In the United States soil taxonomy, soils have been divided by a hierarchical classification system into 10 ORDERS. The orders are based on soil properties as they appear in the field - degree of horizonization, presence and ordering of diagnostic horizons, etc. Although pedogenic factors play a critical role in deteriming the order into which a given soil falls. The actual classification is done on the basis of the appearance of the soil in the field.
Presented here is a collection of images and descriptions of examples of each of the soil orders. Each order has at least one example. Additionally, some of the order descriptions also include images of the landscape containing the soil. Along with each image is a brief description. The order in which the examples are presented and described is the same as that presented in the lecture. The presentation follows a sort of pedogenic sequence with the least developed soils described first and proceeds toward the most weathered soils. At the end, several orders which do not fit the pedogenic sequence logically are included. You may proceed through the orders in sequence, or you may "hop around," examining soil profiles and order descriptions on which you you wish to spend extra time.
Entisols are the pedologically youngest of the soils. Entisols are often formed from freshly deposited or heavily reworkd material. Horizonization is usually very weak, if present at all. Flood deposits (Fluvents) and sand dunes (Psamments) are good examples of entisols.
See some ENTISOLS
Inceptisols (inception, beginning)
Inceptisols represent the earliest horizon development. Small amounts of organic matter that darken the topmost horizon (sometimes forming an ochric epipedon [Ochrept]) may be all that characterizes the inceptisol. Weathering is minimal, and amounts of weatherable minerals in the profile are high.
See some INCEPTISOLS
Mollisols (oll - soft, refers to the high amount of organic material)
Mollisols typically form under grassland vegetation, especially in ustic or udic moisture regimes. These soils dominate the praries of the central U.S. The order is characterised by the presence of a mollic epipedon, a dark, organic rich layer at the surface. The mollic epipedon is usually thick, and is distinguished by a high (>35%) base saturation.
See some MOLLISOLS
Alfisols (nonsense syllable?)
Alfisols are intermediate in maturity between mollisols or spodosols and ultisols. Often, alfisols are found in co-ocurrence with mollisols. These soils are more highly weather than mollisols, and generally have less weatherable material remaining. The % Base Saturation is lower than a mollisol, and the soils are often acidic. Horizons are usually seen, but are not as distinct as in the less-weathered counterparts.
See some ALFISOLS
The Ultisols are the most highly weathered of the temperate zone soils. They are characterized by a thin or absent A horizon, with a thick, strongly expressed B. The soils are deep and productive if well-managed. The soils are often very red or yellow-red and are the dominant soil of the southeastern U.S.
See some ULTISOLS
The oxisols are the highly weathered soils of the tropics. The appear very much like the ultisols, but they have lost most of the weatherable materials. Silicates are usually present as quartz, and the dominant colloids are Fe,Al oxides and hydroxides. The latter material is called laterite and the soils have been classically known as lateritic soils.
See some OXISOLS
Spodosols (spodos - wood ash)
Spodosols are the product of a high degree of podsolization. These soils are typical of both coniferous and deciduous forests in cooler climates. (In warm humid areas, the leaching removal of soil materials is too rapid to allow strong profile development). The profile is characterized by a thin A overlying a well-developed E horizon, which is the most visible feature of the spodosol. The diagnostic spodic horizon is what defines the spodosol, however. The spodic horizon is a zone of accumulation that contains high levels of Fe (and often Al) sesquioxide. Formerly referred to as the B2ir,or B2hir when organic matter also accumulates, the newer designation is Bs and Bhs, respectively. In the pedogenic chronsequence, soils forming under forests would have a spodosol instead of a mollisol in the sequence, with the other orders being similar.
See some SPODOSOLS
Vertisols are soils which contain a high proportion of expanding lattice clays. As a result, these soils tend to swell when they are wet and shrink upon drying. When the soils shrink they often crack open. The cracks can be quite large and deep. Soil from the top of the profile can fall into these cracks, hence the concept of "invert" - top falling to bottom.
See some VERTISOLS
Aridisols (arid - dry)
Aridisols are soils that have developed in very dry conditions. They often show the effects of extreme wetting and drying with a great deal of water-related evidence near the surface, but little if any alteration in the subsoil.
See some ARIDISOLS
Histosols (histos - tissue)
Histosols are organic soils. They have > 20% organic matter to a depth of 1 ft or more. Usually, the soils have a much higher (often nearly 100%) organic matter content. The soils are categorized based on the degree of decomposition of the organic matter present. They generally form in either cool climates or very wet (waterlogged) areas (oten a combination of both). These soils are often associated with bogs or drained swamps.
See some HISTOSOLS
Andisols (and - refers to volcanic)
Andisols are dominated by short-range-order minerals. These soils comprise weakly weathered soils with a high content of volcanic glass, as well as more strongly weathered soils. The content of volcanic glass is a central characteristic used in defining andic soil properties.
Gelisols (The central concept of Gelisols is that of soils that have permafrost within 100 cm of the soil surface and/or have gelic materials (mineral or organic soil materials that have evidence of cryoturbation (frost churning) and/or ice segeration in the active layer (seasonal thaw layer) and/or the upper part of the permafrost) within 100 cm of the soil surface and have permafrost within 200 cm.
Go here for pictures of andisols and gelisols (as well as additional pictures and words on the other orders).