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Soil

From Wikipedia, the free encyclopedia

For other uses, see Soil (disambiguation).

Look up  soil in Wiktionary, the free dictionary.

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

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

Soil is a mix­ture of or­ganic mat­ter, min­er­als, gases, liq­uids, and or­gan­isms that to­gether sup­port life. Earth‘s body of soil, called the pe­dos­phere, has four im­por­tant func­tions:

• as a medium for plant growth
• as a means of water storage, supply and purification
• as a modifier of Earth’s atmosphere
• as a habitat for organisms

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

The pe­dos­phere in­ter­faces with the lithos­phere, the hy­dros­phere, the at­mos­phere, and the bios­phere.^[1] The term pedolith, used com­monly to refer to the soil, trans­lates to ground stone in the sense “fun­da­men­tal stone”.^[2] Soil con­sists of a solid phase of min­er­als and or­ganic mat­ter (the soil ma­trix), as well as a porous phase that holds gases (the soil at­mos­phere) and water (the soil solution).^[3][4][5] Ac­cord­ingly, soil sci­en­tists can en­vis­age soils as a three-state sys­tem of solids, liq­uids, and gases.^[6]

Soil is a prod­uct of sev­eral fac­tors: the in­flu­ence of cli­mate, re­lief (el­e­va­tion, ori­en­ta­tion, and slope of ter­rain), or­gan­isms, and the soil’s par­ent ma­te­ri­als (orig­i­nal min­er­als) in­ter­act­ing over time.^[7] It con­tin­u­ally un­der­goes de­vel­op­ment by way of nu­mer­ous phys­i­cal, chem­i­cal and bi­o­log­i­cal processes, which in­clude weath­er­ing with as­so­ci­ated ero­sion. Given its com­plex­ity and strong in­ter­nal con­nect­ed­ness, soil ecol­o­gists re­gard soil as an ecosys­tem.^[8]

Most soils have a dry bulk den­sity (den­sity of soil tak­ing into ac­count voids when dry) be­tween 1.1 and 1.6 g/cm³, while the soil par­ti­cle den­sity is much higher, in the range of 2.6 to 2.7 g/cm³.^[9] Lit­tle of the soil of planet Earth is older than the Pleis­tocene and none is older than the Ceno­zoic,^[10] al­though fos­silized soils are pre­served from as far back as the Archean.^[11]

Soil sci­ence has two basic branches of study: edaphol­ogy and pedol­ogy. Edaphol­ogy stud­ies the in­flu­ence of soils on liv­ing things.^[12] Pedol­ogy fo­cuses on the for­ma­tion, de­scrip­tion (mor­phol­ogy), and clas­si­fi­ca­tion of soils in their nat­ural environment.^[13] In en­gi­neer­ing terms, soil is in­cluded in the broader con­cept of re­golith, which also in­cludes other loose ma­te­r­ial that lies above the bedrock, as can be found on the Moon and on other ce­les­tial ob­jects as well.^[14] Soil is also com­monly re­ferred to as earth or dirt; some sci­en­tific de­f­i­n­i­tions dis­tin­guish dirt from soil by re­strict­ing the for­mer term specif­i­cally to dis­placed soil.^[15]

Contents

• 1 Functions
• 2 Composition
• 3 Formation
• 4 Physical properties
• 5 Soil moisture
  • 5.1 Water retention
  • 5.2 Water flow
  • 5.3 Water uptake by plants
  • 5.4 Consumptive use and water use efficiency
• 6 Soil gas
• 7 Solid phase (soil matrix)
• 8 Chemistry
  • 8.1 Cation and anion exchange
    • 8.1.1 Cation exchange capacity (CEC)
    • 8.1.2 Anion exchange capacity (AEC)
  • 8.2 Reactivity (pH)
    • 8.2.1 Base saturation percentage
  • 8.3 Buffering
• 9 Nutrients
  • 9.1 Uptake processes
  • 9.2 Carbon
  • 9.3 Nitrogen
    • 9.3.1 Gains
    • 9.3.2 Sequestration
    • 9.3.3 Losses
  • 9.4 Phosphorus
  • 9.5 Potassium
  • 9.6 Calcium
  • 9.7 Magnesium
  • 9.8 Sulfur
  • 9.9 Micronutrients
  • 9.10 Non-essential nutrients
• 10 Soil organic matter
  • 10.1 Humus
  • 10.2 Climatological influence
  • 10.3 Plant residue
• 11 Horizons
• 12 Classification
• 13 Uses
• 14 Degradation
• 15 Reclamation
• 16 History of studies and research
  • 16.1 Studies of soil fertility
  • 16.2 Studies of soil formation
• 17 See also
• 18 References
• 19 Bibliography
• 20 Further reading
• 21 External links

YouTube Encyclopedic 

• 1/5Views:213 245

• ✪ Soil and Soil Dynamics
• ✪ Types of Soil
• ✪ Layers of Soil for Kids | #aumsum #kids #education #science #learn
• ✪ Wonder of Creation: Soil: The Foundation of Life, Part I
• ✪ Safe Bearing Capacity of Soil | Bearing capacity of soil |

Functions

Soil is a major com­po­nent of the Earth‘s ecosys­tem. The world’s ecosys­tems are im­pacted in far-reach­ing ways by the processes car­ried out in the soil, from ozone de­ple­tion and global warm­ing to rain­for­est de­struc­tion and water pol­lu­tion. With re­spect to Earth’s car­bon cycle, soil is an im­por­tant car­bon reser­voir, and it is po­ten­tially one of the most re­ac­tive to human disturbance^[16] and cli­mate change.^[17] As the planet warms, it has been pre­dicted that soils will add car­bon diox­ide to the at­mos­phere due to in­creased bi­o­log­i­cal ac­tiv­ity at higher tem­per­a­tures, a pos­i­tive feed­back (am­pli­fi­ca­tion).^[18] This pre­dic­tion has, how­ever, been ques­tioned on con­sid­er­a­tion of more re­cent knowl­edge on soil car­bon turnover.^[19]

Soil acts as an en­gi­neer­ing medium, a habi­tat for soil or­gan­isms, a re­cy­cling sys­tem for nu­tri­ents and or­ganic wastes, a reg­u­la­tor of water qual­ity, a mod­i­fier of at­mos­pheric com­po­si­tion, and a medium for plant growth, mak­ing it a crit­i­cally im­por­tant provider of ecosys­tem ser­vices.^[20] Since soil has a tremen­dous range of avail­able niches and habi­tats, it con­tains most of the Earth’s ge­netic di­ver­sity. A gram of soil can con­tain bil­lions of or­gan­isms, be­long­ing to thou­sands of species, mostly mi­cro­bial and largely still unexplored.^[21][22] Soil has a mean prokary­otic den­sity of roughly 10⁸ or­gan­isms per gram,^[23] whereas the ocean has no more than 10⁷ prokary­otic or­gan­isms per mil­li­liter (gram) of seawater.^[24] Or­ganic car­bon held in soil is even­tu­ally re­turned to the at­mos­phere through the process of res­pi­ra­tion car­ried out by het­erotrophic or­gan­isms, but a sub­stan­tial part is re­tained in the soil in the form of soil or­ganic mat­ter; tillage usu­ally in­creases the rate of soil res­pi­ra­tion, lead­ing to the de­ple­tion of soil or­ganic matter.^[25] Since plant roots need oxy­gen, ven­ti­la­tion is an im­por­tant char­ac­ter­is­tic of soil. This ven­ti­la­tion can be ac­com­plished via net­works of in­ter­con­nected soil pores, which also ab­sorb and hold rain­wa­ter mak­ing it read­ily avail­able for up­take by plants. Since plants re­quire a nearly con­tin­u­ous sup­ply of water, but most re­gions re­ceive spo­radic rain­fall, the wa­ter-hold­ing ca­pac­ity of soils is vital for plant survival.^[26]

Soils can ef­fec­tively re­move impurities,^[27] kill dis­ease agents,^[28] and de­grade con­t­a­m­i­nants, this lat­ter prop­erty being called nat­ural attenuation.^[29] Typ­i­cally, soils main­tain a net ab­sorp­tion of oxy­gen and methane and un­dergo a net re­lease of car­bon diox­ide and ni­trous 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]

Composition

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

Com­po­nents of a loam soil by per­cent vol­ume  Water (25%)  Gases (25%)  Sand (18%)  Silt (18%)  Clay (9%)  Organic matter (5%)

A typ­i­cal soil is about 50% solids (45% min­eral and 5% or­ganic mat­ter), and 50% voids (or pores) of which half is oc­cu­pied by water and half by gas.^[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] The pore 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, re­duces this space, pre­vent­ing air and water from reach­ing plant roots and soil organisms.^[36]

Given suf­fi­cient time, an un­dif­fer­en­ti­ated soil will evolve a soil pro­file which con­sists of two or more lay­ers, re­ferred to as soil hori­zons, that dif­fer in one or more prop­er­ties such as in their tex­ture, struc­ture, den­sity, poros­ity, con­sis­tency, tem­per­a­ture, color, and reactivity.^[10] The hori­zons dif­fer greatly in thick­ness and gen­er­ally lack sharp bound­aries; their de­vel­op­ment is de­pen­dent on the type of par­ent ma­te­r­ial, the processes that mod­ify those par­ent ma­te­ri­als, and the soil-form­ing fac­tors that in­flu­ence those processes. The bi­o­log­i­cal in­flu­ences on soil prop­er­ties are strongest near the sur­face, while the geo­chem­i­cal in­flu­ences on soil prop­er­ties in­crease with depth. Ma­ture soil pro­files typ­i­cally in­clude three basic mas­ter hori­zons: A, B, and C. The solum nor­mally in­cludes the A and B hori­zons. The liv­ing com­po­nent of the soil is largely con­fined to the solum, and is gen­er­ally more promi­nent in the A horizon.^[37]

The soil tex­ture is de­ter­mined by the rel­a­tive pro­por­tions of the in­di­vid­ual par­ti­cles of sand, silt, and clay that make up the soil. The in­ter­ac­tion of the in­di­vid­ual min­eral par­ti­cles with or­ganic mat­ter, water, gases via bi­otic and abi­otic processes causes those par­ti­cles to floc­cu­late (stick to­gether) to form ag­gre­gates or peds.^[38] 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.^[39] The mix­ture of water and dis­solved or sus­pended ma­te­ri­als that oc­cupy the soil pore space is called the soil so­lu­tion. Since soil water is never pure water, but con­tains hun­dreds of dis­solved or­ganic and min­eral sub­stances, it may be more ac­cu­rately called the soil so­lu­tion. Water is cen­tral to the dis­so­lu­tion, pre­cip­i­ta­tion and leach­ing of min­er­als from the soil pro­file. Fi­nally, water af­fects the type of veg­e­ta­tion that grows in a soil, which in turn af­fects the de­vel­op­ment of the soil, a com­plex feed­back which is ex­em­pli­fied in the dy­nam­ics of banded veg­e­ta­tion pat­terns in semi-arid regions.^[40]

Soils sup­ply plants with nu­tri­ents, most of which are held in place by par­ti­cles of clay and or­ganic mat­ter (col­loids)^[41] The nu­tri­ents may be ad­sorbed on clay min­eral sur­faces, bound within clay min­er­als (ab­sorbed), or bound within or­ganic com­pounds as part of the liv­ing or­gan­isms or dead soil or­ganic mat­ter. These bound nu­tri­ents in­ter­act with soil water to buffer the soil so­lu­tion com­po­si­tion (at­ten­u­ate changes in the soil so­lu­tion) as soils wet up or dry out, as plants take up nu­tri­ents, as salts are leached, or as acids or al­ka­lis are added.^[42][43]

Plant nu­tri­ent avail­abil­ity is af­fected by soil pH, which is a mea­sure of the hy­dro­gen ion ac­tiv­ity in the soil so­lu­tion. Soil pH is a func­tion of many soil form­ing fac­tors, and is gen­er­ally lower (more acid) where weath­er­ing is more advanced.^[44]

Most plant nu­tri­ents, with the ex­cep­tion of ni­tro­gen, orig­i­nate from the min­er­als that make up the soil par­ent ma­te­r­ial. Some ni­tro­gen orig­i­nates from rain as di­lute ni­tric acid and am­mo­nia,^[45] but most of the ni­tro­gen is avail­able in soils as a re­sult of ni­tro­gen fix­a­tion by bac­te­ria. Once in the soil-plant sys­tem, most nu­tri­ents are re­cy­cled through liv­ing or­gan­isms, plant and mi­cro­bial residues (soil or­ganic mat­ter), min­eral-bound forms, and the soil so­lu­tion. Both liv­ing mi­croor­gan­isms and soil or­ganic mat­ter are of crit­i­cal im­por­tance to this re­cy­cling, and thereby to soil for­ma­tion and soil fertility.^[46] Mi­cro­bial ac­tiv­ity in soils 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 by volatil­i­sa­tion (loss to the at­mos­phere as gases) or leach­ing.

Formation

Main article: Pedogenesis

Soil for­ma­tion, or pe­do­ge­n­e­sis, is the com­bined ef­fect of phys­i­cal, chem­i­cal, bi­o­log­i­cal and an­thro­pogenic processes work­ing on soil par­ent ma­te­r­ial. Soil is said to be formed when or­ganic mat­ter has ac­cu­mu­lated and col­loids are washed down­ward, leav­ing de­posits of clay, humus, iron oxide, car­bon­ate, and gyp­sum, pro­duc­ing a dis­tinct layer called the B hori­zon. This is a some­what ar­bi­trary de­f­i­n­i­tion as mix­tures of sand, silt, clay and humus will sup­port bi­o­log­i­cal and agri­cul­tural ac­tiv­ity be­fore that time. These con­stituents are moved from one level to an­other by water and an­i­mal ac­tiv­ity. As a re­sult, lay­ers (hori­zons) form in the soil pro­file. The al­ter­ation and move­ment of ma­te­ri­als within a soil causes the for­ma­tion of dis­tinc­tive soil hori­zons. How­ever, more re­cent de­f­i­n­i­tions of soil em­brace soils with­out any or­ganic mat­ter, such as those re­goliths that formed on Mars^[47] and anal­o­gous con­di­tions in planet Earth deserts.^[48]

An ex­am­ple of the de­vel­op­ment of a soil would begin with the weath­er­ing of lava flow bedrock, which would pro­duce the purely min­eral-based par­ent ma­te­r­ial from which the soil tex­ture forms. Soil de­vel­op­ment would pro­ceed most rapidly from bare rock of re­cent flows in a warm cli­mate, under heavy and fre­quent rain­fall. Under such con­di­tions, plants (in a first stage ni­tro­gen-fix­ing lichens and cyanobac­te­ria then epilithic higher plants) be­come es­tab­lished very quickly on basaltic lava, even though there is very lit­tle or­ganic ma­te­r­ial. The plants are sup­ported by the porous rock as it is filled with nu­tri­ent-bear­ing water that car­ries min­er­als dis­solved from the rocks. Crevasses and pock­ets, local topog­ra­phy of the rocks, would hold fine ma­te­ri­als and har­bour plant roots. The de­vel­op­ing plant roots are as­so­ci­ated with mineral-weath­er­ing my­c­or­rhizal fungi^[49] 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,^[50] in­sel­bergs,^[51] and glacial moraines.^[52]

How soil for­ma­tion pro­ceeds is in­flu­enced by at least five clas­sic fac­tors that are in­ter­twined in the evo­lu­tion of a soil. They are: par­ent ma­te­r­ial, cli­mate, topog­ra­phy (re­lief), or­gan­isms, and time.^[53] When re­ordered to cli­mate, re­lief, or­gan­isms, par­ent ma­te­r­ial, and time, they form the acronym CROPT.^[54]

Physical properties

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

The phys­i­cal prop­er­ties of soils, in order of de­creas­ing im­por­tance for ecosys­tem ser­vices such as crop pro­duc­tion, are tex­ture, struc­ture, bulk den­sity, poros­ity, con­sis­tency, tem­per­a­ture, colour and re­sis­tiv­ity.^[55] Soil tex­ture is de­ter­mined by the rel­a­tive pro­por­tion of the three kinds of soil min­eral par­ti­cles, called soil sep­a­rates: sand, silt, and clay. At the next larger scale, soil struc­tures called peds or more com­monly soil aggregates are cre­ated from the soil sep­a­rates when iron ox­ides, car­bon­ates, clay, sil­ica and humus, coat par­ti­cles and cause them to ad­here into larger, rel­a­tively sta­ble sec­ondary structures.^[56] Soil bulk den­sity, when de­ter­mined at stan­dard­ized mois­ture con­di­tions, is an es­ti­mate of soil com­paction.^[57] Soil poros­ity con­sists of the void part of the soil vol­ume and is oc­cu­pied by gases or water. Soil con­sis­tency is the abil­ity of soil ma­te­ri­als to stick to­gether. Soil tem­per­a­ture and colour are self-defin­ing. Re­sis­tiv­ity refers to the re­sis­tance to con­duc­tion of elec­tric cur­rents and af­fects the rate of cor­ro­sion of metal and con­crete struc­tures which are buried in soil.^[58] These prop­er­ties vary through the depth of a soil pro­file, i.e. through soil hori­zons. Most of these prop­er­ties de­ter­mine the aer­a­tion of the soil and the abil­ity of water to in­fil­trate and to be held within the soil.^[59]

Soil moisture

Further information: Water content and Water potential

Soil moisture refers to the water con­tent of the soil. It can be ex­pressed in terms of vol­umes or weights. Soil mois­ture mea­sure­ment can be based on in situ probes or re­mote sens­ing meth­ods.

Water that en­ters a field is re­moved from a field by runoff, drainage, evap­o­ra­tion or tran­spi­ra­tion.^[60] Runoff is the water that flows on the sur­face to the edge of the field; drainage is the water that flows through the soil down­ward or to­ward the edge of the field un­der­ground; evap­o­ra­tive water loss from a field is that part of the water that evap­o­rates into the at­mos­phere di­rectly from the field’s sur­face; tran­spi­ra­tion is the loss of water from the field by its evap­o­ra­tion from the plant it­self.

Water af­fects soil for­ma­tion, struc­ture, sta­bil­ity and ero­sion but is of pri­mary con­cern with re­spect to plant growth.^[61] Water is es­sen­tial to plants for four rea­sons:

1. It constitutes 80%-95% of the plant’s protoplasm.
2. It is essential for photosynthesis.
3. It is the solvent in which nutrients are carried to, into and throughout the plant.
4. It provides the turgidity by which the plant keeps itself in proper position.^[62]

In ad­di­tion, water al­ters the soil pro­file by dis­solv­ing and re-de­posit­ing min­er­als, often at lower levels.^[63] In a loam soil, solids con­sti­tute half the vol­ume, gas one-quar­ter of the vol­ume, and water one-quar­ter of the volume^[33] of which only half will be avail­able to most plants, with a strong vari­a­tion ac­cord­ing to ma­tric po­ten­tial.^[64]

A flooded field will drain the grav­i­ta­tional water under the in­flu­ence of grav­ity until water’s ad­he­sive and co­he­sive forces re­sist fur­ther drainage at which point it is said to have reached field ca­pac­ity.^[65] At that point, plants must apply suc­tion^[65][66] to draw water from a soil. The water that plants may draw from the soil is called the avail­able water.^[65][67] Once the avail­able water is used up the re­main­ing mois­ture is called un­avail­able water as the plant can­not pro­duce suf­fi­cient suc­tion to draw that water in. At 15 bar suc­tion, wilt­ing point, seeds will not germinate,^[68][65][69] plants begin to wilt and then die. Water moves in soil under the in­flu­ence of grav­ity, os­mo­sis and cap­il­lar­ity.^[70] When water en­ters the soil, it dis­places air from in­ter­con­nected macro­p­ores by buoy­ancy, and breaks ag­gre­gates into which air is en­trapped, a process called slak­ing.^[71]

The rate at which a soil can ab­sorb water de­pends on the soil and its other con­di­tions. As a plant grows, its roots re­move water from the largest pores (macro­p­ores) first. Soon the larger pores hold only air, and the re­main­ing water is found only in the in­ter­me­di­ate- and small­est-sized pores (mi­cro­p­ores). The water in the small­est pores is so strongly held to par­ti­cle sur­faces that plant roots can­not pull it away. Con­se­quently, not all soil water is avail­able to plants, with a strong de­pen­dence on tex­ture.^[72] When sat­u­rated, the soil may lose nu­tri­ents as the water drains.^[73] Water moves in a drain­ing field under the in­flu­ence of pres­sure where the soil is lo­cally sat­u­rated and by cap­il­lar­ity pull to drier parts of the soil.^[74] Most plant water needs are sup­plied from the suc­tion caused by evap­o­ra­tion from plant leaves (tran­spi­ra­tion) and a lower frac­tion is sup­plied by suc­tion cre­ated by os­motic pres­sure dif­fer­ences be­tween the plant in­te­rior and the soil solution.^[75][76] Plant roots must seek out water and grow pref­er­en­tially in moister soil microsites,^[77] but some parts of the root sys­tem are also able to re­moisten dry parts of the soil.^[78] In­suf­fi­cient water will dam­age the yield of a crop.^[79] Most of the avail­able water is used in tran­spi­ra­tion to pull nu­tri­ents into the plant.^[80]

Soil water is also im­por­tant for cli­mate mod­el­ing and nu­mer­i­cal weather pre­dic­tion. Global Cli­mate Ob­serv­ing Sys­tem spec­i­fied soil water as one of the 50 Es­sen­tial Cli­mate Vari­ables (ECVs).^[81] Soil water can be mea­sured in situ with soil mois­ture sen­sor or can be es­ti­mated from satel­lite data and hy­dro­log­i­cal mod­els. Each method ex­hibits pros and cons, and hence, the in­te­gra­tion of dif­fer­ent tech­niques may de­crease the draw­backs of a sin­gle given method.^[82]

Water retention

Further information: Soil water (retention) and Water retention curve

Water is re­tained in a soil when the ad­he­sive force of at­trac­tion that water’s hy­dro­gen atoms have for the oxy­gen of soil par­ti­cles is stronger than the co­he­sive forces that water’s hy­dro­gen feels for other water oxy­gen atoms.^[83] When a field is flooded, the soil pore space is com­pletely filled by water. The field will drain under the force of grav­ity until it reaches what is called field ca­pac­ity, at which point the small­est pores are filled with water and the largest with water and gases.^[84] The total amount of water held when field ca­pac­ity is reached is a func­tion of the spe­cific sur­face area of the soil particles.^[85] As a re­sult, high clay and high or­ganic soils have higher field capacities.^[86] The po­ten­tial en­ergy of water per unit vol­ume rel­a­tive to pure water in ref­er­ence con­di­tions is called water po­ten­tial. Total water po­ten­tial is a sum of ma­tric po­ten­tial which re­sults from cap­il­lary ac­tion, os­motic po­ten­tial for saline soil, and grav­i­ta­tional po­ten­tial when deal­ing with ver­ti­cal di­rec­tion of water move­ment. Water po­ten­tial in soil usu­ally has neg­a­tive val­ues, and there­fore it is also ex­pressed in suc­tion, which is de­fined as the minus of water po­ten­tial. Suc­tion has a pos­i­tive value and can be re­garded as the total force re­quired to pull or push water out of soil. Water po­ten­tial or suc­tion is ex­pressed in units of kPa (10³ pas­cal), bar (100 kPa), or cm H₂O (ap­prox­i­mately 0.098 kPa). Com­mon log­a­rithm of suc­tion in cm H₂O is called pF.^[87] There­fore, pF 3 = 1000 cm = 98 kPa = 0.98 bar.

The forces with which water is held in soils de­ter­mine its avail­abil­ity to plants. Forces of ad­he­sion hold water strongly to min­eral and humus sur­faces and less strongly to it­self by co­he­sive forces. A plant’s root may pen­e­trate a very small vol­ume of water that is ad­her­ing to soil and be ini­tially able to draw in water that is only lightly held by the co­he­sive forces. But as the droplet is drawn down, the forces of ad­he­sion of the water for the soil par­ti­cles pro­duce in­creas­ingly higher suc­tion, fi­nally up to 1500 kPa (pF = 4.2).^[88] At 1500 kPa suc­tion, the soil water amount is called wilt­ing point. At that suc­tion the plant can­not sus­tain its water needs as water is still being lost from the plant by tran­spi­ra­tion, the plant’s turgid­ity is lost, and it wilts, al­though stom­atal clo­sure may de­crease tran­spi­ra­tion and thus may re­tard wilt­ing below the wilt­ing point, in par­tic­u­lar under adap­ta­tion or ac­clima­ti­za­tion to drought.^[89] The next level, called air-dry, oc­curs at 100,000 kPa suc­tion (pF = 6). Fi­nally the oven dry con­di­tion is reached at 1,000,000 kPa suc­tion (pF = 7). All water below wilt­ing point is called un­avail­able water.^[90]

When the soil mois­ture con­tent is op­ti­mal for plant growth, the water in the large and in­ter­me­di­ate size pores can move about in the soil and be eas­ily used by plants.^[72] The amount of water re­main­ing in a soil drained to field ca­pac­ity and the amount that is avail­able are func­tions of the soil type. Sandy soil will re­tain very lit­tle water, while clay will hold the max­i­mum amount.^[86] The avail­able water for the silt loam might be 20% whereas for the sand it might be only 6% by vol­ume, as shown in this table.

Soil Texture	Wilting Point	Field Capacity	Available water
Sand	3.3	9.1	5.8
Sandy loam	9.5	20.7	11.2
Loam	11.7	27.0	15.3
Silt loam	13.3	33.0	19.7
Clay loam	19.7	31.8	12.1
Clay	27.2	39.6	12.4

The above are av­er­age val­ues for the soil tex­tures.

Water flow

Water moves through soil due to the force of grav­ity, os­mo­sis and cap­il­lar­ity. At zero to 33 kPa suc­tion (field ca­pac­ity), water is pushed through soil from the point of its ap­pli­ca­tion under the force of grav­ity and the pres­sure gra­di­ent cre­ated by the pres­sure of the water; this is called sat­u­rated flow. At higher suc­tion, water move­ment is pulled by cap­il­lar­ity from wet­ter to­ward drier soil. This is caused by water’s ad­he­sion to soil solids, and is called un­sat­u­rated flow.^[92][93]

Water in­fil­tra­tion and move­ment in soil is con­trolled by six fac­tors:

1. Soil texture
2. Soil structure. Fine-textured soils with granular structure are most favourable to infiltration of water.
3. The amount of organic matter. Coarse matter is best and if on the surface helps prevent the destruction of soil structure and the creation of crusts.
4. Depth of soil to impervious layers such as hardpans or bedrock
5. The amount of water already in the soil
6. Soil temperature. Warm soils take in water faster while frozen soils may not be able to absorb depending on the type of freezing.^[94]

Water in­fil­tra­tion rates range from 0.25 cm per hour for high clay soils to 2.5 cm per hour for sand and well sta­bi­lized and ag­gre­gated soil structures.^[95] Water flows through the ground un­evenly, in the form of so-called “grav­ity fin­gers”, be­cause of the sur­face ten­sion be­tween water particles.^[96][97]

Tree roots, whether liv­ing or dead, cre­ate pref­er­en­tial chan­nels for rain­wa­ter flow through soil,^[98] mag­ni­fy­ing in­fil­tra­tion rates of water up to 27 times.^[99]

Flood­ing tem­porar­ily in­creases soil per­me­abil­ity in river beds, help­ing to recharge aquifers.^[100]

Water ap­plied to a soil is pushed by pres­sure gra­di­ents from the point of its ap­pli­ca­tion where it is sat­u­rated lo­cally, to less sat­u­rated areas, such as the va­dose zone.^[101][102] Once soil is com­pletely wet­ted, any more water will move down­ward, or per­co­late out of the range of plant roots, car­ry­ing with it clay, humus, nu­tri­ents, pri­mar­ily cations, and var­i­ous con­t­a­m­i­nants, in­clud­ing pes­ti­cides, pol­lu­tants, viruses and bac­te­ria, po­ten­tially caus­ing ground­wa­ter con­t­a­m­i­na­tion.^[103][104] In order of de­creas­ing sol­u­bil­ity, the leached nu­tri­ents are:

• Calcium
• Magnesium, Sulfur, Potassium; depending upon soil composition
• Nitrogen; usually little, unless nitrate fertiliser was applied recently
• Phosphorus; very little as its forms in soil are of low solubility.^[105]

In the United States per­co­la­tion water due to rain­fall ranges from al­most zero cen­time­ters just east of the Rocky Moun­tains to fifty or more cen­time­ters per day in the Ap­palachian Moun­tains and the north coast of the Gulf of Mexico.^[106]

Water is pulled by cap­il­lary ac­tion due to the ad­he­sion force of water to the soil solids, pro­duc­ing a suc­tion gra­di­ent from wet to­wards drier soil^[107] and from macro­p­ores to mi­cro­p­ores. The so-called Richards equa­tion al­lows cal­cu­la­tion of the time rate of change of mois­ture con­tent in soils due to the move­ment of water in un­sat­u­rated soils.^[108] In­ter­est­ingly, this equa­tion at­trib­uted to Richards was orig­i­nally pub­lished by Richard­son in 1922.^[109] The Soil Mois­ture Ve­loc­ity Equa­tion,^[110] which can be solved using the fi­nite wa­ter-con­tent va­dose zone flow method,^[111][112] de­scribes the ve­loc­ity of flow­ing water through an un­sat­u­rated soil in the ver­ti­cal di­rec­tion. The nu­mer­i­cal so­lu­tion of the Richard­son/Richards equa­tion al­lows cal­cu­la­tion of un­sat­u­rated water flow and solute trans­port using soft­ware such as Hy­drus,^[113] by giv­ing soil hy­draulic pa­ra­me­ters of hy­draulic func­tions (water re­ten­tion func­tion and un­sat­u­rated hy­draulic con­duc­tiv­ity func­tion) and ini­tial and bound­ary con­di­tions. Pref­er­en­tial flow oc­curs along in­ter­con­nected macro­p­ores, crevices, root and worm chan­nels, which drain water under grav­ity.^[114][115] Many mod­els based on soil physics now allow for some rep­re­sen­ta­tion of pref­er­en­tial flow as a dual con­tin­uum, dual poros­ity or dual per­me­abil­ity op­tions, but these have gen­er­ally been “bolted on” to the Richards so­lu­tion with­out any rig­or­ous phys­i­cal underpinning.^[116]

Water uptake by plants

Of equal im­por­tance to the stor­age and move­ment of water in soil is the means by which plants ac­quire it and their nu­tri­ents. Most soil water is taken up by plants as pas­sive ab­sorp­tion caused by the pulling force of water evap­o­rat­ing (tran­spir­ing) from the long col­umn of water (xylem sap flow) that leads from the plant’s roots to its leaves, ac­cord­ing to the co­he­sion-ten­sion the­ory.^[117] The up­ward move­ment of water and solutes (hy­draulic lift) is reg­u­lated in the roots by the en­do­der­mis^[118] and in the plant fo­liage by stom­atal con­duc­tance,^[119] and can be in­ter­rupted in root and shoot xylem ves­sels by cav­i­ta­tion, also called xylem embolism.^[120] In ad­di­tion, the high con­cen­tra­tion of salts within plant roots cre­ates an os­motic pres­sure gra­di­ent that pushes soil water into the roots.^[121] Os­motic ab­sorp­tion be­comes more im­por­tant dur­ing times of low water tran­spi­ra­tion caused by lower tem­per­a­tures (for ex­am­ple at night) or high hu­mid­ity, and the re­verse oc­curs under high tem­per­a­ture or low hu­mid­ity. It is these process that cause gut­ta­tion and wilt­ing, respectively.^[122][123]

Root ex­ten­sion is vital for plant sur­vival. A study of a sin­gle win­ter rye plant grown for four months in one cubic foot (0.0283 cubic me­ters) of loam soil showed that the plant de­vel­oped 13,800,000 roots, a total of 620 km in length with 237 square me­ters in sur­face area; and 14 bil­lion hair roots of 10,620 km total length and 400 square me­ters total area; for a total sur­face area of 638 square me­ters. The total sur­face area of the loam soil was es­ti­mated to be 52,000 square meters.^[124] In other words, the roots were in con­tact with only 1.2% of the soil. How­ever, root ex­ten­sion should be viewed as a dy­namic process, al­low­ing new roots to ex­plore a new vol­ume of soil each day, in­creas­ing dra­mat­i­cally the total vol­ume of soil ex­plored over a given growth pe­riod, and thus the vol­ume of water taken up by the root sys­tem over this period.^[125] Root ar­chi­tec­ture, i.e. the spa­tial con­fig­u­ra­tion of the root sys­tem, plays a promi­nent role in the adap­ta­tion of plants to soil water and nu­tri­ent avail­abi­ity, and thus in plant productivity.^[126]

Roots must seek out water as the un­sat­u­rated flow of water in soil can move only at a rate of up to 2.5 cm per day; as a re­sult they are con­stantly dying and grow­ing as they seek out high con­cen­tra­tions of soil moisture.^[127] In­suf­fi­cient soil mois­ture, to the point of caus­ing wilt­ing, will cause per­ma­nent dam­age and crop yields will suf­fer. When grain sorghum was ex­posed to soil suc­tion as low as 1300 kPa dur­ing the seed head emer­gence through bloom and seed set stages of growth, its pro­duc­tion was re­duced by 34%.^[128]

Consumptive use and water use efficiency

Only a small frac­tion (0.1% to 1%) of the water used by a plant is held within the plant. The ma­jor­ity is ul­ti­mately lost via tran­spi­ra­tion, while evap­o­ra­tion from the soil sur­face is also sub­stan­tial, the tran­spi­ra­tion:evap­o­ra­tion ratio vary­ing ac­cord­ing to veg­e­ta­tion type and cli­mate, peak­ing in trop­i­cal rain­forests and dip­ping in steppes and deserts.^[129] Tran­spi­ra­tion plus evap­o­ra­tive soil mois­ture loss is called evap­o­tran­spi­ra­tion. Evap­o­tran­spi­ra­tion plus water held in the plant to­tals to con­sump­tive use, which is nearly iden­ti­cal to evapotranspiration.^[128][130]

The total water used in an agri­cul­tural field in­cludes sur­face runoff, drainage and con­sump­tive use. The use of loose mulches will re­duce evap­o­ra­tive losses for a pe­riod after a field is ir­ri­gated, but in the end the total evap­o­ra­tive loss (plant plus soil) will ap­proach that of an un­cov­ered soil, while more water is im­me­di­ately avail­able for plant growth.^[131] Water use ef­fi­ciency is mea­sured by the tran­spi­ra­tion ratio, which is the ratio of the total water tran­spired by a plant to the dry weight of the har­vested plant. Tran­spi­ra­tion ra­tios for crops range from 300 to 700. For ex­am­ple, al­falfa may have a tran­spi­ra­tion ratio of 500 and as a re­sult 500 kilo­grams of water will pro­duce one kilo­gram of dry alfalfa.^[132]

Soil gas

The at­mos­phere of soil, or soil gas, is very dif­fer­ent from the at­mos­phere above. The con­sump­tion of oxy­gen by mi­crobes and plant roots, and their re­lease of car­bon diox­ide, de­crease oxy­gen and in­crease 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.^[133] Cal­care­ous soils reg­u­late CO₂ con­cen­tra­tion by car­bon­ate buffer­ing, con­trary to acid soils in which all CO₂ respired ac­cu­mu­lates in the soil pore system.^[134] At ex­treme lev­els CO₂ is toxic.^[135] This sug­gests a pos­si­ble neg­a­tive feed­back con­trol of soil CO₂ con­cen­tra­tion through its in­hibitory ef­fects on root and mi­cro­bial res­pi­ra­tion (also called ‘soil res­pi­ra­tion‘).^[136] In ad­di­tion, the soil voids are sat­u­rated with water vapour, at least until the point of max­i­mal hy­gro­scop­ic­ity, be­yond which a vapour-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 by dif­fu­sion from high con­cen­tra­tions to lower, the dif­fu­sion co­ef­fi­cient de­creas­ing with soil com­paction.^[137] 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 (in­clud­ing green­house gases) as well as water.^[138] Soil tex­ture and struc­ture strongly af­fect soil poros­ity and gas dif­fu­sion. It is the total pore space (poros­ity) of soil, not the pore size, and the de­gree of pore in­ter­con­nec­tion (or con­versely pore seal­ing), to­gether with water con­tent, air tur­bu­lence and tem­per­a­ture, that de­ter­mine the rate of dif­fu­sion of gases into and out of soil.^[139][138] Platy soil struc­ture and soil com­paction (low poros­ity) im­pede gas flow, and a de­fi­ciency of oxy­gen may en­cour­age anaer­o­bic bac­te­ria to re­duce (strip oxy­gen) from ni­trate NO₃ to the gases N₂, N₂O, and NO, which are then lost to the at­mos­phere, thereby de­plet­ing the soil of nitrogen.^[140] Aer­ated soil is also a net sink of methane CH₄^[141] but a net pro­ducer of methane (a strong heat-ab­sorb­ing green­house gas) when soils are de­pleted of oxy­gen and sub­ject to el­e­vated temperatures.^[142]

Soil at­mos­phere is also the seat of emis­sions of volatiles other than car­bon and ni­tro­gen ox­ides from var­i­ous soil or­gan­isms, e.g. roots,^[143] bac­te­ria,^[144] fungi,^[145] an­i­mals.^[146] These volatiles are used as chem­i­cal cues, mak­ing soil at­mos­phere the seat of in­ter­ac­tion networks^[147][148] play­ing a de­ci­sive role in the sta­bil­ity, dy­nam­ics and evo­lu­tion of soil ecosystems.^[149] Bio­genic soil volatile or­ganic com­pounds are ex­changed with the above­ground at­mos­phere, in which they are just 1–2 or­ders of mag­ni­tude lower than those from above­ground vegetation.^[150]

We 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,^[151] a bulk prop­erty at­trib­uted in a re­duc­tion­ist man­ner to par­tic­u­lar bio­chem­i­cal com­pounds such as pet­ri­chor or geosmin.

Solid phase (soil matrix)

Main article: Soil matrix

Soil par­ti­cles can be clas­si­fied by their chem­i­cal com­po­si­tion (min­er­al­ogy) as well as their size. The par­ti­cle size dis­tri­b­u­tion of a soil, its tex­ture, de­ter­mines many of the prop­er­ties of that soil, in par­tic­u­lar hy­draulic con­duc­tiv­ity and water po­ten­tial,^[152] but the min­er­al­ogy of those par­ti­cles can strongly mod­ify those prop­er­ties. The min­er­al­ogy of the finest soil par­ti­cles, clay, is es­pe­cially important.^[153]

Chemistry

For the academic discipline, see Soil chemistry.

The chem­istry of a soil de­ter­mines its abil­ity to sup­ply avail­able plant nu­tri­ents and af­fects its phys­i­cal prop­er­ties and the health of its liv­ing pop­u­la­tion. In ad­di­tion, a soil’s chem­istry also de­ter­mines its cor­ro­siv­ity, sta­bil­ity, and abil­ity to ab­sorb pol­lu­tants and to fil­ter water. It is the sur­face chem­istry of min­eral and or­ganic col­loids that de­ter­mines soil’s chem­i­cal properties.^[154] A col­loid is a small, in­sol­u­ble par­ti­cle rang­ing in size from 1 nanome­ter to 1 mi­crom­e­ter, thus small enough to re­main sus­pended by Brown­ian mo­tion in a fluid medium with­out settling.^[155] Most soils con­tain or­ganic col­loidal par­ti­cles called humus as well as the in­or­ganic col­loidal par­ti­cles of clays. The very high spe­cific sur­face area of col­loids and their net elec­tri­cal charges give soil its abil­ity to hold and re­lease ions. Neg­a­tively charged sites on col­loids at­tract and re­lease cations in what is re­ferred to as cation ex­change. Cation-ex­change ca­pac­ity (CEC) is the amount of ex­change­able cations per unit weight of dry soil and is ex­pressed in terms of mil­liequiv­a­lents of pos­i­tively charged ions per 100 grams of soil (or cen­ti­moles of pos­i­tive charge per kilo­gram of soil; cmol_c/kg). Sim­i­larly, pos­i­tively charged sites on col­loids can at­tract and re­lease an­ions in the soil giv­ing the soil anion ex­change ca­pac­ity (AEC).

Cation and anion exchange

Further information: Cation-exchange capacity

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

The neg­a­tive or pos­i­tive charges on col­loid par­ti­cles make them able to hold cations or an­ions, re­spec­tively, to their sur­faces. The charges re­sult from four sources.^[156]

1. Isomorphous substitution occurs in clay during its formation, when lower-valence cations substitute for higher-valence cations in the crystal structure.^[157] Substitutions in the outermost layers are more effective than for the innermost layers, as the electric charge strength drops off as the square of the distance. The net result is oxygen atoms with net negative charge and the ability to attract cations.
2. Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral and octahedral structures are incomplete.^[158]
3. Hydroxyls may substitute for oxygens of the silica layers, a process called hydroxylation. When the hydrogens of the clay hydroxyls are ionised into solution, they leave the oxygen with a negative charge (anionic clays).^[159]
4. Hydrogens of humus hydroxyl groups may also be ionised into solution, leaving, similarly to clay, an oxygen with a negative charge.^[160]

Cations held to the neg­a­tively charged col­loids re­sist being washed down­ward by water and out of reach of plants’ roots, thereby pre­serv­ing the fer­til­ity of soils in areas of mod­er­ate rain­fall and low temperatures.^[161][162]

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

Al³⁺ re­places H⁺ re­places Ca²⁺ re­places Mg²⁺ re­places K⁺ same as NH⁴⁺ re­places Na^+[163]

If one cation is added in large amounts, it may re­place the oth­ers by the sheer force of its num­bers. This is called law of mass ac­tion. This is largely what oc­curs with the ad­di­tion of cationic fer­tilis­ers (potash, lime).^[164]

As the soil so­lu­tion be­comes more acidic (low pH, mean­ing an abun­dance of H⁺, the other cations more weakly bound to col­loids are pushed into so­lu­tion as hy­dro­gen ions oc­cupy ex­change sites (pro­to­na­tion). A low pH may cause hy­dro­gen of hy­droxyl groups to be pulled into so­lu­tion, leav­ing charged sites on the col­loid avail­able to be oc­cu­pied by other cations. This ion­i­sa­tion of hy­droxyl groups on the sur­face of soil col­loids cre­ates what is de­scribed as pH-de­pen­dent sur­face charges.^[165] Un­like per­ma­nent charges de­vel­oped by iso­mor­phous sub­sti­tu­tion, pH-de­pen­dent charges are vari­able and in­crease with in­creas­ing pH.^[43] Freed cations can be made avail­able to plants but are also prone to be leached from the soil, pos­si­bly mak­ing the soil less fertile.^[166] Plants are able to ex­crete H⁺ into the soil through the syn­the­sis of or­ganic acids and by that means, change the pH of the soil near the root and push cations off the col­loids, thus mak­ing those avail­able to the plant.^[167]

Cation exchange capacity (CEC)

Cation ex­change ca­pac­ity should be thought of as 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. CEC is the amount of ex­change­able hy­dro­gen cation (H⁺) that will com­bine with 100 grams dry weight of soil and whose mea­sure is one mil­liequiv­a­lents per 100 grams of soil (1 meq/100 g). Hy­dro­gen ions have a sin­gle charge and one-thou­sandth of a gram of hy­dro­gen ions per 100 grams dry soil gives a mea­sure of one mil­liequiv­a­lent of hy­dro­gen ion. Cal­cium, with an atomic weight 40 times that of hy­dro­gen and with a va­lence of two, con­verts to (40/2) x 1 mil­liequiv­a­lent = 20 mil­liequiv­a­lents of hy­dro­gen ion per 100 grams of dry soil or 20 meq/100 g.^[168] 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, due to leach­ing and de­com­po­si­tion, re­spec­tively, ex­plains the ap­par­ent steril­ity of trop­i­cal soils.^[169] Live plant roots also have some CEC, linked to their spe­cific sur­face area.^[170]

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

Anion exchange capacity (AEC)

Anion ex­change ca­pac­ity should be thought of as the soil’s abil­ity to re­move an­ions (e.g. ni­trate, phos­phate) from the soil water so­lu­tion and se­quester those for later ex­change as the plant roots re­lease car­bon­ate an­ions to the soil water so­lu­tion. Those col­loids which have low CEC tend to have some AEC. Amor­phous and sesquiox­ide clays have the high­est AEC,^[172] fol­lowed by the iron ox­ides. 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.^[173] Phos­phates tend to be held at anion ex­change sites.^[174]

Iron and alu­minum hy­drox­ide clays are able to ex­change their hy­drox­ide an­ions (OH⁻) for other anions.^[175] The order re­flect­ing the strength of anion ad­he­sion is as fol­lows:H₂PO₄⁻ replaces SO₄²⁻ replaces NO₃⁻ replaces Cl⁻

The amount of ex­change­able an­ions is of a mag­ni­tude of tenths to a few mil­liequiv­a­lents per 100 g dry soil.^[171] 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).^[176]

Reactivity (pH)

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

Soil re­ac­tiv­ity is ex­pressed in terms of pH and is a mea­sure of the acid­ity or al­ka­lin­ity of the soil. More pre­cisely, it is a mea­sure of hy­dro­gen ion 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.^[177]

At 25 °C an aque­ous so­lu­tion that has a pH of 3.5 has 10^−3.5 moles H⁺ (hy­dro­gen ions) per litre of so­lu­tion (and also 10^−10.5 mole/litre OH⁻). A pH of 7, de­fined as neu­tral, has 10⁻⁷ moles of hy­dro­gen 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 hy­dro­gen ions per litre of so­lu­tion (and also 10^−2.5 mole per litre OH⁻). A pH of 3.5 has one mil­lion times more hy­dro­gen ions per litre than a so­lu­tion with pH of 9.5 (9.5–3.5 = 6 or 10⁶) and is more acidic.^[178]

The ef­fect of pH on a soil is to re­move from the soil or to make avail­able cer­tain ions. Soils with high acid­ity tend to have toxic amounts of alu­minium and manganese.^[179] As a re­sult of a trade-off be­tween tox­i­c­ity and re­quire­ment most nu­tri­ents are bet­ter avail­able to plants at mod­er­ate pH,^[180] al­though most min­er­als are more sol­u­ble in acid soils. 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.^[181] Given that at low pH toxic met­als (e.g. cad­mium, zinc, lead) are pos­i­tively charged as cations and or­ganic pol­lu­tants are in non-ionic form, thus both made more avail­able to organisms,^[182][183] it has been sug­gested that plants, an­i­mals and mi­crobes com­monly liv­ing in acid soils are pre-adapted to every kind of pol­lu­tion, whether of nat­ural or human origin.^[184]

In high rain­fall areas, soils tend to acid­ity as the basic cations are forced off the soil col­loids by the mass ac­tion of hy­dro­gen ions from the rain 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 in trop­i­cal rain­forests.^[185] Once the col­loids are sat­u­rated with H⁺, the ad­di­tion of any more hy­dro­gen ions or alu­minum hy­droxyl cations dri­ves the pH even lower (more acidic) as the soil has been left with no buffer­ing capacity.^[186] In areas of ex­treme rain­fall and high tem­per­a­tures, the clay and humus may be washed out, fur­ther re­duc­ing the buffer­ing ca­pac­ity of the soil.^[187] In low rain­fall areas, un­leached cal­cium pushes pH to 8.5 and with the ad­di­tion of ex­change­able sodium, soils may reach pH 10.^[188] Be­yond a pH of 9, plant growth is reduced.^[189] High pH re­sults in low mi­cro-nu­tri­ent mo­bil­ity, but wa­ter-sol­u­ble chelates of those nu­tri­ents can cor­rect the deficit.^[190] Sodium can be re­duced by the ad­di­tion of gyp­sum (cal­cium sul­phate) as cal­cium ad­heres to clay more tightly than does sodium caus­ing sodium to be pushed into the soil water so­lu­tion where it can be washed out by an abun­dance of water.^[191][192]

Base saturation percentage

There are acid-form­ing cations (e.g. hy­dro­gen, alu­minium, iron) and there are base-form­ing cations (e.g. cal­cium, mag­ne­sium, sodium). The frac­tion of the neg­a­tively-charged soil col­loid ex­change sites (CEC) that are oc­cu­pied by base-form­ing cations is called base sat­u­ra­tion. If a soil has a CEC of 20 meq and 5 meq are alu­minium and hy­dro­gen cations (acid-form­ing), the re­main­der of po­si­tions on the col­loids (20-5 = 15 meq) are as­sumed oc­cu­pied by base-form­ing cations, so that the base sat­u­ra­tion is 15/20 x 100% = 75% (the com­pli­ment 25% is as­sumed acid-form­ing cations or pro­tons). Base sat­u­ra­tion is al­most in di­rect pro­por­tion to pH (it in­creases with in­creas­ing pH).^[193] 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).^[194] 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.^[195]

Buffering

Further information: Soil conditioner

The re­sis­tance of soil to change in pH, as a re­sult of the ad­di­tion of acid or basic ma­te­r­ial, is a mea­sure of the buffer­ing ca­pac­ity of a soil and (for a par­tic­u­lar soil type) in­creases as the CEC in­creases. Hence, pure sand has al­most no buffer­ing abil­ity, while soils high in col­loids (whether min­eral or or­ganic) have high buffer­ing ca­pac­ity.^[196] Buffer­ing oc­curs by cation ex­change and neu­tral­i­sa­tion. How­ever, col­loids are not the only reg­u­la­tors of soil pH. The role of car­bon­ates should be un­der­lined, too.^[197] More gen­er­ally, ac­cord­ing to pH lev­els, sev­eral buffer sys­tems take prece­dence over each other, from cal­cium car­bon­ate buffer range to iron buffer range.^[198]

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

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

The above are ex­am­ples of the buffer­ing of soil pH. The gen­eral prin­ci­pal is that an in­crease in a par­tic­u­lar cation in the soil water so­lu­tion will cause that cation to be fixed to col­loids (buffered) and a de­crease in so­lu­tion of that cation will cause it to be with­drawn from the col­loid and moved into so­lu­tion (buffered). The de­gree of buffer­ing is often re­lated to the CEC of the soil; the greater the CEC, the greater the buffer­ing ca­pac­ity of the soil.^[199]

Nutrients

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

Sev­en­teen el­e­ments or nu­tri­ents are es­sen­tial for plant growth and re­pro­duc­tion. They are car­bon (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) and chlo­rine (Cl).^{[200][201][202]} Nu­tri­ents re­quired for plants to com­plete their life cycle are con­sid­ered es­sen­tial nu­tri­ents. Nu­tri­ents that en­hance the growth of plants but are not nec­es­sary to com­plete the plant’s life cycle are con­sid­ered non-es­sen­tial. With the ex­cep­tion of car­bon, hy­dro­gen and oxy­gen, which are sup­plied by car­bon diox­ide and water, and ni­tro­gen, pro­vided through ni­tro­gen fix­a­tion,^[202] the nu­tri­ents de­rive orig­i­nally from the min­eral com­po­nent of the soil. The Law of the Min­i­mum ex­presses that when the avail­able form of a nu­tri­ent is not in enough pro­por­tion in the soil so­lu­tion, then other nu­tri­ents can­not be taken up at an op­ti­mum rate by a plant.^[203] A par­tic­u­lar nu­tri­ent ratio of the soil so­lu­tion is thus manda­tory for op­ti­miz­ing plant growth, a value which might dif­fer from nu­tri­ent ra­tios cal­cu­lated from plant composition.^[204]

Plant up­take of nu­tri­ents can only pro­ceed when they are pre­sent in a plant-avail­able form. In most sit­u­a­tions, nu­tri­ents are ab­sorbed in an ionic 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 within pri­mary and sec­ondary min­er­als, they weather too slowly to sup­port rapid plant growth. For ex­am­ple, the ap­pli­ca­tion of finely ground min­er­als, feldspar and ap­atite, to soil sel­dom pro­vides the nec­es­sary amounts of potas­sium and phos­pho­rus at a rate suf­fi­cient for good plant growth, as most of the nu­tri­ents re­main bound in the crys­tals of those minerals.^[205]

The nu­tri­ents ad­sorbed onto the sur­faces of clay col­loids and soil or­ganic mat­ter pro­vide a more ac­ces­si­ble reser­voir of many plant nu­tri­ents (e.g. K, Ca, Mg, P, Zn). As plants ab­sorb the nu­tri­ents from the soil water, the sol­u­ble pool is re­plen­ished from the sur­face-bound pool. The de­com­po­si­tion of soil or­ganic mat­ter by mi­croor­gan­isms is an­other mech­a­nism whereby the sol­u­ble pool of nu­tri­ents is re­plen­ished – this is im­por­tant for the sup­ply of plant-avail­able N, S, P, and B from soil.^[206]

Gram for gram, the ca­pac­ity of humus to hold nu­tri­ents and water is far greater than that of clay min­er­als, most of the soil cation ex­change ca­pac­ity aris­ing from charged car­boxylic groups on or­ganic matter.^[207] How­ever, de­spite the great ca­pac­ity of humus to re­tain water once wa­ter-soaked, its high hy­dropho­bic­ity de­creases its wet­ta­bil­ity.^[208] All in all, small amounts of humus may re­mark­ably in­crease the soil’s ca­pac­ity to pro­mote plant growth.^[209][206]

Element	Symbol	Ion or molecule
Carbon	C	CO2 (mostly through leaves)
Hydrogen	H	H+, HOH (water)
Oxygen	O	O2−, OH −, CO32−, SO42−, CO2
Phosphorus	P	H2PO4 −, HPO42− (phosphates)
Potassium	K	K+
Nitrogen	N	NH4+, NO3 − (ammonium, nitrate)
Sulfur	S	SO42−
Calcium	Ca	Ca2+
Iron	Fe	Fe2+, Fe3+ (ferrous, ferric)
Magnesium	Mg	Mg2+
Boron	B	H3BO3, H2BO3 −, B(OH)4 −
Manganese	Mn	Mn2+
Copper	Cu	Cu2+
Zinc	Zn	Zn2+
Molybdenum	Mo	MoO42− (molybdate)
Chlorine	Cl	Cl − (chloride)

Uptake processes

Nu­tri­ents in the soil are taken up by the plant through its roots, and in par­tic­u­lar its root hairs. To be taken up by a plant, a nu­tri­ent el­e­ment must be lo­cated near the root sur­face; how­ever, the sup­ply of nu­tri­ents in con­tact with the root is rapidly de­pleted within a dis­tance of ca. 2 mm.^[211] There are three basic mech­a­nisms whereby nu­tri­ent ions dis­solved in the soil so­lu­tion are brought into con­tact with plant roots:

1. Mass flow of water
2. Diffusion within water
3. Interception by root growth

All three mech­a­nisms op­er­ate si­mul­ta­ne­ously, but one mech­a­nism or an­other may be most im­por­tant for a par­tic­u­lar nutrient.^[212] For ex­am­ple, in the case of cal­cium, which is gen­er­ally plen­ti­ful in the soil so­lu­tion, ex­cept when alu­minium over com­petes cal­cium on cation ex­change sites in very acid soils (pH less than 4),^[213] mass flow alone can usu­ally bring suf­fi­cient amounts to the root sur­face. How­ever, in the case of phos­pho­rus, dif­fu­sion is needed to sup­ple­ment mass flow. For the most part, nu­tri­ent ions must travel some dis­tance in the soil so­lu­tion to reach the root sur­face. This move­ment can take place by mass flow, as when dis­solved nu­tri­ents are car­ried along with the soil water flow­ing to­ward a root that is ac­tively draw­ing water from the soil. In this type of move­ment, the nu­tri­ent ions are some­what anal­o­gous to leaves float­ing down a stream. In ad­di­tion, nu­tri­ent ions con­tin­u­ally move by dif­fu­sion from areas of greater con­cen­tra­tion to­ward the nu­tri­ent-de­pleted areas of lower con­cen­tra­tion around the root sur­face. That process is due to ran­dom mo­tion, also called Brown­ian mo­tion, of mol­e­cules within a gra­di­ent of de­creas­ing concentration.^[214] By this means, plants can con­tinue to take up nu­tri­ents even at night, when water is only slowly ab­sorbed into the roots as tran­spi­ra­tion has al­most stopped fol­low­ing stom­atal clo­sure. Fi­nally, root in­ter­cep­tion comes into play as roots con­tin­u­ally grow into new, un­de­pleted soil. By this way roots are also able to ab­sorb nano­ma­te­ri­als such as nanopar­tic­u­late or­ganic matter.^[215]

Nutrient	Approximate percentage supplied by:
Mass flow	Root interception	Diffusion
Nitrogen	98.8	1.2	0
Phosphorus	6.3	2.8	90.9
Potassium	20.0	2.3	77.7
Calcium	71.4	28.6	0
Sulfur	95.0	5.0	0
Molybdenum	95.2	4.8	0

In the above table, phos­pho­rus and potas­sium nu­tri­ents move more by dif­fu­sion than they do by mass flow in the soil water so­lu­tion, as they are rapidly taken up by the roots cre­at­ing a con­cen­tra­tion of al­most zero near the roots (the plants can­not tran­spire enough water to draw more of those nu­tri­ents near the roots). The very steep con­cen­tra­tion gra­di­ent is of greater in­flu­ence in the move­ment of those ions than is the move­ment of those by mass flow.^[217] The move­ment by mass flow re­quires the tran­spi­ra­tion of water from the plant caus­ing water and so­lu­tion ions to also move to­ward the roots.^[218] Move­ment by root in­ter­cep­tion is slow­est as the plants must ex­tend their roots.^[219]

Plants move ions out of their roots in an ef­fort to move nu­tri­ents in from the soil, an ex­change process which oc­curs in the root apoplast.^[220] Hy­dro­gen H⁺ is ex­changed for other cations, and car­bon­ate (HCO₃⁻) and hy­drox­ide (OH⁻) an­ions are ex­changed for nu­tri­ent anions.^[221] As plant roots re­move nu­tri­ents from the soil water so­lu­tion, they are re­plen­ished as other ions move off of clay and humus (by ion ex­change or des­orp­tion), are added from the weath­er­ing of soil min­er­als, and are re­leased by the de­com­po­si­tion of soil or­ganic mat­ter. How­ever, the rate at which plant roots re­move nu­tri­ents may not cope with the rate at which they are re­plen­ished in the soil so­lu­tion, stem­ming in nu­tri­ent lim­i­ta­tion to plant growth.^[222] Plants de­rive a large pro­por­tion of their anion nu­tri­ents from de­com­pos­ing or­ganic mat­ter, which typ­i­cally holds about 95 per­cent of the soil ni­tro­gen, 5 to 60 per­cent of the soil phos­pho­rus and about 80 per­cent of the soil sul­fur. Where crops are pro­duced, the re­plen­ish­ment of nu­tri­ents in the soil must usu­ally be aug­mented by the ad­di­tion of fer­til­izer or or­ganic matter.^[216]

Be­cause nu­tri­ent up­take is an ac­tive meta­bolic process, con­di­tions that in­hibit root me­tab­o­lism may also in­hibit nu­tri­ent uptake.^[223] Ex­am­ples of such con­di­tions in­clude wa­ter­log­ging or soil com­paction re­sult­ing in poor soil aer­a­tion, ex­ces­sively high or low soil tem­per­a­tures, and above-ground con­di­tions that re­sult in low translo­ca­tion of sug­ars to plant roots.^[224]

Carbon

Measuring soil respiration in the field using an SRS2000 system.

Plants ob­tain their car­bon from at­mos­pheric car­bon diox­ide through pho­to­syn­thetic car­boxy­la­tion, to which must be added the up­take of dis­solved car­bon from the soil solution^[225] and car­bon trans­fer through my­c­or­rhizal net­works.^[226] About 45% of a plant’s dry mass is car­bon; plant residues typ­i­cally have a car­bon to ni­tro­gen ratio (C/N) of be­tween 13:1 and 100:1. As the soil or­ganic ma­te­r­ial is di­gested by mi­cro-or­gan­isms and saprophagous soil fauna, the C/N de­creases as the car­bona­ceous ma­te­r­ial is me­tab­o­lized and car­bon diox­ide (CO₂) is re­leased as a byprod­uct which then finds its way out of the soil and into the at­mos­phere. Ni­tro­gen turnover (mostly in­volved in pro­tein turnover) is lesser than that of car­bon (mostly in­volved in res­pi­ra­tion) in the liv­ing, then dead mat­ter of de­com­posers, which are al­ways richer in ni­tro­gen than plant lit­ter, and so it builds up in the soil.^[227] Nor­mal CO₂ con­cen­tra­tion in the at­mos­phere is 0.03%, this can be the fac­tor lim­it­ing plant growth. In a field of maize on a still day dur­ing high light con­di­tions in the grow­ing sea­son, the CO₂ con­cen­tra­tion drops very low, but under such con­di­tions the crop could use up to 20 times the nor­mal con­cen­tra­tion. The res­pi­ra­tion of CO₂ by soil mi­cro-or­gan­isms de­com­pos­ing soil or­ganic mat­ter and the CO₂ respired by roots con­tribute an im­por­tant amount of CO₂ to the pho­to­syn­the­sis­ing plants, to which must be added the CO₂ respired by above­ground plant tissues.^[228] Root-respired CO₂ can be ac­cu­mu­lated overnight within hol­low stems of plants, to be fur­ther used for pho­to­syn­the­sis dur­ing the day.^[229] Within the soil, CO₂ con­cen­tra­tion is 10 to 100 times that of at­mos­pheric lev­els but may rise to toxic lev­els if the soil poros­ity is low or if dif­fu­sion is im­peded by flooding.^{[230][200][231]}

Nitrogen

Further information: Nitrogen cycle

Generalization of percent soil nitrogen by soil order

Ni­tro­gen is the most crit­i­cal el­e­ment ob­tained by plants from the soil, to the ex­cep­tion of moist trop­i­cal forests where phos­pho­rus is the lim­it­ing soil nu­tri­ent,^[232] and ni­tro­gen de­fi­ciency often lim­its plant growth.^[233] Plants can use the ni­tro­gen as ei­ther the am­mo­nium cation (NH₄⁺) or the anion ni­trate (NO₃⁻). Plants are com­monly clas­si­fied as am­mo­nium or ni­trate plants ac­cord­ing to their pref­er­en­tial ni­tro­gen nutrition.^[234] Usu­ally, most of the ni­tro­gen in soil is bound within or­ganic com­pounds that make up the soil or­ganic mat­ter, and must be min­er­al­ized to the am­mo­nium or ni­trate form be­fore it can be taken up by most plants. How­ever, sym­bio­sis with my­c­or­rhizal fungi allow plants to get ac­cess to the or­ganic ni­tro­gen pool where and when min­eral forms of ni­tro­gen are poorly available.^[235] The total ni­tro­gen con­tent de­pends largely on the soil or­ganic mat­ter con­tent, which in turn de­pends on tex­ture, cli­mate, veg­e­ta­tion, topog­ra­phy, age and soil man­age­ment.^[236] Soil ni­tro­gen typ­i­cally de­creases by 0.2 to 0.3% for every tem­per­a­ture in­crease by 10 °C. Usu­ally, grass­land soils con­tain more soil ni­tro­gen than for­est soils, be­cause of a higher turnover rate of grass­land or­ganic matter.^[237] Cul­ti­va­tion de­creases soil ni­tro­gen by ex­pos­ing soil or­ganic mat­ter to de­com­po­si­tion by microorganisms,^[238] most losses being caused by den­i­tri­fi­ca­tion,^[239] and soils under no-tillage main­tain more soil ni­tro­gen than tilled soils.^[240]

Some mi­cro-or­gan­isms are able to metabolise or­ganic mat­ter and re­lease am­mo­nium in a process called min­er­al­i­sa­tion. Oth­ers, called ni­tri­fiers, take free am­mo­nium or ni­trite as an in­ter­me­di­ary step in the process of ni­tri­fi­ca­tion, and ox­i­dise it to ni­trate. Ni­tro­gen-fix­ing bac­te­ria are ca­pa­ble of metabolis­ing N₂ into the form of am­mo­nia or re­lated ni­troge­nous com­pounds in a process called ni­tro­gen fix­a­tion. Both am­mo­nium and ni­trate can be im­mo­bi­lized by their in­cor­po­ra­tion into mi­cro­bial liv­ing cells, where it is tem­porar­ily se­questered in the form of amino acids and pro­teins. Ni­trate may be lost from the soil to the at­mos­phere when bac­te­ria metabolise it to the gases NH₃, N₂ and N₂O, a process called den­i­tri­fi­ca­tion. Ni­tro­gen may also be leached from the va­dose zone if in the form of ni­trate, act­ing as a pol­lu­tant if it reaches the water table or flows over land, more es­pe­cially in agri­cul­tural soils under high use of nu­tri­ent fertilizers.^[241] Am­mo­nium may also be se­questered in 2:1 clay min­er­als.^[242] A small amount of ni­tro­gen is added to soil by rain­fall, to the ex­cep­tion of wide areas of North Amer­ica and West Eu­rope where the ex­cess use of ni­tro­gen fer­til­iz­ers and ma­nure has caused at­mos­pheric pol­lu­tion by am­mo­nia emis­sion, stem­ming in soil acid­i­fi­ca­tion and eu­troph­i­ca­tion of soils and aquatic ecosys­tems.^{[243][244][206][245][246][247]}

Gains

In the process of min­er­al­i­sa­tion, mi­crobes feed on or­ganic mat­ter, re­leas­ing am­mo­nia (NH₃), am­mo­nium (NH₄⁺), ni­trate (NO₃⁻) and other nu­tri­ents. As long as the car­bon to ni­tro­gen ratio (C/N) of fresh residues in the soil is above 30:1, ni­tro­gen will be in short sup­ply for the ni­tro­gen-rich mi­crobal bio­mass (ni­tro­gen de­fi­ciency), and other bac­te­ria will up­take am­mo­nium and to a lesser ex­tent ni­trate and in­cor­po­rate them into their cells in the im­mo­bi­liza­tion process.^[248] In that form the ni­tro­gen is said to be im­mo­bilised. Later, when such bac­te­ria die, they too are min­er­alised and some of the ni­tro­gen is re­leased as am­mo­nium and ni­trate. Pre­da­tion of bac­te­ria by soil fauna, in par­tic­u­lar pro­to­zoa and ne­ma­todes, play a de­ci­sive role in the re­turn of im­mo­bi­lized ni­tro­gen to min­eral forms.^[249] If the C/N of fresh residues is less than 15, min­eral ni­tro­gen is freed to the soil and di­rectly avail­able to plants.^[250] Bac­te­ria may on av­er­age add 25 pounds (11 kg) ni­tro­gen per acre, and in an un­fer­tilised field, this is the most im­por­tant source of us­able ni­tro­gen. In a soil with 5% or­ganic mat­ter per­haps 2 to 5% of that is re­leased to the soil by such de­com­po­si­tion. It oc­curs fastest in warm, moist, well aer­ated soil.^[251] The min­er­al­i­sa­tion of 3% of the or­ganic ma­te­r­ial of a soil that is 4% or­ganic mat­ter over­all, would re­lease 120 pounds (54 kg) of ni­tro­gen as am­mo­nium per acre.^[252]

Organic Material	C:N Ratio
Alfalfa	13
Bacteria	4
Clover, green sweet	16
Clover, mature sweet	23
Fungi	9
Forest litter	30
Humus in warm cultivated soils	11
Legume-grass hay	25
Legumes (alfalfa or clover), mature	20
Manure, cow	18
Manure, horse	16–45
Manure, human	10
Oat straw	80
Straw, cornstalks	90
Sawdust	250

In ni­tro­gen fix­a­tion, rhi­zo­bium bac­te­ria con­vert N₂ to am­mo­nia (NH₃), which is rapidly con­verted to amino acids, parts of which are used by the rhi­zo­bia for the syn­the­sis of their own bio­mass pro­teins, while other parts are trans­ported to the xylem of the host plant.^[254] Rhi­zo­bia share a sym­bi­otic re­la­tion­ship with host plants, since rhi­zo­bia sup­ply the host with ni­tro­gen and the host pro­vides rhi­zo­bia with other nu­tri­ents and a safe en­vi­ron­ment. It is es­ti­mated that such sym­bi­otic bac­te­ria in the root nod­ules of legumes add 45 to 250 pounds of ni­tro­gen per acre per year, which may be suf­fi­cient for the crop. Other, free-liv­ing ni­tro­gen-fix­ing di­a­zotroph bac­te­ria and ar­chaea live in­de­pen­dently in the soil and re­lease min­eral forms of ni­tro­gen when their dead bod­ies are con­verted by way of min­er­al­iza­tion.^[255]

Some amount of us­able ni­tro­gen is fixed by light­ning as ni­tric oxide (NO) and ni­tro­gen diox­ide (NO₂⁻).^[256] Ni­tro­gen diox­ide is sol­u­ble in water to form ni­tric acid (HNO₃) dis­so­ci­at­ing in H⁺ and NO₃⁻. Am­mo­nia, NH₃, pre­vi­ously emit­ted from the soil, may fall with pre­cip­i­ta­tion as ni­tric acid at a rate of about five pounds ni­tro­gen per acre per year.^[257]

Sequestration

When bac­te­ria feed on sol­u­ble forms of ni­tro­gen (am­mo­nium and ni­trate), they tem­porar­ily se­quester that ni­tro­gen in their bod­ies in a process called im­mo­bi­liza­tion. At a later time when those bac­te­ria die, their ni­tro­gen may be re­leased as am­mo­nium by the process of min­er­al­iza­tion, sped up by preda­tory fauna.^[258]

Pro­tein ma­te­r­ial is eas­ily bro­ken down, but the rate of its de­com­po­si­tion is slowed by its at­tach­ment to the crys­talline struc­ture of clay and when trapped be­tween the clay layers^[259] or at­tached to rough clay surfaces.^[260] The lay­ers are small enough that bac­te­ria can­not enter.^[261] Some or­gan­isms can exude ex­tra­cel­lu­lar en­zymes that can act on the se­questered pro­teins. How­ever, those en­zymes too may be trapped on the clay crys­tals, re­sult­ing in a com­plex in­ter­ac­tion be­tween pro­teins, mi­cro­bial en­zymes and min­eral surfaces.^[262]

Am­mo­nium fix­a­tion oc­curs mainly be­tween the lay­ers of 2:1 type clay min­er­als such as il­lite, ver­mi­culite or mont­mo­ril­lonite, to­gether with ions of sim­i­lar ionic ra­dius and low hy­dra­tion en­ergy such as potas­sium, but a small pro­por­tion of am­mo­nium is also fixed in the silt frac­tion.^[263] Only a small frac­tion of soil ni­tro­gen is held this way.^[264]

Losses

Us­able ni­tro­gen may be lost from soils when it is in the form of ni­trate, as it is eas­ily leached, con­trary to am­mo­nium which is eas­ily fixed.^[265] Fur­ther losses of ni­tro­gen occur by den­i­tri­fi­ca­tion, the process whereby soil bac­te­ria con­vert ni­trate (NO₃⁻) to ni­tro­gen gas, N₂ or N₂O. This oc­curs when poor soil aer­a­tion lim­its free oxy­gen, forc­ing bac­te­ria to use the oxy­gen in ni­trate for their res­pi­ra­tory process. Den­i­tri­fi­ca­tion in­creases when ox­i­dis­able or­ganic ma­te­r­ial is avail­able, as in or­ganic farm­ing^[265] and when soils are warm and slightly acidic, as cur­rently hap­pen­ing in trop­i­cal areas.^[266] Den­i­tri­fi­ca­tion may vary through­out a soil as the aer­a­tion varies from place to place.^[267] Den­i­tri­fi­ca­tion may cause the loss of 10 to 20 per­cent of the avail­able ni­trates within a day and when con­di­tions are favourable to that process, losses of up to 60 per­cent of ni­trate ap­plied as fer­tiliser may occur.^[268]

Am­mo­nia volatil­i­sa­tion oc­curs when am­mo­nium re­acts chem­i­cally with an al­ka­line soil, con­vert­ing NH₄⁺ to NH₃.^[269] The ap­pli­ca­tion of am­mo­nium fer­tiliser to such a field can re­sult in volatil­i­sa­tion losses of as much as 30 percent.^[270]

All kinds of ni­tro­gen losses, whether by leach­ing or volatiliza­tion, are re­spon­si­ble for a large part of aquifer pol­lu­tion^[271] and air pol­lu­tion, with con­comi­tant ef­fects on soil acid­i­fi­ca­tion and eu­troph­i­ca­tion,^[272] a novel com­bi­na­tion of en­vi­ron­men­tal threats (acid­ity and ex­cess ni­tro­gen) to which ex­tant or­gan­isms are badly adapted, caus­ing se­vere bio­di­ver­sity losses in nat­ural ecosystems.^[273]

Phosphorus

After ni­tro­gen, phos­pho­rus is prob­a­bly the el­e­ment most likely to be de­fi­cient in soils, al­though it often turns to be the most de­fi­cient in trop­i­cal soils where the min­eral pool is de­pleted under in­tense leach­ing and min­eral weath­er­ing while, con­trary to ni­tro­gen, phos­pho­rus re­serves can­not be re­plen­ished from other sources.^[274] The soil min­eral ap­atite is the most com­mon min­eral source of phos­pho­rus, from which it can be ex­tracted by mi­cro­bial and root exudates,^[275][276] with an im­por­tant con­tri­bu­tion of ar­bus­cu­lar my­c­or­rhizal fungi.^[277] The most com­mon form of or­ganic phos­phate is phy­tate, the prin­ci­pal stor­age form of phos­pho­rus in many plant tis­sues. While there is on av­er­age 1000 lb per acre (1120 kg per hectare) of phos­pho­rus in the soil, it is gen­er­ally in the form of or­thophos­phate with low sol­u­bil­ity, ex­cept when linked to am­mo­nium or cal­cium, hence the use of di­ammo­nium phos­phate or mono­cal­cium phos­phate as fertilizers.^[278] Total phos­pho­rus is about 0.1 per­cent by weight of the soil, but only one per­cent of that is di­rectly avail­able to plants. Of the part avail­able, more than half comes from the min­er­al­i­sa­tion of or­ganic mat­ter. Agri­cul­tural fields may need to be fer­tilised to make up for the phos­pho­rus that has been re­moved in the crop.^[279]

When phos­pho­rus does form sol­u­bilised ions of H₂PO₄⁻, if not taken up by plant roots they rapidly form in­sol­u­ble phos­phates of cal­cium or hy­drous ox­ides of iron and alu­minum. Phos­pho­rus is largely im­mo­bile in the soil and is not leached but ac­tu­ally builds up in the sur­face layer if not cropped. The ap­pli­ca­tion of sol­u­ble fer­tilis­ers to soils may re­sult in zinc de­fi­cien­cies as zinc phos­phates form, but soil pH lev­els, partly de­pend­ing on the form of phos­pho­rus in the fer­tiliser, strongly in­ter­act with this ef­fect, in some cases re­sult­ing in in­creased zinc availability.^[280] Lack of phos­pho­rus may in­ter­fere with the nor­mal open­ing of the plant leaf stom­ata, de­creased stom­atal con­duc­tance re­sult­ing in de­creased pho­to­syn­the­sis and res­pi­ra­tion rates^[281] while de­creased tran­spi­ra­tion in­creases plant temperature.^[282] Phos­pho­rus is most avail­able when soil pH is 6.5 in min­eral soils and 5.5 in or­ganic soils.^[270]

Potassium

The amount of potas­sium in a soil may be as much as 80,000 lb per acre-foot, of which only 150 lb is avail­able for plant growth. Com­mon min­eral sources of potas­sium are the mica bi­otite and potas­sium feldspar, KAlSi₃O₈. Rhi­zos­phere bac­te­ria, also called rhi­zobac­te­ria, con­tribute through the pro­duc­tion of or­ganic acids to its solubilization.^[283] When sol­u­bilised, half will be held as ex­change­able cations on clay while the other half is in the soil water so­lu­tion. Potas­sium fix­a­tion often oc­curs when soils dry and the potas­sium is bonded be­tween lay­ers of 2:1 ex­pan­sive clay min­er­als such as il­lite, ver­mi­culite or mont­mo­ril­lonite.^[284] Under cer­tain con­di­tions, de­pen­dent on the soil tex­ture, in­ten­sity of dry­ing, and ini­tial amount of ex­change­able potas­sium, the fixed per­cent­age may be as much as 90 per­cent within ten min­utes. Potas­sium may be leached from soils low in clay.^[285][286]

Calcium

Cal­cium is one per­cent by weight of soils and is gen­er­ally avail­able but may be low as it is sol­u­ble and can be leached. It is thus low in sandy and heav­ily leached soil or strongly acidic min­eral soils, re­sult­ing in ex­ces­sive con­cen­tra­tion of free hy­dro­gen ions in the soil so­lu­tion, and there­fore these soils re­quire liming.^[287] Cal­cium is sup­plied to the plant in the form of ex­change­able ions and mod­er­ately sol­u­ble min­er­als. There are four forms of cal­cium in the soil. Soil cal­cium can be in in­sol­u­ble forms such as cal­cite or dolomite, in the soil so­lu­tion in the form of a di­va­lent cation or re­tained in ex­change­able form at the sur­face of min­eral par­ti­cles. An­other form is when cal­cium com­plexes with or­ganic mat­ter, form­ing co­va­lent bonds be­tween or­ganic com­pounds which con­tribute to struc­tural sta­bil­ity.^[288] Cal­cium is more avail­able on the soil col­loids than is potas­sium be­cause the com­mon min­eral cal­cite, CaCO₃, is more sol­u­ble than potas­sium-bear­ing min­er­als such as feldspar.^[289]

Cal­cium up­take by roots is es­sen­tial for plant nu­tri­tion, con­trary to an old tenet that it was lux­ury con­sump­tion.^[290] Cal­cium is con­sid­ered as an es­sen­tial com­po­nent of plant cell mem­branes, a coun­te­rion for in­or­ganic and or­ganic an­ions in the vac­uole, and an in­tra­cel­lu­lar mes­sen­ger in the cy­tosol, play­ing a role in cel­lu­lar learn­ing and mem­ory.^[291]

Magnesium

Mag­ne­sium is one of the dom­i­nant ex­change­able cations in most soils (after cal­cium and potas­sium). Mag­ne­sium is an es­sen­tial el­e­ment for plants, mi­crobes and an­i­mals, being in­volved in many cat­alytic re­ac­tions and in the syn­the­sis of chloro­phyll. Pri­mary min­er­als that weather to re­lease mag­ne­sium in­clude horn­blende, bi­otite and ver­mi­culite. Soil mag­ne­sium con­cen­tra­tions are gen­er­ally suf­fi­cient for op­ti­mal plant growth, but highly weath­ered and sandy soils may be mag­ne­sium de­fi­cient due to leach­ing by heavy precipitation.^[206][292]

Sulfur

Most sul­fur is made avail­able to plants, like phos­pho­rus, by its re­lease from de­com­pos­ing or­ganic matter.^[292] De­fi­cien­cies may exist in some soils (es­pe­cially sandy soils) and if cropped, sul­fur needs to be added. The ap­pli­ca­tion of large quan­ti­ties of ni­tro­gen to fields that have mar­ginal amounts of sul­fur may cause sul­fur de­fi­ciency by a di­lu­tion effect when stim­u­la­tion of plant growth by ni­tro­gen in­creases the plant de­mand for sulfur.^[293] A 15-ton crop of onions uses up to 19 lb of sul­fur and 4 tons of al­falfa uses 15 lb per acre. Sul­fur abun­dance varies with depth. In a sam­ple of soils in Ohio, United States, the sul­fur abun­dance var­ied with depths, 0–6 inches, 6–12 inches, 12–18 inches, 18–24 inches in the amounts: 1056, 830, 686, 528 lb per acre respectively.^[294]

Micronutrients

The mi­cronu­tri­ents es­sen­tial in plant life, in their order of im­por­tance, in­clude iron,^[295] man­ganese,^[296] zinc,^[297] cop­per,^[298] boron,^[299] chlo­rine^[300] and molyb­de­num.^[301] The term refers to plants’ needs, not to their abun­dance in soil. They are re­quired in very small amounts but are es­sen­tial to plant health in that most are re­quired parts of en­zyme sys­tems which are in­volved in plant me­tab­o­lism.^[302] They are gen­er­ally avail­able in the min­eral com­po­nent of the soil, but the heavy ap­pli­ca­tion of phos­phates can cause a de­fi­ciency in zinc and iron by the for­ma­tion of in­sol­u­ble zinc and iron phosphates.^[303] Iron de­fi­ciency, stem­ming in plant chloro­sis and rhi­zos­phere acid­i­fi­ca­tion, may also re­sult from ex­ces­sive amounts of heavy met­als or cal­cium min­er­als (lime) in the soil.^[304][305] Ex­cess amounts of sol­u­ble boron, molyb­de­num and chlo­ride are toxic.^[306][307]

Non-essential nutrients

Nu­tri­ents which en­hance the health but whose de­fi­ciency does not stop the life cycle of plants in­clude: cobalt, stron­tium, vana­dium, sil­i­con and nickel.^[308] As their im­por­tance is eval­u­ated they may be added to the list of es­sen­tial plant nu­tri­ents, as is the case for silicon.^[309]

Soil organic matter

Main article: Soil organic matter

Soil or­ganic mat­ter is made up of or­ganic com­pounds and in­cludes plant, an­i­mal and mi­cro­bial ma­te­r­ial, both liv­ing and dead. A typ­i­cal soil has a bio­mass com­po­si­tion of 70% mi­croor­gan­isms, 22% macro­fauna, and 8% roots. The liv­ing com­po­nent of an acre of soil may in­clude 900 lb of earth­worms, 2400 lb of fungi, 1500 lb of bac­te­ria, 133 lb of pro­to­zoa and 890 lb of arthro­pods and algae.^[310]

A few per­cent of the soil or­ganic mat­ter, with small res­i­dence time, con­sists of the mi­cro­bial bio­mass and metabo­lites of bac­te­ria, molds, and actin­o­mycetes that work to break down the dead or­ganic matter.^[311][312] Were it not for the ac­tion of these mi­cro-or­gan­isms, 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 to car­bon se­ques­tra­tion in the top­soil through the for­ma­tion of sta­ble humus.^[313] In the aim to se­quester more car­bon in the soil for al­le­vi­at­ing the green­house ef­fect it would be more ef­fi­cient in the long-term to stim­u­late hu­mi­fi­ca­tion than to de­crease lit­ter de­com­po­si­tion.^[314]

The main part of soil or­ganic mat­ter is a com­plex as­sem­blage of small or­ganic mol­e­cules, col­lec­tively called humus or humic sub­stances. The use of these terms, which do not rely on a clear chem­i­cal clas­si­fi­ca­tion, has been con­sid­ered as obsolete.^[315] Other stud­ies showed that the clas­si­cal no­tion of mol­e­cule is not con­ve­nient for humus, which es­caped most at­tempts done over two cen­turies to re­solve it in unit com­po­nents, but still is chem­i­cally dis­tinct from poly­sac­cha­rides, lignins and proteins.^[316]

Most liv­ing things in soils, in­clud­ing plants, an­i­mals, bac­te­ria, and fungi, are de­pen­dent on or­ganic mat­ter for nu­tri­ents and/or en­ergy. Soils have or­ganic com­pounds in vary­ing de­grees of de­com­po­si­tion which rate is de­pen­dent on the tem­per­a­ture, soil mois­ture, and aer­a­tion. Bac­te­ria and fungi feed on the raw or­ganic mat­ter, which are fed upon by pro­to­zoa, which in turn are fed upon by ne­ma­todes, an­nelids and arthro­pods, them­selves able to con­sume and trans­form raw or hu­mi­fied or­ganic mat­ter. This has been called the soil food web, through which all or­ganic mat­ter is processed as in a di­ges­tive sys­tem.^[317] Or­ganic mat­ter holds soils open, al­low­ing the in­fil­tra­tion of air and water, and may hold as much as twice its weight in water. Many soils, in­clud­ing desert and rocky-gravel soils, have lit­tle or no or­ganic mat­ter. Soils that are all or­ganic mat­ter, such as peat (his­tosols), are infertile.^[318] In its ear­li­est stage of de­com­po­si­tion, the orig­i­nal or­ganic ma­te­r­ial is often called raw or­ganic mat­ter. The final stage of de­com­po­si­tion is called humus.

In grass­land, 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 thicker A 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.^[319]

Humus

Humus refers to or­ganic mat­ter that has been de­com­posed by soil mi­croflora and fauna to the point where it is re­sis­tant to fur­ther break­down. Humus usu­ally con­sti­tutes only five per­cent of the soil or less by vol­ume, but it is an es­sen­tial source of nu­tri­ents and adds im­por­tant tex­tural qual­i­ties cru­cial to soil health and plant growth.^[320] Humus also feeds arthro­pods, ter­mites and earth­worms which fur­ther im­prove the soil.^[321] The end prod­uct, humus, is sus­pended in col­loidal form in the soil so­lu­tion and forms a weak acid that can at­tack sil­i­cate minerals.^[322] 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.^[323]

Humic acids and ful­vic 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 en­zymes, re­sult­ing fi­nally in the for­ma­tion of humus.^[324] As the residues break down, only mol­e­cules made of aliphatic and aro­matic hy­dro­car­bons, as­sem­bled and sta­bi­lized by oxy­gen and hy­dro­gen bonds, re­main in the form of com­plex mol­e­c­u­lar as­sem­blages col­lec­tively called humus.^[316] 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.^[323] While the struc­ture of humus has in it­self few nu­tri­ents, to 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.^[325]

Lignin is re­sis­tant to break­down and ac­cu­mu­lates within the soil. It also re­acts with pro­teins,^[326] which fur­ther in­creases its re­sis­tance to de­com­po­si­tion, in­clud­ing en­zy­matic de­com­po­si­tion by microbes.^[327] Fats and waxes 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.^[328] 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.^[329] Pro­teins nor­mally de­com­pose read­ily, to the ex­cep­tion of scle­ro­pro­teins, but when bound to clay par­ti­cles they be­come more re­sis­tant to decomposition.^[330] As for other pro­teins clay par­ti­cles ab­sorb the en­zymes ex­uded by mi­crobes, de­creas­ing en­zyme ac­tiv­ity while pro­tect­ing ex­tra­cel­lu­lar en­zymes from degradation.^[331] 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, while a study showed in­creased soil fer­til­ity fol­low­ing the ad­di­tion of ma­ture com­post to a clay soil.^[332] High soil tan­nin con­tent can cause ni­tro­gen to be se­questered as re­sis­tant tan­nin-pro­tein complexes.^[333][334]

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.^[335] 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 af­fect­ing soil fer­til­ity.^[318] Humus also ab­sorbs water, and ex­pands and shrinks be­tween dry and wet states to a higher ex­tent than clay, in­creas­ing soil poros­ity.^[336] Humus is less sta­ble than the soil’s min­eral con­stituents, as it is re­duced by mi­cro­bial de­com­po­si­tion, 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.^[337] Char­coal is a source of highly sta­ble humus, called black car­bon,^[338] which had been used tra­di­tion­ally to im­prove the fer­til­ity of nu­tri­ent-poor trop­i­cal soils. This very an­cient prac­tice, as as­cer­tained in the gen­e­sis of Ama­zon­ian dark earths, has been re­newed and be­came pop­u­lar under the name of biochar. It has been sug­gested that biochar could be used to se­quester more car­bon in the fight against the green­house ef­fect.^[339]

Climatological influence

The pro­duc­tion, ac­cu­mu­la­tion and degra­da­tion of or­ganic mat­ter are greatly de­pen­dent on cli­mate. Tem­per­a­ture, soil mois­ture and topog­ra­phy are the major fac­tors af­fect­ing the ac­cu­mu­la­tion of or­ganic mat­ter in soils. Or­ganic mat­ter tends to ac­cu­mu­late under wet or cold con­di­tions where de­com­poser ac­tiv­ity is im­peded by low temperature^[340] or ex­cess mois­ture which re­sults in anaer­o­bic conditions.^[341] 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.^[342] 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.^[343]

Plant residue

Typ­i­cal types and per­cent­ages of plant residue com­po­nents  Cellulose (45%)  Lignin (20%)  Hemicellulose (18%)  Protein (8%)  Sugars and starches (5%)  Fats and waxes (2%)

Cel­lu­lose and hemi­cel­lu­lose un­dergo fast de­com­po­si­tion by fungi and bac­te­ria, with a half-life of 12–18 days in a tem­per­ate climate.^[344] Brown rot fungi can de­com­pose the cel­lu­lose and hemi­cel­lu­lose, leav­ing the lignin and phe­no­lic com­pounds be­hind. Starch, which is an en­ergy stor­age sys­tem for plants, un­der­goes fast de­com­po­si­tion by bac­te­ria and fungi. Lignin con­sists of poly­mers com­posed of 500 to 600 units with a highly branched, amor­phous struc­ture, linked to cel­lu­lose, hemi­cel­lu­lose and pectin in plant cell walls. Lignin un­der­goes very slow de­com­po­si­tion, mainly by white rot fungi and actin­o­mycetes; its half-life under tem­per­ate con­di­tions is about six months.^[344]

Horizons

Main article: Soil horizon

A hor­i­zon­tal layer of the soil, whose phys­i­cal fea­tures, com­po­si­tion and age are dis­tinct from those above and be­neath, is re­ferred to as a soil hori­zon. The nam­ing of a hori­zon is based on the type of ma­te­r­ial of which it is com­posed. Those ma­te­ri­als re­flect the du­ra­tion of spe­cific processes of soil for­ma­tion. They are la­belled using a short­hand no­ta­tion of let­ters and num­bers which de­scribe the hori­zon in terms of its colour, size, tex­ture, struc­ture, con­sis­tency, root quan­tity, pH, voids, bound­ary char­ac­ter­is­tics and pres­ence of nod­ules or concretions.^[345] No soil pro­file has all the major hori­zons. Some, called en­ti­sols, may have only one hori­zon or are cur­rently con­sid­ered as hav­ing no hori­zon, in par­tic­u­lar in­cip­i­ent soils from un­re­claimed min­ing waste de­posits,^[346] moraines,^[347] vol­canic cones^[348] sand dunes or al­lu­vial ter­races.^[349] 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.^[350] 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.^[351] 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 in sed­i­ment lay­ers. Sam­pling pollen, 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.^[352] Soil hori­zons can be dated by sev­eral meth­ods such as ra­dio­car­bon, using pieces of char­coal pro­vided they are of enough size to es­cape pe­do­tur­ba­tion by earth­worm ac­tiv­ity and other me­chan­i­cal disturbances.^[353] Fos­sil soil hori­zons from pa­le­osols can be found within sed­i­men­tary rock se­quences, al­low­ing the study of past environments.^[354]

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.^[355] 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 re­lease dis­solved or­ganic mat­ter.^[356] The re­main­ing sur­fi­cial or­ganic layer, called the O hori­zon, pro­duces a more ac­tive soil due to the ef­fect of the or­gan­isms that live within it. Or­gan­isms colonise and break down or­ganic ma­te­ri­als, mak­ing avail­able nu­tri­ents upon which other plants and an­i­mals can live.^[357] After suf­fi­cient time, humus moves down­ward and is de­posited in a dis­tinc­tive or­ganic-min­eral sur­face layer called the A hori­zon, in which or­ganic mat­ter is mixed with min­eral mat­ter through the ac­tiv­ity of bur­row­ing an­i­mals, a process called pe­do­tur­ba­tion. This nat­ural process does not go to com­ple­tion in the pres­ence of con­di­tions detri­men­tal to soil life such as strong acid­ity, cold cli­mate or pol­lu­tion, stem­ming in the ac­cu­mu­la­tion of un­de­com­posed or­ganic mat­ter within a sin­gle or­ganic hori­zon over­ly­ing the min­eral soil^[358] and in the jux­ta­po­si­tion of hu­mi­fied or­ganic mat­ter and min­eral par­ti­cles, with­out in­ti­mate mix­ing, in the un­der­ly­ing min­eral horizons.^[359]

Classification

Main article: Soil classification

Soil is clas­si­fied into cat­e­gories in order to un­der­stand re­la­tion­ships be­tween dif­fer­ent soils and to de­ter­mine the suit­abil­ity of a soil in a par­tic­u­lar re­gion. One of the first clas­si­fi­ca­tion sys­tems was de­vel­oped by the Russ­ian sci­en­tist Vasily Dokuchaev around 1880.^[360] It was mod­i­fied a num­ber of times by Amer­i­can and Eu­ro­pean re­searchers, and de­vel­oped into the sys­tem com­monly used until the 1960s. It was based on the idea that soils have a par­tic­u­lar mor­phol­ogy based on the ma­te­ri­als and fac­tors that form them. In the 1960s, a dif­fer­ent clas­si­fi­ca­tion sys­tem began to emerge which fo­cused on soil mor­phol­ogy in­stead of parental ma­te­ri­als and soil-form­ing fac­tors. Since then it has un­der­gone fur­ther mod­i­fi­ca­tions. The World Ref­er­ence Base for Soil Re­sources (WRB)^[361] aims to es­tab­lish an in­ter­na­tional ref­er­ence base for soil clas­si­fi­ca­tion.

Uses

Soil is used in agri­cul­ture, where it serves as the an­chor and pri­mary nu­tri­ent base for plants. The types of soil and avail­able mois­ture de­ter­mine the species of plants that can be cul­ti­vated. How­ever, as demon­strated by aero­pon­ics, soil ma­te­r­ial is not an ab­solute es­sen­tial for agri­cul­ture.

Soil ma­te­r­ial is also a crit­i­cal com­po­nent in the min­ing, con­struc­tion and land­scape de­vel­op­ment industries.^[362] Soil serves as a foun­da­tion for most con­struc­tion pro­jects. The move­ment of mas­sive vol­umes of soil can be in­volved in sur­face min­ing, road build­ing and dam con­struc­tion. Earth shel­ter­ing is the ar­chi­tec­tural prac­tice of using soil for ex­ter­nal ther­mal mass against build­ing walls. Many build­ing ma­te­ri­als are soil based.

Soil re­sources are crit­i­cal to the en­vi­ron­ment, as well as to food and fibre pro­duc­tion, pro­duc­ing 98.8% of food con­sumed by humans.^[363] Soil pro­vides min­er­als and water to plants. Soil ab­sorbs rain­wa­ter and re­leases it later, thus pre­vent­ing floods and drought. Soil cleans water as it per­co­lates through it. Soil is the habi­tat for many or­gan­isms: the major part of known and un­known bio­di­ver­sity is in the soil, in the form of in­ver­te­brates (earth­worms, woodlice, mil­li­pedes, cen­tipedes, snails, slugs, mites, spring­tails, enchy­traeids, ne­ma­todes, pro­tists), bac­te­ria, ar­chaea, fungi and algae; and most or­gan­isms liv­ing above ground have part of them (plants) or spend part of their life cycle (in­sects) be­low-ground. Above-ground and be­low-ground bio­di­ver­si­ties are tightly interconnected,^[335][364] mak­ing soil pro­tec­tion of para­mount im­por­tance for any restora­tion or con­ser­va­tion plan.

The bi­o­log­i­cal com­po­nent of soil is an ex­tremely im­por­tant car­bon sink since about 57% of the bi­otic con­tent is car­bon. Even on desert crusts, cyanobac­te­ria, lichens and mosses cap­ture and se­quester a sig­nif­i­cant amount of car­bon by pho­to­syn­the­sis. Poor farm­ing and graz­ing meth­ods have de­graded soils and re­leased much of this se­questered car­bon to the at­mos­phere. Restor­ing the world’s soils could off­set the ef­fect of in­creases in green­house gas emis­sions and slow global warm­ing, while im­prov­ing crop yields and re­duc­ing water needs.^{[365][366][367]}

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

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

Geophagy is the prac­tice of eat­ing soil-like sub­stances. Both an­i­mals and human cul­tures oc­ca­sion­ally con­sume soil for med­i­c­i­nal, recre­ational, or re­li­gious pur­poses. It has been shown that some mon­keys con­sume soil, to­gether with their pre­ferred food (tree fo­liage and fruits), in order to al­le­vi­ate tan­nin tox­i­c­ity.^[368]

Soils fil­ter and pu­rify water and af­fect its chem­istry. Rain water and pooled water from ponds, lakes and rivers per­co­late through the soil hori­zons and the upper rock strata, thus be­com­ing ground­wa­ter. Pests (viruses) and pol­lu­tants, such as per­sis­tent or­ganic pol­lu­tants (chlo­ri­nated pes­ti­cides, poly­chlo­ri­nated biphenyls), oils (hy­dro­car­bons), heavy met­als (lead, zinc, cad­mium), and ex­cess nu­tri­ents (ni­trates, sul­fates, phos­phates) are fil­tered out by the soil.^[369] Soil or­gan­isms metabolise them or im­mo­bilise them in their bio­mass and necromass,^[370] thereby in­cor­po­rat­ing them into sta­ble humus.^[371] The phys­i­cal in­tegrity of soil is also a pre­req­ui­site for avoid­ing land­slides in rugged landscapes.^[372]

Degradation

Main articles: Soil retrogression and degradation and Soil conservation

Land degra­da­tion^[373] refers to a hu­man-in­duced or nat­ural process which im­pairs the ca­pac­ity of land to func­tion. Soils degra­da­tion in­volves the acid­i­fi­ca­tion, con­t­a­m­i­na­tion, de­ser­ti­fi­ca­tion, ero­sion or sali­na­tion.

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

Soil con­t­a­m­i­na­tion at low lev­els is often within a soil’s ca­pac­ity to treat and as­sim­i­late waste ma­te­r­ial. Soil biota can treat waste by trans­form­ing it; soil col­loids can ad­sorb the waste ma­te­r­ial. Many waste treat­ment processes rely on this treat­ment ca­pac­ity. Ex­ceed­ing treat­ment ca­pac­ity can dam­age soil biota and limit soil func­tion. Derelict soils occur where in­dus­trial con­t­a­m­i­na­tion or other de­vel­op­ment ac­tiv­ity dam­ages the soil to such a de­gree that the land can­not be used safely or pro­duc­tively. Re­me­di­a­tion of derelict soil uses prin­ci­ples of ge­ol­ogy, physics, chem­istry and bi­ol­ogy to de­grade, at­ten­u­ate, iso­late or re­move soil con­t­a­m­i­nants to re­store soil func­tions and val­ues. Tech­niques in­clude leach­ing, air sparg­ing, chem­i­cal amend­ments, phy­tore­me­di­a­tion, biore­me­di­a­tion and nat­ural degra­da­tion. An ex­am­ple of dif­fuse pol­lu­tion with con­t­a­m­i­nants is the cop­per dis­tri­b­u­tion in agri­cul­tural soils mainly due to fungi­cide ap­pli­ca­tions in vine­yards and other per­ma­nent crops.^[374]

Desertification

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

Erosion control

Ero­sion of soil is caused by water, wind, ice, and move­ment in re­sponse to grav­ity. More than one kind of ero­sion can occur si­mul­ta­ne­ously. Ero­sion is dis­tin­guished from weath­er­ing, since ero­sion also trans­ports eroded soil away from its place of ori­gin (soil in tran­sit may be de­scribed as sed­i­ment). Ero­sion is an in­trin­sic nat­ural process, but in many places it is greatly in­creased by human ac­tiv­ity, es­pe­cially poor land use prac­tices. These in­clude agri­cul­tural ac­tiv­i­ties which leave the soil bare dur­ing times of heavy rain or strong winds, over­graz­ing, de­for­esta­tion, and im­proper con­struc­tion ac­tiv­ity. Im­proved man­age­ment can limit ero­sion. Soil con­ser­va­tion tech­niques which are em­ployed in­clude changes of land use (such as re­plac­ing ero­sion-prone crops with grass or other soil-bind­ing plants), changes to the tim­ing or type of agri­cul­tural op­er­a­tions, ter­race build­ing, use of ero­sion-sup­press­ing cover ma­te­ri­als (in­clud­ing cover crops and other plants), lim­it­ing dis­tur­bance dur­ing con­struc­tion, and avoid­ing con­struc­tion dur­ing ero­sion-prone pe­ri­ods.

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

Soil pip­ing is a par­tic­u­lar form of soil ero­sion that oc­curs below the soil sur­face. It causes levee and dam fail­ure, as well as sink hole for­ma­tion. Tur­bu­lent flow re­moves soil start­ing at the mouth of the seep flow and the sub­soil ero­sion ad­vances up-gradient.^[375] The term sand boil is used to de­scribe the ap­pear­ance of the dis­charg­ing end of an ac­tive soil pipe.^[376]

Soil sali­na­tion is the ac­cu­mu­la­tion of free salts to such an ex­tent that it leads to degra­da­tion of the agri­cul­tural value of soils and veg­e­ta­tion. Con­se­quences in­clude cor­ro­sion dam­age, re­duced plant growth, ero­sion due to loss of plant cover and soil struc­ture, and water qual­ity prob­lems due to sed­i­men­ta­tion. Sali­na­tion oc­curs due to a com­bi­na­tion of nat­ural and hu­man-caused processes. Arid con­di­tions favour salt ac­cu­mu­la­tion. This is es­pe­cially ap­par­ent when soil par­ent ma­te­r­ial is saline. Ir­ri­ga­tion of arid lands is es­pe­cially problematic.^[377] All ir­ri­ga­tion water has some level of salin­ity. Ir­ri­ga­tion, es­pe­cially when it in­volves leak­age from canals and overir­ri­ga­tion in the field, often raises the un­der­ly­ing water table. Rapid sali­na­tion oc­curs when the land sur­face is within the cap­il­lary fringe of saline ground­wa­ter. Soil salin­ity con­trol in­volves wa­tertable con­trol and flush­ing with higher lev­els of ap­plied water in com­bi­na­tion with tile drainage or an­other form of sub­sur­face drainage.^[378][379]

Reclamation

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

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

In 2008, three years after the ini­tial tri­als, IWMI sci­en­tists con­ducted a sur­vey among 250 farm­ers in north­east Thai­land, half of whom had ap­plied ben­tonite to their fields. The av­er­age im­prove­ment for those using the clay ad­di­tion was 18% higher than for non-clay users. Using the clay had en­abled some farm­ers to switch to grow­ing veg­eta­bles, which need more fer­tile soil. This helped to in­crease their in­come. The re­searchers es­ti­mated that 200 farm­ers in north­east Thai­land and 400 in Cam­bo­dia had adopted the use of clays, and that a fur­ther 20,000 farm­ers were in­tro­duced to the new technique.^[380]

If the soil is too high in clay, adding gyp­sum, washed river sand and or­ganic mat­ter will bal­ance the com­po­si­tion. Adding or­ganic mat­ter (like ramial chipped wood for in­stance) to soil which is de­pleted in nu­tri­ents and too high in sand will boost its quality.^[381]

History of studies and research

The his­tory of the study of soil is in­ti­mately tied to hu­mans’ ur­gent need to pro­vide food for them­selves and for­age for their an­i­mals. Through­out his­tory, civ­i­liza­tions have pros­pered or de­clined as a func­tion of the avail­abil­ity and pro­duc­tiv­ity of their soils.^[382]

Studies of soil fertility

Main article: Soil fertility

The Greek his­to­rian Xenophon (450–355 BCE) is cred­ited with being the first to ex­pound upon the mer­its of green-ma­nur­ing crops: “But then what­ever weeds are upon the ground, being turned into earth, en­rich the soil as much as dung.”^[383]

Col­umella‘s “Hus­bandry,” circa 60 CE, ad­vo­cated the use of lime and that clover and al­falfa (green ma­nure) should be turned under, and was used by 15 gen­er­a­tions (450 years) under the Roman Em­pire until its collapse.^[383][384] From the fall of Rome to the French Rev­o­lu­tion, knowl­edge of soil and agri­cul­ture was passed on from par­ent to child and as a re­sult, crop yields were low. Dur­ing the Eu­ro­pean Mid­dle Ages, Yahya Ibn al-‘Awwam‘s handbook,^[385] with its em­pha­sis on ir­ri­ga­tion, guided the peo­ple of North Africa, Spain and the Mid­dle East; a trans­la­tion of this work was fi­nally car­ried to the south­west of the United States when under Span­ish influence.^[386] Olivier de Ser­res, con­sid­ered as the fa­ther of French agron­omy, was the first to sug­gest the aban­don­ment of fal­low­ing and its re­place­ment by hay mead­ows within crop ro­ta­tions, and he high­lighted the im­por­tance of soil (the French ter­roir) in the man­age­ment of vine­yards. His fa­mous book Le Théâtre d’Agri­cul­ture et mes­nage des champs^[387] con­tributed to the rise of mod­ern, sus­tain­able agri­cul­ture and to the col­lapse of old agri­cul­tural prac­tices such as soil im­prove­ment (amend­ment) for crops by the lift­ing of for­est lit­ter and as­sart­ing, which ru­ined the soils of west­ern Eu­rope dur­ing Mid­dle Ages and even later on ac­cord­ing to regions.^[388]

Ex­per­i­ments into what made plants grow first led to the idea that the ash left be­hind when plant mat­ter was burned was the es­sen­tial el­e­ment but over­looked the role of ni­tro­gen, which is not left on the ground after com­bus­tion, a be­lief which pre­vailed until the 19th century.^[389] In about 1635, the Flem­ish chemist Jan Bap­tist van Hel­mont thought he had proved water to be the es­sen­tial el­e­ment from his fa­mous five years’ ex­per­i­ment with a wil­low tree grown with only the ad­di­tion of rain­wa­ter. His con­clu­sion came from the fact that the in­crease in the plant’s weight had ap­par­ently been pro­duced only by the ad­di­tion of water, with no re­duc­tion in the soil’s weight.^[390][391] John Wood­ward (d. 1728) ex­per­i­mented with var­i­ous types of water rang­ing from clean to muddy and found muddy water the best, and so he con­cluded that earthy mat­ter was the es­sen­tial el­e­ment. Oth­ers con­cluded it was humus in the soil that passed some essence to the grow­ing plant. Still oth­ers held that the vital growth prin­ci­pal was some­thing passed from dead plants or an­i­mals to the new plants. At the start of the 18th cen­tury, Jethro Tull demon­strated that it was ben­e­fi­cial to cul­ti­vate (stir) the soil, but his opin­ion that the stir­ring made the fine parts of soil avail­able for plant ab­sorp­tion was erroneous.^[390][392]

As chem­istry de­vel­oped, it was ap­plied to the in­ves­ti­ga­tion of soil fer­til­ity. The French chemist An­toine Lavoisier showed in about 1778 that plants and an­i­mals must [com­bust] oxy­gen in­ter­nally to live and was able to de­duce that most of the 165-pound weight of van Hel­mont‘s wil­low tree de­rived from air.^[393] It was the French agri­cul­tur­al­ist Jean-Bap­tiste Boussin­gault who by means of ex­per­i­men­ta­tion ob­tained ev­i­dence show­ing that the main sources of car­bon, hy­dro­gen and oxy­gen for plants were air and water, while ni­tro­gen was taken from soil.^[394] Jus­tus von Liebig in his book Or­ganic chem­istry in its ap­pli­ca­tions to agri­cul­ture and physiology (pub­lished 1840), as­serted that the chem­i­cals in plants must have come from the soil and air and that to main­tain soil fer­til­ity, the used min­er­als must be replaced.^[395] Liebig nev­er­the­less be­lieved the ni­tro­gen was sup­plied from the air. The en­rich­ment of soil with guano by the Incas was re­dis­cov­ered in 1802, by Alexan­der von Hum­boldt. This led to its min­ing and that of Chilean ni­trate and to its ap­pli­ca­tion to soil in the United States and Eu­rope after 1840.^[396]

The work of Liebig was a rev­o­lu­tion for agri­cul­ture, and so other in­ves­ti­ga­tors started ex­per­i­men­ta­tion based on it. In Eng­land John Ben­net Lawes and Joseph Henry Gilbert worked in the Rotham­sted Ex­per­i­men­tal Sta­tion, founded by the for­mer, and (re)dis­cov­ered that plants took ni­tro­gen from the soil, and that salts needed to be in an avail­able state to be ab­sorbed by plants. Their in­ves­ti­ga­tions also pro­duced the “su­per­phos­phate“, con­sist­ing in the acid treat­ment of phos­phate rock.^[397] This led to the in­ven­tion and use of salts of potas­sium (K) and ni­tro­gen (N) as fer­til­iz­ers. Am­mo­nia gen­er­ated by the pro­duc­tion of coke was re­cov­ered and used as fertiliser.^[398] 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 still awaited dis­cov­ery.

In 1856 J. Thomas Way dis­cov­ered that am­mo­nia con­tained in fer­tilis­ers was trans­formed into nitrates,^[399] and twenty years later Robert War­ing­ton proved that this trans­for­ma­tion was done by liv­ing organisms.^[400] In 1890 Sergei Wino­grad­sky an­nounced he had found the bac­te­ria re­spon­si­ble for this transformation.^[401]

It was known that cer­tain legumes could take up ni­tro­gen from the air and fix it to the soil but it took the de­vel­op­ment of bac­te­ri­ol­ogy to­wards the end of the 19th cen­tury to lead to an un­der­stand­ing of the role played in ni­tro­gen fix­a­tion by bac­te­ria. The sym­bio­sis of bac­te­ria and legu­mi­nous roots, and the fix­a­tion of ni­tro­gen by the bac­te­ria, were si­mul­ta­ne­ously dis­cov­ered by the Ger­man agron­o­mist Her­mann Hell­riegel and the Dutch mi­cro­bi­ol­o­gist Mar­t­i­nus Bei­jer­inck.^[397]

Crop ro­ta­tion, mech­a­ni­sa­tion, chem­i­cal and nat­ural fer­tilis­ers led to a dou­bling of wheat yields in west­ern Eu­rope be­tween 1800 and 1900.^[402]

Studies of soil formation

The sci­en­tists who stud­ied the soil in con­nec­tion with agri­cul­tural prac­tices had con­sid­ered it mainly as a sta­tic sub­strate. How­ever, soil is the re­sult of evo­lu­tion from more an­cient ge­o­log­i­cal ma­te­ri­als, under the ac­tion of bi­otic and abi­otic (not as­so­ci­ated with life) processes. After stud­ies of the im­prove­ment of the soil com­menced, other re­searchers began to study soil gen­e­sis and as a re­sult also soil types and clas­si­fi­ca­tions.

In 1860, 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.^[403] Un­for­tu­nately his work was not con­tin­ued. At about the same time, Friedrich Al­bert Fal­lou was de­scrib­ing soil pro­files and re­lat­ing soil char­ac­ter­is­tics to their for­ma­tion as part of his pro­fes­sional work eval­u­at­ing for­est and farm land for the prin­ci­pal­ity of Sax­ony. His 1857 book, An­fangsgründe der Bodenkunde (First prin­ci­ples of soil sci­ence) es­tab­lished mod­ern soil science.^[404] Con­tem­po­rary with Fal­lou’s work, and dri­ven by the same need to ac­cu­rately as­sess land for eq­ui­table tax­a­tion, Vasily Dokuchaev led a team of soil sci­en­tists in Rus­sia who con­ducted an ex­ten­sive sur­vey of soils, ob­serv­ing that sim­i­lar basic rocks, cli­mate and veg­e­ta­tion types lead to sim­i­lar soil lay­er­ing and types, and es­tab­lished the con­cepts for soil clas­si­fi­ca­tions. Due to lan­guage bar­ri­ers, the work of this team was not com­mu­ni­cated to west­ern Eu­rope until 1914 through a pub­li­ca­tion in Ger­man by Kon­stan­tin Glinka, a mem­ber of the Russ­ian team.^[405]

Cur­tis F. Mar­but, in­flu­enced by the work of the Russ­ian team, trans­lated Glinka’s pub­li­ca­tion into English,^[406] and as he was placed in charge of the U.S. Na­tional Co­op­er­a­tive Soil Sur­vey, ap­plied it to a na­tional soil clas­si­fi­ca­tion system.^[390]