Our planet is sort of like an onion: layer upon layer of different types of soil. The deeper you dig, the more layers you’ll discover, and each layer is different from the one above it. These layers are called horizons, and put together they form a profile.
Each horizon tells a story, and can take us back through time by telling us how it was formed, what it’s made from, and even what kind damage we humans have inflicted upon the Earth in any given area.
The horizons are:
O – (humus or organic) Mostly organic matter such as decomposing leaves. The O horizon is thin in some soils, thick in others, and not present at all in others.
A – (topsoil) Mostly minerals from parent material with organic matter incorporated. A good material for plants and other organisms to live.
E – (eluviated) Leached of clay, minerals, and organic matter, leaving a concentration of sand and silt particles of quartz or other resistant materials – missing in some soils but often found in older soils and forest soils.
B – (subsoil) Rich in minerals that leached (moved down) from the A or E horizons and accumulated here.
C – (parent material) The deposit at Earth’s surface from which the soil developed.
R – (bedrock) A mass of rock such as granite, basalt, quartzite, limestone or sandstone that forms the parent material for some soils – if the bedrock is close enough to the surface to weather. This is not soil and is located under the C horizon.
(Source: Soil Science Society of America)
Steve & Corey Veldheer are organic yard & garden specialists in west Michigan.