Soil is a natural body consisting of layers (soil horizons A soil horizon is a specific layer in the land area which measures parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath. Horizon formation is a function of a range of geological, chemical, and biological processes and occurs over long time periods. Soils vary in the degree to which) of mineral A mineral is a naturally occurring solid chemical substance that is formed through geological processes and that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not have a specific chemical composition constituents of variable thicknesses, which differ from the parent materials in their morphological Geomorphology is the scientific study of landforms and the processes that shape them. Geomorphologists seek to understand why landscapes look the way they do: to understand landform history and dynamics, and predict future changes through a combination of field observation, physical experiment, and numerical modeling. Geomorphology is practiced, physical, chemical, and mineralogical Mineralogy is the study of chemistry, crystal structure, and physical properties of minerals. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization characteristics.^[1]

It is composed of particles of broken rock In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids that have been altered by chemical and environmental processes that include weathering Weathering is the breaking down of Earth's rocks, soils and minerals through direct contact with the planet's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, wind, and gravity and erosion Erosion is the process of weathering and transport of solids in the natural environment or their source and deposits them elsewhere. It usually occurs due to transport by wind, water, or ice; by down-slope creep of soil and other material under the force of gravity; or by living organisms, such as burrowing animals, in the case of bioerosion. Soil differs from its parent rock Parent rock refers to the original rock from which something else was formed. It is mainly used in the context of soil formation where the parent rock will have a large influence on the nature of the resulting soil. The term is also used in the context of metamorphic rocks where again the parent rock refers to the original rock before metamorphism due to interactions between the lithosphere The lithosphere is the rigid outermost shell of a rocky planet. It comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater, hydrosphere A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet, atmosphere An atmosphere is a layer of gases that may surround a material body of sufficient mass,and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low. Some planets consist mainly of various gases, but only their outer layer is their, and the biosphere The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth. From the broadest biophysiological point of view, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere, hydrosphere and atmosphere. The.^[2][3].

It supports a complex ecosystem An ecosystem consists of all the organisms living in a particular area, as well as all the nonliving, physical components of the environment with which the organisms interact, such as air, soil, water, and sunlight. It is all the organisms in a given area, along with the nonliving factors with which they interact; a biological community and its, which supports the plants on the surface and creates new soil by breaking down rocks and sand.^[4]. This microscopic ecosystem has co-evolved with the plants to collect and store water and nutrients in a form usable by plants.^[5]

Soil particles pack loosely, forming a soil structure filled with pore spaces. These pores contain soil solution In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent (liquid) and air (gas).^[6] Accordingly, soils are often treated as a three state States of matter are the distinct forms that different phases of matter take on. Historically, the distinction is made based on qualitative differences in bulk properties. Solid is the state in which matter maintains a fixed volume and shape; liquid is the state in which matter maintains a fixed volume but adapts to the shape of its container; and-system.^[7] Most soils have a density The density of a material is defined as its mass per unit volume. The symbol of density is ρ . In some countries (for instance, in the United States), density is also defined as its weight per unit volume between 1 and 2 g/cm³.^[8] Soil is also known as earth: it is the substance from which our planet takes its name. Little of the soil composition of planet Earth is older than the Tertiary The Tertiary is a term for a geologic period 65 million to 1.8 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Tertiary extinction event, at the start of the Cenozoic era, spanning to the beginning of and most no older than the Pleistocene The Pleistocene is the epoch from 2.588 million to 12,000 years BP covering the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek πλεῖστος (pleistos "most") and καινός (kainos "new").^[9] In engineering Engineering is the discipline, art and profession of acquiring and applying technical, scientific, and mathematical knowledge to design and implement materials, structures, machines, devices, systems, and processes that safely realize a desired objective or invention, soil is referred to as regolith Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other planets, or loose rock material.

Soil forming factors

Soil formation, or pedogenesis Pedogenesis or soil evolution is the process by which soil is created. It is the major topic of the science of pedology, whose other aspects include the soil morphology, classification (taxonomy) of soils, and their distribution in nature, present and past (soil geography and paleopedology),is the combined effect of physical, chemical, biological, and anthropogenic Anthropogenic effects, processes or materials are those that are derived from human activities, as opposed to those occurring in biophysical environments without human influence 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 A soil horizon is a specific layer in the land area which measures parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath. Horizon formation is a function of a range of geological, chemical, and biological processes and occurs over long time periods. Soils vary in the degree to which.

The weathering of bedrock produces the parent material from which soils form. An example of soil development from bare rock occurs on recent lava flows Lava is molten rock expelled by a volcano during an eruption. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at temperatures from 700 °C to 1,200 °C . Up to 100,000 times as viscous as water, lava can flow great distances before in warm regions under heavy and very frequent rainfall. In such climates, plants become established very quickly on basaltic Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey lava, even though there is very little organic material. The plants are supported by the porous rock as it is filled with nutrient A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. Nutrients are the substances that enrich the body. They build and repair tissues, give heat and energy, and regulate body processes. Methods for nutrient intake vary, with animals and-bearing water which carries, for example, dissolved minerals and guano Guano is the excrement (feces and urine) of seabirds, bats, and seals. Guano manure is an effective fertilizer and gunpowder ingredient due to its high levels of phosphorus and nitrogen and also its lack of odor. Superphosphate made from guano is used for aerial topdressing. Soil that is deficient in organic matter can be made more productive by. The developing plant roots, themselves or associated with mycorrhizal A mycorrhiza is a symbiotic (generally mutualistic, but occasionally weakly pathogenic) association between a fungus and the roots of a vascular plant fungi,^[10] gradually break up the porous lava and organic matter soon accumulates.

But even before it does, the predominantly porous broken lava in which the plant roots grow can be considered a soil. 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, they include: parent material, regional climate, topography, biotic potential and the passage of time.^[11]

Parent material

The material from which soils form is called parent material. It includes: weathered primary bedrock; secondary material transported from other locations, e.g. colluvium Colluvium is the name for loose bodies of sediment that have been deposited or built up at the bottom of a low-grade slope or against a barrier on that slope, transported by gravity. The deposits that collect at the foot of a steep slope or cliff are also known by the same name. Colluvium often outerfingers with alluvium . Coarse deposits due to and alluvium Alluvium is loose, unconsolidated (not cemented together into a solid rock), soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel. When this loose alluvial material; deposits that are already present but mixed or altered in other ways - old soil formations, organic material including peat Peat is an accumulation of partially decayed vegetation matter. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world. By volume there are about 4 trillion m³ of peat in the world covering a total of around 2% of global land mass , or alpine humus; and anthropogenic Anthropogenic effects, processes or materials are those that are derived from human activities, as opposed to those occurring in biophysical environments without human influence materials, like landfill A landfill, also known as a dump or rubbish dump , is a site for the disposal of waste materials by burial and is the oldest form of waste treatment. Historically, landfills have been the most common methods of organized waste disposal and remain so in many places around the world or mine waste.^[12] Few soils form directly from the breakdown of the underlying rocks they develop on. These soils are often called “residual soils”, and have the same general chemistry as their parent rocks. Most soils derive from materials that have been transported from other locations by wind, water and gravity.^[13] Some of these materials may have moved many miles or only a few feet. Windblown material called loess Loess is an aeolian sediment formed by the accumulation of wind-blown silt and lesser and variable amounts of sand and clay that are loosely cemented by calcium carbonate. It is usually homogeneous and highly porous and is traversed by vertical capillaries that permit the sediment to fracture and form vertical bluffs is common in the Midwest The Midwestern United States is one of the four geographic regions within the United States of America used by the United States Census Bureau in its reporting of North America North America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific Ocean; South America lies to the southeast and in Central Asia Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.6% of the Earth's total surface area and with approximately 4 billion people, it hosts 60% of the world's current human population. During the 20th century Asia's population nearly quadrupled and other locations. Glacial till Till is unsorted glacial sediment. Glacial drift is a general term for the coarsely graded and extremely heterogeneous sediments of glacial origin. Glacial till is that part of glacial drift which was deposited directly by the glacier. It may vary from clays to mixtures of clay, sand, gravel and boulders. Clay in till may form in spherical shapes is a component of many soils in the northern and southern latitudes Latitude, usually denoted by the Greek letter phi gives the location of a place on Earth (or other planetary body) north or south of the equator. Lines of Latitude are the imaginary horizontal lines shown running east-to-west (or west to east) on maps (particularly so in the Mercator projection) that run either north or south of the equator and those formed near large mountains; till is the product of glacial ice A glacier is a perennial mass of ice which moves over land. A glacier forms in locations where the mass accumulation of snow and ice exceeds ablation over many years. The word glacier comes from French via the Vulgar Latin glacia, and ultimately from Latin glacies meaning ice. The corresponding area of study is called glaciology moving over the ground. The ice can break rock and larger stones into smaller pieces, it also can sort material into different sizes. As glacial ice melts, the melt water also moves and sorts material, and deposits it varying distances from its origin. The deeper sections of the soil profile may have materials that are relatively unchanged from when they were deposited by water, ice or wind.

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 Saprolite is the name for a chemically weathered rock. It is mostly soft or friable and commonly retains the structure of the parent rock since it is not transported, but autochthonously formed in place may form. Saprolite is the result of weathering processes that include: hydrolysis (the replacement of a mineral’s cations with hydrogen ions), chelation Chelation is the formation or presence of two or more separate bindings between a polydentate (multiple bonded) ligand and a single central atom. Usually these ligands are organic compounds, and are called chelants, chelators, chelating agents, or sequestering agents from organic compounds, hydration (the absorption of water by minerals), solution of minerals by water, and physical processes that include freezing and thawing or wetting and drying.^[12] The mineralogical and chemical composition of the primary bedrock material, plus physical features, including grain size and degree of consolidation, plus the rate and type of weathering, transforms it into different soil materials.

Climate

Soil formation greatly depends on the climate, and soils from different climate zones The Köppen climate classification is one of the most widely used climate classification systems. It was first published by the Russian climatologist Wladimir Köppen in 1884, with several later modifications by Köppen himself notably in 1918 and 1936. Later, the German climatologist Rudolf Geiger collaborated with Köppen on changes to the show distinctive characteristics.^[14] Temperature and moisture affect weathering and leaching. Wind moves sand and other particles, especially in arid regions where there is little plant cover. The type and amount of precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that is pulled down by gravity and deposited on the Earth's surface. The main forms of precipitation include rain, snow, ice pellets, and graupel. It occurs when the atmosphere, a large gaseous solution, becomes saturated with water vapour and the water influence soil formation by affecting the movement of ions and particles through the soil, aiding in the development of different soil profiles. Seasonal and daily temperature fluctuations affect the effectiveness of water in weathering parent rock material and affect soil dynamics. The cycle of freezing and thawing is an effective mechanism to break up rocks and other consolidated materials. Temperature and precipitation rates affect biological activity, rates of chemical reactions and types of vegetation cover.

Biological factors

Plants, animals, fungi A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. The Fungi (pronounced /ˈfʌndʒaɪ/ or /ˈfʌŋɡaɪ/) are classified as a kingdom that is separate from plants, animals and bacteria. One major difference is that fungal cells have cell, bacteria The bacteria ( [bækˈtɪəriə] ; singular: bacterium)[α] are a large group of single-celled, prokaryote microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste, and humans affect soil formation. Animals and micro-organisms A microorganism or microbe is an organism that is microscopic (too small to be seen by the naked human eye). The study of microorganisms is called microbiology, a subject that began with Anton van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design mix soils to form burrows A burrow is a hole or tunnel dug into the ground by an animal to create a space suitable for habitation, temporary refuge, or as a byproduct of locomotion. Burrows provide a form of shelter against predation and exposure to the elements, so the burrowing way of life is quite popular among the animals. Burrows are also commonly preserved in the and pores allowing moisture and gases to seep into deeper layers. In the same way, plant roots open channels in the soils, especially plants with deep taproots which can penetrate many meters through the different soil layers to bring up nutrients from deeper in the soil. 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, affect chemical exchanges between roots and soil and act as a reserve of nutrients. Humans can impact soil formation by removing vegetation cover; this removal promotes erosion. They can also mix the different soil layers, restarting the soil formation process as less-weathered material is mixed with and diluting 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.^[15]

Vegetation impacts soils in numerous ways. It can prevent erosion from rain or surface runoff. It shades soils, keeping them cooler and slowing evaporation of soil moisture, or it can cause soils to dry out by transpiration. Plants can form new chemicals that break down or build up soil particles. Vegetation depends on climate, land form topography and biological factors. Soil factors such as soil density, depth, chemistry, pH, temperature and moisture greatly affect the type of plants that can grow in a given location. Dead plants, dropped leaves and stems of plants 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 organic compounds become part of the soil formation process, ultimately shaping the type of soil formed.

Time

Time is a factor in the interactions of all the above factors as they develop soil. Over time, soils evolve features dependent on the other forming factors, and soil formation is a time-responsive process dependent on how the other factors interplay with each other. Soil is always changing. For example, recently-deposited material from a flood exhibits no soil development because there has not been enough time for soil-forming activities. The soil surface is buried, and the formation process begins again for this soil. 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 in 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 away 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.

Characteristics

Soil color is often the first impression one has when viewing soil. Striking colors and contrasting patterns are especially memorable. 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 loessal 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 the extensive and 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 produce various color patterns due to effects by the environment during soil formation. Aerobic conditions produce uniform or gradual color changes, while reducing environments result in disrupted color flow with complex, mottled patterns and points of color concentration.^[16]

Soil structure is the arrangement of soil particles into aggregates. These may have various shapes, sizes and degrees of development or expression.^[17] Soil structure affects aeration, water movement, resistance to erosion and plant root growth. 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.

Soil texture refers to sand, silt and clay composition. Soil content affects soil behavior, including the retention capacity for nutrients and water.^[18] Sand and silt are the products of physical weathering, while clay is the product of chemical weathering. Clay content has retention capacity for nutrients and water. Clay soils resist wind and water erosion better than silty and sandy soils, because the particles are more tightly joined to each other. In medium-textured soils, clay is often translocated downward through the soil profile and accumulates in the subsoil.

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 thereby increase the rate of corrosion.^[19] Soil resistivity values typically range from about 2 to 1000 Ω·m, but more extreme values are not unusual.^[20]

Soil horizons

The naming of soil horizons is based on the type of material the horizons are composed of; these materials reflect the duration of the specific processes used in soil formation. They are labeled using a short hand notation of letters and numbers.^[21] They are described and classified by their color, size, texture, structure, consistency, root quantity, pH, voids, boundary characteristics, and if they have nodules or concretions.^[22] Any one soil profile does not have all the major horizons covered below; soils may have few or many horizons.

The exposure of parent material to favorable conditions produces initial soils that are suitable for plant growth. Plant growth often results in the accumulation of organic residues, the accumulated organic layer is called the O horizon. Biological organisms colonize and break down organic materials, making available nutrients that other plants and animals can live on. After sufficient time a distinctive organic surface layer forms with humus which is 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)^[23] aims to establish an international reference base for soil classification.

Orders

Orders are the highest category of soil classification. Order types end in the letters sol. In the US classification system, there are 10 orders:^[24]

• 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.

Other order schemes may include:

• Andisols - volcanic soils, which tend to be high in glass content.
• Gelisols - permafrost soils.

Organic matter

Most living things in soils, including plants, insects, bacteria and fungi, are dependent on organic matter for nutrients and energy. Soils often have varying degrees of organic compounds in different states of decomposition. 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.^[25]

Humus

Humus refers to organic matter that has decomposed to a point where it is resistant to further breakdown or alteration. Humic acids and fulvic acids are important constituents of humus and typically form from plant residues like foliage, stems and roots. After death, these plant residues begin to decay, starting 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 the decaying plant matter, an increase in 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 by proteins or cause nitrogen immobilization, also making nitrogen unavailable to plants.^[26][27]

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.^[25] 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 other soil constituents, because it is affected by microbial decomposition, and over time its concentration decreases without the addition of new organic matter. However, some forms of humus are highly stable and may persist over centuries if not millennia: they are issued from the slow oxidation of charcoal, also called black carbon, like in Amazonian Terra preta or Black Earths,^[28] or from the sequestration of humic compounds within mineral horizons, like in podzols.^[29]

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^[30] or excess moisture.^[31]

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. 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 exchange with the soil atmosphere and biological organisms.^[32]

In nature

Biogeography is the study of special variations in biological communities. Soils are a restricting factor as to which plants can grow in which environments. Soil scientists survey soils in the hope of understanding controls as to 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 in past ecosystems. 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.

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 man (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 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 used in agriculture (among other things, such as the purported level of moisture in the soil) vary with respect to the species of plants that are cultivated.

Soil material is a critical component in the mining and construction industries. Soil serves as a foundation for most construction projects. 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. 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,^[33][34] 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 the huge increase in greenhouse gases causing global warming while improving crop yields and reducing water needs.^[35][36]

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.^[37][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^[38]. Soil organisms metabolize them or immobilize them in their biomass and necromass,^[39] thereby incorporating them into stable humus.^[40] The physical integrity of soil is also a prerequisite for avoiding landslides in rugged landscapes.^[41]

Degradation

Land degradation 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.^[42] The term sand boil is used to describe the appearance of the discharging end of an active soil pipe.^[43]

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.^[44] 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.^[45] Soil salinity models like SWAP ^[46], DrainMod-S ^[47], UnSatChem ^[48], SaltMod ^{[49] [50]} and SahysMod ^[51] are used to assess the cause of soil salination and to optimize the reclamation of irrigated saline soils.

See also

References

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2. ^ Chesworth, Edited by Ward (2008), Encyclopedia of soil science, Dordrecht, Netherland: Springer, xxiv, ISBN 1402039948
3. ^ James A. Danoff-Burg, Columbia University The Terrestrial Influence: Geology and Soils
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5. ^ http://www.soilsecrets.com/Soil%20Ecology%20and%20the%20Soil%20Food%20Web.pdf Michael Melendrez,2003, Soil Ecology & The Food Web
6. ^ Taylor, S. A., and G. L. Ashcroft. 1972. Physical Edaphology
7. ^ McCarty, David. 1982. Essentials of Soil Mechanics and Foundations
8. ^ Pedosphere.com
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10. ^ Van Schöll, Laura; Smits, Mark M. & Hoffland, Ellis (2006), "Ectomycorrhizal weathering of the soil minerals muscovite and hornblende", New Phytologist 171 (4): 805–814, doi:10.1111/j.1469-8137.2006.01790.x, PMID 16918551
11. ^ University of Wisconsin–Stevens Point
12. ^ ^{a b} NSW Government
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21. ^ Retallack, G. J. (1990), Soils of the past : an introduction to paleopedology, Boston: Unwin Hyman, pp. 32, ISBN 9780044457572, http://books.google.com/?id=YVkVAAAAIAAJ&pg=PA32&dq=Soil+horizons
22. ^ Buol, S.W. (1990), Soil genesis and classification, Ames, Iowe: Iowa State University Press, pp. 36, doi:10.1081/E-ESS, ISBN 0813828732, http://books.google.com/?id=QM0kfIGYMjcC&printsec=frontcover&dq=Soil
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24. ^ University of Virginia
25. ^ ^{a b} Foth, Henry D. (1984), Fundamentals of soil science, New York: Wiley, pp. 151, ISBN 0471889261
26. ^ Verkaik, Eric; Jongkind, Anne G.; Berendse, Frank (2006), "Short-term and long-term effects of tannins on nitrogen mineralization and litter decomposition in kauri (Agathis australis (D. Don) Lindl.) forests", Plant and Soil 287: 337, doi:10.1007/s11104-006-9081-8
27. ^ Fierer, N. (2001), "Influence of balsam poplar tannin fractions on carbon and nitrogen dynamics in Alaskan taiga floodplain soils", Soil Biology and Biochemistry 33: 1827, doi:10.1016/S0038-0717(01)00111-0
28. ^ Solomon, Dawit; Lehmann Johannes, Thies Janice, Schäfer Thorsten, Liang Biqing, Kinyangi James, Neves Eduardo, Petersen James, Luizão Flavio & Skjemstad Jan (2007), "Molecular signature and sources of biochemical recalcitrance of organic C in Amazonian Dark Earths", Geochimica et Cosmochimica Acta 71: 2285–2298, doi:10.1016/j.gca.2007.02.014
29. ^ Nierop, Klaas G. J.; Verstraten Jacobus M. (2003), "Organic matter formation in sandy subsurface horizons of Dutch coastal dunes in relation to soil acidification", Organic Geochemistry 34: 499–513, doi:10.1016/S0146-6380(02)00249-8
30. ^ Wagai, Rota; Mayer Lawrence M., Kitayama Kanehiro & Knicker Heike (2008), "Climate and parent material controls on organic matter storage in surface soils: A three-pool, density-separation approach", Geoderma 147: 23–33, doi:10.1016/j.geoderma.2008.07.010
31. ^ Minayeva, T. Yu.; Trofimov S. Ya., Chichagova O.A., Dorofeyeva E.I., Sirin A.A., Glushkov I.V., Mikhailov N.D. & Kromer B. (2008), "Carbon accumulation in soils of forest and bog ecosystems of southern Valdai in the Holocene", Biology Bulletin 35: 524–532, doi:10.1134/S1062359008050142
32. ^ Dan (2000), Ecology and management of forest soils, New York: John Wiley, pp. 88–92, ISBN 0471194263, http://books.google.com/?id=SAbMIJ_O8dMC&pg=PA91&dq=soils+and+solutions
33. ^ Ponge, Jean-François (2003), "Humus forms in terrestrial ecosystems: a framework to biodiversity", Soil Biology and Biochemistry 35: 935–945, doi:10.1016/S0038-0717(03)00149-4
34. ^ De Deyn, Gerlinde B.; Van der Putten Wim H. (2005), "Linking aboveground and belowground diversity", Trends in Ecology & Evolution 20 (11): 625–633, doi:10.1016/j.tree.2005.08.009, PMID 16701446
35. ^ http://www.sciencemag.org/cgi/content/short/304/5677/1623 r> Lal, 2004, Soil Carbon Sequestration Impacts on Global Climate Change and Food Security
36. ^ http://www.renewableenergyworld.com/rea/news/article/2010/02/greening-deserts-for-carbon-credits Blakeslee, Thomas 2010 Greening Deserts for Carbon Credits
37. ^ Setz, EZF; Enzweiler J, Solferini VN, Amendola MP, Berton RS (1999), "Geophagy in the golden-faced saki monkey (Pithecia pithecia chrysocephala) in the Central Amazon", Journal of Zoology 247: 91–103, doi:10.1111/j.1469-7998.1999.tb00196.x
38. ^ Kohne, John Maximilian; Koehne Sigrid, Simunek Jirka (2009), "A review of model applications for structured soils: a) Water flow and tracer transport", Journal of Contaminant Hydrology 104 (1-4): 4–35, doi:10.1016/j.jconhyd.2008.10.002, PMID 19012994
39. ^ Diplock, EE; Mardlin DP, Killham KS, Paton GI (2009), "Predicting bioremediation of hydrocarbons: laboratory to field scale", Environmental Pollution 157 (6): 1831–1840, doi:10.1016/j.envpol.2009.01.022, PMID 19232804
40. ^ Moeckel, Claudia; Nizzetto Luca, Di Guardo Antonio, Steinnes Eiliv, Freppaz Michele, Filippa Gianluca, Camporini Paolo, Benner Jessica, Jones Kevin C. (2008), "Persistent organic pollutants in boreal and montane soil profiles: distribution, evidence of processes and implications for global cycling", Environmental Science and Technology 42 (22): 8374–8380, doi:10.1021/es801703k, PMID 19068820
41. ^ Rezaei, Khalil; Guest Bernard, Friedrich Anke, Fayazi Farajollah, Nakhaei Mohamad, Aghda Seyed Mahmoud Fatemi, Beitollahi Ali (2009), "Soil and sediment quality and composition as factors in the distribution of damage at the December 26, 2003, Bam area earthquake in SE Iran (M (s)=6.6)", Journal of Soils and Sediments 9: 23–32, doi:10.1007/s11368-008-0046-9
42. ^ Jones, J. A. A. (1976), "Soil piping and stream channel initiation", Water Resources Research 7 (3): 602–610, doi:10.1029/WR007i003p00602.
43. ^ Dooley, Alan (June 2006). "Sandboils 101: Corps has experience dealing with common flood danger". Engineer Update. US Army Corps of Engineers. http://www.hq.usace.army.mil/cepa/pubs/jun06/story8.htm. Retrieved 2008-05-14.
44. ^ ILRI (1989), Effectiveness and Social/Environmental Impacts of Irrigation Projects: a Review, In: Annual Report 1988 of the International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands, p. 18 - 34, http://www.waterlog.info/pdf/irreff.pdf
45. ^ Drainage Manual: A Guide to Integrating Plant, Soil, and Water Relationships for Drainage of Irrigated Lands, Interior Dept., Bureau of Reclamation, 1993, ISBN 0-16-061623-9
46. ^ SWAP model
47. ^ DrainMod-S model
48. ^ UnSatChem model
49. ^ ILRI (1997), SaltMod: a tool for interweaving of irrigation and drainage for salinity control, In: W.B.Snellen (ed.), Towards integration of irrigation, and drainage management. Special report of the International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands, p. 41-43, http://www.waterlog.info/pdf/toolsalt.pdf
50. ^ SaltMod, an agro-hydro-soil salinity model, http://www.waterlog.info/saltmod.htm
51. ^ SahysMod, a spatial agro-hydro-soil salinity cum groundwater model, http://www.waterlog.info/sahysmod.htm

Further reading

• Adams, J.A. 1986. Dirt. College Station, Texas : Texas A&M University Press ISBN 0-89096-301-0
• 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.
• Soil Survey Division Staff. (1999) Soil survey manual. Soil Conservation Service. U.S. Department of Agriculture Handbook 18.
• Logan, W. B., Dirt: The ecstatic skin of the earth. 1995 ISBN 1-57322-004-3
• Faulkner, William. Plowman's Folly. New York, Grosset & Dunlap. 1943. ISBN 0-933280-51-3
• Jenny, Hans, Factors of Soil Formation: A System of Quantitative Pedology 1941
• Why Study Soils?
• Soil notes
• "97 Flood". USGS. http://www.mvm.usace.army.mil/Readiness/97flood/flood.htm. Retrieved 2008-07-08. Photographs of sand boils.
• Soils, Oregon State University
• Soil-Net.com A free schools-age educational site teaching about soil and its importance.
• 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.
• 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 (English), (French)
• Mann, Charles C.: " Our good earth" National Geographic Magazine September 2008

External links

Categories: Soil | Land management | Horticulture and gardening

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