Soil Taxonomy - Classifying Soils and the Soil Orders

In this part of the tutorial, we will introduce you to the soil orders, their predominance in Missouri, and general information distinguishing one order from another. We have chosen to introduce the soil order because it is the most broad and general category in Soil Taxonomy.

Soil scientists use a hierarchical classification system called Soil Taxonomy to differentiate soils based on a variety of properties exhibited within a soil. Nomenclature used in this classification system may seem cumbersome to the novice user, but it communicates a significant amount of information about any particular soil being studied. Within Soil Taxonomy, there are six levels of classification: (1) order (the most general level); (2) suborder; (3) great group; (4) subgroup; (5) family; and (6) soil series (the most specific level).

In all cases, names of soil orders end with the syllable "sol", which is derived from the Latin name solum or soil. The syllable(s) at the beginning of a soil order name convey information to a scientist about a general soil characteristic. Take the soil order Mollisol as an example. Mollisols are soils generally formed under prairie grasses. They contain high organic matter content making them soft in nature and mollis is the Latin word for soft. Thus, when someone mentions that a soil is a Mollisol, a soil scientist has an immediate picture in mind of the vegetation that is or was on this soil during formation (prairie grasses) and what the soil will generally look like and feel like (thick, dark surface horizon that is high in organic matter content and rather soft in nature).

The material presented below will guide you through the soil orders found in Missouri and those that are found elsewhere. It will also provide you some general information typically associated with each soil order.

Soils map of Missouri

Click the map legend to navigate to a soil type.

ultisol inceptisol alfisol mollisol entisol

Soil Orders Found In Missouri

Entisols

 (top)

Entisols are soils that are young or recently formed. They exhibit little soil development as evidenced by the lack of soil horizons found within the soil profile. Entisols are typically found in floodplains where alluvium is deposited during flooding events. They may also be found forming in areas where bedrock has more recently been exposed. Entisols found in floodplains can be highly productive for agriculture and close proximity to large rivers reduces costs associated with food and fiber transport. (Photo Courtesy of MCSS)

 

Inceptisols

 (top)

Inceptisols are more weathered than Entisols and exhibit greater development of soil horizons as noted by formation of a B horizon. Thus, they are often considered to be slightly older than Entisols and may form from Entisols with time and weathering. This soil order ranges from low to high in natural fertility. Inceptisols may be planted with crops, used as rangelands, or managed for timber. (Photo Courtesy of MCSS)

 

Alfisols

 (top)

Alfisols exhibit a greater degree of soil development than Inceptisols and Entisols as noted by evidence that silicate clay has moved from the surface downward into subsoil horizons through a process called illuviation. Subsoil horizons in Alfisols show some evidence of clay illuviation in the form of clay skins (coatings of clay on the face of soil structural units) or increases in percent clay content relative to soil horizons located immediately above the soil horizon where clay has accumulated. Alfisols are generally slightly acidic in nature but contain a higher proportion of exchangeable base cations (calcium - Ca, potassium - K, magnesium - Mg, and sodium - Na) than Ultisols. Alfisols are typically developed under deciduous forests, although the current vegetation may not be deciduous trees (e.g., the trees may have been cleared for row crop agriculture or pasture). Due to the high base status of these soils, Alfisols are fertile soils that can support good crop or tree growth although some liming may be required to increase soil pH to more optimal levels. (Photo Courtesy of MCSS)

 

Ultisols

 (top)

Ultisols are more highly weathered and older soils than Alfisols. They are also more acidic than Alfisols, more red or orange in color due to a higher concentration of iron (Fe) and aluminum (Al) oxide minerals, and they have a lower proportion of base cations (<35% of the soil exchange capacity is saturated with Ca, K, Mg, and Na) due to leaching of these elements out of the soil with time. Evidence of clay illuviation into subsoil horizons may be expressed more greatly in Ultisols than Alfisols. Forests are generally considered to be the primary vegetation growing on Ultisols during development. Large areas of Ultisols can be found in the Southeastern United States. Ultisols may be used to support forested ecosystems. These soils may also be used for row-crop agriculture or pasture. However, use of these soils for agronomic purposes requires liming and fertilizer addition due to the acidic nature of these soils and their low nutrient status. (Photo Courtesy of MCSS)

 

Mollisols

 (top)

Mollisols are formed under native prairie vegetation and in some poorly drained areas in forests where organic matter may accumulate. These soils are high in organic matter content and base cations, particularly Ca, which is attributed to the dense root structure produced by native grassland plants. Structural units of soil (called soil peds) are soft and easily crushed between the forefinger and thumb even when dry. Due to the high productivity of Mollisols, most of the native grasslands in the United States have been converted to agriculture. The use of Mollisols for agronomic purposes is so great that few native grasslands still exist in the U.S. Tucker Prairie east of Columbia, MO on I-70 is an example of untouched native grassland. Efforts are underway in Missouri and elsewhere to reestablish native grasslands, and the Missouri Department of Conservation's Prairie Fork Conservation Area is a fine example of prairie restoration efforts ongoing in Missouri. (Photo Courtesy of MCSS)

 

Vertisols

 (top)

Vertisols are clayey soils with exhibiting high shrink swell potential. Vertisols develop from calcium and magnesium rich parent materials such as limestone or basalt (Brady and Weil, 1999). Soils of this order are found in subhumid and semiarid climates where dry periods last several months. During dry periods, clays within the soil shrink resulting in the formation of deep, wide cracks. Water enters the cracks during precipitation events causing the clays to swell and the cracks to seal shut. Due to high shrink-well properties of Vertisols, construction of roads and buildings is extremely difficult and high maintenance may be needed to ensure the safety associated with these structures. Vertisols are rather fertile and the major land uses associated with this soil order are wetlands, crops and rangeland. (Photo Courtesy of MCSS)

 

Histosols

 (top)

Histosols are organic soils formed in wetland environments (swamps, marshes, and bogs) from vegetative materials that accumulate but decompose rather slowly. Some refer to locales where Histosols are found as peat lands or peat bogs. It should be noted, however, that not all wetland soils are Histosols, but nearly all Histosols occur in wetland ecosystems (Brady and Weil, 1999). Lack of oxygen (anoxic conditions) in saturated environments retards microbial decomposition of the vegetative materials derived from sedges, grasses, mosses, cattails, trees, etc. Subsequently, organic materials accumulate with time resulting in formation of Histosols that may have several feet of organic material overlying mineral soil or bedrock.

Formation of Histosols can occur at nearly all latitudes on Earth, provided that there is sufficient moisture for plant growth and the development of anoxic conditions (Brady and Weil, 1999). However, Histosols are more predominant in colder climatic regions due to reduction in microbial decomposition of organic materials with decreasing temperature. Additionally glaciation of a region enhances Histosol formation by disrupting or truncating drainage ways formed before an area was glaciated. This prevents some areas from draining rapidly or moderately and allows water to accumulate in low lying areas. Relevant examples of Histosols formed due to glacial activity are found around the Great Lakes Region of the U.S and in Canada. Glaciation is not a requirement for Histosol formation as evidenced by the presence of this soil order in Florida and Louisiana.

In many parts of the U.S. and the world, Histosols have been drained for a variety of reasons including agricultural production, mining of landscaping and potting material, and even for use as a fuel source. From an agricultural perspective, Histosols range from moderate to high in soil fertility but they must be drained and carefully managed to ensure continued productivity. The major land uses associated with these soils are wetlands and crop production. (Photo Courtesy of NRCS)

 

Soil Orders Found Outside Missouri

Andisols

 (top)

Andisols are soils formed from volcanic tephra (ash and cinders) or they have properties associated with volcanic ash. As one would expect, the geographic extent of Andisols on Earth is limited to regions having experienced geologically recent (within the past several thousand years) volcanic activity. Similar to Entisols and Inceptisols, Andisols are considered to be relatively young soils that may ultimately weather to other soil orders as the five soil forming factors act upon the soil.

After deposition of tephra ejected from the volcano, volcanic ash begins to weather rather rapidly to form poorly crystalline aluminum silicate minerals such as allophane and imogolite. High contents of poorly crystalline iron oxides are an additional mineralogical feature associated with soils weathered from tephra. The presence of significant quantities of the aforementioned minerals in addition to (1) the presence of volcanic glass and (2) high phosphate (PO43-) retention by the soil are, cumulatively, referred to as andic soil properties. Laboratory tests used to investigate the degree to which a soil expresses andic properties are used as a factor for classifying a soil as an Andisol (Soil Survey Staff, 2003).

Andisols also exhibit some other interesting soil properties. They are often high in organic matter content due to the formation of stable complexes between organic matter and the poorly crystalline minerals present in Andisols. Andisols have a low load bearing capacity, thus, proper engineering of structures built on these soils is critical. Andisols create several problems in the Pacific Northwest with timber harvest activities. For decades, stringent management practices have been enforced to minimize compaction of the soil.

Andisols usually have rather high natural soil fertility. However, fertility is dependent upon the types and chemical composition of ejected tephra (Buol et al., 1989). One of the major issues associated with the fertility of Andisols is the fixation or high retention of phosphate by the poorly crystalline soil minerals present in these soils. Proper soil nutrient management practices are required to ensure adequate plant growth and yield response for crops grown on Andisols. The major land uses for Andisols include forests, crops, and rangelands. (Photo Courtesy of NRCS)

 

Aridisols

 (top)

Aridisols are soils found in deserts or arid environments. A common misnomer associated with Aridisols is that they are devoid or almost devoid of vegetation. That is not true; vegetation present on Aridisols is usually scattered and plants are specifically adapted to living in these harsh environments (e.g., cacti, yucca, agave, mesquite trees, and creosote bush). Scarce vegetation does, however, result in low organic matter content in Aridisols. Temperature is not a requirement for Aridisol formation as evidenced by the formation of these soils in both hot and cool temperature regimes. For example, the Sonoran Desert in the Southwestern U.S. is desert that reaches very high air temperatures (>40 °C) and the Gobi Desert of Northern China and Southern Mongolia is a cold desert with temperatures reaching -30 °C.

Due to the lack of precipitation, soluble components in the soil are often not leached out of the soil profile. This results in an accumulation of these components in subsoil. This can result in high soluble or exchangeable salt content or the formation of minerals such as calcium carbonate or gypsum. It is also possible for carbonates to accumulate to such a degree that the soil becomes cemented. This soil horizon is commonly called caliche or a petrocalcic horizon by soil scientists. Aridisols may also show evidence of clay illuviation in the soil profile.

Aridisols can be very productive for agronomic purposes, provided that they are properly irrigated to prevent buildup of soluble salts in the root zone. Use of Aridisols for rangeland is very common in the Southwestern U.S., however, animal carrying capacity is very low. Thus, large land areas are required for grazing a relatively low number of animals. (Photo Courtesy of NRCS)

 

Gelisols

 (top)

Gelisols are soils containing permafrost (frozen soil) within 1 meter of depth from the soil surface for several consecutive years. Cold and freezing temperatures slow the rate of soil formation, thus these soils are generally considered young (Brady and Weil, 1999). Accumulation of organic matter in Gelisols may be quite substantial due reduced microbial activity caused by low soil temperatures.

Gelisols are know for stability management issues and patterned ground formations caused by freezing and thawing. Short growing seasons prevent substantial agricultural operations from occurring on these soils and most remain vegetated with native tundra species. (Photo Courtesy of NRCS)

 

Oxisols

 (top)

Oxisols are very old and highly weathered soils found predominantly in tropical climates. Due to extensive leaching of silicon (Si) and base cations (K+, Ca2+, Mg2+), these soils are enriched in aluminum and iron found in the form of metal oxides within the soil. The primary land use for Oxisols is forest and cropland. Oxisols contain very low natural fertility, thus they exhibit very efficient nutrient cycling (few nutrients are lost) under forests. Agricultural operations on this soil order often utilize slash and burn agriculture to release nutrients bound in plants back into the soil. (Photo Courtesy of NRCS)

 

Spodosols

 (top)

Spodosols are typically found cool humid or temperate climates and they typically develop in sandy textured soils under acidic coniferous vegetation. Spodosols are formed through a process called podzolization and this process makes this soil order one of the most visually interesting. The podzolization process involves the movement of organic acids, Fe, and Al downward through the soil profile to form a bleached or very light E horizon near the soil surface. Directly below the E horizon a B horizon enriched in organic matter, Fe and Al can be found. During podzolization, organic acids form chemical complexes with Fe and Al and move these elements to deeper depths during leaching events. Eventually, this results in the removal of coloring agents from the soil (E horizon) and translocation of Fe and Al to deeper depths (B horizon). The major land use for Spodosols is forest, although they can be used for cultivation of potatoes, blueberries, and other acidic-loving plant species. (Photo Courtesy of NRCS)