Soil formation, also known aspedogenesis, is the process ofsoil genesis as regulated by the effects of place, environment, and history.Biogeochemical processes act to both create and destroy order (anisotropy) within soils. These alterations lead to the development of layers, termedsoil horizons, distinguished by differences incolor,structure,texture, andchemistry. Thesefeatures occur in patterns ofsoil type distribution, forming in response to differences in soil forming factors.[1]
Pedogenesis is studied as a branch ofpedology, the study of soil in its natural environment. Other branches of pedology are the study ofsoil morphology andsoil classification. The study of pedogenesis is important to understanding soil distribution patterns in current (soil geography) and past (paleopedology) geologic periods.
Soil develops through a series of changes.[2] The starting point isweathering of freshly accumulatedparent material. A variety of soil microbes (bacteria,archaea,fungi) feed on simple compounds (nutrients) released by weathering and produceorganic acids and specializedproteins which contribute in turn to mineral weathering. They also leave behindorganic residues which contribute tohumus formation.[3] Plant roots with their symbioticmycorrhizal fungi are also able to extract nutrients fromrocks.[4]
Soil formation is influenced by at least five classic factors that are intertwined in the evolution of a soil. They are:parent material,climate,topography (relief),soil organisms, and time.[2] When reordered to climate, organisms, relief, parent material, and time, they form the acronym CLORPT.[9]
Soil, on an agricultural field in Germany, which has formed onloess parent material
Parent materials are classified according to how they came to be deposited. Residual materials are mineral materials that have weathered in place from primarybedrock. Transported materials are those that have been deposited by water, wind, ice or gravity. Cumulose material is organic matter that has grown and accumulates in place.[12]
Residual soils are soils that develop from their underlying parent rocks and have the same general chemistry as those rocks.[13] Residual soils can be found onmesas[14] and volcanoes.[15] In the United States as little as three percent of the soils are residual.[16]
Most soils derive from transported materials that have been moved many miles by wind, water, ice and gravity:
Aeolian processes (movement by wind) are capable of movingsilt and finesand many hundreds of miles, formingloess soils (60–90 percent silt),[17] common in theMidwestern United States and Canada, north-western Europe, Argentina andCentral Asia. Clay is seldom moved by wind as it forms stable aggregates.[18]
Water-transported materials are classed as eitheralluvial,lacustrine, or marine. Alluvial materials are those moved and deposited by flowing water.Sedimentary deposits settled in lakes are called lacustrine.Lake Bonneville and many soils around theGreat Lakes are examples.[19] Marine deposits, such as soils along theGulf Coast and in theImperial Valley of California are the beds of ancient seas that have been revealed as the land uplifted.[20]
Ice moves parent material and makes deposits in the form of terminal and lateralmoraines in the case of stationaryglaciers. Retreating glaciers leave smoother ground moraines, and in all casesoutwash plains are left asalluvial deposits are moved downstream from the glacier.[21]
Parent material moved by gravity is obvious at the base of steep slopes astalus cones and is calledcolluvial material.[22]
Cumuloseparent material is not moved but originates from deposited organic material. This includespeat andmuck soils and results from preservation of plant residues by the low oxygen content of a highwater table. While peat may form sterile soils,[23] muck soils may be very fertile.[24]
Theweathering of parent material takes the form of physical weathering (disintegration), chemical weathering (decomposition) and chemical transformation. Weathering is usually confined to the top few meters of geologic material, because physical, chemical, and biological stresses and fluctuations generally decrease with depth.[25] Physical disintegration begins as rocks that have solidified deep in the Earth are exposed to lower pressure near the surface and swell and become mechanically unstable.[26] Chemical decomposition is a function of mineral solubility, the rate of which doubles with each 10 °C rise in temperature but is strongly dependent on water to effect chemical changes.[27] Rocks that will decompose in a few years in tropical climates will remain unaltered for millennia in deserts.[28] Structural changes are the result ofhydration,oxidation, andreduction. Chemical weathering mainly results from the excretion oforganic acids andchelating compounds by bacteria[29] and fungi,[30] and is thought to increase undergreenhouse effect.[31]
Physical disintegration is the first stage in the transformation of parent material into soil. Temperature fluctuations cause expansion and contraction of the rock, splitting it along lines of weakness.[32] Water may then enter the cracks and freeze and cause the physical splitting of material along a path toward the center of the rock,[33] while temperature gradients within the rock can cause exfoliation of "shells".[34] Cycles of wetting and drying cause soil particles to be abraded to a finer size, as does the physical rubbing of material as it is moved by wind, water, and gravity.[35] Organisms may reduce parent material size and create crevices and pores through the mechanical action of plant roots and the digging activity of animals.[36][37]
Chemical decomposition andstructural changes result when minerals are made soluble by water or are changed in structure. The first three of the following list are solubility changes, and the last three are structural changes.[38]
Thesolution of salts in water results from the action of bipolarwater molecules onionic salt compounds producing a solution ofions and water, removing those minerals and reducing the rock's integrity, at a rate depending onwater flow and pore channels.[39]
Hydrolysis is the transformation of minerals intopolar molecules by the splitting of intervening water. This results in solubleacid-base pairs. For example, the hydrolysis oforthoclase-feldspar transforms it to acidsilicate clay and basicpotassium hydroxide, both of which are more soluble.[40]
Hydration is the inclusion of water in a mineral structure, causing it to swell and leaving itstressed and easilydecomposed.[42]
Oxidation of a mineral compound is the inclusion ofoxygen in a mineral, causing it to increase itsoxidation number and swell due to the relatively large size of oxygen, leaving it stressed and more easily attacked by water (hydrolysis) or carbonic acid (carbonation).[43]
Reduction, the opposite of oxidation, means the removal of oxygen, hence the oxidation number of some part of the mineral is reduced, which occurs when oxygen is scarce. The reduction of minerals leaves them electrically unstable, more soluble and internally stressed and easily decomposed. It mainly occurs inwaterlogged conditions.[44]
Of the above,hydrolysis andcarbonation are the most effective, in particular in regions of high rainfall, temperature and physicalerosion.[45] Chemical weathering becomes more effective as thesurface area of the rock increases, thus is favoured by physical disintegration.[46] This stems inlatitudinal andaltitudinal climate gradients inregolith formation.[47][48]
Saprolite is a particular example of a residual soil formed from the transformation ofgranite,metamorphic and other types ofbedrock intoclay minerals. Often called weathered granite, saprolite is the result of weathering processes that include:hydrolysis,chelation from organic compounds,hydration and physical processes that includefreezing andthawing. The mineralogical and chemical composition of the primarybedrock material, its physical features (includinggrain size and degree of consolidation), and the rate and type of weathering transforms the parent material into a different mineral. The texture, pH and mineral constituents of saprolite are inherited from itsparent material. This process is also calledarenization, resulting in the formation of sandy soils, thanks to the much higher resistance of quartz compared to other mineral components of granite (e.g.,mica,amphibole, feldspar).[49]
The principalclimatic variables influencing soil formation are effectiveprecipitation (i.e., precipitation minusevapotranspiration) andtemperature, both of which affect the rates of chemical, physical, and biological processes.[50] Temperature andmoisture both influence theorganic matter content of soil through their effects on the balance betweenprimary production anddecomposition: the colder or drier the climate the lesseratmospheric carbon is fixed as organic matter while the lesser organic matter isdecomposed.[51] Climate also indirectly influences soil formation through the effects of vegetation cover and biological activity, which modify the rates ofchemical reactions in the soil.[52]
Climate is the dominant factor in soil formation, and soils show the distinctive characteristics of theclimate zones in which they form, with afeedback to climate through transfer ofcarbon stocked insoil horizons back to theatmosphere.[53] If warm temperatures and abundant water are present in the profile at the same time, the processes of weathering,leaching, andplant growth will be maximized. According to the climatic determination ofbiomes, humid climates favor the growth of trees. In contrast, grasses are the dominant native vegetation insubhumid andsemiarid regions, while shrubs and brush of various kinds dominate inarid areas.[54]
Water is essential for all the major chemicalweathering reactions. To be effective in soil formation, water must penetrate theregolith. The seasonal rainfall distribution, evaporative losses, sitetopography, andsoil permeability interact to determine how effectively precipitation can influence soil formation. The greater the depth of water penetration, the greater the depth of weathering of the soil and its development.[55] Surplus waterpercolating through thesoil profile transportssoluble andsuspended materials from the upper layers (eluviation) to the lower layers (illuviation), includingclay particles[56] anddissolved organic matter.[57] It may also carry away soluble materials in the surfacedrainage waters. Thus, percolating water stimulates weathering reactions and helps differentiate soil horizons.
Likewise, a deficiency of water is a major factor in determining the characteristics of soils of dry regions. Soluble salts are not leached from these soils, and in some cases they build up to levels that curtail plant[58] and microbial growth.[59] Soil profiles in arid and semi-arid regions are also apt to accumulatecarbonates and certain types of expansive clays (calcrete orcaliche horizons).[60][61] In tropical soils, when the soil has been deprived of vegetation (e.g. bydeforestation) and thereby is submitted to intenseevaporation, the upwardcapillary movement of water, which has dissolvediron andaluminium salts, is responsible for the formation of a superficialhardpan oflaterite orbauxite, respectively, which is improper for cultivation, a known case of irreversiblesoil degradation.[62]
Very intense chemical weathering, leaching, and erosion in warm and humid regions where soil does not freeze
Climate directly affects the rate ofweathering andleaching. Wind moves sand and smaller particles (dust), especially in arid regions where there is littleplant cover, depositing it close to[64] or far from the entrainment source.[65] The type and amount of precipitation influence soil formation by affecting the movement of ions and particles through the soil, and aid in the development of different soil profiles. Soil profiles are more distinct in wet and cool climates, where organic materials may accumulate, than in wet and warm climates, where organic materials are rapidly consumed.[66] The effectiveness of water in weathering parent rock material depends on seasonal and daily temperature fluctuations, which favourtensile stresses in rock minerals, and thus their mechanical disaggregation, a process calledthermal fatigue.[67] By the same processfreeze-thaw cycles are an effective mechanism which breaks up rocks and other consolidated materials.[68]
Thetopography, orrelief, is characterized by the inclination (slope),elevation, and orientation of the terrain (aspect). Topography determines the rate of precipitation orrunoff and the rate of formation orerosion of the surfacesoil profile. The topographical setting may either hasten or retard the work of climatic forces.[69]
Steep slopes encourage rapid soil loss by erosion and allow less rainfall to enter the soil before running off and hence, little mineral deposition in lower profiles (illuviation). In semiarid regions, the lower effective rainfall on steeper slopes also results in less complete vegetative cover, so there is less plant contribution to soil formation.[70] For all of these reasons, steep slopes prevent the formation of soil from getting very far ahead of soil destruction. Therefore, soils on steep terrain tend to have rather shallow, poorly developed profiles in comparison to soils on nearby, more level sites.[71]
Topography determines exposure toweather,fire, and other forces of man and nature. Mineral accumulations, plantnutrients, type of vegetation, vegetation growth, erosion, and water drainage are dependent on topographic relief.[72] Soils at the bottom of a hill will get more water than soils on the slopes, and soils on the slopes that face thesun's path (south aspect) will be drier than soils on slopes that do not.[73]
Inswales and depressions where runoff water tends to concentrate, theregolith is usually more deeply weathered, and soil profile development is more advanced.[74] However, in the lowest landscape positions, water may saturate the regolith to such a degree thatdrainage andaeration are restricted. Here, the weathering of some minerals and the decomposition of organic matter are retarded, while the loss of iron and manganese is accelerated. In such low-lying topography, special profile features characteristic ofwetland soils may develop. Depressions allow the accumulation of water, minerals and organic matter, and in the extreme, the resulting soils will besaline marshes orpeat bogs.[75]
Each soil has a unique combination of microbial, plant, animal and human influences acting upon it.Microorganisms are particularly influential in the mineral transformations critical to the soil forming process. Additionally, some bacteria canfix atmospheric nitrogen, and some fungi are efficient at extracting deep soilphosphorus and increasingsoil carbon levels in the form ofglomalin.[78] Plants hold soil against erosion,[79] and accumulated plant material build soilhumus levels.[80] Plantroot exudation supportsmicrobial activity.[81] Animals serve to decompose plant materials and mix soil throughbioturbation.[82]
Soil is the most speciose (species-rich)ecosystem on Earth, but the vast majority of organisms in soil aremicrobes, a great many of which have not been described so far.[83][84] There may be a microbial population limit of around one billion cells per gram of soil, but estimates of the number of species vary widely from 50,000 per gram to over a million per gram of soil.[85][86] The number of organisms and species can vary widely according to soil type, location, and depth.[84][86]
Plants,animals, fungi, bacteria and humans affect soil formation (seesoil biomantle andstonelayer). Soil animals, including soilmacrofauna (e.g.earthworms,termites,tenebrionids,gophers,moles) andmesofauna (e.g.enchytraeids,springtails,mites), mix soils as they formburrows andpores, allowing moisture and gases to move about, a process calledbioturbation.[87] In the same way, plant roots penetrate soil horizons and open channels upon decomposition.[88] Plants with deeptaproots can penetrate many metres through the different soil layers to bring up nutrients from deeper in the profile.[89] Plants have fine roots that excrete organic compounds (sugars,organic acids,mucilage), slough off cells (in particular at their tip), and are easily decomposed, adding organic matter to soil, a process calledrhizodeposition.[90]
Microorganisms, including fungi and bacteria, effect chemical exchanges between roots and soil and act as a reserve of nutrients in a soil biological hotspot calledrhizosphere.[91] The growth of roots through the soil stimulates microbial populations, stimulating in turn the activity of their predators (notablyamoeba and free-livingnematodes), thereby increasing themineralization rate, and in last turn root growth, apositive feedback called the soilmicrobial loop.[92] Out of root influence, in thebulk soil most bacteria are in a quiescent stage, forming micro-aggregates, i.e.mucilaginous colonies to which clay particles are glued, offering them a protection againstdesiccation and predation by soilmicrofauna (bacteriophagousprotozoa andnematodes).[93] Microaggregates (20–250 μm) are ingested by soil fauna, and bacterial bodies are partly or totally digested in their guts.[94]
Humans impact soil formation by removing vegetation cover throughtillage, application ofherbicides, fire,deforestation, and leaving soils bare. This can lead toerosion,waterlogging,lateritization orpodzolization (according to climate and topography).[95] Tillage mixes the different soil layers, restarting the soil formation process as less weathered material is mixed with the more developed upper layers, resulting in net increased rate of mineralweathering.[96]
Earthworms,ants,termites,moles,gophers, as well as somemillipedes andtenebrionid beetles, mix the soil as they burrow, significantly affecting soil formation.[97] Earthworms ingest soil particles and organic residues, enhancing the availability of plant nutrients in the material that passes through their bodies.[98] They aerate and stir the soil and create stablesoil aggregates, after having disrupted links between soil particles during the intestinal transit of ingested soil,[99] thereby assuring ready infiltration of water.[100] As ants andtermites buildmounds, earthworms transport soil materials from one horizon to another.[101] Other important functions are fulfilled by earthworms in thesoil ecosystem, in particular their intensemucus production, both within theintestine and as a lining in their galleries (burrows),[102] exert apriming effect on soil microflora,[103] giving them the status ofecosystem engineers, which they share with ants and termites.[104]
In general, the mixing of the soil by the activities of animals, sometimes calledpedoturbation, tends to undo or counteract the tendency of other soil-forming processes that create distinct horizons.[105] Termites and ants may also retard soil profile development by denuding large areas of soil around their nests, leading to increased loss of soil by erosion,[106] the same for the deposition of casts at the soil surface by earthworms.[107] Large animals such as gophers, moles, andprairie dogs bore into the lower soil horizons, bringing materials to the surface.[108] Their tunnels are often open to the surface, encouraging the movement of water and air into thesubsurface layers. In localized areas, they enhance mixing of the lower and upper horizons by creating and later refilling the tunnels. Old animal burrows in the lower horizons often become filled with soil material from the overlyingA horizon, creating profile features known ascrotovinas orkrotovinas.[109]
Vegetation impacts soils in numerous ways. It can prevent erosion caused by excessive rain that might result fromsurface runoff.[110] Plants shade soils, keeping them cooler[111] and slowingevaporation ofsoil moisture.[112] Conversely, by way oftranspiration, plants can cause soils to lose moisture, resulting in complex and highly variable relationships betweenleaf area index (measuring light interception) and moisture loss: more generally plants prevent soil fromdesiccation during driest months while they dry it during moister months, thereby acting as a buffer against strong moisture variation.[113] Plants can form new chemicals that can break down minerals, both directly[114] and indirectly throughmycorrhizal fungi[30] and rhizosphere bacteria,[115] and improve thesoil structure.[116] The type and amount of vegetation depend on climate, topography, soil characteristics and biological factors, mediated or not by human activities.[117][118] Soil factors such as density, depth, chemistry, pH, temperature and moisture greatly affect the type of plants that can grow in a given location.[119] Dead plants and fallen leaves and stems begin their decomposition on the soil surface. There, organisms feed on them and mix the organic material with the upper soil layers; these added organic compounds become part of the soil formation process.[120]
The influence of humans, and by association, fire, are state factors placed within the organisms state factor.[121] Humans can import or extract nutrients and energy in ways that dramatically change soil formation. Accelerated soil erosion fromovergrazing, andPre-Columbianterraforming in theAmazon basin resulting interra preta are two examples of the effects of human management.[122][123]
It is believed thatNative Americans regularly set fires to maintain several large areas ofprairie grasslands inIndiana andMichigan, although climate and mammaliangrazers (e.g.bisons) are also advocated to explain the maintenance of theGreat Plains of North America.[124] In more recent times, human destruction of natural vegetation and subsequenttillage of the soil forcrop production has abruptly modified soil formation.[125] Likewise,irrigating soil in anarid region drastically influences soil-forming factors,[126] as does addingfertilizer andlime to soils of lowfertility.[127]
Distinct ecosystems produce distinct soils, sometimes in easily observable ways. For example, three species ofland snails in the genusEuchondrus in theNegev desert are noted for eatinglichens growing under the surfacelimestone rocks and slabs (endolithic lichens). The grazing activity of these ecosystem engineers disrupts the limestone, resulting in the weathering and the subsequent formation of soil.[128] They have a significant effect on the region: the population of snails is estimated to process between 0.7 and 1.1 metric ton per hectare per year oflimestone in theNegev desert.[128]
The effects of ancient ecosystems are not as easily observed, and this challenges the understanding of soil formation. For example, thechernozems of the North Americantallgrass prairie have ahumus fraction nearly half of which ischarcoal. This outcome was not anticipated because the antecedentprairiefires capable of producing these distinct deep richblack soils are not easily observed.[129] It is now accepted thatNeolithic human-causedwildfires enriched soils incharcoal (also calledblack carbon) and played a prominent role in the formation of the fertilechernozems andterra preta, actively used for the sake of agriculture.[130][131]
It has been questioned whether soil formation can take place in the absence of organisms. Some researchers considered that life was not a prerequisite, taking as an exampleMartian soils,[132] while others considered that these 'soils' were in factregoliths.[133]
Time is a factor in the interactions of all the above.[2] While a mixture ofsand,silt andclay constitute thetexture of a soil and theaggregation of those components producespeds, the development of a distinctB horizon marks the development of a soil or pedogenesis.[134] With time, soils will evolve features that depend on the interplay of the prior listed soil-forming factors.[2] It takes decades[135] to several thousand years for a soil to develop a profile,[136] although the notion of soil development has been criticized, soil being in a constant state-of-change under the influence of fluctuating soil-forming factors.[137] That time period depends strongly on climate, parent material, relief, and biotic activity.[138][139] For example, recently deposited material from a flood exhibits no soil development as there has not been enough time for the material to form a structure that further defines soil.[140] The original soil surface is buried, and the formation process must begin anew for this deposit. Over time the soil will develop a profile that depends on the intensities of biota and climate. While a soil can achieve relative stability of its properties for extended periods,[136] the soil life cycle ultimately ends in soil conditions that leave it vulnerable to erosion.[141] Despite the inevitability ofsoil retrogression and degradation, most soil cycles are long.[136]
Soil-forming factors continue to affect soils during their existence, even on stable landscapes that are long-enduring, some for millions of years.[136] Materials are deposited on top[142] or are blown or washed from the surface.[143] With additions, removals and alterations, soils are always subject to new conditions. Whether these are slow or rapid changes depends on climate, topography and biological activity.[144]
Time as a soil-forming factor may be investigated by studying soilchronosequences, in which soils of different ages but with minor differences in other soil-forming factors can be compared.[137]Paleosols are soils formed during previous soil-forming conditions which can be deduced from comparisons with present-day soil development.[145]
Jenny's state equation inFactors of Soil Formation differs from Vasily Dokuchaev's equation, treating time (t) as a factor, adding topographic relief (r), and pointedly leaving the ellipsisopen for more factors (state variables) to be added as our understanding becomes more refined.
There are two principal methods by which the state equation may be solved: first in a theoretical or conceptual manner by logical deductions from certain premises, and second empirically byexperimentation orfield observation. The empirical method is still mostly employed today, and soil formation can be defined by varying a single factor and keeping the other factors constant. This had led to the development of empirical models to describe pedogenesis, such asclimofunctions,biofunctions,topofunctions,lithofunctions, andchronofunctions. Since Jenny published his formulation in 1941, it has been used by innumerablesoil surveyors all over the world as a qualitative list for understanding the factors that may be important for producing the soil pattern within a region.[148]
^Oldeman, L. Roel (1992)."Global extent of soil degradation".ISRIC Bi-Annual Report 1991–1992. Wageningen, The Netherlands:ISRIC. pp. 19–36. Retrieved14 January 2026.
Stanley W. Buol, F.D. Hole and R.W. McCracken. 1997. Soil Genesis and Classification, 4th ed. Iowa State Univ. Press, AmesISBN0-8138-2873-2
C. Michael Hogan. 2008.Makgadikgadi, The Megalithic Portal, ed. A. Burnham[1]
Francis D. Hole and J.B. Campbell. 1985. Soil landscape analysis. Totowa Rowman & Allanheld, 214 p.ISBN0-86598-140-X
Hans Jenny. 1994.Factors of Soil Formation. A System of Quantitative Pedology. New York: Dover Press. (Reprint, with foreword by R. Amundson, of the 1941 McGraw-Hill publication). pdf file format.
Ben van der Pluijm et al. 2005.Soils, Weathering, and Nutrients from the Global Change 1 Lectures. University of Michigan. Url last accessed on 2007-03-31
.jpg)
