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- The O horizon, also known as the organic horizon, is the topmost layer of soil. It is made up of decomposed plant material, such as leaves and twigs, and is dark in color due to the high concentration of organic matter. The O horizon plays an important role in soil fertility by providing nutrients to plants and retaining moisture.
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Feb 17, 2023 · Soil Horizons. The soil is the topmost layer of the earth’s crust consisting of air, water, inorganic minerals (rock, sand, clay, and slit), and organic matter (dead plants and animals). It forms the source of food for plants. It provides shelter for many animals such as insects, centipedes, burrowing animals, microorganisms, and many others.
Most soils have three major horizons (A, B, C) and some have an organic horizon (O). 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.
The horizon description system begins by splitting soil horizons into two distinct groups: organic and mineral horizons. Organic horizons are those that contain 17% or more organic carbon; mineral horizons have less than 17% organic carbon.
The O horizon is rich in microorganisms and contributes to soil health by enhancing nutrient availability for plants. This layer acts as a protective cover for the underlying horizons, preventing erosion and maintaining soil structure.
Soil profiles can show distinct or indistinct layers called soil horizons (see Figure 11). Soil profiles can be used to quickly compare ground characteristics between different locations and to assess whether or not a location is suitable for tasks such as farming.
The soil profile has four distinct layers: 1) O horizon; 2) A horizon; 3) B horizon, or subsoil; and 4) C horizon, or soil base (Figure \(\PageIndex{2}\)). The O horizon has freshly decomposing organic matter—humus—at its surface, with decomposed vegetation at its base.
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A typical soil has a soil profile consisting of four horizons, which are designated: O, A, B and C. The O horizon is the top layer at the earth's surface. It consists of surface litter, such as fallen leaves (duff), sticks and other plant material, animal waste and dead organisms.
