Horizons in soils

Price, G.H. 2001. New horizons in soil fertility management. Australian Journal of Soil Research 39 683-710. 

Biological, chemical, and physical processes produce distinctive layers, or horizons, in soils. There are numerous types of soil horizons (Birkeland, 1984). However, the most common from the top of a soil profile to the bottom are the O, A, B, C, and R (Table 3.19). Not all of the horizons are present in every soil. Furthermore, the specific characteristics of each horizon are affected by climate, bedrock composition, the types of vegetation, pH, redox conditions, and time (Faure, 1998), 354-355. 

Rabenhorst, M.C., West, L.T. Wilding, L.P. (1991) Genesis of calcic and petrocalcic horizons in soils over carbonate rocks. In Nettleton, W.D. (Ed.) Occurrence, Characteristics and Genesis of Carbonate, Gypsum and Silica Accumulations in Sods. Special Publication 26. Madison, WI Soil Science Society of America, pp. 61-74. 

Fig. 4.23 Schematic diagram explaining the formation of argillic horizons in soils. The resulting horizons are comparable to those in the alfisol (luvisol) shown in Plate 4.2(a). The master horizons are typically centimetres to tens of centimetres thick.

Horizon. In soil science, a layer of soil approximately parallel to the land surface and differing from adjacent layers in chemical, physical, and biological properties, and in characteristics such as color, structure. 

Forstner, 1984 Forstner, 1988). Usually, the chemical compositions of these sediments are close to those of the regoliths and C-horizons in soil profiles of a given area. As in soils, trace elements can be contained in the mineral phases or adsorbed on them (Bourg, 1988). In this paper, we discuss river sediments in connection with naturally contaminated catchments (Section 3). In this field, a good knowledge of typical natural trace element ranges is particularly important in order to assess a possible anthropogenic contribution. 

The horizon that has been significantly leached of its mineral and/or organic content, leaving a pale layer largely composed of silicates. These are present only in older, well-developed soils, and generally occur between the A and B horizons. In soils that contain gravels, due to animal bioturbation and stonelayer commonly forms near or at the base of the E horizon. 

Movement of raw and transformed materials can take place within the soil and results in zones of accumulation, depletion, or mixing. Formation, migration, and accumulation of different elements, clays, oxides, and organic matter can occur in different parts of the soil. These different zones or layers in soil that are approximately parallel to the surface are called soil horizons. Depleted or enriched soil horizons result in different depths in the soil having different chemical and physical properties. Translocations are caused by a combination of physical, chemical, and biological processes. 

Biogeochemical compartment. The biogeochemical compartment consists of the O, A, E, and B horizons of soils, in which substantial organic matter can be an integral component of the soil. The load of the water leaving the soil from the biogeochemical compartment consists mainly of soluble compounds because the matrix of the soil acts as a filter that retains particulate matter. In some cases, however, clay and particulate humic substances are also car-... 

Normally, clay in soil is not present as individual particles but is clustered to aggregates that consist wholly of clay or of a mixture of clay and other mineral and/or organic soil material. Mass transport of soil material along cracks and pores, common in cracking soils in regions with alternating wet and dry periods, does not necessarily enrich the subsoil horizons with clay. 

Arid soils that lack diagnostic subsurface horizons are generally classified as Entisols, which fall into the azonal concept of Sibirtsev. These include many of the younger soils on floodplains, dunes, and erosional surfaces. In Soil Taxonomy, floodplain soils are mainly classified as... 

Fig. 3. Soil inventory of carbon in soil organic matter (SOM) (a), A14C of SOM (b), noncrystalline minerals (c), and crystalline minerals (d) versus age of soil substrate. Filled circles, total profiles filled triangle, surface (O and A) horizons (Tom et al. 1997).

In soils, lead concentrates in organic-rich surface horizons (NRCC 1973). In one instance, only 17 mg of soluble Pb/kg was found in soils 3 days after the addition of 2784 mg of lead (as lead nitrate)/kg (NRCC 1973). The estimated residence time of lead in soils is about 20 years complete turnover in topsoil is expected every few decades (Nriagu 1978a). In forest litter, however, the mean residence time of lead is lengthy estimates range from 220 years (Turner et al. 1985) to more than 500 years (Friedland and Johnson 1985). 

Collecting soils from the A and C horizons eliminates variability resulting from mixing of soil horizons in random... 

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