![]() ![]() This horizon is typically darkly colored because of the presence of organic matter. ![]() The A horizon consists of a mixture of organic material with inorganic products of weathering, and it is therefore the beginning of true mineral soil. Topsoil is rich in organic material microbial processes occur there, and it is the “workhorse” of plant production. For instance, river deltas like the Mississippi River delta have deep layers of topsoil. Topsoil-the top layer of soil-is usually two to three inches deep, but this depth can vary considerably. Humus enriches the soil with nutrients and enhances soil moisture retention. The O horizon has freshly decomposing organic matter-humus-at its surface, with decomposed vegetation at its base. 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 ( ). Soils are named and classified based on their horizons. Five factors account for soil formation: parent material, climate, topography, biological factors, and time. A horizon is a soil layer with distinct physical and chemical properties that differ from those of other layers. Within the soil profile, soil scientists define zones called horizons. ![]() Soil distribution is not homogenous because its formation results in the production of layers together, the vertical section of a soil is called the soil profile. Most soil microorganisms-bacteria, algae, or fungi-are dormant in dry soil, but become active once moisture is available. The organic component of soil serves as a cementing agent, returns nutrients to the plant, allows soil to store moisture, makes soil tillable for farming, and provides energy for soil microorganisms. About one-half of the pore space should contain water, and the other half should contain air. Soil should ideally contain 50 percent solid material and 50 percent pore space. Soil formation is the consequence of a combination of biological, physical, and chemical processes. Mineral soils are predominant in terrestrial ecosystems, where soils may be covered by water for part of the year or exposed to the atmosphere. Soils can be divided into two groups: organic soils are those that are formed from sedimentation and primarily composed of organic matter, while those that are formed from the weathering of rocks and are primarily composed of inorganic material are called mineral soils. Soil develops very slowly over long periods of time, and its formation results from natural and environmental forces acting on mineral, rock, and organic compounds. In agriculture, the history of the soil, such as the cultivating practices and previous crops, modify the characteristics and fertility of that soil. Soil quality depends not only on the chemical composition of the soil, but also the topography (regional surface features) and the presence of living organisms. Soil quality is a major determinant, along with climate, of plant distribution and growth. Soil is the outer loose layer that covers the surface of Earth. Plants obtain inorganic elements from the soil, which serves as a natural medium for land plants. By the end of this section, you will be able to: ![]()
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