All sorts of things go into natural and commercially available soil. Natural soil is, by volume, mostly inorganic, made up of rocks ground up into sand and powder. When plants and animals die, they decompose into this rock and sand substrate to become organic soil, which provides nutrients for plant life. Commercially available soil generally contains a mix of organic and inorganic soils balanced to provide both nutrient support and drainage.
Soil has three basic regions. The parent material is the lowest of the three, made of weathered and ground-up bedrock and materials transported from other places. This layer of soil provides a foundation for the secondary layer. The second layer is made up almost entirely of deposits imported from other locations through glacial movement and geological events. The topsoil contains most or all of the nutrients and oxygen from decayed organic matter.
A certain amount of mixture between organic soil and inorganic soil is inevitable and is beneficial to plant life. A bit of inorganic soil in the topsoil helps promote drainage and keeps the plant roots from drowning. The organic matter and water that leaches down into the second layer acts as a reserve in case the topsoil runs dry of moisture and nutrients.
The peat bogs of Scotland are organic soil in almost pure form, being composed of little more than dead and decayed plant matter. On the opposite extreme, Death Valley is almost nothing but inorganic soil. Either conditions will limit what kinds of plants can grow there. An excess of inorganic soil on top of even the richest organic soil can create a desert as well. Take as an example the Makgadikgadi Pan in Botswana: Even though this dry lakebed contains massive amounts of nutrient-rich organic soil, that soil is covered with a thin crust of dry sand and limestone blown in from the surrounding areas. The end result is a desert plain where there should be a forest.
Fertilizer and Nutrients
Fertilizing soil is essentially the process of making it more organic by introducing nutrients in pure form. These nutrients generally include nitrogen, phosphorous and potassium, which are the most important to plant growth and are typically lacking in sandy and rocky soils. However, magnesium, zinc, sulfur, copper, iron and manganese are vital to producing energy-providing chlorophyll and the enzymes that produce chlorophyll. All plants need molybdenum to convert inorganic phosphates to useful organic phosphates, and bean plants need the nutrient to produce the legume modules. Boron is necessary for cell wall formation and pollen production, and a bit of chlorine can help certain small grains mature faster in some soils.