soil

A represents soil; B represents laterite, a regolith; C represents saprolite, a less-weathered regolith; the bottommost layer represents bedrock

Loess field in Germany

Surface-water-gley developed in glacial till, Northern Ireland

Soil is a natural body consisting of layers (soil horizons) of primarily mineral constituents of variable thicknesses, which differ from the parent materials in their texture, structure, consistence, color, chemical, biological and other physical characteristics.^[1] In engineering, soil is referred to as regolith, or loose rock material. Strictly speaking, soil is the depth of regolith that influences and has been influenced by plant roots.

Soil is composed of particles of broken rock that have been altered by chemical and mechanical processes that include weathering and erosion. Soil is altered from its parent rock due to interactions between the lithosphere, hydrosphere, atmosphere, and the biosphere.^[2] It is a mixture of mineral and organic constituents that are in solid, gaseous and aqueous states.^[3][4] Soil is commonly referred to as earth or dirt; technically, the term dirt should be restricted to displaced soil.^[5]

Soil forms a structure that is filled with pore spaces, and can be thought of as a mixture of solids, water and air (gas).^[6] Accordingly, soils are often treated as a three state system.^[7] Most soils have a density between 1 and 2 g/cm³.^[8] Little of the soil of planet Earth is older than the Tertiary and most no older than the Pleistocene.^[9]

Darkened topsoil and reddish subsoil layers are typical in some regions.

On a volume basis a good quality soil is one that is 45% minerals, 25% water, 25% air, and 5% organic material, both live and dead. The mineral and organic components are considered a constant with the percentages of water and air the only variable parameters where the increase in one is balanced by the reduction in the other.

A soil profile consists of two or more layers called horizons that differ in one or more properties such as texture, structure, color, porosity, consistence, and reaction. The horizons differ greatly in thickness and generally lack sharp boundaries. Most soil profiles include three master horizons A, B and C. The A and B horizons are called the solum or “true soil” as most of the chemical and biological activity that has altered the soil takes place in these two profiles.^[10]

History of the study of soil

The history of the study of soil is intimately tied to our urgent need to provide food for ourselves and forage for our animals. Throughout history, civilizations have prospered or declined as a function of availability and productivity of soils.

The Greek historian Xenophon (450-355 B.C.) is credited with being the first to expound upon the merits of green-manuring crops, "But then whatever weeds are upon the ground, being turned into earth, enrich the soil as much a dung."^[11]

Columella’s Husbandry, circa 60 A.D. was used by 15 generations (450 years) of those encompassed by the Roman Empire until its collapse. From the fall of Rome to the French Revolution, knowledge of soil and agriculture was passed on from parent to child and as a result, crop yields were low. During the Dark Ages for Europe, Yahya Ibn_al-'Awwam’s handbook guided the people of North Africa, Spain and the Middle East with its emphasis on irrigation, a translation of which was finally carried to the southwest of the United States.

Jethro Tull, an English gentleman, introduced in 1701 an improved grain drill that systemized the planting of seed and invented a horse-drawn weed hoe, the two of which allowed fields once choked with weeds to be brought back to production and seed to be used more economically. Tull, however, also introduced the mistaken idea that manure introduced weed seeds, and that fields should be plowed in order to pulverize the soil and so release the locked up nutrients. His ideas were taken up and carried to their extremes in the 20th century, when farmers repeatedly plowed fields far beyond what was necessary to control weeds. During a period of drought, the repeated plowing resulted in the Dust Bowl in the prairie region of the Central United States and Canada.

The "two-course system" of a year of wheat followed by a year of fallow was replaced in the 18th century by the Norfolk four-course system, in which wheat was grown in the first year, turnips the second, followed by barley, with clover and ryegrass together, in the third. The taller barley was harvested in the third year while the clover and ryegrass were grazed or cut for feed in the fourth. The turnips fed cattle and sheep in the winter. The fodder crops produced large supplies of animal manure, which returned nutrients to the soil.^[12]

Experiments into what made plants grow first led to the idea that the ash left behind when plant matter was burnt was the essential element and overlooked the role of nitrogen, which is not left on the ground after combustion. Jan Baptist van Helmont thought he had proved water to be the essential element from his famous experiment with a willow tree grown in carefully controlled conditions in which only water was added, which after five years of growth was removed and weighed, roots and all, and found to weigh 165 pounds. The oven-dried soil, originally 200 pounds, was again dried and weighed and found to have lost only two ounces, which van Helmont reasonably explained as experimental error and assumed that the soil had in fact lost nothing. As rain water was the only thing added by the experimenter, he concluded that water was the essential element in plant life. In fact the two ounces lost from the soil were the minerals taken up by the willow tree during its growth.

John Woodward experimented with various types of water ranging from clean to muddy and found muddy water the best, and so he concluded earthy matter was the essential element. Others concluded it was humus in the soil that passed some essence to the growing plant.

The French chemist Antonine Lavoisier showed that plants and animals must “combust” oxygen internally to live and was able to deduce that most of the 165-pound weight of Van Helmont’s willow tree derived from air. The chemical basis of nutrients delivered to the soil in manure was emphasized and in the mid-19th century chemical fertilizers were used, but the dynamic interaction of soil and its life forms awaited discovery.

It was known that nitrogen was essential for growth and in 1880 the presence of Rhizobium bacteria in the roots of legumes explained the increase of nitrogen in soils so cultivated. The importance of life forms in soil was finally recognized.

Crop rotation, mechanization, chemical and natural fertilizers led to a doubling of wheat yields in western Europe between 1800 to 1900.^[13]

Soil forming factors

Soil formation, or pedogenesis, is the combined effect of physical, chemical, biological, and anthropogenic processes on soil parent material. Soil genesis involves processes that develop layers or horizons in the soil profile. These processes involve additions, losses, transformations and translocations of material that compose the soil. Minerals derived from weathered rocks undergo changes that cause the formation of secondary minerals and other compounds that are variably soluble in water. These constituents are moved (translocated) from one area of the soil to other areas by water and animal activity. The alteration and movement of materials within soil causes the formation of distinctive soil horizons.

How the soil "life" cycle proceeds is influenced by at least five classic soil forming factors that are dynamically intertwined in shaping the way soil is developed: parent material, climate, topography, organisms and the passage of time. When reordered to climate, relief, organisms, parent material and time they form the acronym CROPT.^[14] An example of soil development would begin with the weathering of lava flow bedrock which would produce the purely mineral-based parent material from which soils form. Soil development would start from bare rock of recent flows in warm regions under heavy and very frequent rainfall. In such climates, plants become established very quickly on basaltic lava, even though there is very little organic material. The plants are supported by the porous rock as it is filled with nutrient-bearing water that carries dissolved minerals from rocks and guano. Crevasses and pockets, local topography of the rocks, would hold fine materials and harbor plant roots. The developing plant roots themselves are associated with mycorrhizal fungi^[15] that gradually break up the porous lava, and by these means organic matter and a finer mineral soil accumulate with time.

Parent material

The material from which soil forms is called parent material. It includes: weathered primary bedrock; secondary material transported from other locations, namely colluvium and alluvium; deposits that are already present but mixed or altered in other ways—old soil formations, organic material; and anthropogenic materials, such as landfill or mine waste.^[16]

Soils that develop from their underlying parent rocks are called “residual soils”, and have the same general chemistry as their parent rocks. The soils found on mesas, plateaus and plains are residual soils but few other soils are residual.

Most soils derive from transported parent materials that have been moved many miles by wind, water and gravity.^[17]. Aeolian processes are capable of moving fine sand and silt many hundreds of miles forming loess soils, common in the Midwest of North America and in Central Asia Clay is seldom moved by wind as it forms stable aggregates.

Cumulose parent material includes peats and mucks and may develop in place from plant residues that have been preserved by the low oxygen content of a high water table.

Weathering is the first stage in the transforming of parent material into soil material. In soils forming from bedrock, a thick layer of weathered material called saprolite may form. Saprolite is the result of weathering processes that include: hydrolysis (the division of a mineral into acid and base pairs by the splitting of intervening water molecules), chelation from organic compounds, hydration (the solution of minerals in water with resulting cation, anion pairs), and physical processes that include freezing and thawing.^[16] The mineralogical and chemical composition of the primary bedrock material, its physical features, including grain size and degree of consolidation, plus the rate and type of weathering, transforms the parent material into the different mineral components of soils. Texture, pH and mineral constituents are inherited by a soil from its parent material.

Climate

Soil formation greatly depends on the climate, and soils show the distinctive characteristics of the climate zones in which they form.^[18] Temperature and moisture affect the rate of weathering and leaching. Wind moves sand and smaller particles, especially in arid regions where there is little plant cover. The type and amount of precipitation influence soil formation by affecting the movement of ions and particles through the soil, and aid in the development of different soil profiles. The effectiveness of water in weathering parent rock material depends on seasonal and daily temperature fluctuations. Cycles of freezing and thawing constitute an effective mechanism that breaks up rocks and other consolidated materials. Temperature and precipitation rates affect vegetation cover, biological activity, and the rates of chemical reactions in the soil.

Topography

The topography or relief characterized by the inclination of the surface determines the rate of precipitation runoff and rate of formation and erosion of the surface soil profiles. Steep slopes allow rapid runoff and erosion of the top soil profiles and little mineral deposition in lower profiles. Depressions allow the accumulation of water, minerals and organic matter and in the extreme, the resulting soils will be saline marshes or peat bogs. Intermediate topography affords the best conditions for the formation of an agriculturally productive soil.

Organisms

Plants, animals, fungi, bacteria and humans affect soil formation (see soil biomantle and stonelayer). Animals and micro-organisms mix soils as they form burrows and pores, allowing moisture and gases to move about. In the same way, plant roots open channels in soils. Plants with deep taproots can penetrate many meters through the different soil layers to bring up nutrients from deeper in the profile. Plants with fibrous roots that spread out near the soil surface have roots that are easily decomposed, adding organic matter. Micro-organisms, including fungi and bacteria, effect chemical exchanges between roots and soil and act as a reserve of nutrients. Humans can impact soil formation by removing vegetation cover with erosion as the result. They can also mix the different soil layers, restarting the soil formation process as less weathered material is mixed with the more developed upper layers. Some soils may contain up to one million species of microbes per gram, most of those species being unknown, making soil the most abundant ecosystem on Earth.^[19]

Vegetation impacts soils in numerous ways. It can prevent erosion caused by excessive rain and the resulting surface runoff. Plants shade soils, keeping them cooler and slowing evaporation of soil moisture, or conversely, by way of transpiration, plants can cause soils to lose moisture. Plants can form new chemicals that can break down or build up soil particles. The type and amount of vegetation depends on climate, land form topography, soil characteristics, and biological factors. Soil factors such as density, depth, chemistry, pH, temperature and moisture greatly affect the type of plants that can grow in a given location. Dead plants and dropped leaves and stems fall to the surface of the soil and decompose. There, organisms feed on them and mix the organic material with the upper soil layers; these added organic compounds become part of the soil formation process.

Time

Time is a factor in the interactions of all the above. Over time, soils evolve features dependent on the other forming factors. Soil formation is a time-responsive process that is dependent on how the other factors interplay with each other. Soil is always changing. It takes about 800 to 1000 years for a 2.5 cm thick layer of fertile soil to be formed in nature. For example, recently deposited material from a flood exhibits no soil development because there has not been enough time for soil-forming activities. The original soil surface is buried, and the formation process must begin anew for this deposit. The long periods over which change occurs and its multiple influences mean that simple soils are rare, resulting in the formation of soil horizons. While soil can achieve relative stability of its properties for extended periods, the soil life cycle ultimately ends in soil conditions that leave it vulnerable to erosion. Despite the inevitability of soil retrogression and degradation, most soil cycles are long and productive.

Soil-forming factors continue to affect soils during their existence, even on “stable” landscapes that are long-enduring, some for millions of years. Materials are deposited on top and materials are blown or washed from the surface. With additions, removals and alterations, soils are always subject to new conditions. Whether these are slow or rapid changes depend on climate, landscape position and biological activity.

Physical Properties of Soils

The physical properties of soils are its texture, structure, density, porosity, consistency, temperature and color. These determine the availability of oxygen in the soil and ability of water to infiltrate and be held in the soil. Soil texture, characterized by the different soil particles, called soil "separates" sand, silt and clay is the relative proportion of those three. Larger soil structures are created from the separates when iron oxides, carbonates, clay and silica with the organic constituent humus, coat particles and cause them to adhere into relatively stable secondary structures called "peds". Soil density, particularly bulk density, is a measure of the soil compaction. Soil porosity consists of the part of the volume occupied by air and water. Consistency is the ability of soil to stick together. Soil temperature and color are self defining.

Texture

Soil types by clay, silt and sand composition as used by the USDA

Iron rich soil near Paint Pots in Kootenay National Park of Canada.

The mineral components of soil consist of a mixture of sand, silt and clay. In the illustrated textural classification triangle the only soil that does not exhibit one of those predominately is called "loam." While even pure sand, silt or clay may be considered a soil, from the perspective of food production a loam soil with a small amount of organic material is considered ideal. The mineral constituents of a loam soil might be 40% sand, 40% silt and the balance 20% clay by weight. Soil texture affects soil behavior, including the retention capacity for nutrients and water.^[20]

Sand and silt are the products of physical weathering, while clay is the product of chemical weathering. Silt is finely powdered parent material. Clay on the other hand is a product of chemical weathering and forms as a secondary mineral from dissolved minerals that precipitate out of solution. It is the specific surface area of soil particles and unbalanced ionic charges in the soil particle that determine their role in the cation exchange potential of soil hence its fertility. Sand is least active followed by silt; clay is the most active. Sand has its greatest benefit to soil by resisting compaction. Silt, with its higher specific surface area, is more chemically active than sand and the clay content, with its very high specific surface area and generally large number of negative charges, gives clay its great retention capacity for nutrients and water. Clay soils resist wind and water erosion better than silty and sandy soils, as the particles are bonded to each other.

Sand is the most stable of the mineral components of soil; it consists of rock fragments, primarily quartz particles, ranging in size from 2.0 mm to 0.05 mm. Sand is largely inert but plays an important part in holding open soil. Silt ranges in size from 0.05 mm to 0.002 mm. Silt is mineralogically like sand but is more active than sand due to its larger surface area. Clay is the most important component of mineral soil due to its net negative charge and ability to hold cations. Clay cannot be resolved by optical microscopes; it ranges in size from 0.002 mm or less.^[21] In medium-textured soils, clay is often washed downward through the soil profile and accumulates in the subsoil. Components larger than 2.0 mm are classed as rock and gravel and are removed before determining the percentages of the remaining components and the texture class of the soil but are included in the name. For example, a sandy loam soil with 20% gravel would be called gravely sandy loam.

When the organic component of a soil is substantial, the soil is called organic soil rather than mineral soil. The limits are as follows:

1. Mineral fraction is 0% clay and organic matter is 20% or more.
2. Mineral fraction is 0% to 50% clay and organic matter is between 20% to 30%.
3. Mineral fraction is 50% or more clay and organic matter 30% or more.^[22]

Structure

Soil structure is the aggregation of the sand, silt, and clay soil components into larger units. It is caused by the adhesion of those particles into larger aggregates (peds) by organic substances, iron oxides, carbonates, clays and silica, and by the breakage of the aggregates due to expansion and contraction from freezing and thawing, wetting and drying cycles of the soil. These peds evolve into units that may have various shapes, sizes and degrees of development.^[23] A soil clod is not a ped but rather a mass of soil that results from tillage. The soil structure affects aeration, water movement, resistance to erosion and plant root growth.

Soil structure often gives clues to texture, organic matter content, biological activity, past soil evolution, human use, and chemical and mineralogical conditions under which the soil formed. While texture, defined by the mineral component of a soil, is an innate property of the soil and does not change with agricultural activities, soil structure can be improved or destroyed by our choice and timing of farming practices.

Soil Structural Classes:^[24]

1. Types: Shape and arrangement of peds

a. Platy: Peds are flattened one atop the other 1-10 mm thick.

Found in the A-horizon of forest soils and lake sedimentation.

b. Prismatic and Columnar: Prismlike peds are long in the

vertical dimension, 10-100 mm wide. Prismatic peds have flat

tops, columnar peds have rounded tops. Tend to form in the B-

horizon in high sodium soil where clay has accumulated.

c. Angular and subangular: Blocky peds are imperfect cubes,

5-50 mm, angular have sharp edges, subangular have rounded

edges. Tend to form in the B-horizon where clay has

accumulated and indicate poor water penetration.

d. Granular and Crumb: Spheroid peds of polyhedrons, 1-10 mm,

often found in the A-horizon in the presence of organic

material. Crumb peds are more porous and are considered ideal.

2.Classes: Size of peds whose ranges depend upon the above type

a. Very fine or very thin: <1 mm platy and spherical; <5 mm

blocky; <10 mm prismlike.

b. Fine or thin: 1-2 mm platy, and spherical; 5-10 mm blocky;

10-20 mm prismlike.

c. Medium: 2-5 mm platy, granular; 10-20 mm blocky; 20-50

prismlike.

d. Coarse or thick: 5-10 mm platy, granular; 20-50 mm blocky;

50-100 mm prismlike.

e. Very coarse or very thick: >10 mm platy, granular; >50 mm

blocky; >100 mm prismlike.

3. Grades: Is a measure of the degree of cementation within the

peds that results in their strength and stability.

a. Weak: Weak cementation allows peds to fall apart into the

three constituents of sand, silt and clay.

b. Moderate: Peds are not distinct in undisturbed soil but when

removed they break into aggregates, some broken aggregates and

little unaggregated material. This is considered ideal.

c. Strong:Peds are distinct before removed from the profile and

do not break apart easily.

d. Structureless: Soil is entirely cemented together in one

great mass such as slabs of clay or no cementation at all such

as with sand.

The forces that result from swelling and shrinkage initially tend to act horizontally, causing vertically oriented prismatic peds. Clayey soil will induce horizontal cracks reducing columns to blocky peds. Roots, rodents, worms and freezing further break the peds into a spherical shape.

Plant roots extend into voids and remove water causing the open space to shrink, further reinforcing physical aggregation. At the same time roots, fungal hyphea and earthworms create microscopic tunnels that break up peds. Plant roots, bacteria and fungi exude sticky polysaccharides that bind soil into small peds. The addition of the raw organic matter that bacteria and fungi feed upon encourages the formation of desirable soil structure.

Polyvalent cations such as Ca²⁺, Mg²⁺ and Al³⁺ have the ability to infiltrate negatively charged plates of clay and hold them together and so neutralize them overall thereby allowing them to gather together or flocculate. On the other hand, monovalent ions such as sodium tend to destroy peds by displacing the polyvalent cations leaving a net negative charge on the layers that compose clay; this allows them to push apart and so prevents the flocculation of clay particles into random assemblages. As a result, the clay disperses and settles into voids between peds causing them to close. In this way the soil is made impenetrable to air and water. Such sodic soil tends to form columnar structures near the surface.^[25]

Density

Density is the weight per unit volume of an object. Particle density is the density of the mineral particles that make up a soil i.e. excluding pore space and organic material. Particle density averages approximately 2.65 g/cc (165 lbm/ft³. Soil bulk density, a dry weight, includes air space and organic materials of the soil volume. A high bulk density indicates either compaction of the soil or high sand content. The bulk density of cultivated loam is about 1.1 to 1.4 g/cc (for comparison water is 1.0 g/cc).^[26] A lower bulk density by itself does not indicate suitability for plant growth due to the influence of soil texture and structure.

Table: Representative bulk densities of soils. The percentage pore space was calculated using 2.7 g/cc for particle density except for the peat soil, which is estimated.^[27]

Porosity

Pore space is that part of the bulk volume not occupied by either mineral or organic matter but is open space occupied by either air or water. The air space is needed to supply oxygen to organisms decomposing organic matter, humus and plant roots. Pore space allows the movement and storage of water and dissolved nutrients.

There are four categories of pores:

1. Coarse pores: 0.2 mm -200 microns
2. Medium pores: 200-20 microns
3. Fine pores: 20-2 microns
4. Very fine pores: < 2 microns

When pore space is less than 30 microns, the forces of attraction that hold water in place are greater than those acting to drain the water. At that point, soil becomes water logged and it cannot breathe. For a growing plant, pore size is of greater importance than total pore space. A medium textured loam provides the ideal balance of pore sizes. Having large pore spaces that allow rapid air and water movement is superior to total percentage pore space.^[28]

Consistency

Consistency is the ability of soil to stick together and resist fragmentation. It is of use in predicting cultivation problems and engineering of foundations. Consistency is measured at three moisture conditions: air-dry, moist and wet. The measures of consistency border on subjective as they employ the "feel" of the soil in those states. A soil's resistance to fragmentation and crumbling is made in the dry state by rubbing the sample. Its resistance to shearing forces is made in the moist state by thumb and finger pressure. Finally, a soils plasticity is measured in the wet state by molding with the hand.

The terms used to describe soil in those three moisture states and a last state of no agricultural value are as follows:

1. Consistency of Dry Soil: loose, soft, hard, extremely hard.
2. Consistency of Moist Soil: loose, friable, firm, extremely firm.
3. Consistency of Wet Soil: non-sticky, sticky or non-plastic, plastic
4. Consistency of Cemented Soil: weakly cemented, indurated (cemented)

Soil consistency is useful in estimating the ability of soil to support buildings and roads. More precise measures of soil strength are often made prior to construction.

Temperature

Soil temperature regulates germination, root growth and availability of nutrients. Soil temperatures range from permafrost at a few inches below the surface to 38 C (100 F) in Hawaii on a warm day. The color of the ground cover and insulating ability have a strong influence on soil temperature. Snow cover and heavy mulching will reflect light and slow the warming of the soil, but at the same time reduce the fluctuations in the surface temperature.

Below 50 cm (20 in) soil temperature seldom changes and can be approximated by adding 1.8 C (2 F) degrees to the mean annual air temperature

Most often soil temperatures must be accepted and agricultural activities adapted to them.

1. To maximize germination and growth by timing of planting.
2. To optimize use of anhydrous ammonia by applying to soil below 10 C (50 F).
3. To prevent heaving and thawing of frosts from damaging shallow rooted crops.
4. To prevent damage to soil tilth by freezing of saturated soils.
5. To improve uptake of phosphorus by plants.

Otherwise soil temperatures can be raised by drying soils or using clear plastic mulches. Organic mulches slow the warming of the soil

Color

In general, color is determined by organic matter content, drainage conditions, and the degree of oxidation. Soil color, while easily discerned, has little use in predicting soil characteristics. It is of use in distinguishing boundaries, as an indication of wetness and waterlogged conditions, and as a qualitative means of measuring organic and salt contents of soils.

Soil color is often the first impression one has when viewing soil. Striking colors and contrasting patterns are especially noticeable. The Red River (Mississippi watershed) carries sediment eroded from extensive reddish soils like Port Silt Loam in Oklahoma. The Yellow River in China carries yellow sediment from eroding loess soils. Mollisols in the Great Plains are darkened and enriched by organic matter. Podsols in boreal forests have highly contrasting layers due to acidity and leaching.

Soil color is primarily influenced by soil mineralogy. Many soil colors are due to various iron minerals. The development and distribution of color in a soil profile result from chemical and biological weathering, especially redox reactions. As the primary minerals in soil parent material weather, the elements combine into new and colorful compounds. Iron forms secondary minerals with a yellow or red color, organic matter decomposes into black and brown compounds, and manganese, sulfur and nitrogen can form black mineral deposits. These pigments can produce various color patterns within a soil. Aerobic conditions produce uniform or gradual color changes, while reducing environments (anaerobic) result in disrupted color flow with complex, mottled patterns and points of color concentration.^[29]

Resistivity

Soil resistivity is a measure of a soil's ability to retard the conduction of an electric current. The electrical resistivity of soil can affect the rate of galvanic corrosion of metallic structures in contact with the soil. Higher moisture content or increased electrolyte concentration can lower the resistivity and increase the conductivity thereby increasing the rate of corrosion.^[30][31] Soil resistivity values typically range from about 2 to 1000 Ω·m, but more extreme values are not unusual.^[32]

Soil horizons

Horizontal layers of the soil, whose physical features, composition and age are distinct from the ones above and beneath, are referred to as soil horizons. The naming of horizons is based on the type of material of which they are composed; these materials reflect the duration of specific processes in soil formation. They are labeled using a short hand notation of letters and numbers.^[33] They are described and classified by their color, size, texture, structure, consistency, root quantity, pH, voids, boundary characteristics, and presence of nodules or concretions.^[34] Few soil profiles have all the major horizons; soils may have few or many horizons.

The exposure of parent material to favorable conditions produces mineral soils that are marginally suitable for plant growth. Plant growth often results in the accumulation of organic residues. The accumulated organic layer called the O horizon produces a more active soil due to the effect of the organisms that live within it. Biological organisms colonize and break down organic materials, making available nutrients upon which other plants and animals can live. After sufficient time, humus is deposited in a distinctive organic surface layer called the A horizon.

Classification

Soil is classified into categories in order to understand relationships between different soils and to determine the usefulness of a soil for a particular use. One of the first classification systems was developed by the Russian scientist Dokuchaev around 1880. It was modified a number of times by American and European researchers, and developed into the system commonly used until the 1960s. It was based on the idea that soils have a particular morphology based on the materials and factors that form them. In the 1960s, a different classification system began to emerge, that focused on soil morphology instead of parental materials and soil-forming factors. Since then it has undergone further modifications. The World Reference Base for Soil Resources (WRB)^[35] aims to establish an international reference base for soil classification.

USDA Soil Taxonomy

In the United States, soil orders are the highest hierarchical level of soil classification in the USDA Soil Taxonomy classification system. Names of the orders end with the suffix -sol. There are 12 soil orders in Soil Taxonomy:^[36]

• Entisol - recently formed soils that lack well-developed horizons. Commonly found on unconsolidated sediments like sand, some have an A horizon on top of bedrock.
• Vertisol - inverted soils. They tend to swell when wet and shrink upon drying, often forming deep cracks that surface layers can fall into.
• Inceptisol - young soils. They have subsurface horizon formation but show little eluviation and illuviation.
• Aridisol - dry soils forming under desert conditions. They include nearly 20% of soils on Earth. Soil formation is slow, and accumulated organic matter is scarce. They may have subsurface zones (calcic horizons) where calcium carbonates have accumulated from percolating water. Many aridiso soils have well-developed Bt horizons showing clay movement from past periods of greater moisture.
• Mollisol - soft soils with very thick A horizons.
• Spodosol - soils produced by podsolization. They are typical soils of coniferous and deciduous forests in cooler climates.
• Alfisol - soils with aluminium and iron. They have horizons of clay accumulation, and form where there is enough moisture and warmth for at least three months of plant growth.
• Ultisol - soils that are heavily leached.
• Oxisol - soil with heavy oxide content.
• Histosol - organic soils.
• Andisols - volcanic soils, which tend to be high in glass content.
• Gelisols - permafrost soils.

Organic matter

The organic soil matter includes all the dead plant material and all creatures live and dead. The living component of an acre of soil may contain 900 pounds of earthworms, 2400 pounds of fungi, 1500 pounds of bacteria, 133 pounds of protozoa, 890 pounds of arthropods and algae.^[37]

Most living things in soils, including plants, insects, bacteria and fungi, are dependent on organic matter for nutrients and energy. Soils have varying organic compounds in varying degrees of decomposition. Organic matter holds soils open, allowing the infiltration air and water and may hold as much twice its weight in water. Many soils, including desert and rocky-gravel soils, have no or little organic matter. Soils that are all organic matter, such as peat (histosols), are infertile.^[38] In its earliest stage of decomposition the original organic material is often called raw organic matter. The final stage of decomposition is called humus.

Humus

Humus refers to organic matter that has been decomposed by bacteria, fungi and protozoa to the final point where it is resistant to further breakdown. Humus usually constitutes only 5 percent of the soil or less by volume but it is an essential source of nutrients and adds important textural qualities to soil critical to plant growth. Humus also hold bits of un-decomposed organic matter which feed arthropods and worms that further improve the soil. Humus has high cation exchange capacity that on a dry weight basis is many times greater than clay colloids and acts as a buffer against changes in pH.

Humic acids and fulvic acids are important constituents of humus that originate from decomposed foliage, stems and roots. After death, these plant residues begin to decay, resulting finally in the formation of humus. Humus formation involves changes within the soil and plant residue. There is a reduction of water soluble constituents including cellulose and hemicellulose; as the residues are deposited and break down, humin, lignin and lignin complexes accumulate within the soil; as microorganisms live and feed on decaying plant matter, an increase in these proteins occurs.

Lignin is resistant to breakdown and accumulates within the soil; it also chemically reacts with amino acids which add to its resistance to decomposition, including enzymatic decomposition by microbes. Fats and waxes from plant matter have some resistance to decomposition and persist in soils for a while. Clay soils often have higher organic contents that persist longer than soils without clay. Proteins normally decompose readily, but when bound to clay particles they become more resistant to decomposition. Clay particles also absorb enzymes that would break down proteins. The addition of organic matter to clay soils can render the organic matter and any added nutrients inaccessible to plants and microbes for many years, since they can bind strongly to the clay. High soil tannin (polyphenol) content from plants can cause nitrogen to be sequestered in proteins or cause nitrogen immobilization, also making nitrogen unavailable to plants.^[39][40]

Humus formation is a process dependent on the amount of plant material added each year and the type of base soil; both are affected by climate and the type of organisms present. Soils with humus can vary in nitrogen content but have 3 to 6 percent nitrogen typically; humus, as a reserve of nitrogen and phosphorus, is a vital component affecting soil fertility.^[38] Humus also absorbs water, acting as a moisture reserve, that plants can utilize; it also expands and shrinks between dry and wet states, providing pore spaces. Humus is less stable than the soil's mineral constituents, because it is affected by microbial decomposition, and over time its concentration decreases without the addition of new organic matter. However, humus may persist over centuries if not millennia.

Climate and organics

The production and accumulation or degradation of organic matter and humus is greatly dependent on climate conditions. Temperature and soil moisture are the major factors in the formation or degradation of organic matter, they along with topography, determine the formation of organic soils. Soils high in organic matter tend to form under wet or cold conditions where decomposer activity is impeded by low temperature^[41] or excess moisture.^[42]

Soil solutions

Soils retain water that can dissolve a range of molecules and ions. These solutions exchange gases with the soil atmosphere, contain dissolved sugars, fulvic acids and other organic acids, plant nutrients such as nitrate, ammonium, potassium, phosphate, sulfate and calcium, and micronutrients such as zinc, iron and copper. These nutrients are exchanged with the mineral and humic component, that retains them in its ionic state, by adsorption. Some arid soils have sodium solutions that greatly impact plant growth. Soil pH can affect the type and amount of anions and cations that soil solutions contain and that be exchanged between the soil substrate and biological organisms.^[43]

In nature

Biogeography is the study of special variations in biological communities. Soils determine which plants can grow in which environments. Soil scientists survey soils in the hope of understanding the parameters that determine what vegetation can and will grow in a particular location.

Geologists also have a particular interest in the patterns of soil on the surface of the earth. Soil texture, color and chemistry often reflect the underlying geologic parent material, and soil types often change at geologic unit boundaries. Buried paleosols mark previous land surfaces and record climatic conditions from previous eras. Geologists use this paleopedological record to understand the ecological relationships that existed in the past. According to the theory of biorhexistasy, prolonged conditions conducive to forming deep, weathered soils result in increasing ocean salinity and the formation of limestone.

Geologists use soil profile features to establish the duration of surface stability in the context of geologic faults or slope stability. An offset subsoil horizon indicates rupture during soil formation and the degree of subsequent subsoil formation is relied upon to establish time since rupture occurred.

A homeowner tests soil to apply only the nutrients needed.

Due to their thermal mass, rammed earth walls fit in with environmental sustainability aspirations.

A homeowner sifts soil made from his compost bin in background. Composting is an excellent way to recycle organic wastes.

Sediment in the Yellow River.

Soil examined in shovel test pits is used by archaeologists for relative dating based on stratigraphy (as opposed to absolute dating). What is considered most typical is to use soil profile features to determine the maximum reasonable pit depth than needs to be examined for archaeological evidence in the interest of cultural resources management.

Soils altered or formed by humans (anthropic and anthropogenic soils) are also of interest to archaeologists, such as terra preta soils.

Uses

Soil is used in agriculture, where it serves as the anchor and primary nutrient base for plants; however, as demonstrated by hydroponics, it is not essential to plant growth if the soil-contained nutrients could be dissolved in a solution. The types of soil and available moisture determine the species of plants that can be cultivated.

Soil material is a critical component in the mining and construction industries. Soil serves as a foundation for most construction projects. The movement of massive volumes of soil can be involved in surface mining, road building and dam construction. Earth sheltering is the architectural practice of using soil for external thermal mass against building walls.

Soil resources are critical to the environment, as well as to food and fiber production. Soil provides minerals and water to plants. Soil absorbs rainwater and releases it later, thus preventing floods and drought. Soil cleans the water as it percolates through it. Soil is the habitat for many organisms: the major part of known and unknown biodiversity is in the soil, in the form of invertebrates (earthworms, woodlice, millipedes, centipedes, snails, slugs, mites, springtails, enchytraeids, nematodes, protists), bacteria, archaea, fungi and algae; and most organisms living above ground have part of them (plants) or spend part of their life cycle (insects) belowground. Above-ground and below-ground biodiversities are tightly interconnected,^[44][45] making soil protection of paramount importance for any restoration or conservation plan.

The biological component of soil is an extremely important carbon sink since about 57% of the biotic content is carbon. Even on desert crusts, cyanobacteria lichens and mosses capture and sequester a significant amount of carbon by photosynthesis. Poor farming and grazing methods have degraded soils and released much of this sequestered carbon to the atmosphere. Restoring the world's soils could offset some of the huge increase in greenhouse gases causing global warming while improving crop yields and reducing water needs.^[46][47][48]

Waste management often has a soil component. Septic drain fields treat septic tank effluent using aerobic soil processes. Landfills use soil for daily cover. Land application of wastewater relies on soil biology to aerobically treat BOD.

Organic soils, especially peat, serve as a significant fuel resource; but wide areas of peat production, such as sphagnum bogs, are now protected because of patrimonial interest.

Both animals and humans in many cultures occasionally consume soil. It has been shown that some monkeys consume soil, together with their preferred food (tree foliage and fruits), in order to alleviate tannin toxicity.^[49][1]

Soils filter and purify water and affect its chemistry. Rain water and pooled water from ponds, lakes and rivers percolate through the soil horizons and the upper rock strata, thus becoming groundwater. Pests (viruses) and pollutants, such as persistent organic pollutants (chlorinated pesticides, polychlorinated biphenyls), oils (hydrocarbons), heavy metals (lead, zinc, cadmium), and excess nutrients (nitrates, sulfates, phosphates) are filtered out by the soil.^[50] Soil organisms metabolize them or immobilize them in their biomass and necromass,^[51] thereby incorporating them into stable humus.^[52] The physical integrity of soil is also a prerequisite for avoiding landslides in rugged landscapes.^[53]

Degradation

Land degradation^[54] is a human-induced or natural process which impairs the capacity of land to function. Soils are the critical component in land degradation when it involves acidification, contamination, desertification, erosion or salination.

While soil acidification of alkaline soils is beneficial, it degrades land when soil acidity lowers crop productivity and increases soil vulnerability to contamination and erosion. Soils are often initially acid because their parent materials were acid and initially low in the basic cations (calcium, magnesium, potassium and sodium). Acidification occurs when these elements are removed from the soil profile by normal rainfall, or the harvesting of forest or agricultural crops. Soil acidification is accelerated by the use of acid-forming nitrogenous fertilizers and by the effects of acid precipitation.

Soil contamination at low levels is often within soil capacity to treat and assimilate. Many waste treatment processes rely on this treatment capacity. Exceeding treatment capacity can damage soil biota and limit soil function. Derelict soils occur where industrial contamination or other development activity damages the soil to such a degree that the land cannot be used safely or productively. Remediation of derelict soil uses principles of geology, physics, chemistry and biology to degrade, attenuate, isolate or remove soil contaminants to restore soil functions and values. Techniques include leaching, air sparging, chemical amendments, phytoremediation, bioremediation and natural attenuation.

Desertification is an environmental process of ecosystem degradation in arid and semi-arid regions, often caused by human activity. It is a common misconception that droughts cause desertification. Droughts are common in arid and semiarid lands. Well-managed lands can recover from drought when the rains return. Soil management tools include maintaining soil nutrient and organic matter levels, reduced tillage and increased cover. These practices help to control erosion and maintain productivity during periods when moisture is available. Continued land abuse during droughts, however, increases land degradation. Increased population and livestock pressure on marginal lands accelerates desertification.

Soil erosional loss is caused by wind, water, ice and movement in response to gravity. Although the processes may be simultaneous, erosion is distinguished from weathering. Erosion is an intrinsic natural process, but in many places it is increased by human land use. Poor land use practices including deforestation, overgrazing and improper construction activity. Improved management can limit erosion by using techniques like limiting disturbance during construction, avoiding construction during erosion prone periods, intercepting runoff, terrace-building, use of erosion-suppressing cover materials, and planting trees or other soil binding plants.

A serious and long-running water erosion problem occurs in China, on the middle reaches of the Yellow River and the upper reaches of the Yangtze River. From the Yellow River, over 1.6-billion tons of sediment flow each year into the ocean. The sediment originates primarily from water erosion (gully erosion) in the Loess Plateau region of northwest China.

Soil piping is a particular form of soil erosion that occurs below the soil surface. It is associated with levee and dam failure, as well as sink hole formation. Turbulent flow removes soil starting from the mouth of the seep flow and subsoil erosion advances upgradient.^[55] The term sand boil is used to describe the appearance of the discharging end of an active soil pipe.^[56]

Soil salination is the accumulation of free salts to such an extent that it leads to degradation of soils and vegetation. Consequences include corrosion damage, reduced plant growth, erosion due to loss of plant cover and soil structure, and water quality problems due to sedimentation. Salination occurs due to a combination of natural and human caused processes. Arid conditions favor salt accumulation. This is especially apparent when soil parent material is saline. Irrigation of arid lands is especially problematic.^[57] All irrigation water has some level of salinity. Irrigation, especially when it involves leakage from canals and overirrigation in the field, often raises the underlying water table. Rapid salination occurs when the land surface is within the capillary fringe of saline groundwater. Soil salinity control involves watertable control and flushing with higher levels of applied water in combination with tile drainage or another form of subsurface drainage.^[58][59]

Soil salinity models like SWAP,^[60] DrainMod-S,^[61] UnSatChem,^[62] SaltMod^[63][64] and SahysMod^[65] are used to assess the cause of soil salination and to optimize the reclamation of irrigated saline soils.

Reclamation

Soils that contain high levels of particular clays, such as smectites, are often very fertile. For example, the smectite-rich clays of Thailand's Central Plains are among the most productive in the world.

Many farmers in tropical areas, however, struggle to retain organic matter in the soils they work. In recent years, for example, productivity has declined in the low-clay soils of northern Thailand. Farmers initially responded by adding organic matter from termite mounds, but this was unsustainable in the long-term. Scientists experimented with adding bentonite, one of the smectite family of clays, to the soil. In field trials, conducted by scientists from the International Water Management Institute in cooperation with Khon Kaen University and local farmers, this had the effect of helping retain water and nutrients. Supplementing the farmer's usual practice with a single application of 200kg bentonite per rai (6.26 rai = 1 hectare) resulted in an average yield increase of 73%. More work showed that applying bentonite to degraded sandy soils reduced the risk of crop failure during drought years.

In 2008, three years after the initial trials, IWMI scientists conducted a survey among 250 farmers in northeast Thailand, half who had applying bentonite to their fields and half who had not. The average output for those using the clay addition was 18% higher than for non-clay users. Using the clay had enabled some farmers to switch to growing vegetables, which need more fertile soil. This helped to increase their income. The researchers estimated that 200 farmers in northeast Thailand and 400 in Cambodia had adopted the use of clays, and that a further 20,000 farmers were introduced to the new technique.^[66]

If the soil is too high in clay, adding gypsum, washed river sand and organic matter will balance the composition. Adding organic matter to soil that is depleted in nutrients and too high in sand will boost the quality.^[67]

See also

Wikiquote has a collection of quotations related to: Soil

Wikimedia Commons has media related to: Soils

References

Further reading

• Soil-Net.com A free schools-age educational site teaching about soil and its importance.
• Adams, J.A. 1986. Dirt. College Station, Texas : Texas A&M University Press ISBN 0-89096-301-0
• Certini, G., Scalenghe, R., 2006. Soils: Basic concepts and future challenges. Cambridge Univ Press, Cambridge UK.
• David R. Montgomery, Dirt: The Erosion of Civilizations, ISBN 978-0-5202-5806-8
• Faulkner, Edward H. Plowman's Folly. New York, Grosset & Dunlap. 1943. ISBN 0-933280-51-3
• LandIS Free Soilscapes Viewer Free interactive viewer for the Soils of England and Wales
• Geo-technological Research Paper, IIT Kanpur, Dr P P Vitkar - Strip footing on weak clay stabilized with a granular pile National Research Council Canada: From Discovery to Innovation / Conseil national de recherches Canada : de la découverte à l'innovation (English), (French)
• Jenny, Hans, Factors of Soil Formation: A System of Quantitative Pedology 1941 http://www.soilandhealth.org/01aglibrary/010159.Jenny.pdf
• Logan, W. B., Dirt: The ecstatic skin of the earth. 1995 ISBN 1-57322-004-3
• Mann, Charles C.: " Our good earth" National Geographic Magazine September 2008
• "97 Flood". USGS. http://www.mvm.usace.army.mil/Readiness/97flood/flood.htm. Retrieved 2008-07-08.  Photographs of sand boils.
• Soil Survey Division Staff. (1999) Soil survey manual. Soil Conservation Service. U.S. Department of Agriculture Handbook 18.
• Soil Survey Staff. (1975) Soil Taxonomy: A basic system of soil classification for making and interpreting soil surveys. USDA-SCS Agric. Handb. 436. United States Government Printing Office, Washington, DC.
• Soils, Oregon State University
• Why Study Soils?
• Soil notes
• LandIS Soils Data for England and Wales a pay source for GIS data on the soils of England and Wales and soils data source; they charge a handling fee to researchers.

External links