Soil is the very topmost portion of the earth's crust; it supports all plant and animal life. It is composed of minerals and organic material resulting from millions of years of geologic activity and of generations of animal life. Each geographic area possesses soil that is the unique result of the base or "parent" material that lies below it in the lower crust and upper mantle of the earth. It may have been moved by rivers and seas, blown by winds or born in volcanic explosions. It has received the remains of plants and animals since the beginning of time and allowed them to rest and decompose, contributing carbon and bacteria to the surrounding mineral material. As the millennia passed, this layer of jumbled mineral and organic materials became home to ever-more complex plant life and hordes of bacteria, insects and simple life forms. We call this top "horizon" of the earth's surface topsoil.
Topsoil composition is based on "parent" material (the geological base), topography, climate, biological activity and age. It is composed of different-sized particles, the largest of which, sand, allows fast drainage of water because of the air-filled spaces between particles. The smallest particle, clay, fits with others so tightly that little drainage is allowed; wet clay soils remain wet for long periods, making for excellent fossil preservation but poor agriculture. Soil that will provide a good medium for plant growth must have a large percentage of sand and a very small percentage of clay. The remainder is made up of a medium-sized particle called silt that, unlike clay, is large enough to capture oxygen and shift about. Unlike sand, however, it is small enough to hold some moisture in place. Topsoils vary in "balance" according to percentages of these three types. When 2 percent or more of the total is composed of organic material, a balanced soil becomes "loam," a perfect soil for gardens, lawns and agriculture.
Improving on Nature
Not all topsoil is properly balanced, drains well (friable) and has enough organic matter to qualify as loam. Human beings have improved soil since the first farmer buried the first fish in a garden. Today, a thriving industry produces topsoil to make thin layers deeper, correct unbalanced soil---or to replace topsoil removed by developers. Sand is harvested from ocean shores and silt from river basins to balance local clays. Compost, peat moss, manure or bird guano is added for fertility. Lime is added to raise pH of acidic soil or sulfur is added to lower pH of an alkaline mix. The entire batch is then heated to destroy undesirable bacteria before shipping. The "engineered" topsoil is then sold by the cubic measure (yard or meter) to homeowners and landscapers to add to their own.