Soil is a mixture of minerals, organic matter, water, and air.  It has a definite structure and composition, and forms on the surface of land.  Many soils have three major layers: top soil, subsoil, and parent matter.  These layers make up the soil profile.  The soil profile represents the succession of distinctive soil layers from the surface down to the unchanged parent material beneath it. 

We are mainly concerned here with the top soil since it is here that the living and nonliving link together to support the plants in the bioretention system.  High quality top soil is usually dark brown, dark gray, or black because of the humus it contains.  Humus is the highly decomposed plant and animal residue that makes up much of the organic nutrients composing the soil.  Because of its texture, a good topsoil will hold adequate moisture and allow air to circulate, which are required for photosynthesis to take place in plants.  Decomposers break down humus to free the nitrates, phosphates, and potassium that are required by plants.  Fertile soils contain and are able to supply a complete set of nutrients required by growing plants. 

The major essential nutrient elements supplied through soil are nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur.  Soil pH is an important property since it affects the availability of nutrients to plants. The nutrients described above tend to be less available to plants in soils with low pH. Other major nutrients like carbon, hydrogen, and oxygen come from water and atmospheric carbon.  Nitrogen, phosphorus, and potassium are of particular interest to farmers and other plant growers since plants require relatively large quantities of these elements.  When plant materials are harvested over and over from the same area, the soils are quickly depleted of these elements.  This is why the main components of fertilizers are nitrogen, phosphorus, and potassium.  However, excess amounts of these three components in water systems can be detrimental and lead to the premature aging of ponds and lakes (eutrophication).