Soil typically consists of layers of material, called horizons, which differ in both texture and appearance. A soil profile is a cross section of these layers, and it measures the different characteristics of each layer. Although every soil from around the world has a different soil profile, most soils consist of three or more layers, including the topsoil, subsoil, and bedrock. The top layer is generally finer and contains less rocks than the deeper layers.
Topsoil is the uppermost part of a soil profile, and it is the ground on which people and animals walk. Plants will also typically lay the majority of their roots in the topsoil. It can be as thin as two inches (5.1 cm) or as thick as 5 feet (1.5 m), and it is often a dark color, sometimes even black.
In uncultivated areas, it may be littered with such organic matter as leaves, twigs, or dead animals that serve to help prevent erosion, hold moisture, and produce nutrient-rich soil. When organic matter decays, it is often referred to as humus, and it contains vital nutrients. It is this layer of the soil profile from which plants get most of their nutrients.
The subsoil is the layer of the soil profile that lies directly beneath the topsoil. There is usually no litter or debris present in this soil layer, and it is often lighter in color. Subsoil often consists of clay, silt, pebbles, and sand, depending on the area, and it generally contains an abundance of minerals that have leached down from the upper layers of the soil.
As a person digs deeper and deeper into the soil, he will find that it gets rockier and rockier. Some scientists consider the next layer of the soil profile, called the regolith, to be part of the subsoil, while others consider it to be a completely separate layer. This layer almost never contains plant roots or other organic matter, but is made up primarily of soil and small, weathered rocks.
The bedrock layer is present in just about every different type of soil profile. This layer is made of hard, solid rock, which is eroded and weathered to produce most of the soil above it. Bedrock can be as little as 5 feet (1.5 m) below the surface, or it can even be exposed in some areas. In situations where much of the upper soil has been deposited from somewhere else, however, the bedrock can lay hundreds of feet beneath the surface.
