Chapter 3 Soil taxonomy and classification.
F. D. Hole, A Rainbow of Soil Words, 1985
In Chapter 2 you examined how the soil-forming factors interact to create soil, and how the manifestation of that activity in pedogenesis is apparent in the soil profile. Soil profiles develop distinct horizons depending on the processes of addition, loss, translocation, and transformation.
In this chapter you are going to examine the design and operation of a system to take the information evident in a soil profile, as well as other physical and chemical measurements, and use that information to classify soils into identifiable groups. Much of this chapter deals with terminology, and it may sound like a foreign language (in many cases it is). This is unfortunate, but the reward is that once you understand the terminology, and the basis behind why the terminology is used, you will have a much better grasp of the differences between soils, why they developed in different ways, and how that knowledge can be used for management.
After reading this chapter, you should be able to:
* Understand the basic terminology used in soil classification and taxonomy.
* Outline how soil classification schemes have developed in the United States.
* Interpret the approximate meaning of most soil taxonomic names.
* Identify the twelve soil orders of Soil Taxonomy and provide basic characteristics of each.
* Identify the basic properties of soils to the suborder level, and identify where they are likely to be found.
* Use a Munsell color chart to determine the diagnostic color of a soil.
WHAT IS CLASSIFICATION?
Classification organizes knowledge in meaningful ways.
Classification organizes knowledge in a meaningful way. On one hand it helps to characterize what's present, and on the other hand it helps to predict what to expect from similar examples. So, classifying soils is a way of cataloging the available soil resources, and at the same time helping land use managers (especially farmers) predict how a soil will look and behave in a given field when they only have specific knowledge about part of that field.
Classification systems are artificial; they are logical for a certain place and time.
Classification systems are completely artificial. There is no "natural" system of soil classification. Rather, there are classification systems that have a certain logic for their place and time. No classification system is better than any other; those that are valuable prove most adequate at meeting the purpose for their development. If there are multiple purposes, it makes sense to have multiple types of classification. The most useful classification system permits the largest number of the most important statements about a given class of objects--in this case different soils.
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BASIS OF CLASSIFICATION
The basis of soil classification is the natural body, which owes its properties to the five soil-forming factors of climate, organisms (vegetation), relief, parent material, and time. Thus, soil classification begins at the landscape, where all five of those features are manifested. Within each landscape are polypedons, collections of soil units that share the same properties. The smallest classifiable unit in each area is called the pedon and reflects a surface area of approximately one square meter (Figure 3-2).
The U.S. system of soil classification is outlined in the USDA publication Soil Taxonomy.
Using the pedon as the smallest classifiable natural body of soil, the U.S. system of soil classification outlined in the United States Department of Agriculture (USDA) publication Soil Taxonomy uses a hierarchical approach to classify the soil. There are six categories of classification in Soil Taxonomy ranging from very broad and encompassing to the field scale: (1) order, (2) suborder, (3) great group, (4) subgroup, (5) family, (6) series. A seventh layer of classification is often considered to be "type." There are 12 orders, 63 suborders, 319 great groups, 2484 subgroups, approximately 8000 families, and approximately 19,000 soil series at present. The relationship between the classification levels in Soil Taxonomy and another familiar classification scheme is given in Figure 3-3.
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One of the most important features in Soil Taxonomy, developed by C. E. Kellogg and Guy Smith, is that it attempts to classify the soils based on observable and quantifiable soil properties that can be viewed and sampled in an exposed pedon. For example, are certain horizons present or not? At what depth do they occur? How thick are they? What are properties of temperature and climate? Thus, the first step in classification in this system relies on interpreting some basic information from diagnostic surface and subsurface horizons.
Classification is like a visit to a doctor's office--diagnosis is based on observations and tests.
Classifying soils has been likened to a visit to the doctor's office. The doctor can make some characterizations based on obvious appearance (fat/thin, tall/short, pale/tan, black/white), symptoms (coughing, sneezing, bleeding, rash), and measurements (temperature, heartbeat, raspy lungs), but for a real diagnosis some additional clinical tests have to be made. This is analogous to the surface and subsurface diagnoses that occur in soil taxonomy, which are always followed up by more thorough laboratory analysis before a classification is made.
Diagnostic Surface Horizons-The Epipedon
The surface horizon of the soil is called the epipedon.
In Soil Taxonomy the entrance to classification is made by diagnosing the presence and depth of the surface horizon of soil or epipedon. There are seven diagnostic epipedons, which are outlined in Table 3-1. Each diagnostic horizon has a rigid definition to which it must be held, but as a general rule, most comparisons are made on the basis of its similarity or dissimilarity to the mollic epipedon.
The formal definition of a mollic epipedon is a thick, dark-colored upper horizon usually associated with grassland soils. It must contain at least 0.6 percent organic carbon and have at least 50 percent base saturation (at least 50 percent of the cation exchange capacity must be saturated by cations such as [Ca.sup.2+], [Mg.sup.2+], and [K.sup.+]). The minimum thickness ranges from 10 cm over bedrock through one-third of the solum to 25 cm in a deep soil. Mollic epipedons typically have granular structure and easy workability. Hence, the name derives from the Latin word mollus, meaning "soft."
The formal definition of an umbric epipedon is one that in most respects mirrors that of the mollic epipedon except that the base saturation is < 50 percent. The name comes from the Latin word umbra, meaning "shade," which refers to the dark color of the horizon. Umbric epipedons are found in environments where more weathering has occurred.
Ochric epipedons show more weathering than mollic epipedons.
The formal definition of an ochric epipedon describes a horizon that is too thin, too light in color, or has too low a carbon content to be a mollic epipedon. It reflects more soil weathering. The ochric epipedon is the most common type described. The name comes from the Greek word ochros, meaning "pale." Typical plowed fields with an ochric epipedon have a grayish or yellowish-brown appearance.
Histic epipedons show the accumulation of organic matter.
The histic epipedon reflects the accumulation of organic matter on the soil surface. When this develops into a layer 20 to 40 cm deep (8 to 16 inches) it is called a histic epipedon. Histic comes from the Greek word histose, meaning "tissue."
Melanic, Anthropic, and Plaggen Epipedons
Three other less common epipedons are the melanic, anthropic, and plaggen epipedons. The melanic epipedon is a dark upper horizon dominated by volcanic ash and allophane, which has high anion exchange capacity. Anthropic epipedons show the influence of human activities such as fertilization or irrigation. Plaggen epipedons are also artificial epipedons formed through long-term plowing and heavy manure application.
Diagnostic Subsurface Horizons
Diagnostic subsurface horizons focus on distinctive soil properties, some of which have to be measured.
There are eighteen common subsurface diagnostic horizons. Some of these are briefly described in Table 3-2. As before, more formal definitions for each exist in Soil Taxonomy. While the epipedons focus on such observable features as soil color, appearance, and fertility, the diagnostic subsurface horizons focus more on properties associated with translocation and transformation: weathering, accumulation of clays, presence of iron and aluminum, bleaching, salt accumulation, and so on. Other features that may be examined are the presence of impermeable layers of various types.
Among the most important subsurface diagnostic horizons to remember are the albic, cambic, argillic, spodic, and oxic horizons.
Albic Diagnostic Horizons
Albic horizons are eluvial horizons that form below the A horizon. They are at least 1 cm thick and at least 85 percent of this volume is filled by bleached materials. The bleaching is due to intense leaching, often because of water saturation. Albic comes from the Latin word albus, meaning "white."
Cambic Diagnostic Horizons
Cambic horizons indicate change within the horizon.
A cambic horizon formed by weathering of materials within the horizon rather than by gaining materials through illuviation. Cambic horizons can have bright colors, but are too weakly developed to be considered either argillic or spodic horizons. Cambic is from the Latin word cambriare, meaning "to change."
Argillic Diagnostic Horizons
Argillic horizons typically reflect the accumulation of translocated clay.
An argillic horizon is an illuvial horizon that is at least 10 percent as thick as the overlying A horizon and contains 3 to 8 percent more clay. There should also be the accumulation of clay films on soil peds, pores, and sand grains. Argillic comes from the Latin word argilla, meaning "white clay." One type of argillic horizon is a natric horizon, which contains sodium and causes soil sealing. Another type of argillic horizon is the kandic horizon, which is rich in kaolinite clays that do not retain nutrients well.
The spodic horizon represents an accumulation of illuviated organic matter and iron and aluminum oxides lying underneath a bleached horizon. The colors of the horizon can be very bright. The name comes from the Greek word spodos, meaning "wood ash."
Oxic horizons reflect extreme weathering.
The oxic diagnostic horizon is an impoverished subsoil layer so highly weathered that almost no minerals other than quartz, kaolinite, and metal oxides persist. Oxic horizons do not appear distinct from the other subsurface horizons in these soils. Most oxic horizons have very low cation exchange capacity.
FOCUS ON ...CLASSIFICATION AND DIAGNOSTIC HORIZONS
1. Are soil classification systems unique to modern agricultural societies?
2. Where was the first scientific soil classification system developed?
3. How did Soil Taxonomy become the basis of soil classification in the United States?
4. What is a diagnostic horizon?
5. How does a mollic epipedon differ from an umbric epipedon?
6. What does an argillic subsurface horizon refer to?
7. Which two surface epipedons reflect human influence?
The U.S. soil classification system has twelve soil orders.
Based on information collected from surface and subsurface diagnostic horizons, and some additional information on organic matter content, moisture content, clay type, and parent material identified in Figure 3-4, soils can be placed in one of twelve soil orders, which are the broadest classification in Soil Taxonomy (Table 3-3).
Details about the soil orders will be given later in the chapter. From the perspective of weathering and soil development, the soil orders can be arranged as in Table 3-4, which also gives their relative distribution.
Classification into Suborders
Suborders provide information about climatic regime.
To classify soils to the suborder level requires some additional information, mostly about climate. Other descriptive names associated with the nomenclature of suborders are given in Table 3-5.
Soil Moisture Regimes
Soil moisture regimes reflect water availability in the soil and, indirectly, how much water is likely to affect leaching.
Aquic soils are saturated with water for at least part of the year.
Udic soils are in humid climates that provide enough water to meet most plant growth needs. Extremely wet regions are perudic.
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Ustic climates are intermediate between udic and aridic environments; some periods of drought occur.
Aridic environments are arid and moist for less than ninety consecutive days. Torric soils are both hot and dry.
Xeric soils are found in Mediterranean-type climates that have hot dry summers and cool wet winters. Long periods of drought may occur in summer.
Soil Temperature Regimes
Torric and cryic refer to hot and cold temperature regimes, respectively.
Torric soils are both hot and dry. Cryic soils, in contrast, are extremely cold. These are the two most common temperature terms used to categorize suborders.
FOCUS ON ... SOIL ORDERS AND SUBORDERS
1. What is a simple device for remembering the soil orders?
2. Which soil order is most likely found only in the far north or at high elevation?
3. Which soil order is most prevalent in the United States?
4. Which soil order is least prevalent in the United States?
5. What are the most important factors used to designate the dominant suborders for each soil order?
NOMENCLATURE AND ORDERING IN SOIL TAXONOMY
Constructing soil taxonomic names is systematic
The sequence of constructing taxonomic names in Soil Taxonomy makes a considerable amount of sense once you work with it a little. The naming of the four highest categories (order, suborder, great group, subgroup) is systematic. Each addition to the name adds a little more information about the soil properties of the soil involved. For example, take Woolper silty clay loam, taxonomically described as a Typic Argiudoll. What does that mean?
Order--Mollisol (This is a Mollisol, it has a mollic epipedon and probably formed in grassland.)
Suborder--Udoll (This Mollisol formed in a udic or humid climate regime.)
Great Group--Argiudoll (This Mollisol also has an argillic horizon in the subsurface in addition to the mollic epipedon. Some weathering and translocation of clay particles has occurred.)
Subgroup--Typic Argiudoll (This is a typical soil for this great group.)
So, each new name in the taxonomy, or the formative syllable, tells you something more about the soil when you add it. The hardest thing is learning what all the formative names refer to. Examples of formative names to give to subgroups are given in Table 3-6. Examples of formative names to add for great groups are given in Table 3-7.
Soil names at the family level have adjectives describing important properties.
Even more information can be given at the family level, although the emphasis here, rather than on soil-forming factors, is on descriptive adjectives indicating properties of texture, mineralogy, and fertility, to name a few. A list of some of the common adjectives used is given in Table 3-8.
Soil series names come from a town or community close to where a soil was first described.
Ultimately a series name appears. There are approximately 19,000 soil series names. These series names are given to soils with very similar properties. Note that it is very similar properties that determine classification, not identical properties. Soil series names are derived from a town or community near where the soil was first officially described. Obviously, a similarly named series in an adjacent state will not be precisely alike, but it will be sufficiently close for management purposes. The Woolper series described earlier, for example, was established in Bath Co., Kentucky in 1960 and is distributed in Kentucky, Ohio, and Tennessee. The complete description of the Woolper soil is given in Figure 3-5, which gives you another look at how taxonomic names are constructed.
FOCUS ON ... NOMENCLATURE IN SOIL TAXONOMY
1. What is the order of classification in Soil Taxonomy?
2. What is the smallest level of classification in Soil Taxonomy?
3. Does "Cryoll" refer to a great group or suborder?
4. In what kind of environment did an aquent develop?
5. Soils in xeric climate regimes have what type of winters?
6. In the name "typic Hapludoll," which term refers to the great group?
THE SOIL ORDERS IN SOIL TAXONOMY
What are the characteristics and distribution of the soil orders classified in Soil Taxonomy? Each soil order has a specific suite of diagnostic surface and subsurface features that allow you to distinguish one from the other. However, our survey will just take an overview of the distinguishing characteristics of each soil type.
Alfisols are soils of deciduous forests.
Alfisols are soils that typically occur under deciduous forests. Although they usually have an ochric epipedon because the humus returned to soil by leaf litter is not thick, Alfisols can sometimes have an umbric epipedon. Alfisols are generally fertile, with base saturation greater than 35 percent (Figure 3-6). The subsurface is characterized by an argillic or natric horizon. The mean soil temperature in which Alfisols form is usually > 8[degrees]C (47[degrees]F). About 13.9 percent of the surface area in the United States consists of Alfisols (Table 3-4). Once cleared of forest they are usually considered to be fertile cropland.
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Dominant Suborders of Alfisols
There are five dominant suborders of Alfisols: Aqualfs, Cryalfs, Udalfs, Ustalfs, and Xeralfs (Table 3-5).
* Aqualfs form in warm, wet conditions (Figure 3-6). Most Aqualfs are believed to have originally been in forest, and drainage or water control are usually necessary to bring them into cultivation.
* Cryalfs are cold Alfisols that occur at high elevations (Figure 3-6). Because these regions have short growing seasons they usually remain in forest.
* Udalfs are more extensive than the other suborders of Alfisols (Figure 3-6). They have udic moisture regimes. All are believed to have supported forest growth prior to any cultivation.
* Ustalfs, as the name implies, have ustic moisture regimes and form in drier environments than Udalfs (Figure 3-6). They can have pronounced dry seasons. Ustalfs support savanna-type vegetation and grassland and most are used for either grazing or cropping.
* Xeralfs are the driest of the Alfisols. Most Xeralfs occur in California (Figure 3-6). The most common cultivated crops are small grains. The original vegetation of Xeralfs was a mixture of grasses, forbs, and woody shrubs, although coniferous forest grew on the cooler and moister Xeralfs.
Andisols are soils of volcanic regions.
Andisols are volcanic soils. They developed in volcanic ejecta of volcanoclastic materials. Andisols represent less than 2 percent of the surface land in the United States (Table 3-4). Andisols are dominated by rapidly weathering materials and volcanic glass, the proportion of which is a measure of how long weathering has proceeded. The layers of volcanic material can be thick. Andisols typically have a low bulk density, high carbon content, and a unique capacity to immobilize phosphorus. They are often very dark and can be extremely fertile (Figure 3-7).
Dominant Suborders of Andisols
The dominant suborders of Andisols are Aquands, Cryands, Torrands, Udands, Ustands, Vitrands, and Xerands (Table 3-5).
* Aquands occur in aquic conditions in lower elevations with forest or grass vegetation (Figure 3-7). Some Aquands are drained for agriculture.
* Cryands are found in cold climates in the mountains of the Pacific Northwest and in Alaska (Figure 3-7). Most formed beneath coniferous forest and are used as forest.
* Torrands are warmer and drier Andisols that formed under grassy or shrub vegetation (Figure 3-7).
* Udands, although they can have a high water content, hold it too tightly for plants to use, and so most of these soils remain in the original forest (Figure 3-7).
* Ustands are very similar to Udands in terms of water availability, although of much less extent. They formed under savanna or forest-type vegetation and are mostly found in Hawaii (Figure 3-7).
* Vitrands can have udic or ustic moisture regimes and are coarse-textured Andisols. They typically formed under coniferous forest in Oregon and Washington (Figure 3-7). Most Vitrands are still used as forest land but they can be cropped.
* Xerands have xeric moisture regimes and temperature regimes that range from frigid to thermic. Xerands with frigid and mesic temperature regimes typically formed beneath coniferous forest, and those with thermic temperature regimes formed under grass and shrub vegetation (Figure 3-7).
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Aridisols are desert soils.
Aridisols are desert soils and represent about 9 percent of the surface soils in the United States (Table 3-4). Some but not all Aridisols are salty, and most contain lime (Figure 3-8). Aridisols will have one or more of the following features within 100 cm of the soil surface: a calcic, cambic, gypsic, natric, petrocalcic, petrogypsic, or a salic horizon, or a duripan or an argillic horizon. In some Aridisols, for example, the lime has cemented in the subsoil to form a petrocalcic horizon. One common name for this phenomenon is caliche. In other cases the soil has cemented with silica to form a duripan.
Black alkali soils are Aridisols that have so much sodium that any organic matter has dispersed over the soil surface and colored it dark. White alkali soils lack the sodium and remain light, but they are also salty. Aridisols are highly productive if given water, but extremely fragile in their natural state. In their native state they are too dry for mesophytic vegetation to survive. Overgrazing of desert soils has been a major environmental problem.
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Dominant Suborders of Aridisols
Aridisols are extensive throughout the American southwest and throughout Nevada, where there is no period of plant-available water longer than three months (Figure 3-8). The dominant suborders are Argids, Calcids, Cambids, Cryids, Durids, Gypsids, and Salids (Table 3-5).
* Argids have an argillic or natric horizon, but not a duripan or a gypsic, petrocalcic, petrogypsic, or salic horizon.
* Calcids have calcic or petrocalcic horizons and have calcium carbonate in the layers above. The parent materials have a high carbonate content and the lack of precipitation prevents carbonates from leaching through the soil profile to any great extent.
* Cambids are Aridisols with the least degree of soil development.
* Cryids are cold desert soils such as those found in the soils at high elevations in mountain valleys and basins in Idaho.
* Durids are Aridisols with a duripan; they are typically found in Idaho and Nevada.
* Gypsids are Aridisols with a gypsic or petrogypsic horizon.
* Salids are salty Aridisols commonly found in depressions (playa). They are unsuitable for agriculture unless the salts are leached.
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Entisols are very young soils that are just developing.
Entisols are primitive, early soils with little or no evidence that pedogenic horizons have developed. This may be because the parent material is very resistant to weathering and soil formation is very slow, or because the newly developing soil is either eroded or buried by new deposits before distinctive horizons have a chance to develop.
Many Entisols have an ochric epipedon and a few have an anthropic epipedon. Many are sandy or very shallow. Entisols have no diagnostic horizons within one meter (39 in.) of the soil surface. They typically occur on steep slopes that are actively eroding and in floodplains or glacial outwash plains (Figure 3-9). About 12 percent of the surface land in the United States consists of Entisols (Table 3-4). Entisols are widely distributed throughout the United States because the processes that form them are also widely distributed (Figure 3-9).
Dominant Suborders of Entisols
The dominant suborders of Entisols are Aquents, Arents, Fluvents, Orthents, and Psamments (Table 3-5).
* Aquents are wet Entisols that form in coastal regions and floodplains from recent sediments. They support vegetation that tolerates temporary flooding. Cropping occurs, but most Aquents are used as pasture or wildlife habitat.
* Arents are anthropogenic soils. They do not have diagnostic features because they have been plowed, spaded, or unearthed by human activity. With irrigation they are valuable cropland, particularly in California.
* Fluvents are freely draining Entisols typically found in floodplains, alluvial fans, and deltas. They show evidence of stratification. Some can be used as cropland if protected from flooding by dikes and levees.
* Orthents reflect recent erosional surfaces.
* Psamments are sandy in all layers. They can be productive rangeland, but bare psamments are subject to significant wind erosion and drifting.
Gelisols are soils of cold regions.
Gelisols are frozen soils of the north (although these soils can also be found in the southern hemisphere and at high elevation). Although Gelisols make up 7.5 percent of the ice-free surface land in the United States and 8.6 percent globally, those areas are almost exclusively in Alaska, Canada, Scandinavia, and Siberia. Gelisols are characterized by permafrost (permanently frozen soil) within 100 cm of the soil surface (Figure 3-10). Gelisols will have evidence of cryoturbation (frost heaving and churning) in the soil above the permafrost. The permafrost acts as a barrier to soil development by preventing the downward movement of solutes.
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Dominant Suborders of Gelisols
The dominant suborders of Gelisols are Histels, Orthels, and Turbels (Table 3-5).
* Histels are Gelisols with large amounts of organic carbon. They are found in Alaska. The organic carbon comes from the tundra vegetation in this region, which consists of mosses, sedges, and shrubs. The short growing season precludes agricultural activity, so these soils mostly sustain wildlife.
* Orthels show little or no evidence of cryoturbation. In addition to mosses, sedges, and shrubs they also contain some black and white spruce.
* Turbels show cryoturbation, which appears as irregular, broken, or distorted horizon boundaries, ice wedges, or oriented rock fragments on top of the permafrost. Turbels occur on slopes that receive more sunlight or areas where fire and land clearing have altered the thermal properties of the soil so that the permafrost can thaw periodically.
Histosols are dominantly organic.
Histosols are soils that are dominantly organic. They make up a small (1.3 percent) but important part of the soil resources in the United States from the north to the Everglades and in some mountain regions (Figure 3-11). They are commonly called bogs, moors, peats, or mucks. The organic matter accumulates because these soils are too wet or cold to allow organic matter decomposition to keep up with organic matter deposition. A soil is classified as a Histosol if it has more than 25 percent organic matter and does not have permafrost (Figure 3-11). A general rule is that if half or more of the upper 80 cm of soil is organic, the soil is classified as a Histosol.
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Many Histosols form in old glacial lakes that have filled with dead plants. Although Histosols have great agricultural potential, drainage causes some unique problems. When Histosols are drained and aerated, decomposition can accelerate, and the soil can drop (subside) as much as 30 cm (about a foot) in ten years.
Dominant Suborders of Histosols
The dominant suborders of Histosols reflect the extent of organic matter decomposition that has occurred within them and consist of Fibrists, Folists, Hemists, and Saprists (Table 3-5).
* Fibrists are wet, slightly decomposed Histosols often referred to as peat. Most are found in southern Alaska. There is a long tradition of harvesting peat for fuel in countries such as Ireland.
* Folists are freely drained Histosols with horizons consisting of leaf litter, twigs, and branches resting on bedrock or rock fragments. They mostly occur in Alaska and Hawaii. The high organic matter content prevents them from being classified as Entisols.
* Hemists are wet Histosols in which the organic matter is slightly decomposed.
* Saprists are wet Histosols in which the organic matter is highly decomposed. These are also called muck soils and may appear very black.
Inceptisols are immature, developing soils.
Inceptisols are immature soils of humid and subhumid regions that have altered horizons with evidence of chemical and physical change, but still have some weatherable material and lack a clear illuviated zone (Figure 3-12). Most Inceptisols have an ochric A horizon and a cambic B horizon. They can have other diagnostic horizons, but because they are early in development they lack argillic, natric, kandic, spodic, or oxic horizons that would be indicative of weathering and solute movement.
Inceptisols are widely distributed across the United States (Figure 3-12) and make up about 10 percent of the surface soils. They are found in all climate zones except the desert. Common locations for Inceptisols in a landscape are depressions.
Dominant Suborders of Inceptisols
The dominant suborders of Inceptisols are Anthrepts, Aquepts, Cryepts, Udepts, Ustepts, and Xerepts (Table 3-5).
* Anthrepts are freely drained Inceptisols that have an anthropic or plaggen epipedon. They are not known to occur in the United States.
* Aquepts are wet Inceptisols with poor drainage and groundwater at or near the soil surface at least some parts of the year. They can have any type of vegetation. Water is a limitation for agricultural use.
* Cryepts are cold Inceptisols occurring in mountains or high latitudes. The native vegetation is often conifers or mixed conifers and hardwood trees.
* Udepts are freely draining Inceptisols with udic moisture regimes. Most are used as forest but many have been cleared for cropland.
* Ustepts have ustic moisture regimes. They are drier Inceptisols and are common in the Great Plains (Figure 3-12). The native vegetation was originally mostly grass.
* Xerepts have xeric moisture regimes and are found in the western United States. The temperature regime can range from frigid to thermic.
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Mollisols are grassland soils and among the most productive soils in the world.
Mollisols are grassland soils extensively distributed throughout the United States, but primarily west of the Mississippi River (Figure 3-13). The dominant feature of Mollisol is a deep, dark-colored A horizon that is base rich and extremely fertile. Other diagnostic horizons are argillic, natric, cambic, or calcic horizons. Mollisols are among the most productive agricultural soils in the world. Small grains are grown in drier regions and maize (corn) and soybeans in the warmer, humid regions. Mollisols were among the first soils scientifically described in Russia, where they were called Chernozems (black earth).
Dominant Suborders of Mollisols
In the United States the dominant suborders of Mollisols appear as sequential bands across the country reflecting the moisture regimes in which each Mollisol developed: Albolls, Aquolls, Cryolls, Rendolls, Udolls, Ustolls, and Xerolls (Table 3-5; Figure 3-13).
* Albolls have an albic horizon and fluctuating groundwater table. They generally occur on gentle slopes.
* Aquolls are wet Mollisols, most of which have been drained for agricultural activity.
* Cryolls are cool or cold Mollisols, generally freely draining, that occur in high elevations or Alaska. Their agricultural use is limited by short growing seasons.
* Rendolls are Mollisols that formed in humid regions in highly calcareous parent materials such as limestone, chalk, and drift in places such as Florida, tropical islands, and a few mountainous regions in the western United States.
* Udolls are freely draining Mollisols in udic (humid) moisture regimes in the eastern Great Plains. The native vegetation was originally tall grass prairie.
* Ustolls are Mollisols formed in subhumid to semiarid climates in the western Great Plains. The original vegetation was grass, but most of these soils have been converted to cropland. Rainfall can be periodic, and drought severe, and in the absence of permanent soil cover wind erosion can be significant.
* Xerolls are Mollisols that form in an environment with a Mediterranean climate (hot, dry summers; cool, wet winters). The original vegetation was bunch grasses, shrubs, and trees in environments with mesic or frigid temperatures and savanna with perennial oak and fir in areas with thermic temperature regimes. Many Xerolls are irrigated, and many are used as rangeland.
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Oxisols are highly weathered soils.
Oxisols are highly weathered soils of great age and are virtually absent from the continental United States (Figure 3-14). They form in tropical and subtropical environments. Oxisols are characterized by a high content of quartz, inert clays, and oxides of iron and aluminum that give these soils their distinctive red color (Figure 3-14). The only type of layered clay remaining in these soils is kaolinite. Horizon designations are usually arbitrary.
Although the structure of Oxisols is granular and good for root growth, leaching is severe and nutrient retention is low. Traditional global management for Oxisols has been slash-and-burn, wherein after a short period of cultivation trees and shrubs are allowed to return to recycle nutrients from lower profiles and form at least some organic residue near the soil surface that can be exploited after the overlying vegetation is burned. More intensive agricultural use requires substantial inputs.
Dominant Suborders of Oxisols
The dominant suborders of Oxisols are Aquox, Torrox, Udox, and Ustox (Table 3-5).
* Aquox are wet Oxisols occurring only in Puerto Rico and Hawaii.
* Torrox are Oxisols of arid regions.
* Udox are well-drained Oxisols with a udic moisture regime. They have a year-round growing season and generally sufficient precipitation to support nonirrigated agriculture year-round.
* Ustox are Oxisols that have a ustic moisture regime and can support one rain-fed crop. Otherwise there is a period of at least ninety days in most years when rainfall is inadequate to support crop growth.
[FIGURE 3-15 OMITTED]
Spodosols are soils of acid coniferous regions.
Spodosols have very discrete distribution within the United States (Figure 3-15) and make up about 3.3 percent of the land area (Table 3-4). Spodosols are soils of acid sandy pine land occurring in either the north, south, or far northwest. The original Russian term for these soils was Podzol, meaning "ash beneath," because they have a distinctive, nearly white, bleached horizon (Figure 3-15). The spodic horizon, immediately beneath this bleached zone, consists of accumulated organic matter and amorphous aluminum and/or iron. Many Spodosols originally harvested for their abundant timber proved to be poor agricultural land and have since been returned to timber production as their major use.
Dominant Suborders of Spodosols
The dominant suborders of Spodosols are Aquods, Cryods, Humods, and Orthods (Table 3-5).
* Aquods are wet Spodosols characterized by a shallow, fluctuating water table, particularly in Florida and along the Atlantic coast. They support water-loving plants. Although the native fertility is low, they can be managed successfully for agriculture.
* Cryods are cold Spodosols at high elevations or high latitudes. They consist primarily of coniferous forest.
* Humods are freely drained Spodosols that have accumulated significant organic matter in the spodic horizon. They mostly occur in the Pacific northwest (Figure 3-15).
* Orthods are Spodosols that are freely draining and have accumulated a modest amount of organic matter in the spodic horizon. They are found extensively in the northeast and Great Lakes states (Minnesota, Michigan, Wisconsin). These regions were extensively harvested for timber.
Ultisols are soils of warm, humid regions.
Ultisols are low-base-status forest soils of warm, humid regions. Base (e.g., calcium, magnesium, potassium) cycling occurs in the forest cover but is not as great as in Alfisols because the parent material usually has less limestone and the weathering and leaching has been more severe. Ultisols are usually older than Alfisols by tens of thousands of years. Because of the iron and aluminum content, which is unmasked by lower organic matter levels, Ultisols tend to be colorful soils ranging from red and yellow to gray, depending on drainage. Native fertility is limited, but the growing season can be long. The history of Ultisols in the southern United States is one of lost fertility and erosion. But current agricultural practices have made many of these soils extremely productive.
Ultisols are extensive in the United States (Figure 3-16), primarily in the southeast (about 10 percent of the surface area, Table 3-4). Ultisols have B horizons that contain considerable amounts of translocated clay (argillic or kandic horizons), base saturation < 35 percent, and an ochric epipedon (Figure 3-16). The base saturation decreases with depth. The original vegetation was mixed conifers and hardwoods.
Dominant Suborders of Ultisols
The dominant suborders of Ultisols are Aquults, Humults, Udults, Ustults, and Xerults (Table 3-5).
* Aquults are Ultisols forming in wet areas in which groundwater is close to the surface during part of the year (usually winter and spring). Most Aquults are on coastal plains with gentle slopes.
* Humults are freely drained, humus-rich Ultisols. They receive considerable rainfall, but can be periodically dry.
* Udults form in humid environments. They are freely drained but humus-poor.
* Ustults form in ustic moisture regimes. They have low organic matter content.
* Xerults form in Mediterranean climates with xeric moisture regimes. The vegetation is typically coniferous forest.
Vertisols are shrinking and swelling soils.
Vertisols are rich in clay. They make up about 2 percent of the land surface in the United States with spotty distribution (Figure 3-17, Table 3-4). Vertisols are described as cracking, dark clay soils; they shrink when dry and swell when wet. They occur in warm temperate and tropical environments, but do not appear leached because they tend to have a lime content because shrinking and swelling of the indigenous clays churns up fresh limey material from the C horizon to replenish that which was lost or leached (Figure 3-17).
[FIGURE 3-16 OMITTED]
The shrinking and swelling action causes Vertisols to seal and shed water in wet seasons. In summer the shrinking soils form lens-shaped blocks. These blocks can slide past one another causing polished surfaces called slickensides. The shrinking and swelling also causes buckling, which shapes the landscape into mounds and hollows called gilgai. Although these soil properties do not prevent agricultural use, as long as adequate precipitation and irrigation exists to maintain relatively constant moisture content, they pose some significant engineering problems.
Dominant Suborders of Vertisols
The dominant suborders of Vertisols are Aquerts, Cryerts, Torrerts, Uderts, Usterts, and Xererts (Table 3-5).
* Aquerts are wet Vertisols, although cracks can open during some parts of the year. Drainage on these soils is a problem because the saturated hydraulic conductivity is low.
* Cryerts do not typically occur in the United States. They exist in cold environments where there is just enough of a summer thaw to allow cracking to occur.
* Torrerts are Vertisols of arid environments. The cracks formed stay open for most of the year, but periodically close in winter. Saturated hydraulic conductivity is low despite the soil cracking, and this can lead to salt accumulation at the soil surface.
* Uderts form in udic moisture regimes. Soil cracks open and close in response to precipitation. Subsurface irrigation is usually used to convert these soils to agricultural use.
* Usterts pose many of the same problems as Uderts, and for the same reasons, although the opening and closing of cracks is more limited because of the greater periodicity of rainfall they experience.
* Xererts form in Mediterranean climates that have cool wet winters and warm dry summers. Consequently, these soils have cracks that regularly open and close, which can damage overlying structures such as roads. Their main use is for rangeland.
[FIGURE 3-7 OMITTED]
FOCUS ON ... SOIL ORDERS
1. Why aren't there any Andisols in the eastern United States?
2. What other parts of the world would you predict would have Andisols?
3. What soil orders often have duripans?
4. There is a curious gap between two sections of Ultisols in Figure 3-17. Can you explain it?
5. Why does the suborder classification of Mollisols continuously change as you go from east to west?
6. Why is designating horizon boundaries in Oxisols mostly arbitrary?
OTHER SOIL CLASSIFICATION SYSTEMS
There are, of course, many classification systems throughout the world. Canada, for example, has a unique system for classifying its soils. The FAO (Food and Agriculture Organization) has developed a worldwide soil classification system. The FAO, for example, has twenty-six distinct soil orders, which are roughly described in Table 3-9. Although some of the names overlap with similar designations in Soil Taxonomy, many do not, and instead reflect the much greater diversity of soils than can be found outside of the United States when global-soil forming environments are considered.
This chapter started with a poem about soil color, and so the last section appropriately ends with that topic. Soil color is a manifestation of physical and chemical properties. The presence of brown and black is indicative of organic matter content. The presence of reds and yellows is indicative of oxidized iron. The presence of blues, grays, and greens is indicative of reducing environments. The absence of color or presence of albic (white) layers is indicative of leaching.
Soil color is one of soil's most diagnostic features.
Soil color, however, also plays a diagnostic role, which you can hardly doubt if you read a list of some of the names in the 1938 classification system that preceded Soil Taxonomy: brown soils, chestnut soils, gray-brown podzolics, red desert soils, reddish brown soils, and red-yellow podzolics, for example. The Russian terms from Dokuchaiev's original classification often refer to color: Sierozem (gray soil), Brunizem (brown soil), Chernozem (black soil). Soil colors are among the most obvious and consistent properties related to climatic and vegetation factors affecting soil.
Remember that Soil Taxonomy aims to quantify soil properties to assist in classification. How do you quantify soil color so that it can be used as a diagnostic tool? Soil scientists use three quantifiable soil properties--hue, value, and chroma--and a series of standardized color chips to assist them in this process (Figure 3-18).
Hue reflects the dominant spectral (rainbow) color in terms of the five cardinal colors (blue, green, yellow, red, and purple). In most
soils in the United States the hue is yellow-red (YR), which reflects the dominance of iron oxides in determining soil color. The range for each hue goes from 0 to 10. Within the YR hue, for example, the hue becomes more yellow and less red as the numbers increase.
[FIGURE 3-18 OMITTED]
Hue, value, and chroma are used to quantify soil color.
Value is a measure of the lightness or darkness of a material and ranges from 0 (black) to 10 (white).
Chroma reflects the strength or dullness of a color. A pure, brilliant color would have a chroma of about 20. The chroma in most soils is < 8. Actual soil colors can be made by mixing pure colors with neutral gray colors, which have a chroma of 0.
How to Determine Soil Color
The Munsell soil color chart is used to help describe soil color.
The Munsell (2000) nomenclature for soil color combines hue, value, and chroma in a standard set of symbols such as 10YR 5/4 (Hue Value/Chroma). The Munsell soil color chart contains 322 chips that reflect the typical colors found in soil (Figure 3-19). A moist soil sample is prepared and compared to the hue page that most closely matches the color. Then the soil is moved up and down (to determine value) and left to right (to determine chroma) until as close a match as possible is made. The usual practice is to give the color name and the appropriate Munsell designation (Troeh and Thompson, 1993). Gleyed soils, soils that have been saturated, and in which the iron is reduced, have a grayish-green color, and their color is described by a second set of color charts.
FOCUS ON ... SOIL COLOR
1. What does hue refer to?
2. What can soil color tell us about soil chemical and physical properties?
3. If a soil color is designated as 5YR 4/3, what does the 4/3 refer to?
4. How does chroma affect soil color?
5. What is the dominant hue of most soils in the United States?
[FIGURE 3-19 OMITTED]
In this chapter we learned the rationale for soil classification systems and reviewed how the soil classification system evolved in the United States. The pedon is the smallest classifiable unit of soil that is recognized. The guidelines in Soil Taxonomy, which was adopted in 1965 as the standard method for classifying soils in the United States, uses a diagnostic and quantitative approach to classifying soils. The first step in classifying soils in this system is to identify the diagnostic epipedon (surface horizon) and diagnostic subsurface horizons. Thereafter, the soil can be placed into one of twelve soil orders. The soils can be further classified to suborder, great group, subgroup, family, and soil series by progressively adding more information to the soil description, such as climatic regime, distinctive horizons, texture, and mineralogy. The addition of distinctive syllables to the soil order gives each soil its unique taxonomic name. Soil series are typically named after a nearby community but can be located in many other areas. There are over 19,000 described soil series.
Although soil color reflects physical and chemical properties, it has been made quantifiable so that it can be used for diagnostic purposes in classification. Soil color is one of the first differences you notice between representative soil profiles of each of the soil orders.
In the next chapter we enter the soil profile and learn how the soil is actually put together.
END OF CHAPTER QUESTIONS
1. What is a unit of natural soil called in classification?
2. What was V. V. Dokuchaiev's great contribution to soil science?
3. What is the basis for soil classification in Soil Taxonomy and how does it differ from earlier classification systems?
4. Why is the Mollic epipedon a standard against which other epipedons are measured?
5. What are the levels of classification in Soil Taxonomy?
6. Is there a "natural" system of soil classification? Why or why not?
7. Which two soil orders most likely developed under forest vegetation?
8. What most distinguishes Entisols from Inceptisols?
9. What are the most weathered soil orders you are likely to find in the United States? What color will they have? Why?
10. Are duripans and fragipans desirable soil characteristics?
11. Why do Vertisols represent unique engineering problems?
12. What conditions cause Entisols to form?
13. When keying out soil orders, which are the easiest to identify?
14. Where are you most likely to find a spodic horizon? What chemical would be found in it?
15. What is base saturation, and does a low or high base saturation indicate high fertility?
16. Why have many Ultisols suffered from severe erosion?
17. What is one problem you might face in draining a Histosol and building a house on it?
18. A soil that has a hue of 2.5YR will appear to be what color?
19. What are the characteristics of a fine-loamy, mixed-mesic, typic Haludoll?
20. What is the taxonomic name of your state soil?
The ideal gateway into soil taxonomy and classification is through the USDA-NRCS Web site at http://www.nrcs.usda.gov. There you will be able to open an electronic version of Soil Taxonomy as well as explore the properties of the over 19,000 soil series currently classified in the United States. There are also transcribed interviews with Guy Smith detailing the history and philosophy of how soil taxonomy and classification evolved in the United States from 1938 to the present.
Brady, N. C., and R. R. Weil. 2002. The nature and property of soils, 13th ed. Upper Saddle River, NJ: Prentice Hall.
Callahan, G. N. 2003. Eating dirt. Emerging and Infectious Diseases 9: 1016-1021.
Harpstead, M. I., T. J. Sauer, and W. F. Bennett. 1997. Soil science simplified, 3rd edition. Ames: Iowa State University Press.
Munsell (r)Color. 2000. Munsell soil color charts. New Windsor, NY: Gretag Macbeth.
Sandor, J. A., and L. Furbee. 1996. Indigenous knowledge and classification of soils in the Andes of southern Peru. Soil Science Society of America Journal 60: 1502-1512.
Troeh, F. R., and L. M. Thompson. 1993. Soils and soil fertility, 5th ed. New York: Oxford University Press, New York.
INDIGENOUS SOIL CLASSIFICATION SYSTEMS--LARI AND COLCA VALLEYS, SOUTHERN PERU
Soil classification systems are not unique to industrial agricultural societies. Many if not all indigenous agricultural cultures have some sort of system that allows their members to assess either the quality or value of land.
The residents of Lari and Colca Valleys in southern Peru have inherited an agricultural tradition at least 1500 years old. The farmers in these communities recognize different types of soil and different types of soil materials, including those that are beneficial after consumption (some of the minerals in these soils may adsorb glycoalkaloid phytotoxins in the potatoes that are an important food in the region; Callahan, 2003), those that are poorly drained, those that are excessively drained, those that are infertile, those that have different horizons, and those that have impeding layers.
Classification by these farmers reflects their agricultural needs. So in this case, the most important characteristics of soil are texture, drainage, workability, fertility, and location. Soil color plays an important role in distinguishing differences in soil. An example of one farmer's classification system is given in Figure 3-1. In this example, the farmer recognizes four categorical levels of soil, including several types and colors of clay. At the second categorical level the five classes of agricultural soil are arranged from left to right in terms of decreasing productivity. At the far right are soils that are nonagricultural or serve some other purpose besides agriculture.
GUY SMITH AND THE DEVELOPMENT OF SOIL TAXONOMY
The first scientific system of soil classification is credited to the Russian V. V. Dokuchaiev. His work was published in 1879 and introduced to the United States via translation in 1927 by C. F. Marbut, who partially adapted Dokuchaiev's "zonal" concept of soil development and created several "great groups" reflecting soil-forming conditions in the United States. Marbut's classification system was modified by C. E. Kellogg and others for the 1938 Yearbook of Agriculture at the bequest of Secretary of Agriculture Henry Wallace. Because less than a year was given to develop a classification system, they borrowed heavily from the original Russian system. The 1938 soil classification system lacked real definitions of any of the great groups and contained mostly broad definitions. In addition, the scientists involved realized that the system was flawed because they were unable to find any single soil property that included all the soils collected into the largest groups.
In response to these limitations, Guy Smith (1902-1981) of the USDA spent his career trying to devise a manageable soil classification system that could be based on specific quantifiable soil properties rather than observational notes about soil color or potential genesis. The first Soil Taxonomy was published in 1960 by the Soil Survey Staff and officially adopted by the Soil Conservation Service in 1965. The seventh approximation (and the one officially published as Soil Taxonomy) appeared in 1975. The size of Soil Taxonomy has grown tremendously since its inception. There are now approximately 19,000 different soil series classified by Soil Taxonomy. The most recent soil order added is the Gelisol.
TABLE 3-1 Diagnostic surface horizons (epipedons). Diagnostic Horizon Major Features Anthropic Human-modified, mollic-like horizon Histic Very high in organic matter, periodically wet Melanic Thick, black, 6% organic C, common in volcanic soils Mollic Thick, dark-colored, well structured, high base saturation Ochric Too light-colored, too little organic matter, or too thin to be mollic Plaggen Human-made sodlike horizon created by years of manuring Umbric Same as mollic except for low base saturation TABLE 3-2 Diagnostic subsurface horizons and features. Diagnostic Horizon (typical location and designation) Major Features Albic (E) Light-colored, clay and Fe and Al oxides mostly removed Agric (A or B) Organic matter and clay accumulation just below the plow layer Argillic (Bt) Zone of silicate clay accumulation Calcic (Bk) Accumulation of CaC[O.sub.3] and MgC[O.sub.3] Cambric (Bw, Bg) Nonilluvial physical or chemical change Duripan (Bqm) Hardpan cemented by silica Fragipan (Bx) Dense, brittle, loamy-textured pan Gypsic (By) Accumulation of gypsum Natric (Btn) Argillic horizon high in sodium Oxic (Bo) Highly weathered horizon with a mixture of Fe and Al oxides and silicate clays Petrocalcic (Ckm) Cemented calcic horizon Petrogypsic (Cym) Cemented gypsic horizon Salic (Bz) Spodic (Bh, Bs) Organic matter and Al and Fe oxide accumulation Sulfuric (Cj) Highly acidic with Jarosite mottles TABLE 3-3 Names of the soil orders in Soil Taxonomy and their major characteristics. Order Major Characteristics Inceptisol Immature soils with some development Few diagnostic features, ochric or umbic epipedon, cambic horizon Alfisol Fertile soils of hardwood forests Medium to high base saturation, some argillic or natric horizons Mollisol Fertile grassland soils Dark soils with mollic epipedons and high base saturation, some argillic or natric horizons Andisol Volcanic soils Formed from volcanic ejecta, high in allophane or Al-humic materials Spodosol Acid soils of coniferous pine forests Spodic horizons with iron and aluminum oxides and humus accumulation Ultisol Low fertility forest soils of warm humid regions Argillic horizons present, low base saturation Aridisol Desert soils Ochric epipedons, sometimes argillic or natric horizons Vertisol Cracking, dark clay soils High in swelling clays Entisol Primitive (young) developing soils Little profile development, ochric epipedon common Histosol Organic soils Peat or bog soils, > 20% organic matter Oxisol Highly weathered tropical soils Oxic epipedons, no argillic horizons Gelisol Frozen soils Permafrost and frost churning visible A useful mnemonic device to remember the names of the 12 soil orders is to take the first letter of each order, as represented here, I AM A SUAVE HOG. TABLE 3-4 Global and U.S. distribution of major soil types classified by Soil Taxonomy. (Adapted from Brady and Weil, 2002) Order Environment Least Weathered Entisols Recently deposited Least Weathered Inceptisols Various conditions Least Weathered Andisols Mildly weathered ejecta Least Weathered Histosols Wet, organic Least Weathered Gelisols Very cold Least Weathered Aridisols Dry, desert shrubs, grass, alkaline Least Weathered Vertisols Wet/dry seasons Least Weathered Alfisols Moist, mildly acid Least Weathered Mollisols Semiarid to moist Least Weathered Ultisols Wet tropical and subtropical Least Weathered Spodosols Cool, acid coniferous Most Weathered Oxisols Hot, wet tropical Regolith/Sand % of Ice-Free Land Order Global (a) Least Weathered Entisols 16.3 Least Weathered Inceptisols 9.9 Least Weathered Andisols 0.7 Least Weathered Histosols 1.2 Least Weathered Gelisols 8.6 Least Weathered Aridisols 12.1 Least Weathered Vertisols 2.4 Least Weathered Alfisols 9.6 Least Weathered Mollisols 6.9 Least Weathered Ultisols 8.5 Least Weathered Spodosols 2.6 Most Weathered Oxisols 7.6 Regolith/Sand 14.1 Order United States (b) Least Weathered Entisols 12.2 Least Weathered Inceptisols 9.1 Least Weathered Andisols 1.7 Least Weathered Histosols 1.3 Least Weathered Gelisols 7.5 Least Weathered Aridisols 8.8 Least Weathered Vertisols 1.7 Least Weathered Alfisols 13.9 Least Weathered Mollisols 22.4 Least Weathered Ultisols 9.6 Least Weathered Spodosols 3.3 Most Weathered Oxisols 0.1 Regolith/Sand 7.8 (a) Data from FAO world database. (b) Data from USDA/NRCS Soil Survey Division. The advantages of the United States in terms of its soil resources are abundantly clear in this table; almost 40 percent of the ice-free land is classified in soil types that have inherently moderate to high fertility. TABLE 3-5 Order and suborder names in Soil Taxonomy. Characteristic Environment, Order Suborder Climate, or Feature Alfisols Aqualfs Wet Cryalfs Cold climates Udalfs Humid climates Ustalfs Semiarid climates Xeralfs Mediterranean climates Andisols Aquands Wet Cryands Cold climates Torrands Hot and dry climates Udands Humid climates Ustands Semiarid climates Vitrands Volcanic glass Xerands Mediterranean climates Aridisols Argids Clay accumulation Calcids Carbonate accumulation Cambids Typical Cryids Cold climates Durids Duripans present Gypsids Gypsum accumulation Salids Salty Entisols Aquents Wet Arents Mixed horizons Fluvents Alluvial deposits Orthents Typical Psamments Sandy Gelisols Histels High organic content Orthels Typical Turbels Cryoturbation Histosols Fibrists Partially decomposed (peats) Folists Undecomposed leaf and twig litter mats Hemists Moderately decomposed Saprists Highly decomposed (mucks) Inceptisols Anthrepts Human-influenced Aquepts Wet Cryepts Cold climates Udepts Humid climates Ustepts Semiarid climates Xerepts Mediterranean climates Mollisols Albolls Albic horizons Aquolls Wet Cryolls Cold climates Rendolls Calcareous Udolls Humid climates Ustolls Semiarid climates Xerolls Mediterranean climates Oxisols Aquox Wet Perox Very humid climates Torrox Very hot and dry climates Udox Humid climates Ustox Semiarid climates Spodosols Aquods Wet Cryods Cold climates Humods Humus accumulation Orthods Typical Ultisols Aquults Wet Humults Humus accumulation Udults Humid climates Ustults Semiarid climates Xerults Mediterranean climates Vertisols Aquerts Wet Cryerts Cold climates Uderts Humid climates Usterts Semiarid climates Xererts Mediterranean climates TABLE 3-6 Formative element names in the suborders of Soil Taxonomy. Formative Element Connotes Denotes alb White Albic (bleached eluvial) horizon anthr Human Anthropic or plaggen (plowed or tilled) horizon aqu Water Characteristics of wetness ar Plowed Mixed horizons arg White clay Argillic horizon (horizon with illuvial clay) calc Lime Presence of calcic horizon camb Change Presence of cambric horizon cry Cold Characteristics of cold dur Hard Presence of a duripan fibr Fibrous Least decomposed stage of organic materials fluv River Floodplains fol Leaf Mass of leaves gyps Gypsum Presence of gypsic horizon hem Half Intermediate stage of decomposition hist Tissue Presence of histic epipedon hum Earth/Humus Presence of organic matter orth True Common per Throughout time Year-round climates psamm Sand Sand textures rend Rendzina High in carbonate sal Salty Salic (saline) horizon sapr Rotton Most decomposed torr Torrid Hot and dry turb Turbulent Perturbed (such as by frost heaving) ud Humid Of humid climates ust Burnt Of dry climates vitr Glass Resembling glass xer Dry Dry summers, moist winters TABLE 3-7 Formative elements for names of great groups in Soil Taxonomy. Formative Element Connotation acr Extreme weathering pale Old development agr Agric horizon alb Albic horizon argi Argillic horizon calc, calci Calcic horizon camb Cambic horizon fulv Light melanic horizon gyps Gypsic horizon hapl Minimum horizon natr Natric horizon petr Cemented horizon plagg Plaggen horizon sal Salic horizon somb Dark horizon al High Al, low Fe dystr, dys Low base saturation eutr High base saturation ferr Iron hal Salty kand Low activity 1:1 silicate clay plinth Plinthite sulf Sulfur vitr Glass dur Duripan fragi Fragipan plac Thin pan psamm Sand texture quartz High quartz fibr Least decomposed fol Mass of leaves hem Intermediate decomposition hist Presence of organic material hum Humus sapr Most decomposed sphagn Sphagnum moss verm Wormy or mixed by animals and Ando-like chrom High chroma lithic Near stone luv, lu Illuvial melan Melanic epipedon molli Mollic epipedon rhod Dark red colors umbr Umbric epipedon cry Cold torr Usually dry and hot ud Humid climates ust Dry climate xer Dry summers, moist winters anhy Anhydrous aqu Water saturated endo Fully water saturated epi Perched water table fluv Floodplain hydr Water TABLE 3-8 Characteristics used to distinguish families. Particle Size Mineralogy CEC Class Class (a) Class (b) Ashy Mixed Superactive Fragmental Micaceous Active Sandy-skeletal Silicaceous Semiactive Sandy Kaolinitic Subactive Loamy Smectitic Clayey Gibbsitic Fine-silty Carbonic Fine-loamy Gypsic Temperature Regime Particle Size Class (Mean annual Class temperature, [degrees]C) Ashy Hypergelic (<-10) Fragmental Pergelic (-10 to -4) Sandy-skeletal Subgelic (-4 to + 1) Sandy Cryic (< +8) Loamy Frigid (< +8) Clayey Mesic (+ to +15) Fine-silty Thermic (+15 to +22) Fine-loamy Hyperthermic (> +22) (a) Mixed = mixed mineralogy, Micaceous = dominated by mica; Silicaceous = dominated by silica/quartz; Kaolinitic/Smectitic = dominated by 1:1 or 2:1 silicate clays, respectively; Gibbsitic = dominated by aluminum oxides and hydroxides; Carbonic = carbonate deposits; Gypsic = gypsum deposits. (b) Reflects the amount of CEC relative to % clay content. Superactive is highest while subactive is lowest. TABLE 3-9 Soil order names in the FAO system of classification. Order Characteristic Histosol Organic soils Lithosol Hard rock within 10 cm of soil surface Vertisol Clay cracking soils Fluvisol Recent alluvial deposits without development Solonchaks Saline soils Gleysols Hydromorphic features within 50 cm of the soil surface Andosols Volcanic soils Arenosols Coarse-textured soils with albic material Regosols Developing soils without diagnostic horizons--ochric epipedon Rankers Developing soils without diagnostic horizons--umbric epipedon Rendzinas Developing soils without diagnostic horizons--mollic epipedon Podzols Spodic B horizon Ferralsols Oxic B horizon Planosols Impermeable horizons such as a fragipan Solonetz Natric (high sodium) B horizon Greyzems Mollic A epipedon with bleaching Chernozems Deep, dark mollic epipedon Kastanozems Mollic epipedon Phaeozems Mollic epipedon Podzoluvisols Irregular broken horizons Xerosols Weak A and aridic moisture regime Vermosols Weak ochric A and aridic moisture regime Nitosols Having an argillic B horizon Acrisols Having an argillic B horizon Luvisols Having an argillic B horizon Cambisols Having a cambric B or umbric A horizon 25 cm thick
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|Title Annotation:||Section 1 Describing Soil|
|Publication:||Fundamental Soil Science|
|Date:||Jan 1, 2006|
|Previous Article:||Chapter 2 Soil geography and genesis.|
|Next Article:||Chapter 4 Soil solids: particle size and texture.|