Soil consists of a solid collection ofminerals andorganic matter (thesoil matrix), as well as aporous phase that holdsgases (thesoil atmosphere) and a liquid phase that holds water and dissolved substances both organic and inorganic, inionic or inmolecular form (thesoil solution).[1][2] Accordingly, soil is acomplex three-state system of solids, liquids, and gases.[3] Soil is a product of several factors: the influence ofclimate,relief (elevation, orientation, and slope of terrain), organisms, and the soil'sparent materials (original minerals) interacting over time.[4] It continually undergoes development by way of numerous physical, chemical and biological processes, which includeweathering with associatederosion.[5] Given its complexity and strong internalconnectedness,soil ecologists regard soil as anecosystem.[6]
Most soils have a drybulk density (density of soil taking into account voids when dry) between 1.1 and 1.6 g/cm3, though the soilparticle density is much higher, in the range of 2.6 to 2.7 g/cm3.[7] Little of the soil ofplanet Earth is older than thePleistocene and none is older than theCenozoic,[8] althoughfossilized soils are preserved from as far back as theArchean.[9]
All of these functions, in their turn, modify the soil and its properties.
Soil science has two basic branches of study:edaphology andpedology.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] Inengineering terms, soil is included in the broader concept ofregolith, which also includes other loose material that lies above thebedrock, as can be found on theMoon and othercelestial objects.[13]
Soil acts as anengineering medium, a habitat forsoil organisms, a recycling system fornutrients andorganic wastes, a regulator ofwater quality, a modifier ofatmospheric composition, and a medium forplant growth, making it a critically important provider ofecosystem services.[19] Since soil has a tremendous range of availableniches andhabitats, it contains a prominent part of the Earth'sgenetic diversity. A gram of soil can contain billions of organisms, belonging to thousands of species, mostly microbial and largely still unexplored.[20][21] Soil has ameanprokaryotic density of roughly 108 organisms per gram,[22] whereas the ocean has no more than 107 prokaryotic organisms per milliliter (gram) of seawater.[23]Organic carbon held in soil is eventually returned to the atmosphere through the process ofrespiration carried out byheterotrophic organisms, but a substantial part is retained in the soil in the form ofsoil organic matter;tillage usually increases the rate ofsoil 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 interconnectedsoil pores, which also absorb and hold rainwater making it readily available for uptake by plants.[25] Since plants require a nearly continuous supply of water, but most regions receive sporadic rainfall, thewater-holding capacity of soils is vital for plant survival.[26]
Soils can effectively remove impurities,[27] kill disease agents,[28] and degradecontaminants, this latter property being callednatural attenuation.[29] Typically, soils maintain a net absorption ofoxygen andmethane and undergo a net release ofcarbon dioxide andnitrous oxide.[30] Soils offer plants physical support, air, water, temperature moderation, nutrients, and protection from toxins.[31] Soils provide readily available nutrients to plants and animals by converting dead organic matter into various nutrient forms.[32]
A, B, and C represent thesoil profile, a notation firstly coined byVasily Dokuchaev (1846–1903), the father of pedology. Here, A is thetopsoil; B is aregolith; C is asaprolite (a less-weathered regolith); the bottom-most layer represents thebedrock.
Components of a silt loam soil by percent volume
Water (25.0%)
Gases (25.0%)
Sand (18.0%)
Silt (18.0%)
Clay (9.00%)
Organic matter (5.00%)
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.[33] The percent soil mineral and organic content can be treated as a constant (in the short term), while the percent soil water and gas content is considered highly variable whereby a rise in one is simultaneously balanced by a reduction in the other.[34] Thepore space allows for the infiltration and movement of air and water, both of which are critical for life existing in soil.[35]Compaction, a common problem with soils, in particular under heavy machinery traffic,[36] reduces this space, preventing air and water from reaching plant roots and soil organisms.[37]
Given sufficient time, an undifferentiated soil will evolve asoil profile that consists of two or more layers, referred to assoil horizons. These differ in one or more properties such as in theirtexture,structure,density,porosity,consistency, temperature, color, andreactivity.[8] The horizons differ greatly in thickness and generally lack sharp boundaries; their development is dependent on the type ofparent material, the processes that modify those parent materials (e.g.mineral weathering), and thesoil-forming factors that influence those processes. The biological influences on soil properties (e.g.bioturbation) are strongest near the surface, while thegeochemical influences on soil properties increase with depth. Mature soil profiles typically include three basic master horizons: A, B, and C. Thesolum 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.[38] It has been suggested that thepedon, a column of soil extending vertically from the surface to the underlyingparent material and large enough to show the characteristics of all its horizons, could be subdivided in thehumipedon (the living part, where most soil organisms are dwelling, corresponding to thehumus form), thecopedon (in intermediary position, where mostweathering of minerals takes place) and thelithopedon (in contact with the subsoil).[39]
The soiltexture is determined by the relative proportions of the individual particles ofsand,silt, andclay that make up the soil.
Asoil texture triangle plot is a visual representation of the proportions of sand, silt, and clay in a soil sample.
The interaction of the individual mineral particles with organic matter, water, gases viabiotic andabiotic processes causes those particles toflocculate (stick together) to formaggregates orpeds.[40] Where these aggregates can be identified, a soil can be said to be developed, and can be described further in terms of color, porosity, consistency, reaction (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.[41] The mixture of water and dissolved or suspended materials that occupy the soilpore space is called thesoil solution. Sincesoil water is never pure water, but contains hundreds of dissolved organic and mineral substances, it may be more accurately called thesoil solution. Water is central to thedissolution,precipitation andleaching of minerals from thesoil 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.[42]
Soils supplyplants withnutrients, most of which are held in place by particles ofclay and organic matter (colloids)[43] The nutrients may beadsorbed onclay mineral surfaces, bound within clay minerals (absorbed), or bound within organic compounds as part of the livingorganisms or dead soil organic matter (humus).[44] These bound nutrients interact with soil water tobuffer 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.[45]
Most plant nutrients, with the exception ofnitrogen,fixed from the atmosphere, originate from the minerals that make up the soilparent material. Some nitrogen also originates from rain as dilutenitric acid andammonia,[47] but most of the nitrogen is available in soils as a result ofnitrogen fixation bydiazotrophbacteria (e.g.cyanobacteria withheterocysts,Clostridium). Once in the soil-plant system, most nutrients are recycled through living organisms, plant and microbial residues (soil organic matter), mineral-bound forms (humus), and the soil solution. Both living soil organisms (microbes, animals and plant roots) and soil organic matter are of critical importance to this recycling, and thereby tosoil formation andsoil fertility.[48] Microbialenzymes may release nutrients from minerals or organic matter for use by plants and other microorganisms, sequester (incorporate) them into living cells, or cause their loss from the soil byvolatilisation (loss to the atmosphere as gases) or leaching.[49]
Soil is said to be formed when organic matter has accumulated and colloids are washed downward, leaving deposits ofclay,humus,iron oxide,carbonate, andgypsum, producing a distinct layer called the Bhorizon. This is a somewhat arbitrary definition as mixtures ofsand,silt, clay and humus will support biological and agricultural activity before that time.[50] These constituents are moved from one level to another by water (leaching) and animal activity (bioturbation). As a result, layers (horizons) form in the soil profile. The alteration (weathering) and movement of materials within a soil causes the formation of distinctivesoil horizons. However, more recent definitions of soil embrace soils without any organic matter, such as thoseregoliths that formed on Mars[51] and analogous conditions in planetEarth deserts.[52]
An example of the development of a soil would begin with theweathering of lava flowbedrock, which would produce the purely mineral-based parent material from which thesoil 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 stagenitrogen-fixinglichens andcyanobacteria thenepilithichigher plants) become established very quickly onbasaltic lava, even though there is very little organic material.[53] Basaltic minerals commonly weather relatively quickly, according to theGoldich dissolution series.[54] 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-weatheringmycorrhizal fungi[55] 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,[56] inselbergs,[57] and glacial moraines.[58]
Soil formation is governed by five interrelated factors —parent material,climate, topography (relief),organisms, andtime — which together drive the development and evolution of soil.[59] When reordered to climate, relief, organisms, parent material, and time, they form the acronym CROPT.[60]
The physical properties of soils, in order of decreasing importance forecosystem services such ascrop production, aretexture,structure,bulk density,porosity, consistency,temperature,colour andresistivity.[61] Soil texture is determined by the relative proportion of the three kinds of soil mineral particles, called soil separates:sand,silt, andclay. At the next larger scale, soil structures calledpeds or more commonlysoil aggregates are created from the soil separates wheniron oxides,carbonates, clay,silica andhumus, coat particles and cause them to adhere into larger, relativelystable secondary structures.[62] Soilbulk density, when determined at standardizedmoisture conditions, is an estimate ofsoil compaction.[63] Soilporosity 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 ofelectric currents and affects the rate ofcorrosion ofmetal andconcrete structures which are buried in soil.[64] These properties vary through the depth of a soil profile, i.e. throughsoil horizons. Most of these properties determine theaeration of the soil and the ability of water toinfiltrate and to beheld within the soil.[65]
Soil moisture measurement—measuring the water content of the soil, as can be expressed in terms of volume or weight—can be based onin situ probes (e.g.,capacitance probes,neutron probes), orremote sensing methods. Soil moisture measurement is an important factor in determining changes in soil biological activity.[66]
The atmosphere of soil, orsoil gas, is very different from theatmosphere above. The consumption ofoxygen by microbes and plant roots, and their release ofcarbon dioxide, decreases oxygen and increases carbon dioxide concentration. Atmospheric CO2 concentration is 0.04%, but in the soil pore space it may range from 10 to 100 times that level, thus potentially contributing to the inhibition of root respiration.[70]Calcareous soils regulate CO2 concentration bycarbonatebuffering, contrary to acid soils in which all CO2 respired accumulates in the soil pore system.[71] At extreme levels, CO2 is toxic.[72] This suggests a possiblenegative feedback control of soil CO2 concentration through its inhibitory effects on root and microbial respiration (also calledsoil respiration).[73] In addition, the soil voids are saturated withwater vapour, at least until the point of maximalhygroscopicity, beyond which avapour-pressure deficit occurs in the soil pore space.[35] Adequate porosity is necessary, not just to allow the penetration of water, but also to allow gases to diffuse in and out. Movement of gases is bydiffusion from high concentrations to lower, thediffusion coefficient decreasing withsoil compaction.[74] Oxygen from above atmosphere diffuses in the soil where it is consumed and levels of carbon dioxide in excess of above atmosphere diffuse out with other gases (includinggreenhouse gases) as well aswater vapor.[75]Soil texture andstructure 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, airturbulence and temperature, that determine the rate of diffusion of gases into and out of soil.[76][75]Platy soil structure and soil compaction (low porosity) impede gas flow, and a deficiency of oxygen may encourageanaerobic bacteria to reduce (strip oxygen) fromnitrate NO3 to the gases N2, N2O, and NO, which are then lost to the atmosphere, thereby depleting the soil of nitrogen, a detrimental process calleddenitrification.[77] Aerated soil is also a net sink ofmethane (CH4)[78] but a net producer ofmethane (a strong heat-trappinggreenhouse gas) when soils are depleted of oxygen and subject to elevated temperatures.[79]
Soil atmosphere is also the seat of emissions ofvolatiles other than carbon and nitrogen oxides from various soil organisms, e.g. roots,[80] bacteria,[81] fungi,[82] animals.[83] These volatiles are used as chemical cues, making soil atmosphere the seat of interaction networks[84][85] playing a decisive role in the stability, dynamics and evolution of soil ecosystems.[86]Biogenic soilvolatile organic compounds are exchanged with the aboveground atmosphere, in which they are just 1–2 orders of magnitude lower than those from aboveground vegetation.[87]
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,[88] a bulk property attributed in areductionist manner to particularbiochemical compounds such aspetrichor orgeosmin.
Soil particles can be classified by their chemical composition (mineralogy) as well as their size. Theparticle size distribution of a soil, itstexture, determines many of the properties of that soil, in particularhydraulic conductivity andwater potential,[89] but themineralogy of those particles can strongly modify those properties. The mineralogy of the finest soil particles,clay, is especially important.[90]
Large numbers ofmicrobes,animals,plants andfungi are living in soil.[91] 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 (potworms) have the greatest percentage of their species living 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% ofinsects, and close to 50% ofarachnids.[92] While mostvertebrates live above ground (ignoring aquatic species), many species arefossorial, that is, they live in soil (e.g.moles,pocket gophers,voles,blind snakes), an adaptation to subterranean life thought to be inherited from past globalecological crises.[93]
The chemistry of a soil determines its ability to supply availableplant nutrients and affects its physical properties and the health of its living population. In addition, a soil's chemistry also determines itscorrosivity, stability, and ability toabsorbpollutants and to filter water. It is thesurface chemistry of mineral and organiccolloids that determines soil's chemical properties.[94] A colloid is a small, insoluble particle ranging in size from 1nanometer to 1micrometer, thus small enough to remain suspended byBrownian motion in a fluid medium without settling.[95] Most soils contain organic colloidal particles calledhumus as well as the inorganic colloidal particles ofclays. The very highspecific surface area of colloids and their netelectrical charges give soil its ability to hold and releaseions. Negatively charged sites on colloids attract and releasecations in what is referred to ascation exchange.Cation-exchange capacity is the amount of exchangeablecations per unit weight of dry soil and is expressed in terms ofmilliequivalents ofpositively charged ions per 100 grams of soil (or centimoles of positive charge per kilogram of soil;cmolc/kg). Similarly, positively charged sites on colloids can attract and releaseanions in the soil, giving the soilanion-exchange capacity.
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.[99]
Isomorphous substitution occurs in clay during its formation, when lower-valence cations substitute for higher-valence cations in thecrystal structure.[100] Substitutions in the outermost layers are more effective than for the innermost layers, as theelectric 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.[101]
Edge-of-clay oxygen atoms are not in balance ionically as thetetrahedral andoctahedral structures are incomplete.[102]
Hydroxyls may substitute for oxygens of the silica layers, a process calledhydroxylation. When the hydrogens of the clay hydroxyls are ionised into solution, they leave the oxygen with a negative charge (anionic clays).[103]
Hydrogens of humus hydroxyl groups may also be ionised into solution, leaving, similarly to clay, an oxygen with a negative charge.[104]
Cations held to the negatively charged colloids resist being washed downward by water and are at first out of reach of plant roots, thereby preserving thesoil fertility in areas of moderate rainfall and low temperatures.[105][106]
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:
If one cation is added in large amounts, it may replace the others by thesheer force of its numbers. This is calledlaw of mass action. This is largely what occurs with the addition of cationicfertilisers (potash,lime).[108]
As the soil solution becomes more acidic (lowpH, 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. Thisionisation ofhydroxy groups on the surface of soil colloids creates what is described as pH-dependentsurface charges.[109] Unlike permanent charges developed byisomorphous substitution, pH-dependent charges are variable and increase with increasing pH.[110] Freed cations can be made available to plants but are also prone to be leached from the soil, possibly making the soil less fertile.[111] Plants are able to excrete H+ into the soil through the synthesis oforganic 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.[112]
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.[113] CEC is the amount of exchangeable hydrogen cations (H+) that will combine with 100 grams dry weight of soil and whose measure is onemilliequivalent per 100 grams of soil (1 meq/100 g). Hydrogen ions have a single charge and one-thousandth of a gram (1 mg) 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 avalence of two, converts to(40 ÷ 2) × 1 milliequivalent = 20 milliequivalents of hydrogen ion per 100 grams of dry soil or 20 meq/100 g.[114] 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 astropical rainforests), due to fast leaching and decomposition, respectively, explains the apparent lack offertility of tropical soils.[115] Live plant roots also have some CEC, linked to theirspecific surface area.[116]
Cation exchange capacity for soils; soil textures; soil colloids[117]
Anion exchange capacity is the soil's ability to remove anions (such asnitrate,phosphate) from the soil water solution and sequester those for later exchange as the plant roots release carbonate anions to the soil water solution.[118] Those colloids which have lowCEC tend to have some AEC.Amorphous andsesquioxide clays have the highest AEC,[119] followed by the iron oxides.[120] 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.[121] Phosphates tend to be held at anion exchange sites.[122]
Iron and aluminumhydroxide clays are able to exchange their hydroxide anions (OH−) for other anions.[118] The order reflecting the strength of anion adhesion is as follows:
H 2PO− 4 replacesSO2− 4 replacesNO− 3 replaces Cl−
The amount of exchangeable anions is of a magnitude of tenths to a few milliequivalents per 100 g dry soil.[117] 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).[123]
Soil reactivity is expressed in terms of pH and is a measure of theacidity oralkalinity of the soil. More precisely, it is a measure ofhydronium 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.[124]
At 25 °C an aqueous solution that has a pH of 3.5 has 10−3.5moles 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 thus more acidic.[125]
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 ofaluminium andmanganese.[126] As a result of a trade-off betweentoxicity and requirement most nutrients are better available to plants at moderate pH,[127] although most minerals are more soluble (weatherable) in acid soils.[128] Soil organisms are hindered by high acidity, and most agricultural crops do best with mineral soils of pH 6.5 and organic soils of pH 5.5.[129] Given that at low pH toxic metals (e.g.aluminium,cadmium,zinc,lead) are positively charged as cations and organic pollutants are in non-ionic form, thus both are made more available to organisms,[130][131] it has been suggested that plants, animals and microbes commonly living in acid soils arepre-adapted to every kind of pollution, whether of natural or human origin.[132]
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 unusualrain acidity against those attached to the colloids. High rainfall rates can then wash the nutrients out, leaving the soil inhabited only by those organisms which are particularly efficient to uptake nutrients in very acid conditions, like intropical rainforests.[133] Once the colloids are saturated with H3O+, the addition of any more hydronium ions oraluminum hydroxyl cations drives the pH even lower (more acidic) as the soil has been left with nobuffering capacity.[134] In areas of extreme rainfall and high temperatures, clay and humus may be washed out, further reducing the buffering capacity of the soil.[135] In low rainfall areas, unleached calcium pushes pH to 8.5 and with the addition of exchangeable sodium, soils may reach pH 10.[136] Beyond a pH of 9, plant growth is reduced.[137] High pH results in lowmicro-nutrient mobility, but water-solublechelates of those nutrients can correct the deficit.[138]Sodium can be reduced by the addition ofgypsum (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.[139][140]
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 calledbase 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 is15 ÷ 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).[141] 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).[142] 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.[143]
The resistance of soil to change in pH, as a result of the addition of acid or basic material, is a measure of thebuffering capacity of a soil and (for a particular soil type) increases as theCEC increases. Hence, pure sand has almost no buffering ability, though soils high incolloids (whether mineral or organic) have high buffering capacity.[144] Buffering occurs by cation exchange andneutralisation. However, colloids are not the only regulators of soil pH. The role ofcarbonates should be underlined, too.[145] More generally, according to pH levels, several buffer systems take precedence over each other, fromcalcium carbonatebuffer range to iron buffer range.[146]
Seventeen elements or nutrients are essential for plant growth and reproduction. They arecarbon (C),hydrogen (H),oxygen (O),nitrogen (N),phosphorus (P),potassium (K),sulfur (S),calcium (Ca),magnesium (Mg),iron (Fe),boron (B),manganese (Mn),copper (Cu),zinc (Zn),molybdenum (Mo),nickel (Ni) andchlorine (Cl).[154][155][156] Nutrients required for plants to complete their life cycle are consideredessential nutrients. Nutrients that enhance the growth of plants but are not necessary to complete the plant's life cycle are considered non-essential. Except for carbon, hydrogen, and oxygen, which are supplied by carbon dioxide and water, and nitrogen, provided through nitrogen fixation,[156] the nutrients derive originally from the mineral component of the soil. Thelaw of the minimum expresses that when the available form of a nutrient is not in enough proportion in the soil solution, other nutrients cannot be taken up at an optimum rate by a plant.[157] A particular nutrient ratio (stoichiometry) of the soil solution is thus mandatory for optimizing plant growth, a value which might differ from nutrient ratios calculated from plant composition.[158]
Plant uptake of nutrients can only proceed when present in a plant-available form. In most situations, nutrients are absorbed in anionic form from (or together with) soil water. Although minerals are the origin of most nutrients, and the bulk of most nutrient elements in the soil is held in crystalline form withinprimary andsecondary minerals, they weather too slowly to support rapid plant growth. For example, the application of finely ground minerals,feldspar andapatite, 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.[159] However, plants are able to stimulatemineral weathering, and thus the availability of mineral-bound nutrients, through various processes, both direct (e.g. weathering agents such ascarbon dioxide,organic acids andligands) and indirect (e.g.mycorrhizal fungi,rhizosphere bacteria).[160][161][162]
The nutrients adsorbed onto the surfaces ofclay colloids andsoil 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. Thedecomposition of soil organic matter bymicroorganisms 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.[163]
Gram for gram, the capacity ofhumus to hold nutrients and water is far greater than that of clay minerals, most of the soilcation exchange capacity arising from chargedcarboxylic groups on organic matter.[164] However, despite the remarkable capacity of humus to retain water once water-soaked, its highhydrophobicity decreases itswettability once dry, a non-reversible process calledhysteresis.[165] Small amounts of humus may remarkably increase the soil's capacity to promote plant growth.[166][163]
The organic material in soil is made up oforganic 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 ofearthworms, 2400 lb offungi, 1500 lb ofbacteria, 133 lb ofprotozoa and 890 lb ofarthropods andalgae.[167]
A few percent of thesoil organic matter, with smallresidence time, consists of the microbialbiomass andmetabolites of bacteria,molds, andactinomycetes that work to break down the dead organic matter.[168][169] Were it not for themineralizing action of these microorganisms, the entire carbon dioxide part of the atmosphere would be sequestered as organic matter in the soil. However, in the same time soil microbes contribute tocarbon sequestration in the topsoil through the formation of stablehumus.[170] In the aim to sequester more carbon in the soil for alleviating thegreenhouse effect it would be more efficient in the long-term to stimulatehumification than to decrease litterdecomposition.[171]
The main part of soil organic matter is a complex assemblage of small organic molecules, collectively calledhumus orhumic substances. The use of these terms, which do not rely on a clear chemical classification, has been considered as obsolete.[172] Other studies showed that the classical notion ofmolecule is not convenient for humus, which escaped most attempts done over two centuries to resolve it in unit components, but still is chemically distinct frompolysaccharides,lignins andproteins.[173]
Most living things in soils, including plants, animals, bacteria, and fungi, transform nutrients and energy in organic matter and in turn are dependent on it for their requirements.[174] Soils have organic compounds in varying degrees ofdecomposition, the rate of which is dependent on temperature, soil moisture, and aeration.[175] Bacteria and fungi feed on raw organic matter, which are fed upon byprotozoa, which in turn are fed upon bynematodes,annelids andarthropods, themselves able to consume and transform raw or humified organic matter. This has been called thesoil food web, through which all organic matter is processed as in adigestive system.[176] Organic matter holds soils open, allowing the infiltration of air[177] and water,[178] and may hold as much as twice its weight in water.[179] Many soils, including desert and rocky-gravel soils, have little or no organic matter. Soils that are all organic matter, such aspeat (histosols), are infertile.[180] In its earliest stage of decomposition, the original organic material is often called raw organic matter. The final stage of decomposition is called humus.
Ingrassland, much of the organic matter added to the soil is from the deep, fibrous, grass root systems. By contrast, tree leaves falling on the forest floor are the principal source of soil organic matter in the forest. Another difference is the frequent occurrence in the grasslands of fires that destroy large amounts of aboveground material but stimulate even greater contributions from roots. Also, the much greater acidity under any forests inhibits the action of certain soil organisms that otherwise would mix much of the surface litter into the mineral soil. As a result, the soils under grasslands generally develop a thickerA horizon with a deeper distribution of organic matter than in comparable soils under forests, which characteristically store most of their organic matter in the forest floor (O horizon) and thin A horizon.[181]
Humus refers to organic matter that has been decomposed by soil microflora and fauna to the point where it is resistant to further breakdown.[44] 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 tosoil health and plant growth.[182] Humus also feeds arthropods,termites andearthworms which further improve the soil.[183] The end product, humus, is suspended incolloidal form in the soil solution and forms aweak acid that can attack silicate minerals bychelating their iron and aluminum atoms.[184] 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.[185]
Humic acids andfulvic acids, which begin as raw organic matter, are important constituents of humus. After the death of plants, animals, and microbes, microbes begin to feed on the residues through their production of extra-cellular soil enzymes, resulting finally in the formation of humus.[186] As the residues break down, only molecules made ofaliphatic andaromatic hydrocarbons, assembled and stabilized by oxygen andhydrogen bonds, remain in the form of complex molecular assemblages collectively called humus.[173] 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.[185] 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.[187]
Lignin is resistant to breakdown and accumulates within the soil. It also reacts withproteins,[188] which further increases its resistance to decomposition, including enzymatic decomposition by microbes.[189]Fats andwaxes from plant matter have still more resistance to decomposition and persist in soils for thousand years, hence their use as tracers of past vegetation in buried soil layers.[190] Clay soils often have higher organic contents that persist longer than soils without clay as the organic molecules adhere to and are stabilised by the clay.[191] Proteins normally decompose readily, to the exception ofscleroproteins, but when bound to clay particles they become more resistant to decomposition.[192] As for other proteins clay particles absorb the enzymes exuded by microbes, decreasingenzyme activity while protectingextracellular enzymes from degradation.[193] 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.[194] A study showed increased soil fertility following the addition of mature compost to a clay soil.[195] High soiltannin content can cause nitrogen to be sequestered as resistant tannin-protein complexes.[196][197]
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.[181] Soils with humus can vary in nitrogen content but typically have 3 to 6 percent nitrogen. Raw organic matter, as a reserve of nitrogen and phosphorus, is a vital component affectingsoil fertility.[180] Humus also absorbs water, and expands and shrinks between dry and wet states to a higher extent than clay, increasing soil porosity.[198] Humus is less stable than the soil's mineral constituents, as it is reduced by microbialdecomposition, and over time its concentration diminishes without the addition of new organic matter. However, humus in its most stable forms may persist over centuries if not millennia.[199]Charcoal is a source of highly stable humus, calledblack carbon,[200] which had been used traditionally to improve the fertility of nutrient-poor tropical soils. This very ancient practice, as ascertained in the genesis ofAmazonian dark earths, has been renewed and became popular under the name ofbiochar. It has been suggested that biochar could be used to sequester more carbon in the fight against the greenhouse effect.[201]
The production, accumulation and degradation of organic matter are greatly dependent on climate. For example, when athawing event occurs, the flux ofsoil gases with atmospheric gases is significantly influenced.[202] Temperature, soil moisture andtopography are the major factors affecting the accumulation of organic matter in soils. Organic matter tends to accumulate under wet or cold conditions wheredecomposer activity is impeded by low temperature[203] or excess moisture which results in anaerobic conditions.[204] 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.[205] 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.[206]
Typical types and percentages of plant residue components
Cellulose (45.0%)
Lignin (20.0%)
Hemicellulose (18.0%)
Protein (8.00%)
Sugars and starches (5.00%)
Fats and waxes (2.00%)
Cellulose andhemicellulose undergo fast decomposition by fungi and bacteria, with a half-life of 12–18 days in a temperate climate.[207]Brown rot fungi can decompose the cellulose and hemicellulose, leaving thelignin andphenolic compounds behind.Starch, which is anenergy storage system for plants, undergoes fast decomposition by bacteria and fungi. Lignin consists ofpolymers composed of 500 to 600 units with a highly branched,amorphous structure, linked to cellulose, hemicellulose andpectin inplant cell walls. Lignin undergoes very slow decomposition, mainly bywhite rot fungi andactinomycetes; its half-life under temperate conditions is about six months.[207]
A horizontal layer of the soil, whose physical features, composition and age are distinct from those above and beneath, is referred to as asoil 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 ofnodules orconcretions.[208] No soil profile has all the major horizons. Some, calledentisols, may have only one horizon or are currently considered as having no horizon, in particular incipient soils from unreclaimedmining waste deposits,[209]moraines,[210]volcanic cones[211]sand dunes oralluvial terraces.[212] Upper soil horizons may be lacking in truncated soils following wind or water ablation, with concomitant downslope burying of soil horizons, a natural process aggravated by agricultural practices such as tillage.[213] The growth of trees is another source of disturbance, creating a micro-scale heterogeneity which is still visible in soil horizons once trees have died.[214] By passing from a horizon to another, from the top to the bottom of the soil profile, one goes back in time, with past events registered in soil horizons like insediment layers. Samplingpollen,testate amoebae and plant remains in soil horizons may help to reveal environmental changes (e.g. climate change,land use change) which occurred in the course of soil formation.[215] Soil horizons can be dated by several methods such asradiocarbon, using pieces of charcoal provided they are of enough size to escapepedoturbation byearthworm activity and other mechanical disturbances.[216] Fossil soil horizons frompaleosols can be found withinsedimentary rock sequences, allowing the study of past environments.[217]
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.[218] The growth of vegetation results in the production of organic residues which fall on the ground as litter for plant aerial parts (leaf litter) or are directly produced belowground for subterranean plant organs (root litter), and then releasedissolved organic matter.[219] The remaining surficial organic layer, called theO 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.[220] After sufficient time,humus moves downward and is deposited in a distinctive organic-mineral surface layer called theA horizon, in which organic matter is mixed with mineral matter through the activity of burrowing animals, a process calledpedoturbation. 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[221] and in the juxtaposition of humified organic matter and mineral particles, without intimate mixing, in the underlying mineral horizons.[222]
One of the first soil classification systems was developed by Russian scientistVasily Dokuchaev around 1880.[223] 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 onsoil morphology instead of parental materials and soil-forming factors. Since then, it has undergone further modifications. TheWorld Reference Base for Soil Resources[224] aims to establish an international reference base for soil classification.
In the United States, the system ofSoil Taxonomy[225] is used. This system was established by the United States Department of Agriculture: Natural Resource Conversation Service and is currently on its second edition, released in 1999 by the Soil Survey Staff.[226]
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 ofpedology in the 19th century. However, as demonstrated byaeroponics,aquaponics andhydroponics, 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.[227]
Soil material is also a critical component inmining,construction and landscape development (also calledlandscape architecture) industries.[228] Soil serves as a foundation for most construction projects. The movement of massive volumes of soil can be involved insurface mining,road building anddam construction.Earth sheltering is the architectural practice of using soil for externalthermal mass against building walls. Manybuilding 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 ofsubsistence agriculture.[229]
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 andmosses formbiological soil crusts which capture and sequester a significant amount of carbon byphotosynthesis. 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 ingreenhouse gas emissions and slow global warming, while improving crop yields and reducing water needs.[235][236][237]
Waste management often has a soil component.Septic drain fields treatseptic tank effluent usingaerobic soil processes. Land application ofwaste water relies onsoil biology to aerobically treatBOD. Alternatively,landfills use soil fordaily cover, isolating waste deposits from the atmosphere and preventing unpleasant smells.Composting is now widely used to treat aerobically solid domestic waste and dried effluents ofsettling 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.[238]
Organic soils, especiallypeat, serve as a significant fuel andhorticultural resource. Peat soils are also commonly used for the sake of agriculture in Nordic countries, because peatland sites, when drained, provide fertile soils for food production.[239] However, wide areas of peat production, such as rain-fedsphagnumbogs, also calledblanket bogs orraised bogs, are now protected because of their patrimonial interest. As an example,Flow Country, covering 4,000 square kilometres of rolling expanse of blanket bogs in Scotland, is now recognized as aUNESCOWorld Heritage Site. 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 andcarbon dioxide) and increased temperature,[240] a contention which is still under debate when replaced at field scale and including stimulated plant growth.[241]
Geophagy is the practice of eating soil-like substances. Both animals and humans occasionally consume soil for medicinal, recreational, or religious purposes.[242] It has been shown that somemonkeys consume soil, together with their preferred food (treefoliage andfruits), in order to alleviate tannin toxicity.[243]
Soil acidification is beneficial in the case ofalkaline soils, but it degrades land when it lowerscrop productivity, soil biological activity and increases soil vulnerability tocontamination anderosion. Soils are initially acid and remain such when their parent materials are low in basiccations (calcium, magnesium, potassium andsodium). On parent materials richer inweatherable minerals acidification occurs when basic cations areleached from the soil profile by rainfall or exported by the harvesting of forest or agricultural crops. Soil acidification is accelerated by the use of acid-formingnitrogenous fertilizers and by the effects ofacid precipitation.Deforestation is another cause of soil acidification, mediated by increased leaching of soil nutrients in the absence oftree canopies.[250]
Soilcontamination at low levels is often within a soil's capacity to treat and assimilatewaste material.Soil biota can treat waste by transforming it, mainly through microbialenzymatic activity.[251] Soil organic matter and soil minerals can adsorb the waste material and decrease itstoxicity,[252] although when in colloidal form they may transport the adsorbed contaminants to subsurface environments.[253] Many waste treatment processes rely on this naturalbioremediation 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 restoresoil functions and values. Techniques includeleaching,air sparging,soil conditioners,phytoremediation,bioremediation andMonitored Natural Attenuation. An example of diffuse pollution with contaminants iscopper accumulation invineyards andorchards to whichfungicides are repeatedly applied, even inorganic farming.[254]
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 ortwine, as well asirrigation water in which clothes had been washed.[255]
The application ofbiosolids fromsewage sludge andcompost can introducemicroplastics 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.[255]
Desertification, an environmental process of ecosystem degradation in arid and semi-arid regions, is often caused by badly adapted human activities such asovergrazing or excess harvesting offirewood. It is a common misconception thatdrought causes desertification.[256] While droughts are common inarid andsemiarid 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.[257] These practices help to controlerosion and maintain productivity during periods when moisture is available. Continued land abuse during droughts, however, increasesland degradation. Increased population andlivestock pressure onmarginal lands accelerates desertification.[258] It is now questioned whether present-dayclimate 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.[259]
Erosion of soil is caused bywater,wind,ice, andmovement in response to gravity. More than one kind of erosion can occur simultaneously. Erosion is distinguished fromweathering, since erosion also transports eroded soil away from its place of origin (soil in transit may be described assediment). Erosion is an intrinsic natural process, but in many places it is greatly increased by human activity, especially unsuitableland use practices.[260] These includeagricultural activities which leave the soil bare during times of heavy rain or strong winds,overgrazing,deforestation, and improperconstruction activity. Improved management can limit erosion.Soil conservation techniques which are employed include changes of land use (such as replacing erosion-pronecrops withgrass or other soil-binding plants), changes to the timing or type of agricultural operations,terrace building, use of erosion-suppressing cover materials (includingcover crops and other plants), limiting disturbance during construction, and avoiding construction during erosion-prone periods and in erosion-prone places such as steep slopes.[261] Historically, one of the best examples of large-scale soil erosion due to unsuitable land-use practices is wind erosion (the so-calleddust bowl) which ruined American and Canadian prairies during the 1930s, when immigrant farmers, encouraged by the federal government of both countries, settled and converted the originalshortgrass prairie toagricultural crops andcattle ranching.
A serious and long-running water erosion problem occurs inChina, on the middle reaches of theYellow River and the upper reaches of theYangtze 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 theLoess Plateau region of northwest China.[262]
Soil piping is a particular form of soil erosion that occurs below the soil surface.[263] It causeslevee anddam failure, as well assink hole formation. Turbulent flow removes soil starting at the mouth of theseep flow and thesubsoil erosion advances up-gradient.[264] The termsand boil is used to describe the appearance of the discharging end of an active soil pipe.[265]
Soil salination is the accumulation of freesalts to such an extent that it leads to degradation of the agricultural value of soils and vegetation. Consequences includecorrosion damage, reduced plant growth, erosion due to loss ofplant cover andsoil structure, andwater quality problems due tosedimentation. Salination occurs due to a combination of natural and human-caused processes. Arid conditions favour salt accumulation. This is especially apparent when soil parent material is saline.Irrigation of arid lands is especially problematic.[266] All irrigation water has some level of salinity. Irrigation, especially when it involves leakage from canals and overirrigation in the field, often raises the underlyingwater table. Rapid salination occurs when the land surface is within thecapillary fringe of salinegroundwater.Soil salinity control involveswatertable control andflushing with higher levels of applied water in combination withtile drainage or another form ofsubsurface drainage.[267][268]
Soils which contain high levels of particular clays with high swelling properties, such assmectites, are often very fertile. For example, the smectite-richpaddy soils of Thailand'sCentral Plains are among the most productive in the world. However, the overuse of mineral nitrogenfertilizers and pesticides inirrigated intensiverice production has endangered these soils, forcing farmers to implementintegrated practices based on Cost Reduction Operating Principles.[269]
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 ofshifting cultivation for a more permanent land use.[270] Farmers initially responded by adding organic matter and clay fromtermite mound material, but this wasunsustainable in the long-term because of rarefaction of termite mounds. Scientists experimented with addingbentonite, one of thesmectite family of clays, to the soil. In field trials, conducted by scientists from theInternational Water Management Institute (IWMI) in cooperation withKhon Kaen University and local farmers, this had the effect of helping retain water and nutrients. Supplementing the farmer's usual practice with a single application of 200kilograms perrai (1,300 kg/ha; 1,100 lb/acre) of bentonite resulted in an average yield increase of 73%.[271] Other studies showed that applying bentonite to degraded sandy soils reduced the risk of crop failure during drought years.[272]
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 inCambodia had adopted the use of clays, and that a further 20,000 farmers were introduced to the new technique.[273]
Adding organic matter, likeramial chipped wood orcompost, to soil which is depleted in nutrients and too high in sand will boost its quality and improve production.[275][276]
Special mention must be made of the use ofcharcoal, and more generallybiochar to improve nutrient-poor tropical soils, a process based on the higher fertility of anthropogenicpre-Columbian AmazonianDark Earths, also calledTerra Preta de Índio, due to interesting physical and chemical properties of soilblack carbon as a source of stable humus.[277] However, the uncontrolled application ofcharred waste products of all kinds may endanger soil life and human health.[278]
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.[279]
The Greek historianXenophon (450–355 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.'[280]
As chemistry developed, it was applied to the investigation of soil fertility. The French chemistAntoine Lavoisier showed in about 1778 that plants and animals mustcombust 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.[291] It was the French agriculturalistJean-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.[292]Justus von Liebig in his bookOrganic 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.[293] Liebig nevertheless believed the nitrogen was supplied from the air. The enrichment of soil with guano by the Incas was rediscovered in 1802, byAlexander 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.[294]
The work of Liebig was a revolution for agriculture, and so other investigators started experimentation based on it. In EnglandJohn Bennet Lawes andJoseph Henry Gilbert worked in theRothamsted 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 thesuperphosphate, consisting in the acid treatment of phosphate rock.[295] This led to the invention and use of salts of potassium (K) and nitrogen (N) asfertilizers. Ammonia generated by the production ofcoke was recovered and used as fertiliser.[296] 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 intonitrate,[297] and twenty years laterRobert Warington proved that this transformation was done by living organisms.[298] In 1890Sergei Winogradsky announced he had found the bacteria responsible for this transformation.[299]
It was known that certainlegumes 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 agronomistHermann Hellriegel and the Dutch microbiologistMartinus Beijerinck.[295]
Crop rotation, mechanisation, chemical and natural fertilisers led to a doubling of wheat yields in western Europe between 1800 and 1900.[300]
Scientists who studied soil in connection with agricultural practices considered it mainly a static substrate. However, the soil is the result of evolution from more ancient geological materials under the action of biotic and abiotic processes. After studies of soil improvement commenced, other researchers began to study soil genesis and, as a result, 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.[301] His work was not continued. At about the same time,Friedrich Albert Fallou described soil profiles and related soil characteristics to their formation as part of his professional work evaluating forest and farmland for the principality ofSaxony. His 1857 book,Anfangsgründe der Bodenkunde (First principles of soil science), established modern soil science.[302] 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 byKonstantin Glinka, a member of the Russian team.[303]
Curtis F. Marbut, influenced by the work of the Russian team, translated Glinka's publication into English,[304] and, as he was placed in charge of the U.S.National Cooperative Soil Survey, applied it to a national soil classification system.[156]
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