chemical and/or physical breakdown of minerals to form the initial material that overlies the bedrock substrate.
Biology quickens this by secreting acidic compounds that help break rock apart. Particular biologic pioneers are lichen, mosses and seed bearing plants, but many other inorganic reactions take place that diversify the chemical makeup of the early soil layer. Once weathering and decomposition products accumulate, a coherent soil body allows the migration of fluids both vertically and laterally through the soil profile, causing ion exchange between solid, fluid and gaseous phases. As time progresses, the bulk geochemistry of the soil layer will deviate away from the initial composition of the bedrock and will evolve to a chemistry that reflects the type of reactions that take place in the soil.