Soil taxonomy

Soil classification is the arrangement of soils into groups of soils on the basis of soil characteristics. 

There are 12 orders of soils, which are sorted by degree of horizon development and the kinds of horizon present. The description of soil orders presented here was obtained from http //soils.usda. gov/technical/classification/orders/, http //soils.ag.uidaho.edu/soilorders/orders.htm, andhttps // 

Entisols. Mineral soils that have no distinct subsurface diagnostic horizons within 1 m of the soil surface. Entisols have little or no evidence of the development of pedogenic horizons. Most Entisols have no diagnostic horizons other than an ochric epipedon. Entisols are divided into five suborders Aquents, Arents, Psamments, Fluvents, and Orthents. 

Oxisols. Mineral soils that have an oxic horizon within 2 m of the surface or plinthite as a continuous phase within 30 cm of the surface, and that do not have a spodic or argellic horizon above the oxic horizon. Oxisols are very highly weathered soils that are found primarily in the intertropical regions of the world. These soils contain few weatherable minerals and are often rich in Fe and A1 oxide minerals. Oxisols are divided into live suborders Aquox, Torrox, Ustox, Perox, and Udox. 

Ultisols. Mineral soils that have an argillic horizon with a base saturation of 35% when measured at pH 8.2. Ultisols are strongly leached, acid forest soils with relatively low native fertility. They are found primarily in humid temperate and tropical areas of the world, typically on older, stable landscapes. These soils are characterized by an accumulation of clays in the subsurface horizon and exhibit strong yellowish or reddish colors resulting from the presence of iron oxides. Ultisols are divided into five suborders Aquults, Humults, Udults, Ustults, and Xerults. 

Soil taxonomy distinguishes between mineral and organic soils. This distinction is not so easily applied in practice, and somewhat complicated rules need to be established. The different strata in soils are called horizons and practically all soils have both mineral and organic components. Those horizons that have less than 20-35% organic matter show properties that are closer to mineral soils, and vice versa. 

Some examples of soil composition, arranged in decreasing order of organic content, are given in Table 5.2. 

Soil type Total organic Soluble organic Moisture, carbon, TOC, % matter, SOM, % % pH  

Plant roots give evidence of plant root activity and penetration. For example, it is important to record if roots only penetrate through cracks, being retarded by waterlogged layers or cemented layers. Other reasons for limited root penetration can be soil compaction or the absence of nutrients. If there is no obstacle to root growth in the soil the roots may be distributed evenly in a soil. It is important to record the quantity per unit area and the diameter of roots. 

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See "Keys to Soil Taxonomy" (Soil Survey Staff, 1998) for a complete listing. 

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... 

Table 1.3. Global extent of arid and semi-arid soils (km2) based on the Soil Taxonomy system) (after Monger et al., 2004)...

Figure 1.9. Illustration of soil taxonomy suborders and great groups that have aridic moisture regimes (shaded) and their moisture counterparts that have ustic and xeric moisture regimes (after Monger, 2002. Reprinted from Encyclopedia of Soil Science, H. Tan, ed., Arid Soils, Monger, p. 86, Copyright (2002), with permission from Marcel Dekker)...

Fenton T.E. Mollisols. In Pedogenesis and Soil Taxonomy, II. The soil orders, L.P. 

Nettleton W.D., Peterson F.F. Aridisols. In Pedogenesis and Soil Taxonomy. II. The Soil Orders, P. Wilding, N.E. Smeck, G.F. Hall, eds. Amsterdam Elsevier, 1983. 

Soil Survey Staff. Soil taxonomy, a basic system of soil classification for making and interpreting soil surveys. U.S. Dept. Agric. Handbook, 436. Washington D.C. Soil Conserv Serv, 1975. 

Bellon MR, Taylor JE (1993) Folk soil taxonomy and the partial adoption of new seed varieties. Econo Dev Cult Change 41(4) 762-786... 

Two basic concepts are used in developing the naming of different soils. First is the idea that a soil s characteristics will determine which group it falls into and its name. Another idea is that soils will be related to a reference soil. In both cases, the natural soil horizons are used in the soil description. In Soil Taxonomy, horizons used for naming the soil are called diagnostic horizons. In the reference soils, the horizons are called reference horizons. The general concepts are similar in all systems, and so are many names for soil characteristics. The names are often descriptive in that they give an idea of the characteristic of the soil. 

Soil Survey Staff. Soil Taxonomy A Basic System of Soil Classification for Making and Interpreting Soil Surveys. Washington, DC U.S. Government Printing Office 1975. 

Except for the top soil where the colour caused by Fe oxides is often masked by hu-mics, most of the soil profile receives its brown, yellow or red colour from Fe oxides (Bigham Ciolkosz, 1993). Because this is so obvious to the naked eye, soils have been named according to colour in most national classification systems, e. g. red-yellow podzols (USA), sol ochreux (France), Braunerde (Germany), krasnozem (Russia), terra rossa (Italy), and even the current modern international systems (U.S. Soil Taxonomy system and World Reference Base for Soil Resources, WRB) use colour connotations such as Rhodic (red) and Xanthic (yellow). 

U.S. Department of Agriculture. Natural Resources Conservation Service. Keys to Soil Taxonomy, 8th ed. 1998. www.nssc.nrcs.usda.gov... 

The Brazilian Latossolos correspond to well drained Oxisols in the U.S. Soil Taxonomy, and the FAO-UNESCO soil map legend identifies them as Ferralsols. In the French classification they are defined as Sols Ferrallitiques, commonly fortement d at-ures, typiques. The Podzolicos belong to the Alfisols (when eutrofic) and to the Kandisols... 

Brazilian Classification U.S. Soil Taxonomy Mean Median SD SE... 

Beinroth, F. H. 1975- Relationships between U.S. soil taxonomy, the Brazilian soil classification system, and FAO/UNESCO soil units. In Soil Management in Tropical America, eds. E. Bomemisza and A. Alvarado (North Carolina State Univ., Raleigh), pp. 92-108. 

Spodosols are one of the 12 soil orders in the USDA Soil Taxonomy. The soil orders, and their key properties, are listed in Table 4. The distribution of the soil orders in the world is illustrated in Figure 14. With respect to Spodosols, which are common to the NE USA, Canada, Scandinavia, and Russia, the key... 

Table 4 The soil orders of the US Soil Taxonomy and a brief definition of their characteristics.

Soil Survey Staff (1999) Keys to Soil Taxonomy. Pocahontas Press, Blacksburg, Virginia. 

McFarlane, 1976, 1983a), through to mineralogical and textural based schemes (e.g. Aleva, 1986 Bardossy and Aleva, 1990), to a system incorporated within soil taxonomy (Soil Survey Staff, 1975), as well as more quantitative, chemically based genetic classifications (Schellmann, 1982). It is not the intention of this chapter to discuss the merits of these schemes, nor to outline the others that have been proposed this has been reviewed previously (e.g. McFarlane, 1976, 1983a Aleva, 1994 Bourman and Ollier, 2002). Instead, a simple distinction is made between laterite and ferricrete (Aleva, 1994), and a chemical classification for defining lateritic materials (Schellmann, 1982, 1986) is adopted. 

Soil Survey Staff (1975) Soil Taxonomy. USDA Handbook 436. Washington, DC United States Department of Agriculture. 

Table 3 compares the most commonly used peat classification systems based primarily on degree of decomposition. The U.S. system (Soil Taxonomy, USDA, 1975) and the Canadian system (Day, 1968) are essentially similar except for terminology and are based on the amount of unrubbed or rubbed fiber content. 

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