If you took a cross section of dirt ranging from the surface of soil all the way down to bedrock, you would notice that the soil has several layers. The soil layers range from a dark, aerated material at the surface of the ground known as topsoil. Between the topsoil and bedrock are several layers known as subsoil. Subsoil is denser than top soil for several reasons.
The surface soil is exposed to more oxygen than the subsoil. Because of this, oxygen has more opportunity to permeate the topsoil regions than it does the subsoil regions. Additionally, humans practice a number of cultural processes that aerate the soil ranging from ploughing to core aeration. Soil that is more oxygenated is less dense.
Topsoil has more organic material than subsoil due to the constant incorporation of organic material into it. This layer of soil is coarse and loosely compacted. Because of this, water can flow freely through topsoil. As you descend in layers closer to the bedrock layer, organic material thins out and the soil has more fine-particle clay. Clay particles pack more closely together and water will not easily move through it. This makes it denser than topsoil.
Soil forms its layers from the decomposition of animals and the weathering of rocks. The composition of soil will differ because of the presence or absence of these materials. In areas located near mountains, such as the Piedmont region of the Southeast, topsoil is shallow and the clay subsoil is abundant due to the weathering processes of the nearby mountain ranges. In the Midwest, the topsoil is deep and fertile due to extra deposits of topsoil brought in from northern areas by glaciers during the last ice age.
The least dense soil is sandy, loose soil. Soil of this nature contains very little organic material. Desert topsoils and farmland that has been repeatedly tilled without organic amendments added are good examples of these types of soils. As a general rule, when more air is incorporated into the soil, oxygen can break down more organic material.
Most scientists measure soil in terms of porosity. A soil's porosity is a function of the soil's bulk density and particle density. Bulk density is the dry mass of soils per unit volume. It can be found through measuring on a balance scale. Scientists rate particle density as a constant 2.65 g/cm3. To figure the porosity of a soil, multiply the particle density by 100 and divide it by the bulk density. Then subtract by 100 to find the percentage of porosity.