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In soil science the various distinct strata within the soil are referred to as horizons. Some strata are very common, and are particularly useful in determining soil classification. It is important to keep in mind that a soil classification does not refer to 'sandy' or 'loamy', but refers to a layered sample involving multiple horizons, which may extend from a surface layer of compost all the way down to the bedrock, passing through any number of horizons along the way.

The A horizon is usually the second strata down, the top layer usually being the O horizon. While the O horizon is the layer of organic material actively undergoing decomposition, the A horizon is a more stable layer comprised primarily of organic matter that is not actively decomposing, and is most commonly called humus or topsoil. It is generally dark brown or black, and it is not possible to tell what organic matter it decomposed from.

The A horizon can be anywhere from centimeters to meters thick, and is generally thought of as being the core of the soil biomantle, where the majority of biological activity takes place. It is comparatively nutrient rich, and while root systems will usually push deeper than the A horizon, they may get the majority of their nutrients here. This is also home to burrowing mammals, insects, worms, nematodes, fungi, and many other organisms. The health of the A horizon is also an important factor in how successful an attempt to farm the land may be.

Below the A horizon is usually the B horizon, which is commonly celled subsoil; it is primarily comprised of clay, sand, or other mineral-based soils. However, organic material often leeches into the B horizon from the A horizon. Together with the O horizon, the A and B horizons make up the solem.

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