Soils are classified on the basis of soil profile and soil formation. They can be grouped according to a number of characteristics, including agronomic use, color, organic matter content, texture, and moisture condition.
Typical soil is about 45 percent minerals and about 5 percent organic matter. The other 50 percent of soil consists of pores that hold either water or air. The liquid portion of soil contains dissolved minerals and organic compounds, produced by plants and microorganisms.
The gases found in soil often are the same as those found in the air above it. Soil can support plant life if climate and moisture are suitable. It is a changing and dynamic body, adjusting to conditions of climate, topography, and vegetation.
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In turn, soil influences plant and root growth, available moisture, and the nutrients available to plants. While “the soil” is a collective term for all soils, “a soil”means one individual soil body with a particular length, depth, and breadth.
Soil Profile
In a typical soil, the top layer is usually dark with decomposing organic matter; the layers below are sand, silt, clay, or some combination of the three. Soil scientists classify soils on the basis of soil profile and soil formation.
Typically the top soil layer is called the O horizon, or organic matter horizon. It has rotten logs, leaf litter, and other recognizable bits of plants and animals. Underneath the O horizon is the A horizon. It is characterized by thoroughly decomposed organic matter.
Water passing through the A horizon carries clay particles and organic acids through it into the B horizon. Clay or organic substances passing into the B horizon glue soil particles together, forming soil aggregates.
Soil aggregates—granular, columnar, and so on—are indicators of amature, healthy soil. The lowest level of the soil profile is the C horizon. It contains bedrock or soil parent material that shows little or no evidence of plant growth or soil formation.
Soil Formation
Soil formation takes hundreds, even thousands, of years. Parent material, climate, organisms, topography, and time all contribute.
Sources of parent material include igneous, sedimentary, and metamorphic rocks (fragments of which may be deposited by water, wind, and ice), and plant and animal deposits. Soil formation is the result of the physical, chemical, and biochemical breakdown of parent material.
It also reflects the processes of weathering and change within the soil mass. Many substances are added to soil—rain,water from irrigation, nitrogen from bacteria, sediment, salts, organic residues, and a variety of substances created by humans.
However, many substances are also removed from the soil—water-soluble minerals, clay, plants, carbon dioxide, and nitrogen. Other transformations also are occurring: Organic matter is decomposing, and minerals are solubilizing and changing chemical form. Clays and soluble salts that move along with the soil water cause color and chemical changes in the soil.
Parent material is a primary determinant of soil type or soil classification. All soils at the lowest category of soil classification are distinct if the parent material differs.
The differences in parent materials—weathering rates, the plant nutrient content, and soil texture resulting from parent material breakdown—contribute to the formation of distinctive soils. For example, sandstone yields sandy soil with low fertility.
Chemical Weathering
Soils slowly change color and density as a result of wetting and drying, warming and cooling, and freezing and thawing. During weathering—the rubbing, grinding, and moving of rocks by water, wind, and gravity—rocks are split into smaller and smaller fragments. Soil is composed of fragments 2 millimeters or less in diameter.
The expansion force of water as it freezes is sufficient to split minerals. However, water also is involved in chemical weathering—solution, hydrolysis, carbonation, reduction, oxidation, and hydration. A simple example of solution, the dissolving of minerals in liquid, is the dissolving of salt in water.
The salts then move along with the liquid. In hot arid climates, salts can move to the surface as water evaporates, creating salt flats. In wetter climates, salts can move through the soil, depleting it of necessary plant nutrients and contaminating ground water.
Hydrolysis is the splitting of a water molecule to form hydroxides and soluble hydroxide compounds, such as sodium hydroxide. Hydration is the addition of water to minerals in rock.
When a mineral such as hematite (an oxide of iron) hydrates, it expands, softens, and changes color. Carbonation is the reaction of a compound with carbonic acid, a weak acid produced when carbon dioxide dissolves in water.
Water often contains carbonic acid and other organic acids produced by organic matter decomposition. These acids increase the power of the water to disintegrate rock. Oxidation is the addition of oxygen to a mineral, and reduction is the removal of oxygen from a mineral.
Biological Weathering
Biological weathering is a combination of physical and chemical disintegration of rocks to produce soil. The roots of plants can crack rocks and break them apart.
Plant roots also produce carbon dioxide, which combines with water to produce carbonic acid. Carbonic acid dissolves certain minerals, speeding the breakdown of parent material and chemically changing the soil.
Plants and animals also add humus (organic matter) to soil, increasing its fertility and water-holding capacity and speeding rock weathering.
Animals such as earthworms, ants, prairie dogs, gophers, and moles also contribute to soil aeration and fertility by mixing the soil. In areas where animal populations are large, they can influence both the formation and destruction of soil.
Climate and Topography
Climate also influences soil formation indirectly through its action on vegetation. Soils in arid climates have sparse vegetation, less organic matter, and little soil profile development. Wet soil, however, usually has thick vegetation and high organic matter, and therefore a deep soil profile.
The shape of the land is referred to as its topography. Each land form—valleys, plains, hills, and mountains—is covered with a crazy quilt of different soil types.
For example, the steep sides of the Sandia Mountains near Albuquerque, New Mexico, which are severely eroded by wind and summer rains, contain a variety of soil types—forest soils, sandy soils, and rocky soils.
Sand, silt, and clay eroded from the mountains and nearby extinct volcanoes combine in the moist and fertile Rio Grande Valley. The valley has deep sandy soils, layered sand and clay soils, and soils eroded by flash floods.
Soils located in similar climates that develop from similar parent material on steep hillsides usually have thin A and B horizons because less water moves through the soil. Similar materials on shallower slopes allow more water to pass through them.
Topography and climate work together either to allow or to prohibit plant growth and organic matter deposition. Without moisture, plants cannot grow to impede soil erosion, and soil development is slow. With moisture, plants can grow, hold the soil in place, add organic matter to the soil, and speed soil development.
The age of a soil may be reckoned in tens, hundreds, or thousands of years. Under ideal conditions, a soil profile may develop in two hundred years; however, under less favorable conditions soil development may take several thousand years.
Soil Classification
Scientists have identified and classified soils for hundreds of years. Soils can be grouped according to agronomic use, color, organicmatter content, texture, moisture condition, and other characteristics.
Each of these groupings serves a particular purpose. U.S. soil scientists adopted a system of soil classification on January 1, 1965, that was based on the knowledge they had about soil genesis, morphology, and classification.
The U.S. system is divided into six categories: order, suborder, great group, subgroup, family, and series. Soil taxonomy is patterned after the worldwide system of plant and animal taxonomy, which contains phylum, class, order, family, genus, and species.
| Soil Order | Features |
| Alfisols | Soils in humid and subhumid climates with precipitation from 500 to 1,300 millimeters (20 to 50 inches), frequently under forest vegetation. Clay accumulation in the B horizon and available water most of the growing season. Slightly to moderately acid soils. |
| Andisols | Soils with greater than 60 percent volcanic ash, cinders, pumice, and basalt. They have a dark A horizon as well as high absorption and immobilization of phosphorus and very high cation exchange capacity. |
| Aridisols | Aridisols exist in dry climates. Some have horizons of lime or gypsum accumulations, salty layers, and A and slight B horizon development |
| Entisols | Soils with no profile development except a shallow A horizon. Many recent river floodplains, volcanic ash deposits, severely eroded areas, and sand are entisols. |
| Gelisols | Soils that commonly have a dark organic surface layer and mineral layers underlain by permafrost, which forms a barrier to downward movement of soil solution. Common in tundra regions of Alaska. Alternate thawing and freezing of ice layers results in special features in the soil; slow decomposition of the organic matter due to cold temperatures results in a peat layer at the surface in many gelisols. |
| Histosols | Organic soils of variable depths of accumulated plant remains in bogs, marshes, and swamps. |
| Inceptisols | Soils found in humid climates that have weak to moderate horizon development. Horizon development may have been delayed because of cold climate or waterlogging. |
| Mollisols | Mostly grassland soils, but with some broadleaf forest-covered soils with relatively deep, dark A horizons, a possible B horizon, and lime accumulation. |
| Oxisols | Excessively weathered soils. Oxisols are over 3 meters (10 feet) deep, have low fertility, have dominantly iron and aluminum oxide clays, and are acid. Oxisols are found in tropical and subtropical climates. |
| Spodosols | Sandy leached soils of the cool coniferous forests, usually with an organic or O horizon and a strongly acidic profile. The distinguishing feature of spodosols is a B horizon with accumulated organic matter plus iron and aluminum oxides. |
| Ultisols | Strongly acid and severely weathered soils of tropical and subtropical climates. They have clay accumulation in the B horizon |
| Vertisols | Soils with a high clay content that swell when wet and crack when dry. Vertisols exist in temperate and tropical climates with distinct dry and wet seasons. Usually vertisols have only a deep self-mixing A horizon. When the topsoil is dry, it falls into the cracks, mixing the soil to the depth of the cracks. |
Changes to the system have proceeded through a number of major revisions or approximations. The system can be used to classify soils anywhere in the world, especially with the addition of a new soil order, the andisols. The new soil classification continues to be tested, and minor modifications may be anticipated.
The approximation being used as of 1997 treated soil as a collection of three dimensional entities that can be grouped based on similar physical, chemical, and mineralogical properties. The minimum volume of soil that scientists consider when they classify a soil is the pedon, which can range from 1 to 10 meters square and is as deep as roots extend into a soil.
By 1999, the U.S. soil classification system recognized twelve soil orders. The differences among orders reflect the dominant soil-forming processes and the degree of soil formation. Each order is identified by a word ending in “-sol.”
Each order is divided into suborders, primarily on the basis of properties that influence soil genesis, are important to plant growth, and reflect the most important variables within the orders. The last syllable in the name of a suborder indicates the order. An example is “aquent,” meaning water, plus “-ent,” from “entisol.”
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Suborders are distinctive to each order and are not interchangeable between orders. Each suborder is divided into great groups on the basis of additional soil properties and horizons resulting from differences in soil moisture and soil temperature. Great groups are denoted by a prefix that indicates a property of the soil.
An example is “psammaquents” (“psamm” referring to sandy texture and “aquent” being the suborder of the entisols that has an aquic moisture regime). Soil scientists have identified more than three hundred great groups in the United States.
Great groups are distinguished on the basis of differing horizons and soil features. The differing soil horizons include those with accumulated clay, iron, or organic matter and those hardened or cemented by soil cultivation or other human activities.
The differentiating soil features include self-mixing of soil due to clay content, soil temperature, and differences in content of calcium, magnesium, sodium, potassium, gypsum, and other salts.
There are more than twenty-four hundred subgroups, and each great group is divided into three kinds of subgroups:a typic subgroup, an intergrade subgroup, and an extragrade subgroup.
The typic subgroup represents the central spectrum of a soil group. The intergrade subgroup represents soils with properties like those of other orders, suborders, or great groups.
The extra grade subgroup represents soils with some properties that are not representative of the great group but do not indicate transitions to any other known kind of soil. Each subgroup is identified by one or more adjectives preceding the name of the great group.
Families are established within a subgroup on the basis of physical and chemical properties and other characteristics that are important to plant growth or that are related to the behavior of soils that are important for engineering concerns.
Among the properties and characteristics considered are particle size, mineral content, temperature regime, depth of the root zone, moisture, slope, and permanent cracks.
A family name consists of the name of a subgroup preceded by terms that indicate soil properties. Several thousand families have been identified in the United States.
Finally, the series is the lowest soil category, with more than nineteen thousand recognized in the United States as of 1999. A series might share one or more properties with those of an entire family, but for at least one property only a narrower range is permitted.
Texture, Structure, and Consistency
Soil texture is determined by the percent of sand, silt, and clay in a soil sample. Most fertile or productive soils have a loam texture, or about equal amounts of sand, silt, and clay, and a high organic matter content (about 5 to 10 percent). Soil texture determines the water-holding and nutrient-holding capacity of a soil.
Thus, clay soils have a high nutrient-holding capacity, but they water log easily. Sandy soils have a lower nutrient-holding capacity but dry out easily. Farmers base their plans of how to fertilize and irrigate their crops partly on the texture of the soil.
Soil structure refers to how soil particles are glued together to form aggregates.During soil formation, soil particles are glued together with clay, dead microorganisms, earth worm slime, and plant roots, and they form air and water channels.
Plants need these channels so they can absorb nutrients, water, and air. Soil structure may be destroyed when farmers cultivate wet or waterlogged soils with heavy farm machinery. Destroying soil structure makes a soil unsuitable for plant growth.
Soil consistency is the “feel” of a soil and the ease with which a lump can be crushed in one’s fingers. Common soil consistencies are loose, friable, firm, plastic, sticky, hard, and soft.
Clay soils, for example, are sticky or plastic when they are wet, but they become hard or harsh when they are dry. The best time to work a clay soil is when it is soft or friable. Sandy soils, on the other hand, do not become plastic or sticky when they are wet or hard or harsh when they are dry.
They have a tendency to stay loose, which makes them easier to work. Loam and silt loam soils are intermediate in behavior. When farmers are trying to determine whether to work the soil or wait for better soil moisture conditions, they usually check the soil consistency.
Aeration and Moisture
Soil aeration relates to the exchange of soil air with atmospheric air. Growing roots need oxygen and are constantly expiring carbon dioxide. Unless there is a continuous flow of oxygen into soil and carbon dioxide out of the soil, oxygen becomes depleted. When their oxygen supply is cut off, the roots will die.
Soil moisture refers to water held in soil pores. A plant draws water from soil the same way a child draws water from a cup with a straw. When the cup is full, it is easy for the child to draw up the water, but as the cup empties, the child must work harder to get water.
Similarly, plants draw water from soil easily when the soil has plenty of water. As the soil dries, however, plants must work harder to pull water out of the soil until they reach wilting point.
Fertility
Plants absorb many of the nutrients they need from soil, including phosphorus, potassium, calcium, magnesium, sulfur, boron, chlorine, cobalt, copper, iron, manganese, molybdenum, and zinc. They may obtain carbon, hydrogen, and nitrogen from the air and water.
Soil testing services give farmers specific fertilizer and lime recommendations based on soil texture and chemical analysis. Farmers use soil tests to determine if their soil has enough essential nutrients for a crop to grow.
The absence of one essential nutrient can limit overall crop growth. Nitrogen, phosphorus, and potassium are commonly applied to the soil as commercial fertilizer andmanure.
Calcium and magnesium are applied as lime, which is also used to reduce the acidity of soil and to increase the solubility of some minerals. Manure and other organic matter added to soils increase water-holding and nutrient-holding capacity and therefore boost crop yields.
Agricultural extension services offer guidelines for the maximum amounts of manure, sewage sludge, fertilizer, and other chemicals that farmers should apply to soils.
Farmers are encouraged to apply nitrogen fertilizer in small applications at times when plants are growing rapidly. This soil management practice decreases deep percolation losses that could pollute groundwater.
With an understanding of soil characteristics, farmers and gardeners can learn to manage a wide variety of soils. Some soils are naturally fertile and need few amendments to promote high crop yields.
Other soils, whether because of their parent material or climate, are naturally infertile and might best be used for purposes other than agriculture.
Like the water and the air, the soil is a crucial natural resource. From an airplane, all soils look about the same, but from an ant’s view, soils are all different. Differences in soil type make huge differences to plants, animals, humans, and the environment.
