Soil

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For other uses, see Soil (disambiguation).
glacial till in Northern Ireland

Soil, also commonly referred to as earth or

minerals, gases, liquids, and organisms that together support the life of plants and soil organisms
. Some scientific definitions distinguish dirt from soil by restricting the former term specifically to displaced soil.

Soil measuring and surveying device

Soil consists of a solid phase of minerals and organic matter (the soil matrix), as well as a porous phase that holds gases (the soil atmosphere) and water (the soil solution).[1][2] Accordingly, soil is a three-state system of solids, liquids, and gases.[3] Soil is a product of several factors: the influence of climate, relief (elevation, orientation, and slope of terrain), organisms, and the soil's parent materials (original minerals) interacting over time.[4] It continually undergoes development by way of numerous physical, chemical and biological processes, which include weathering with associated erosion.[5] Given its complexity and strong internal connectedness, soil ecologists regard soil as an ecosystem.[6]

Most soils have a dry

planet Earth is older than the Pleistocene and none is older than the Cenozoic,[8] although fossilized soils are preserved from as far back as the Archean.[9]

Collectively the Earth's body of soil is called the pedosphere. The pedosphere interfaces with the lithosphere, the hydrosphere, the atmosphere, and the biosphere.[10] Soil has four important functions:

All of these functions, in their turn, modify the soil and its properties.

Soil science has two basic branches of study: edaphology and pedology. Edaphology studies the influence of soils on living things.[11] Pedology focuses on the formation, description (morphology), and classification of soils in their natural environment.[12] In engineering terms, soil is included in the broader concept of regolith, which also includes other loose material that lies above the bedrock, as can be found on the Moon and other celestial objects.[13]

Processes

Soil is a major component of the

rainforest destruction and water pollution. With respect to Earth's carbon cycle, soil acts as an important carbon reservoir,[14] and it is potentially one of the most reactive to human disturbance[15] and climate change.[16] As the planet warms, it has been predicted that soils will add carbon dioxide to the atmosphere due to increased biological activity at higher temperatures, a positive feedback (amplification).[17] This prediction has, however, been questioned on consideration of more recent knowledge on soil carbon turnover.[18]

Soil acts as an engineering medium, a habitat for

heterotrophic organisms, but a substantial part is retained in the soil in the form of soil organic matter; tillage usually increases the rate of soil respiration, leading to the depletion of soil organic matter.[24] Since plant roots need oxygen, aeration is an important characteristic of soil. This ventilation can be accomplished via networks of interconnected soil pores, which also absorb and hold rainwater making it readily available for uptake by plants. Since plants require a nearly continuous supply of water, but most regions receive sporadic rainfall, the water-holding capacity of soils is vital for plant survival.[25]

Soils can effectively remove impurities,

natural attenuation.[28] Typically, soils maintain a net absorption of oxygen and methane and undergo a net release of carbon dioxide and nitrous oxide.[29] Soils offer plants physical support, air, water, temperature moderation, nutrients, and protection from toxins.[30] Soils provide readily available nutrients to plants and animals by converting dead organic matter into various nutrient forms.[31]

Composition

This is a diagram and related photograph of soil layers from bedrock to soil.
A, B, and C represent the soil profile, a notation firstly coined by Vasily Dokuchaev (1846–1903), the father of pedology. Here, A is the topsoil; B is a regolith; C is a saprolite (a less-weathered regolith); the bottom-most layer represents the bedrock.

Components of a silt loam soil by percent volume

  Water (25%)
  Gases (25%)
  Sand (18%)
  Silt (18%)
  Clay (9%)
  Organic matter (5%)

A typical soil is about 50% solids (45% mineral and 5% organic matter), and 50% voids (or pores) of which half is occupied by water and half by gas.

pore space allows for the infiltration and movement of air and water, both of which are critical for life existing in soil.[34] Compaction, a common problem with soils, reduces this space, preventing air and water from reaching plant roots and soil organisms.[35]

Given sufficient time, an undifferentiated soil will evolve a soil profile that consists of two or more layers, referred to as soil horizons. These differ in one or more properties such as in their texture, structure, density, porosity, consistency, temperature, color, and reactivity.[8] The horizons differ greatly in thickness and generally lack sharp boundaries; their development is dependent on the type of parent material, the processes that modify those parent materials, and the soil-forming factors that influence those processes. The biological influences on soil properties are strongest near the surface, though the geochemical influences on soil properties increase with depth. Mature soil profiles typically include three basic master horizons: A, B, and C. The solum normally includes the A and B horizons. The living component of the soil is largely confined to the solum, and is generally more prominent in the A horizon.[36] It has been suggested that the pedon, a column of soil extending vertically from the surface to the underlying parent material and large enough to show the characteristics of all its horizons, could be subdivided in the humipedon (the living part, where most soil organisms are dwelling, corresponding to the humus form), the copedon (in intermediary position, where most weathering of minerals takes place) and the lithopedon (in contact with the subsoil).[37]

The soil texture is determined by the relative proportions of the individual particles of

acidity
), etc.

Water is a critical agent in soil development due to its involvement in the dissolution, precipitation, erosion, transport, and deposition of the materials of which a soil is composed.

soil profile. Finally, water affects the type of vegetation that grows in a soil, which in turn affects the development of the soil, a complex feedback which is exemplified in the dynamics of banded vegetation patterns in semi-arid regions.[40]

Soils supply

organisms or dead soil organic matter. These bound nutrients interact with soil water to buffer the soil solution composition (attenuate changes in the soil solution) as soils wet up or dry out, as plants take up nutrients, as salts are leached, or as acids or alkalis are added.[42]

Plant nutrient availability is affected by soil pH, which is a measure of the hydrogen ion activity in the soil solution. Soil pH is a function of many soil forming factors, and is generally lower (more acidic) where weathering is more advanced.[43]

Most plant nutrients, with the exception of

volatilisation (loss to the atmosphere as gases) or leaching.[46]

Formation

Main article: Soil formation
Further information: Soil mechanics § Genesis

Soil is said to be formed when organic matter has accumulated and colloids are washed downward, leaving deposits of clay,

soil horizons. However, more recent definitions of soil embrace soils without any organic matter, such as those regoliths that formed on Mars[48] and analogous conditions in planet Earth deserts.[49]

An example of the development of a soil would begin with the weathering of lava flow bedrock, which would produce the purely mineral-based parent material from which the soil texture forms. Soil development would proceed most rapidly from bare rock of recent flows in a warm climate, under heavy and frequent rainfall. Under such conditions, plants (in a first stage

higher plants) become established very quickly on basaltic lava, even though there is very little organic material.[50] Basaltic minerals commonly weather relatively quickly, according to the Goldich dissolution series.[51] The plants are supported by the porous rock as it is filled with nutrient-bearing water that carries minerals dissolved from the rocks. Crevasses and pockets, local topography of the rocks, would hold fine materials and harbour plant roots. The developing plant roots are associated with mineral-weathering mycorrhizal fungi[52] that assist in breaking up the porous lava, and by these means organic matter and a finer mineral soil accumulate with time. Such initial stages of soil development have been described on volcanoes,[53] inselbergs,[54] and glacial moraines.[55]

How soil formation proceeds is influenced by at least five classic factors that are intertwined in the evolution of a soil: parent material, climate, topography (relief), organisms, and time.[56] When reordered to climate, relief, organisms, parent material, and time, they form the acronym CROPT.[57]

Physical properties

Main article: Physical properties of soil
For the academic discipline, see Soil physics.

The physical properties of soils, in order of decreasing importance for ecosystem services such as

silica and humus, coat particles and cause them to adhere into larger, relatively stable secondary structures.[59] Soil bulk density, when determined at standardized moisture conditions, is an estimate of soil compaction.[60] Soil porosity consists of the void part of the soil volume and is occupied by gases or water. Soil consistency is the ability of soil materials to stick together. Soil temperature and colour are self-defining. Resistivity refers to the resistance to conduction of electric currents and affects the rate of corrosion of metal and concrete structures which are buried in soil.[61] These properties vary through the depth of a soil profile, i.e. through soil horizons. Most of these properties determine the aeration of the soil and the ability of water to infiltrate and to be held within the soil.[62]

Soil moisture

Main article: Soil moisture

Soil

wilting point, air dry, and oven dry. Field capacity describes a drained wet soil at the point water content reaches equilibrium with gravity. Irrigating soil above field capacity risks percolation losses. Wilting point describes the dry limit for growing plants. During growing season, soil moisture is unaffected by functional groups or specie richness.[63]

Available water capacity is the amount of water held in a soil profile available to plants. As water content drops, plants have to work against increasing forces of adhesion and sorptivity to withdraw water. Irrigation scheduling avoids moisture stress by replenishing depleted water before stress is induced.[64][65]

Capillary action is responsible for moving groundwater from wet regions of the soil to dry areas. Subirrigation designs (e.g., wicking beds, sub-irrigated planters) rely on capillarity to supply water to plant roots. Capillary action can result in an evaporative concentration of salts, causing land degradation through salination.

Soil moisture measurement—measuring the water content of the soil, as can be expressed in terms of volume or weight—can be based on in situ probes (e.g., capacitance probes, neutron probes), or remote sensing methods. Soil moisture measurement is an important factor in determining changes in soil activity.[63]

Soil gas

Main article: Soil gas

The atmosphere of soil, or

greenhouse gases) as well as water.[71] Soil texture and structure strongly affect soil porosity and gas diffusion. It is the total pore space (porosity) of soil, not the pore size, and the degree of pore interconnection (or conversely pore sealing), together with water content, air turbulence and temperature, that determine the rate of diffusion of gases into and out of soil.[72][71] Platy soil structure and soil compaction (low porosity) impede gas flow, and a deficiency of oxygen may encourage anaerobic bacteria to reduce (strip oxygen) from nitrate NO3 to the gases N2, N2O, and NO, which are then lost to the atmosphere, thereby depleting the soil of nitrogen, a detrimental process called denitrification.[73] Aerated soil is also a net sink of methane (CH4)[74] but a net producer of methane (a strong heat-absorbing greenhouse gas) when soils are depleted of oxygen and subject to elevated temperatures.[75]

Soil atmosphere is also the seat of emissions of volatiles other than carbon and nitrogen oxides from various soil organisms, e.g. roots,[76] bacteria,[77] fungi,[78] animals.[79] These volatiles are used as chemical cues, making soil atmosphere the seat of interaction networks[80][81] playing a decisive role in the stability, dynamics and evolution of soil ecosystems.[82] Biogenic soil volatile organic compounds are exchanged with the aboveground atmosphere, in which they are just 1–2 orders of magnitude lower than those from aboveground vegetation.[83]

Humans can get some idea of the soil atmosphere through the well-known 'after-the-rain' scent, when infiltering rainwater flushes out the whole soil atmosphere after a drought period, or when soil is excavated,

reductionist manner to particular biochemical compounds such as petrichor or geosmin
.

Solid phase (soil matrix)

Main article: Soil matrix

Soil particles can be classified by their chemical composition (mineralogy) as well as their size. The particle size distribution of a soil, its texture, determines many of the properties of that soil, in particular hydraulic conductivity and water potential,[85] but the mineralogy of those particles can strongly modify those properties. The mineralogy of the finest soil particles, clay, is especially important.[86]

Soil biodiversity

Large numbers of microbes, animals, plants and fungi are living in soil. However, biodiversity in soil is much harder to study as most of this life is invisible, hence estimates about soil biodiversity have been unsatisfactory. A recent study suggested that soil is likely home to 59 ± 15% of the species on Earth. Enchytraeidae (worms) have the greatest percentage of species in soil (98.6%), followed by fungi (90%), plants (85.5%), and termites (Isoptera) (84.2%). Many other groups of animals have substantial fractions of species living in soil, e.g. about 30% of insects, and close to 50% of arachnids.[87] While most vertebrates live above ground (ignoring aquatic species), many species are fossorial, that is, they live in soil, such as most blind snakes.

Chemistry

For the academic discipline, see Soil chemistry.

The chemistry of a soil determines its ability to supply available

anions
in the soil, giving the soil anion exchange capacity.

Cation and anion exchange

Further information: Cation-exchange capacity

The cation exchange, that takes place between colloids and soil water, buffers (moderates) soil pH, alters soil structure, and purifies percolating water by adsorbing cations of all types, both useful and harmful.

The negative or positive charges on colloid particles make them able to hold cations or anions, respectively, to their surfaces. The charges result from four sources.[90]

  1. Isomorphous substitution occurs in clay during its formation, when lower-valence cations substitute for higher-valence cations in the crystal structure.[91] Substitutions in the outermost layers are more effective than for the innermost layers, as the electric charge strength drops off as the square of the distance. The net result is oxygen atoms with net negative charge and the ability to attract cations.
  2. Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral and octahedral structures are incomplete.[92]
  3. Hydroxyls may substitute for oxygens of the silica layers, a process called hydroxylation. When the hydrogens of the clay hydroxyls are ionised into solution, they leave the oxygen with a negative charge (anionic clays).[93]
  4. Hydrogens of humus hydroxyl groups may also be ionised into solution, leaving, similarly to clay, an oxygen with a negative charge.[94]

Cations held to the negatively charged colloids resist being washed downward by water and are out of reach of plant roots, thereby preserving the soil fertility in areas of moderate rainfall and low temperatures.[95][96]

There is a hierarchy in the process of cation exchange on colloids, as cations differ in the strength of adsorption by the colloid and hence their ability to replace one another (ion exchange). If present in equal amounts in the soil water solution:

Al3+ replaces H+ replaces Ca2+ replaces Mg2+ replaces K+ same as NH+
4 replaces Na+[97]

If one cation is added in large amounts, it may replace the others by the sheer force of its numbers. This is called law of mass action. This is largely what occurs with the addition of cationic fertilisers (potash, lime).[98]

As the soil solution becomes more acidic (low pH, meaning an abundance of H+), the other cations more weakly bound to colloids are pushed into solution as hydrogen ions occupy exchange sites (protonation). A low pH may cause the hydrogen of hydroxyl groups to be pulled into solution, leaving charged sites on the colloid available to be occupied by other cations. This ionisation of hydroxy groups on the surface of soil colloids creates what is described as pH-dependent surface charges.[99] Unlike permanent charges developed by isomorphous substitution, pH-dependent charges are variable and increase with increasing pH.[100] Freed cations can be made available to plants but are also prone to be leached from the soil, possibly making the soil less fertile.[101] Plants are able to excrete H+ into the soil through the synthesis of organic acids and by that means, change the pH of the soil near the root and push cations off the colloids, thus making those available to the plant.[102]

Cation exchange capacity (CEC)

Cation exchange capacity is the soil's ability to remove cations from the soil water solution and sequester those to be exchanged later as the plant roots release hydrogen ions to the solution.[103] CEC is the amount of exchangeable hydrogen cation (H+) that will combine with 100 grams dry weight of soil and whose measure is one milliequivalents per 100 grams of soil (1 meq/100 g). Hydrogen ions have a single charge and one-thousandth of a gram of hydrogen ions per 100 grams dry soil gives a measure of one milliequivalent of hydrogen ion. Calcium, with an atomic weight 40 times that of hydrogen and with a valence of two, converts to (40 ÷ 2) × 1 milliequivalent = 20 milliequivalents of hydrogen ion per 100 grams of dry soil or 20 meq/100 g.[104]
The modern measure of CEC is expressed as centimoles of positive charge per kilogram (cmol/kg) of oven-dry soil.

Most of the soil's CEC occurs on clay and humus colloids, and the lack of those in hot, humid, wet climates (such as tropical rainforests), due to leaching and decomposition, respectively, explains the apparent sterility of tropical soils.[105] Live plant roots also have some CEC, linked to their specific surface area.[106]

Cation exchange capacity for soils; soil textures; soil colloids[107]
Soil State CEC meq/100 g
Charlotte fine sand Florida 1.0
Ruston fine sandy loam Texas 1.9
Glouchester loam New Jersey 11.9
Grundy silt loam Illinois 26.3
Gleason clay loam California 31.6
Susquehanna clay loam Alabama 34.3
Davie mucky fine sand Florida 100.8
Sands — 1–5
Fine sandy loams — 5–10
Loams and silt loams — 5–15
Clay loams — 15–30
Clays — over 30
Sesquioxides — 0–3
Kaolinite — 3–15
Illite — 25–40
Montmorillonite — 60–100
Vermiculite (similar to illite) — 80–150
Humus — 100–300

Anion exchange capacity (AEC)

Anion exchange capacity is the soil's ability to remove anions (such as nitrate, phosphate) from the soil water solution and sequester those for later exchange as the plant roots release carbonate anions to the soil water solution.[108] Those colloids which have low CEC tend to have some AEC. Amorphous and sesquioxide clays have the highest AEC,[109] followed by the iron oxides.[110] Levels of AEC are much lower than for CEC, because of the generally higher rate of positively (versus negatively) charged surfaces on soil colloids, to the exception of variable-charge soils.[111] Phosphates tend to be held at anion exchange sites.[112]

Iron and aluminum hydroxide clays are able to exchange their hydroxide anions (OH−) for other anions.[108] The order reflecting the strength of anion adhesion is as follows:

H
2PO−
4 replaces SO2−
4 replaces NO−
3 replaces Cl−

The amount of exchangeable anions is of a magnitude of tenths to a few milliequivalents per 100 g dry soil.[107] As pH rises, there are relatively more hydroxyls, which will displace anions from the colloids and force them into solution and out of storage; hence AEC decreases with increasing pH (alkalinity).[113]

Reactivity (pH)

Main articles: Soil pH and Soil pH § Effect of soil pH on plant growth

Soil reactivity is expressed in terms of pH and is a measure of the acidity or alkalinity of the soil. More precisely, it is a measure of hydronium concentration in an aqueous solution and ranges in values from 0 to 14 (acidic to basic) but practically speaking for soils, pH ranges from 3.5 to 9.5, as pH values beyond those extremes are toxic to life forms.[114]

At 25 °C an aqueous solution that has a pH of 3.5 has 10−3.5 moles H3O+ (hydronium ions) per litre of solution (and also 10−10.5 moles per litre OH−). A pH of 7, defined as neutral, has 10−7 moles of hydronium ions per litre of solution and also 10−7 moles of OH− per litre; since the two concentrations are equal, they are said to neutralise each other. A pH of 9.5 has 10−9.5 moles hydronium ions per litre of solution (and also 10−2.5 moles per litre OH−). A pH of 3.5 has one million times more hydronium ions per litre than a solution with pH of 9.5 (9.5 − 3.5 = 6 or 106) and is more acidic.[115]

The effect of pH on a soil is to remove from the soil or to make available certain ions. Soils with high acidity tend to have toxic amounts of

pre-adapted to every kind of pollution, whether of natural or human origin.[121]

In high rainfall areas, soils tend to acidify as the basic cations are forced off the soil colloids by the mass action of hydronium ions from usual or unusual

chelates of those nutrients can correct the deficit.[127] Sodium can be reduced by the addition of gypsum (calcium sulphate) as calcium adheres to clay more tightly than does sodium causing sodium to be pushed into the soil water solution where it can be washed out by an abundance of water.[128][129]

Base saturation percentage

There are acid-forming cations (e.g. hydronium, aluminium, iron) and there are base-forming cations (e.g. calcium, magnesium, sodium). The fraction of the negatively-charged soil colloid exchange sites (CEC) that are occupied by base-forming cations is called

base saturation. If a soil has a CEC of 20 meq and 5 meq are aluminium and hydronium cations (acid-forming), the remainder of positions on the colloids (20 − 5 = 15 meq) are assumed occupied by base-forming cations, so that the base saturation is 15 ÷ 20 × 100% = 75% (the compliment 25% is assumed acid-forming cations). Base saturation is almost in direct proportion to pH (it increases with increasing pH).[130] It is of use in calculating the amount of lime needed to neutralise an acid soil (lime requirement). The amount of lime needed to neutralize a soil must take account of the amount of acid forming ions on the colloids (exchangeable acidity), not just those in the soil water solution (free acidity).[131] The addition of enough lime to neutralize the soil water solution will be insufficient to change the pH, as the acid forming cations stored on the soil colloids will tend to restore the original pH condition as they are pushed off those colloids by the calcium of the added lime.[132]

Buffering

Further information: Soil conditioner

The resistance of soil to change in pH, as a result of the addition of acid or basic material, is a measure of the buffering capacity of a soil and (for a particular soil type) increases as the CEC increases. Hence, pure sand has almost no buffering ability, though soils high in colloids (whether mineral or organic) have high

buffer range to iron buffer range.[135]

The addition of a small amount of highly basic aqueous ammonia to a soil will cause the ammonium to displace hydronium ions from the colloids, and the end product is water and colloidally fixed ammonium, but little permanent change overall in soil pH.

The addition of a small amount of lime, Ca(OH)2, will displace hydronium ions from the soil colloids, causing the fixation of calcium to colloids and the evolution of CO2 and water, with little permanent change in soil pH.

The above are examples of the buffering of soil pH. The general principal is that an increase in a particular cation in the soil water solution will cause that cation to be fixed to colloids (buffered) and a decrease in solution of that cation will cause it to be withdrawn from the colloid and moved into solution (buffered). The degree of buffering is often related to the CEC of the soil; the greater the CEC, the greater the buffering capacity of the soil.[136]

Redox

Main article: Redox § Redox_reactions_in_soils
See also: Table of standard reduction potentials for half-reactions important in biochemistry

Soil chemical reactions involve some combination of proton and electron transfer. Oxidation occurs if there is a loss of electrons in the transfer process while reduction occurs if there is a gain of electrons. Reduction potential is measured in volts or millivolts. Soil microbial communities develop along electron transport chains, forming electrically conductive biofilms, and developing networks of bacterial nanowires.

Redox factors in soil development, where formation of

soil-based microbial fuel cells
.

Nutrients

Plant nutrients, their chemical symbols, and the ionic forms common in soils and available for plant uptake[137]
Element Symbol Ion or molecule
Carbon C CO2 (mostly through leaves)
Hydrogen H H+, H2O (water)
Oxygen O O2−, OH−, CO2−
3, SO2−
4, CO2
Phosphorus P H
2PO−
4, HPO2−
4 (phosphates)
Potassium K K+
Nitrogen N NH+
4, NO−
3 (ammonium, nitrate)
Sulfur S SO2−
4
Calcium Ca Ca2+
Iron Fe Fe2+, Fe3+ (ferrous, ferric)
Magnesium Mg Mg2+
Boron B H3BO3, H
2BO−
3, B(OH)−
4
Manganese Mn Mn2+
Copper Cu Cu2+
Zinc Zn Zn2+
Molybdenum Mo MoO2−
4 (molybdate)
Chlorine Cl Cl− (chloride)
Main articles: Plant nutrients in soil, Plant nutrition, and Soil pH § Effect of soil pH on plant growth

Seventeen elements or nutrients are essential for plant growth and reproduction. They are

Law of the Minimum expresses that when the available form of a nutrient is not in enough proportion in the soil solution, then other nutrients cannot be taken up at an optimum rate by a plant.[141] A particular nutrient ratio of the soil solution is thus mandatory for optimizing plant growth, a value which might differ from nutrient ratios calculated from plant composition.[142]

Plant uptake of nutrients can only proceed when they are present in a plant-available form. In most situations, nutrients are absorbed in an

secondary minerals, they weather too slowly to support rapid plant growth. For example, the application of finely ground minerals, feldspar and apatite, to soil seldom provides the necessary amounts of potassium and phosphorus at a rate sufficient for good plant growth, as most of the nutrients remain bound in the crystals of those minerals.[143]

The nutrients adsorbed onto the surfaces of clay colloids and soil organic matter provide a more accessible reservoir of many plant nutrients (e.g. K, Ca, Mg, P, Zn). As plants absorb the nutrients from the soil water, the soluble pool is replenished from the surface-bound pool. The decomposition of soil organic matter by microorganisms is another mechanism whereby the soluble pool of nutrients is replenished – this is important for the supply of plant-available N, S, P, and B from soil.[144]

Gram for gram, the capacity of

wettability once dry.[146] All in all, small amounts of humus may remarkably increase the soil's capacity to promote plant growth.[147][144]

Soil organic matter

Main article:
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)

The organic material in soil is made up of

organic compounds and includes plant, animal and microbial material, both living and dead. A typical soil has a biomass composition of 70% microorganisms, 22% macrofauna, and 8% roots. The living component of an acre of soil may include 900 lb of earthworms, 2400 lb of fungi, 1500 lb of bacteria, 133 lb of protozoa and 890 lb of arthropods and algae.[148]

A few percent of the soil organic matter, with small

humification than to decrease litter decomposition.[152]

The main part of soil organic matter is a complex assemblage of small organic molecules, collectively called humus or

humic substances. The use of these terms, which do not rely on a clear chemical classification, has been considered as obsolete.[153] Other studies showed that the classical notion of molecule is not convenient for humus, which escaped most attempts done over two centuries to resolve it in unit components, but still is chemically distinct from polysaccharides, lignins and proteins.[154]

Most living things in soils, including plants, animals, bacteria, and fungi, are dependent on organic matter for nutrients and/or energy. Soils have organic compounds in varying degrees of decomposition which rate is dependent on temperature, soil moisture, and aeration. Bacteria and fungi feed on the raw organic matter, which are fed upon by

histosols), are infertile.[156]
In its earliest stage of decomposition, the original organic material is often called raw organic matter. The final stage of decomposition is called humus.

In

O horizon) and thin A horizon.[157]

Humus

Humus refers to organic matter that has been decomposed by soil microflora and fauna to the point where it is resistant to further breakdown. Humus usually constitutes only five percent of the soil or less by volume, but it is an essential source of nutrients and adds important textural qualities crucial to

weak acid that can attack silicate minerals by chelating their iron and aluminum atoms.[160] Humus has a high cation and anion exchange capacity that on a dry weight basis is many times greater than that of clay colloids. It also acts as a buffer, like clay, against changes in pH and soil moisture.[161]

aromatic hydrocarbons, assembled and stabilized by oxygen and hydrogen bonds, remain in the form of complex molecular assemblages collectively called humus.[154] Humus is never pure in the soil, because it reacts with metals and clays to form complexes which further contribute to its stability and to soil structure.[161] Although the structure of humus has in itself few nutrients (with the exception of constitutive metals such as calcium, iron and aluminum) it is able to attract and link, by weak bonds, cation and anion nutrients that can further be released into the soil solution in response to selective root uptake and changes in soil pH, a process of paramount importance for the maintenance of fertility in tropical soils.[163]

extracellular enzymes from degradation.[169] The addition of organic matter to clay soils can render that organic matter and any added nutrients inaccessible to plants and microbes for many years.[170] A study showed increased soil fertility following the addition of mature compost to a clay soil.[171] High soil tannin content can cause nitrogen to be sequestered as resistant tannin-protein complexes.[172][173]

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.

Amazonian dark earths, has been renewed and became popular under the name of biochar. It has been suggested that biochar could be used to sequester more carbon in the fight against the greenhouse effect.[177]

Climatological influence

The production, accumulation and degradation of organic matter are greatly dependent on climate. For example, when a thawing event occurs, the flux of soil gases with atmospheric gases is significantly influenced.[178] Temperature, soil moisture and topography are the major factors affecting the accumulation of organic matter in soils. Organic matter tends to accumulate under wet or cold conditions where decomposer activity is impeded by low temperature[179] or excess moisture which results in anaerobic conditions.[180] Conversely, excessive rain and high temperatures of tropical climates enables rapid decomposition of organic matter and leaching of plant nutrients. Forest ecosystems on these soils rely on efficient recycling of nutrients and plant matter by the living plant and microbial biomass to maintain their productivity, a process which is disturbed by human activities.[181] Excessive slope, in particular in the presence of cultivation for the sake of agriculture, may encourage the erosion of the top layer of soil which holds most of the raw organic material that would otherwise eventually become humus.[182]

Plant residue

Typical types and percentages of plant residue components

  Cellulose (45%)
  Lignin (20%)
  Hemicellulose (18%)
  Protein (8%)
  Sugars and starches (5%)
  Fats and waxes (2%)

actinomycetes; its half-life under temperate conditions is about six months.[183]

Horizons

Main article: Soil horizon

A horizontal layer of the soil, whose physical features, composition and age are distinct from those above and beneath, is referred to as a soil horizon. The naming of a horizon is based on the type of material of which it is composed. Those materials reflect the duration of specific processes of soil formation. They are labelled using a shorthand notation of letters and numbers which describe the horizon in terms of its colour, size, texture, structure, consistency, root quantity, pH, voids, boundary characteristics and presence of nodules or concretions.

paleosols can be found within sedimentary rock sequences, allowing the study of past environments.[193]

The exposure of parent material to favourable conditions produces mineral soils that are marginally suitable for plant growth, as is the case in eroded soils.

dissolved organic matter.[195] The remaining surficial organic layer, called the O horizon, produces a more active soil due to the effect of the organisms that live within it. Organisms colonise and break down organic materials, making available nutrients upon which other plants and animals can live.[196] After sufficient time, humus moves downward and is deposited in a distinctive organic-mineral surface layer called the A horizon, in which organic matter is mixed with mineral matter through the activity of burrowing animals, a process called pedoturbation. This natural process does not go to completion in the presence of conditions detrimental to soil life such as strong acidity, cold climate or pollution, stemming in the accumulation of undecomposed organic matter within a single organic horizon overlying the mineral soil[197] and in the juxtaposition of humified organic matter and mineral particles, without intimate mixing, in the underlying mineral horizons.[198]

Classification

Main article: Soil classification

One of the first soil classification systems was developed by Russian scientist Vasily Dokuchaev around 1880.[199] It was modified a number of times by American and European researchers and was 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 which 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[200] aims to establish an international reference base for soil classification.

Uses

Soil is used in agriculture, where it serves as the anchor and primary nutrient base for plants. The types of soil and available moisture determine the species of plants that can be cultivated. Agricultural soil science was the primeval domain of soil knowledge, long time before the advent of pedology in the 19th century. However, as demonstrated by aeroponics, aquaponics and hydroponics, soil material is not an absolute essential for agriculture, and soilless cropping systems have been claimed as the future of agriculture for an endless growing mankind.[201]

Soil material is also a critical component in mining, construction and landscape development industries.

Earth sheltering is the architectural practice of using soil for external thermal mass against building walls. Many building materials are soil based. Loss of soil through urbanization is growing at a high rate in many areas and can be critical for the maintenance of subsistence agriculture.[203]

Soil resources are critical to the environment, as well as to food and fibre production, producing 98.8% of food consumed by humans.

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 in deserts, cyanobacteria, lichens and mosses form biological soil crusts which 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 effect of increases in greenhouse gas emissions and slow global warming, while improving crop yields and reducing water needs.[209][210][211]

Composting is now widely used to treat aerobically solid domestic waste and dried effluents of settling basins. Although compost is not soil, biological processes taking place during composting are similar to those occurring during decomposition and humification of soil organic matter.[212]

Organic soils, especially peat, serve as a significant fuel and

World Heritage List. Under present-day global warming peat soils are thought to be involved in a self-reinforcing (positive feedback) process of increased emission of greenhouse gases (methane and carbon dioxide) and increased temperature,[214] a contention which is still under debate when replaced at field scale and including stimulated plant growth.[215]

fruits), in order to alleviate tannin toxicity.[217]

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

metabolise them or immobilise them in their biomass and necromass,[219] thereby incorporating them into stable humus.[220] The physical integrity of soil is also a prerequisite for avoiding landslides in rugged landscapes.[221]

Degradation

Main articles: Soil retrogression and degradation and Soil conservation

Land degradation is a human-induced or natural process which impairs the capacity of land to function.[222] Soil degradation involves acidification, contamination, desertification, erosion or salination.[223]

Acidification

Soil acidification is beneficial in the case of

tree canopies.[224]

Contamination

Soil

Soil biota can treat waste by transforming it, mainly through microbial enzymatic activity.[225] Soil organic matter and soil minerals can adsorb the waste material and decrease its toxicity,[226] although when in colloidal form they may transport the adsorbed contaminants to subsurface environments.[227] Many waste treatment processes rely on this natural bioremediation 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, soil conditioners, phytoremediation, bioremediation and Monitored Natural Attenuation. An example of diffuse pollution with contaminants is copper accumulation in vineyards and orchards to which fungicides are repeatedly applied, even in organic farming.[228]

Microfibres from synthetic textiles are another type of plastic soil contamination, 100% of agricultural soil samples from southwestern China contained plastic particles, 92% of which were microfibres. Sources of microfibres likely included string or twine, as well as irrigation water in which clothes had been washed.[229]

The application of biosolids from sewage sludge and compost can introduce microplastics to soils. This adds to the burden of microplastics from other sources (e.g. the atmosphere). Approximately half the sewage sludge in Europe and North America is applied to agricultural land. In Europe it has been estimated that for every million inhabitants 113 to 770 tonnes of microplastics are added to agricultural soils each year.[229]

Desertification

Desertification

Desertification, an environmental process of ecosystem degradation in arid and semi-arid regions, is often caused by badly adapted human activities such as overgrazing or excess harvesting of firewood. It is a common misconception that drought causes desertification.[230] 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.[231] 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.[232] It is now questioned whether present-day climate warming will favour or disfavour desertification, with contradictory reports about predicted rainfall trends associated with increased temperature, and strong discrepancies among regions, even in the same country.[233]

Erosion

Erosion control

cattle ranching
.

A serious and long-running water erosion problem occurs in

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.[236]

Soil piping is a particular form of soil erosion that occurs below the soil surface.[237] It causes levee and dam failure, as well as sink hole formation. Turbulent flow removes soil starting at the mouth of the seep flow and the subsoil erosion advances up-gradient.[238] The term sand boil is used to describe the appearance of the discharging end of an active soil pipe.[239]

Salination

flushing with higher levels of applied water in combination with tile drainage or another form of subsurface drainage.[241][242]

Reclamation

Main article: Soil regeneration

Soils which contain high levels of particular clays with high swelling properties, such as smectites, are often very fertile. For example, the smectite-rich paddy soils of Thailand's Central Plains are among the most productive in the world. However, the overuse of mineral nitrogen fertilizers and pesticides in irrigated intensive rice production has endangered these soils, forcing farmers to implement integrated practices based on Cost Reduction Operating Principles.[243]

Many farmers in tropical areas, however, struggle to retain organic matter and clay in the soils they work. In recent years, for example, productivity has declined and soil erosion has increased in the low-clay soils of northern Thailand, following the abandonment of

rai (1,300 kg/ha; 1,100 lb/acre) of bentonite resulted in an average yield increase of 73%.[245] Other studies showed that applying bentonite to degraded sandy soils reduced the risk of crop failure during drought years.[246]

In 2008, three years after the initial trials, IWMI scientists conducted a survey among 250 farmers in northeast Thailand, half of whom had applied bentonite to their fields. The average improvement 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.[247]

If the soil is too high in clay or salts (e.g.

saline sodic soil), adding gypsum, washed river sand and organic matter (e.g.municipal solid waste) will balance the composition.[248]

Adding organic matter, like ramial chipped wood or compost, to soil which is depleted in nutrients and too high in sand will boost its quality and improve production.[249][250]

Special mention must be made of the use of

Terra Preta de Índio, due to interesting physical and chemical properties of soil black carbon as a source of stable humus.[251] However, the uncontrolled application of charred waste products of all kinds may endanger soil life and human health.[252]

History of studies and research

The history of the study of soil is intimately tied to humans' urgent need to provide food for themselves and forage for their animals. Throughout history, civilizations have prospered or declined as a function of the availability and productivity of their soils.[253]

Studies of soil fertility

Main article: Soil fertility

The Greek historian

BCE) was 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 as dung.'[254]

forest litter and assarting, which ruined the soils of western Europe during the Middle Ages and even later on according to regions.[260]

Experiments into what made plants grow first led to the idea that the ash left behind when plant matter was burned was the essential element but overlooked the role of nitrogen, which is not left on the ground after combustion, a belief which prevailed until the 19th century.[261] In about 1635, the Flemish chemist Jan Baptist van Helmont thought he had proved water to be the essential element from his famous five years' experiment with a willow tree grown with only the addition of rainwater. His conclusion came from the fact that the increase in the plant's weight had apparently been produced only by the addition of water, with no reduction in the soil's weight.[262][263][264] John Woodward (d. 1728) experimented with various types of water ranging from clean to muddy and found muddy water the best, and so he concluded that earthy matter was the essential element. Others concluded it was humus in the soil that passed some essence to the growing plant. Still others held that the vital growth principal was something passed from dead plants or animals to the new plants. At the start of the 18th century, Jethro Tull demonstrated that it was beneficial to cultivate (stir) the soil, but his opinion that the stirring made the fine parts of soil available for plant absorption was erroneous.[263][265]

As chemistry developed, it was applied to the investigation of soil fertility. The French chemist Antoine Lavoisier showed in about 1778 that plants and animals must combust oxygen internally to live. He was able to deduce that most of the 165-pound (75 kg) weight of van Helmont's willow tree derived from air.[266] It was the French agriculturalist Jean-Baptiste Boussingault who by means of experimentation obtained evidence showing that the main sources of carbon, hydrogen and oxygen for plants were air and water, while nitrogen was taken from soil.[267] Justus von Liebig in his book Organic chemistry in its applications to agriculture and physiology (published 1840), asserted that the chemicals in plants must have come from the soil and air and that to maintain soil fertility, the used minerals must be replaced.[268] Liebig nevertheless believed the nitrogen was supplied from the air. The enrichment of soil with guano by the Incas was rediscovered in 1802, by Alexander von Humboldt. This led to its mining and that of Chilean nitrate and to its application to soil in the United States and Europe after 1840.[269]

The work of Liebig was a revolution for agriculture, and so other investigators started experimentation based on it. In England John Bennet Lawes and Joseph Henry Gilbert worked in the Rothamsted Experimental Station, founded by the former, and (re)discovered that plants took nitrogen from the soil, and that salts needed to be in an available state to be absorbed by plants. Their investigations also produced the superphosphate, consisting in the acid treatment of phosphate rock.[270] This led to the invention and use of salts of potassium (K) and nitrogen (N) as fertilizers. Ammonia generated by the production of coke was recovered and used as fertiliser.[271] Finally, the chemical basis of nutrients delivered to the soil in manure was understood and in the mid-19th century chemical fertilisers were applied. However, the dynamic interaction of soil and its life forms was still not understood.

In 1856, J. Thomas Way discovered that ammonia contained in fertilisers was transformed into nitrates,[272] and twenty years later Robert Warington proved that this transformation was done by living organisms.[273] In 1890 Sergei Winogradsky announced he had found the bacteria responsible for this transformation.[274]

It was known that certain legumes could take up nitrogen from the air and fix it to the soil but it took the development of bacteriology towards the end of the 19th century to lead to an understanding of the role played in nitrogen fixation by bacteria. The symbiosis of bacteria and leguminous roots, and the fixation of nitrogen by the bacteria, were simultaneously discovered by the German agronomist Hermann Hellriegel and the Dutch microbiologist Martinus Beijerinck.[270]

Crop rotation, mechanisation, chemical and natural fertilisers led to a doubling of wheat yields in western Europe between 1800 and 1900.[275]

Studies of soil formation

See also: Soil formation

The scientists who studied the soil in connection with agricultural practices had considered it mainly as a static substrate. However, soil is the result of evolution from more ancient geological materials, under the action of biotic and abiotic processes. After studies of the improvement of the soil commenced, other researchers began to study soil genesis and as a result also soil types and classifications.

In 1860, while in Mississippi, Eugene W. Hilgard (1833–1916) studied the relationship between rock material, climate, vegetation, and the type of soils that were developed. He realised that the soils were dynamic, and considered the classification of soil types.[276] His work was not continued. At about the same time, Friedrich Albert Fallou was describing soil profiles and relating soil characteristics to their formation as part of his professional work evaluating forest and farm land for the principality of Saxony. His 1857 book, Anfangsgründe der Bodenkunde (First principles of soil science), established modern soil science.[277] Contemporary with Fallou's work, and driven by the same need to accurately assess land for equitable taxation, Vasily Dokuchaev led a team of soil scientists in Russia who conducted an extensive survey of soils, observing that similar basic rocks, climate and vegetation types lead to similar soil layering and types, and established the concepts for soil classifications. Due to language barriers, the work of this team was not communicated to western Europe until 1914 through a publication in German by Konstantin Glinka, a member of the Russian team.[278]

Curtis F. Marbut, influenced by the work of the Russian team, translated Glinka's publication into English,[279] and, as he was placed in charge of the U.S. National Cooperative Soil Survey, applied it to a national soil classification system.[263]

See also

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Sources

 This article incorporates text from a free content work. Licensed under Cc BY-SA 3.0 IGO (license statement/permission). Text taken from Drowning in Plastics – Marine Litter and Plastic Waste Vital Graphics​, United Nations Environment Programme.

Bibliography

Further reading

External links
Look up soil in Wiktionary, the free dictionary.
Wikiversity has learning resources about Soil Formation
The Wikibook Historical Geology has a page on the topic of: Soils and paleosols
Wikimedia Commons has media related to Soils.
Wikiquote has quotations related to Soil.

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