Most soils consist of three main vertical layers called horizons, one above the other, with different colors and other properties. The horizons are produced by the longtime effects of climate and vegetation on mineral matter. Collectively, these horizons make up a soil profile.
Uncovering these hidden depths helps gardeners decide what type of plants can be grown in their areas. You don’t have to see all the layers to have a good garden, but taking a look increases knowledge of soil makeup. Highway cuts and nearby residential construction sites are good places to find exposed soil layers.
If you dig through the lower layers of soil and find no definite bands, it’s reasonable to assume that the soil has been moved around and mixed up recently. Some good examples include man-made landfills, soils that have been deposited in floodplains along streams, and newly formed sand dunes.
The first soil horizon is the topsoil, called by soil scientists the A horizon, which is generally 4 to 6 inches thick. More fertile than underlying soils, topsoil is more enriched with organic matter than the layers beneath it. This is the soil layer that most concerns gardeners, since it’s the bed for plants. When soil improvement is mentioned, the topsoil and perhaps the subsoil (see below) are the ones to be amended.
The horizon under the topsoil is called the subsoil (or B horizon) and may reach a depth of 30 inches. It’s finer and more even textured than topsoil; it’s also more compact, partly because it contains less organic matter. Subsoil supports surface soil and stores water and nutrients leached from the topsoil.
When exposed during construction, subsoil must be incorporated into a garden. If too compact, it can inhibit root and water penetration. Ways to amend its texture and increase its fertility are described at Exposed Subsoil.
The deepest soil layer, the C horizon, consists of decomposed rock, parent material that has characteristics of the subsoil above it and the bedrock beneath it. This parent soil is mostly responsible for the texture, natural fertility, rate of formation, acidity, and depth of the soil horizons above.