Soil profiles

A soil profile is the vertical section of a soil extending through its horizons into the unweathered parent material. Scientists use the different arrangements of horizons to give classification names to the soil profiles, which can be used to interpret landscape history and determine future land use.

Horizon Nomenclature

The term “soil” has many definitions. To the soil scientist, it is anything that will support plant growth. To the civil engineer, it is an unconsolidated surficial material that can be penetrated by a shovel. To the geologist, it is an unconsolidated layer of weathered mineral matter arranged in layers at the earth’s surface. A soil profile is a vertical arrangement of soil horizons down to and including the parent material in which the soil has developed. Two sets of horizon nomenclature are used to describe soil profiles: symbols for field descriptions and diagnostic horizon names for classification. The field description symbols have been in use since the nineteenth century, whereas the diagnostic horizon names came into use only in the 1960s.

The field description symbols used to identify various kinds of horizons were developed in Russia in the 1880s. There, soil scientists such as V. V. Dokuchayev applied the letters A, B, and C to the main horizons of the black soils of the steppes of Russia. The A horizon was designated the zone of maximum organic material accumulation, the C horizon was the unaltered parent material, and the B horizon was the layer in between the A and the C. These concepts spread to the rest of the world, with the B horizon concept being modified to be a zone of the accumulation of iron, aluminum, and clays that had moved down the profile. Horizons may be prominent or so weak that they can be detected only in the laboratory; they can be thick or thin. The use of the A, B, and C master horizons has remained the backbone of horizon symbols. Three additional master horizons, however, have also been included: O, for organic-rich layers; E, for layers that have been intensively leached; and R, for rock. Soil terms like “podzol” (from pod, meaning “underneath,” and zola, meaning “ash”) and “chernozem” (from cherny, meaning “black,” and zemlya, meaning “earth”) are still used and reflect the Russian roots of soil classification.

Since the 1930s, many sets of horizon symbols have been developed around the world. With the United States and many other countries adopting a horizon nomenclature similar to that produced by the Food and Agricultural Organization (FAO) of the United Nations, the world has come closer to a universally accepted set of horizon symbols.

Refining Horizon Nomenclature

There are three kinds of symbols used in soil horizon descriptions: capital letters, lowercase letters, and Arabic numerals. The capital letters describe the master horizons, the lowercase letters depict some specific characteristic of a master horizon, and the numeral characterizes a further subdivision or parent material layering. An example of a horizon symbol might be Bg1.

The “g” describes a “gleyed” horizon, or one where the iron has been removed during soil-forming processes, or saturation with stagnant water in a high water table has preserved the reduced state of the iron, giving the layer a neutral or gray color. This letter also can describe a soil with red and gray specks from the oxidized and reduced forms of iron. A horizon that is more than 90 percent cemented is given the “m” designation. Horizons with abundant silica are described with a “q” and abundant sodium with an “n.” If the horizon is very densely packed, which commonly occurs in silt-rich soils, the “x” designation is used. A “b” denotes a buried soil horizon. A “c” signifies that weathering has formed concretions, or nodules, in the soil.

Transitions between master horizons are commonly found in nature. Where the properties of both horizons are mixed in the same layer, both capital letters are used, with the first letter denoting the horizon whose properties dominate. The term BA horizon would be a transition between the A and B horizons, where the B horizon characteristics prevail. Where the horizon has distinct parts of both horizons, the two capital letters are separated by a slash mark (A/B, E/B, B/C).

Organic Soil Horizons

The O horizon is a master horizon in which there is an accumulation of mainly organic matter that overlies a mineral soil. It is dominated by fresh or partly decomposed organic litter such as twigs and needles, and many times the original form of most of the vegetative matter is visible to the naked eye. This horizon contains between 20 and 30 percent organic material, sometimes more depending upon the clay content of the underlying mineral horizons. Three subdivisions of this master horizon are based on the amount of decomposition in the organic material and range from the Oi (least decomposition) to the Oe (intermediate decomposition) and the Oa (most decomposition).

The A horizon is a master horizon in which decomposed organic matter, humus, is mixed with mineral sediments. The organic matter content is not great enough to be classified as an O horizon. This horizon is generally a surface horizon, except when located below an O horizon. Because of the presence of organic material, this layer can be darker than underlying horizons. The organic material is assumed to be derived from the decomposition of plant and animal remains deposited on the surface. This layer is the zone of maximum biological activity. A subdivision is the Ap, or “plow horizon,” where the A horizon has been disturbed by cultivation.

Mineral Soil Horizons

The E horizon is the master horizon commonly located below the O or A horizons in forest environments where leaching is dominant in the soil profile. Abundant water passing through the O horizon may become very acidic; this water leaches iron, aluminum, and organics from the A horizon as it passes to the lower part of the profile. The remaining mineral matrix is light-colored because of the color of the primary mineral sand grains that remain. Sometimes the horizon is almost pure white. For many years, this horizon was called an A2 horizon but has been removed from the category, as it is not a zone of organic material accumulation. The E horizon sometimes is light enough to look like ash, explaining the Russian term “podzol”; the process that forms such soils is still called “podzolization.”

The B horizon is the master zone of accumulation of materials that have been moved down by water from the O, A, and/or E horizons. These suspended materials include clay, iron, aluminum, and organic matter. The soil horizon shows little or no evidence of the original sediment or rock structure. Several kinds of B horizons are recognized depending upon the materials moved into them and, of course, not all horizons are present in any given soil. A Bh horizon is an accumulation of abundant organic material and therefore is very black in color. In a Bs horizon, the deep red color depicts the accumulation of abundant iron and aluminum (the latter of which is colorless). A Bk horizon has an abundant amount of calcium carbonate and a white color throughout. A Bt horizon includes a large amount of clay that has moved down from the horizons above. Clay films are noted on the sediments, and the soil samples are very sticky. A By horizon is an accumulation of gypsum. A Bz horizon contains salts more soluble than gypsum. A Bw horizon is a weakly developed B horizon in which a reddish color has developed through weathering, but few or no apparent materials have been moved into it from above.

The C master horizon is the subsurface layer of partially weathered parent material. Soil-forming processes, in which particles and chemicals move downward from the O, A, E, and B horizons, have not affected this horizon, yet weathering of the sediments has slightly changed the color of the parent material. If the horizon material has the structure of the parent rock but is weathered enough to get a shovel through it, the horizon is called a Cr horizon; geologists also refer to this horizon as a saprolite. The R master horizon is the unweathered, consolidated bedrock underlying the soil.

Soil Taxonomy

A second set of diagnostic horizon nomenclature was developed in the latter half of the twentieth century in the United States, using a new system called Soil Taxonomy. Information from diagnostic surface horizons (epipedons), subsurface horizons, soil-moisture, and temperature regimes together are used to determine the soil classification. Soil Taxonomy is based mainly on measurable soil properties based in seventeen epipedons and subsurface horizons rather than on theories of soil formation, which form the basis for older classification systems. The classification system is very exact and offers exotic combinations of Latin and Greek syllables, such as Pachic Cryumbrepts and Natraqualfic Mazaquerts. Each syllable is a descriptive term for some aspect of the soil. For example, “natraqualfic” means sodium-rich (natr-), wet (aq-), and having a B horizon rich in iron and aluminum (alfic). Field observations must be supplemented by laboratory analyses of properties plus climatic data. Similar approaches using this type of horizon nomenclature have also been developed in Canada and by the FAO (the UN Food and Agriculture Organization). In the United States, soils are classified into twelve soil orders on the basis of their horizon materials and structures. Most soil types are defined by the distinctness of their horizons and degree of evolution, but some, like soils developed from peat, volcanic ash, swelling clays, and on permafrost, are defined by those special characteristics.

Soil Taxonomy describes four different epipedons. The Mollic epipedon is a thick, dark-colored surface horizon rich in mineral nutrients. The Umbric epipedon is similar to the Mollic visually but is low in mineral nutrients. The Ochric epipedon is a thin surface horizon that does not meet the requirements of the Mollic or Umbric. The Histic epipedon is basically an O horizon.

The additional Soil Taxonomy diagnostic horizons are subsurface horizons. The Albic horizon is basically an E horizon, as it has been highly leached. An Argillic horizon is a B horizon that has been enriched significantly with clay from higher in the profile. The Natric horizon contains a high abundance of salt. The Spodic horizon is a B horizon with abundant iron, aluminum, and/or organics that have been moved from above. The Cambic horizon is a B horizon that is slightly enriched in red color but, unlike the Spodic and Argillic horizons, lacks an accumulation of products from above. The Oxic horizon is a B horizon with an abundance of highly weathered minerals. A Calcic horizon has an abundance of calcium carbonate in it. A Petrocalcic horizon is a cemented Calcic horizon, often called caliche. The Gypsic horizon contains an abundance of gypsum. A Petrogypsic horizon is a cemented Gypsic horizon. A Duripan is cemented with silica. A Fragipan horizon is very dense and is many times formed in silt particles.

Studying a Soil

The first step in the study of a soil is producing a description of the soil profile. A scientist studying a particular soil exposes a soil profile by either digging a soil pit or finding a road cut. First, the soil horizons are delineated (A, B, C, and so on). Second, each horizon is characterized by thickness, color, presence of films, and boundaries. The horizon texture (the relative amount of sediment sizes) is recorded as well. For example, a sandy loam soil has an abundance of sand with some silt in it. The structure of that horizon, or how the sediments are put together, is described as well. Third, samples from each horizon are gathered to be analyzed in the laboratory. Finally, an approximate classification is made based on the minimal information available in the soil pit.

The soil samples are returned to the laboratory for a chemical and physical characterization. The pH (degree of acidity) and amounts of chemical nutrients are generally determined using pH meters and titrations. The exact texture is determined using sieves and water-settling columns. The percentage of organic matter is calculated using titration. Additional information can be obtained using an X-ray machine to determine the types of clays in the soil.

Once the field and laboratory data have been assembled, diagnostic horizons can be selected and the exact classification is determined using Soil Taxonomy. Twelve major classification groupings, or orders, are used in this system. Entisols are young soils that have no B horizon, whereas Inceptisols have a weak B horizon. In humid climates the major mature soil types are Mollisols, Alfisols, and Spodosols. The Mollisols are soils with a Mollic epipedon and are often grassland soils. Alfisols have an Argillic horizon and abundant nutrients; they are often deciduous forest soils. Spodosols are highly leached soils that have Spodic (E) horizons and are common in conifer forests. Ultisols are highly weathered soils of warm climates, and Oxisols are extremely weathered soils of the tropics. Aridisols are arid climate soils. Histosols are peaty soils that have thick Histic epipedons. Vertisols are high in clays that shrink and swell with seasonal moisture variation and exhibit extensive cracking during dry seasons. The Andisols are volcanically produced soils. Gelisols are soils formed above permafrost. Using the proper classification, scientists can interpret the soil’s age, future land use, the crops it can best support, and its past history.

A Fundamental Tool

For years, the soil profile has been the basic descriptive tool of scientists. This cross-section constitutes the format from which scientists make their soil classifications. Without a proper soil profile description, one cannot perform the soil classification required before environmental interpretations and soil maps can be produced. The symbols that are used to describe the horizons of the soil profile may have changed, but the basic A-B-C concept that was developed during the nineteenth century has not changed much. Different systems of description of the soil profile and the resulting soil classification systems are present around the world, but with time they are becoming more similar.

In the United States, the Soil Taxonomy system has been officially used since 1975. This new classification offers a very exact hierarchical system similar to what biologists use to classify plants and animals. The twelve major orders are divided into suborders, great groups, subgroups, and finally soil series. The subgroup name, such as “Typic Paleudult,” is similar to the genus/species designation of a plant or animal. Soil Taxonomy is the most comprehensive soil classification system in the world and therefore is used more than any other approach. It is such an exact system that one can give a scientist familiar with the taxonomy a subgroup name and he or she can construct a complete soil profile description very close to the actual one without even seeing the soil. Soil classification may be the most difficult classification problem in science, as a large number of factors must be considered and a wide variety of client groups want to apply the results to very different problems.

Principal Terms

horizon: a layer of soil material approximately parallel to the surface of the land that differs from adjacent related layers in physical, chemical, and biological properties

leaching: the dissolving or removal of soluble materials from a soil horizon by percolating water

sediment: rock fragments such as clay, silt, sand, gravel, and cobbles

structure: the arrangement of primary soil particles into secondary units called peds

texture: the relative proportions of varying sediment sizes in a soil

weathering: the mechanical disintegration and chemical decomposition of rocks and sediments

Bibliography

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Paton, T. R., G. S. Humphreys, and P. B. Mitchell. Soils: A New Global View. New Haven, Conn.: Yale University Press, 1995. Aimed at readers at the university level. Offers many good examples of soils and soil formation from around the world. Color illustrations, maps, bibliography, and index.

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