Soil Profile

During formation soils develop horizontal layers, or horizons, that look different from one another (Fig. 2.3).

The horizons within a soil profile vary in thickness depending on the intensity of the soil-forming factors, though their boundaries are not always easy to distinguish. Uppermost layers of mineral soils are most altered during soil formation, whereas the deeper layers are most similar to the original parent material. Horizon differences in the solum, the parent material most altered during soil formation, involve: (i) organic matter from plant residues and roots in the surface mineral horizons decaying and forming humus, which gives these horizons a dark color— the organic-matter-enriched horizons nearest the soil surface are called A horizons; (ii) movement of soluble and colloidal inorganic and organic constituents from surface layers; and (iii) accumulation of varying amounts of inorganic and organic precipitates. These underlying, enriched layers in mineral soils are referred to as B horizons. The C horizons are the least weathered part of the mineral soil profile. Organic soils are commonly saturated with water and consist mainly of mosses, sedges, or other hydrophytic vegetation; the upper material is referred to as the O layer. In upland areas where drainage is better and forest vegetation supported, folic-derived organic materials accumulate to form the L-F-H layer. In both types of organic soils, it is the residual organic matter in the surface layer that most resembles the vegetation from which it is derived.

The vadose zone is the underlying, unsaturated, parent material extending (from the soil surface) downward to where it reaches the water table and the soil becomes saturated. Below the solum, this zone contains relatively unweathered parent material, low in organic matter and nutrients and often deficient in O2. The thickness of the vadose zone can fluctuate considerably during the season, depending on soil texture, soil water content, and height of the soil water table. When the water table is near the surface, for example as in wetlands, it may be narrow or nonexistent. But in arid or semiarid areas where soils are well drained, the vadose zone can extend for several meters and even be hundreds of meters deep.

physical aspects of soil

Dimensions of physical features commonly encountered when considering the soil habitat range from meters (pedon, soil landscape, and watershed), down through a few millimeters (the fine-earth fraction), to a few micrometers (microorganisms and clay minerals) and nanometers (humic molecules) (Table 2.1).

FIGURE 2.3 Natural (undisturbed) mineral and organic soils showing the soil profile: mollisol (top left), spodosol (top right), oxisol (bottom left), and histosol (bottom right). (Soil profiles reproduced with permission: spodosol and oxisol from University of Nebraska Press, mollisol from American Society of Agronomy; histosol from University of Idaho.)

FIGURE 2.3 Natural (undisturbed) mineral and organic soils showing the soil profile: mollisol (top left), spodosol (top right), oxisol (bottom left), and histosol (bottom right). (Soil profiles reproduced with permission: spodosol and oxisol from University of Nebraska Press, mollisol from American Society of Agronomy; histosol from University of Idaho.)

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