Soil Clasification
Soil Taxonomy System
The Soil Taxonomy system employs a specific nomenclature that both classifies the soil and confers a distinctive name to the individual soil. Names are constructed from various “formative elements” (generally originating from Greek or Latin), which are used in specific combinations to provide a highly descriptive name to a specific soil type. The Soil Taxonomy system is a hierarchical scheme consisting of 6 classification levels. In order from broadest to narrowest, the levels of classification are: (1) Order, (2) Suborder, (3) Great Group, (4) Subgroup, (5) Family, and (6) Series. The defining characteristics of the broadest levels of classification are based on soil-forming processes and parent materials, whereas the narrower levels become much more specific and consider the arrangement of horizons, colors, textures, etc.
Soil Orders
At the highest level of the classification system, twelve soil orders are recognized. The names of soil orders end with the formative element “sol” (from Latin solum, soil or ground). They are differentiated by the presence or absence of diagnostic horizons or features that reflect soil-forming processes. If the soils in a given taxon are thought to have had significantly different genesis, the intent has been to sort out the differences in the next lower category. To some degree, most of the soil orders occur in climatic regions that can be described by moisture and temperature regimes. The geographic distribution of the soil orders across the United States is depicted in Figure 3.1.
Alfisols
Alfisols are moderately leached soils that have relatively high native fertility (Figure 3.2). These soils have mainly formed under a forest and have a subsurface horizon in which clays have accumulated. Alfisols are primarily found in temperate humid and subhumid regions of the world.
Andisols
Andisols are usually formed on volcanic ash and cinders deposited in recent geological times (Figure 3.3). They are commonly found near the volcano source or in areas downwind from the volcano, where a sufficiently thick layer of ash has been deposited during eruptions. Andisols have not had time to become highly weathered. The principal soil-forming process has been the rapid weathering (transformation) of volcanic ash to produce amorphous or poorly crystallized silicate minerals.
Aridisols
Aridisols are commonly associated with semiarid and arid regions (Figure 3.4). These regions have a low mean annual rainfall. The lack of moisture in the soil affects the soil development and weathering process. Therefore, these soils are primarily affected by physical weathering, not chemical weathering (weathering processes discussed in Chapter 2).
Entisols
Entisols are soils of recent origin (Figure 3.5). Entisols are either young in years or their parent materials have not reacted to soil-forming factors. The central concept is soils developed in unconsolidated parent material with usually no horizons except an A horizon.
Gelisols
Gelisols are also “young” soils in regard to geologic time and were developed under cold temperatures or frozen conditions and contain permafrost within two yards of the surface (Figure 3.6). These soils are limited geographically to the high-latitude polar regions and localized areas at high mountain elevations. The principal defining feature of these soils is the presence of a permafrost layer. Permafrost is a layer of material that remains at temperatures below 32 degrees F (0°C) for more than 2 consecutive years.
Histosols
Histosols are dominantly composed of organic material in their upper portion (Figure 3.7). The Histosol order mainly contains soils commonly called bogs, moors, peat lands, muskegs, fens, or peats and mucks. These soils form when organic matter, such as leaves, mosses, or grasses, decomposes more slowly than it accumulates due to a decrease in microbial decay rates.
Inceptisols
Inceptisols are soils that exhibit minimal horizon development, lacking significant clay accumulation in the subsoil (Figure 3.8). They are more developed than Entisols, but still lack the features that are characteristic of other soil orders.
Mollisols
Mollisols are the soils of grassland ecosystems. They are characterized by a thick, dark surface horizon, are highly fertile, and are rich in chemical “bases” such as calcium and magnesium (Figure 3.9). The dark surface horizon comes from the yearly addition of organic matter to the soil from the roots of prairie plants and is often 150 to 200 inches in depth ( 60–80cm).
Oxisols
Oxisols are the most highly weathered soil order in the U.S. classification system (Figure 3.10). Oxisols are very highly weathered soils that are found primarily in tropical and subtropical regions of the world such as Hawaii, Puerto Rico, South America, and Africa. Their most important diagnostic feature is a deep oxic subsurface horizon. This horizon is generally very high in clay-size particles dominated by hydrous oxides of iron and aluminum.
Spodosols
Spodosols occur mostly on coarse-textured, acid parent materials subject to ready leaching (Figure3.11). They often have a dark surface underlain by an ashy gray layer, which is subsequently underlain by a reddish, rusty, coffee-colored, or black subsoil horizon. Spodosols formed under forest vegetation, especially under coniferous species whose needles are low in non-acid cations such as calcium and high in add resins. As this acid pine litter decomposes, strongly acid organic compounds are released and carried down into the permeable profile by percolating waters.
Ultisols
Ultisols are strongly leached, acid forest soils with relatively low native fertility (Figure 3.12). They are found primarily in humid temperate and tropical areas of the world, typically on old land surfaces, usually under forest vegetation, although savanna or even swamp vegetation is also common. Intense weathering of primary minerals has occurred, and much calcium, magnesium, and potassium has been leached from these soils.
Vertisols
Vertisols are soils that lack profile development due to the expansion and contraction of clay-rich soil (Figure 3.13). These processes cause the soil to mix, which does not allow for clear soil profile development. Vertisols form large cracks that may be more than three feet deep (1m) deep and several inches or centimeters, wide.
Suborder, Great Group, Subgroup, Family, and Series
Orders can be further broken down into suborders, great groups, subgroups, families, and series. Suborders of an order are differentiated by the presence or absence of properties associated with wetness, soil moisture regimes, major parent material, and vegetation. For example, the Ultisol order has 5 suborders: Aquults, Humults, Udults, Ustults, and Xerults. The ult comes from Ultisols. The aqu in Aquults refers to a nearly constant wet soil condition. The ud in Udults refers to a moist soil with generally moist conditions but some short dry periods. The ust in Ustults refers to an ustic moisture regime, or a moisture regime that is limited for much of the year but sufficient at times for plant growth.
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