Definitions of Soil

8/10/2019 Definitions of Soil

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DEFINITIONS OF SOIL

This definition is from the Soil Science Society of America.

soil- (i) The unconsolidated mineral or organic material on the immediate surface of the earth

that serves as a natural medium for the growth of land plants. (ii) The unconsolidated mineral

or organic matter on the surface of the earth that has been subjected to and shows effects of

genetic and environmental factors of: climate (including water and temperature effects) and

macro- and microorganisms conditioned by relief acting on parent material over a period of

time. A product-soil differs from the material from which it is derived in many physical

chemical biological and morphological properties and characteristics.

This definition is from Soil Ta!onomy second edition.

soil- Soil is a natural body comprised of solids (minerals and organic matter) li"uid and

gases that occurs on the land surface occupies space and is characteri#ed by one or both of

the following: hori#ons or layers that are distinguishable from the initial material as a result

of additions losses transfers and transformations of energy and matter or the ability to

support rooted plants in a natural environment.

The upper limit of soil is the boundary between soil and air shallow water live plants or

plant materials that have not begun to decompose. Areas are not considered to have soil if the

surface is permanently covered by water too deep (typically more than $.% meters) for the

growth of rooted plants.

The lower boundary that separates soil from the nonsoil material is most difficult to define.

Soil consists of hori#ons near the earth&s surface that in contrast to the underlying parent

material have been altered by the interactions of climate relief and living organisms over

time. 'ommonly soil grades at its lower boundary to hard roc or to earthy materials virtually

devoid of animals roots or other mars of biological activity. or purposes of classification

the lower boundary of soil is arbitrarily set at $** cm.


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SOIL FORMING FACTORS

Although many of us don&t thin about the ground beneath us or the soil that we wal on each

day the truth is soil is a very important resource. +rocesses tae place over thousands of years

to create a small amount of soil material. ,nfortunately the most valuable soil is often used

for building purposes or is unprotected and erodes away. To protect this vital natural resourceand to sustain the world&s growing housing and food re"uirements it is important to learn

about soil how soil forms and natural reactions that occur in soil to sustain healthy plant

growth and purify water. Soil is important to the livelihood of plants animals and humans.

owever soil "uality and "uantity can be and is adversely affected by human activity and

misuse of soil.

'ertain soils are best used for growing crops that humans and animals consume and for

building airports cities and roads. ther types of soil have limitations that prevent them from

being built upon and must be left alone. ften these soils provide habitats for living creatures

both in the soil and atop the soil. ne e!ample of soils that have use limitations are those that

hold laes rivers streams and wetlands. umans don&t normally establish their homes in

these places but fish and waterfowl find homes here as do the wildlife that live around these

bodies of water.

/atural processes that occur on the surface of 0arth as well as alterations made to earth

material over long periods of time form thousands of different soil types. 1n the ,nited States

alone there are over %**** different soils2 Specific factors are involved in forming soil and

these factors vary worldwide creating varied soil combinations and soil properties

worldwide:

The Five Soil Forming Factors

3. +arent material: The primary material from which the soil is formed. Soil parent material

could be bedroc or organic material. or a deposit from water wind volcanoes or material

moving down a slope.

$. 'limate: 4eatheringforces such as heat rain ice snow wind sunshine and other

environmental forces brea down parent material and affect how fast or slow soil formation

processes go.

5. rganisms: All plants and animals living in or on the soil (including micro-organisms and

humans2). The amount of water and nutrients plants need affects the way soil forms. The wayhumans use soils affects soil formation. Also animals living in the soil affect decomposition

of waste materials and how soil materials will be moved around in the soil profile. n the soil

surface remains of dead plants and animals are wored by microorganisms and eventually

become organic matter that is incorporated into the soil and enriches the soil.

6. Topography: The location of a soil on a landscape can affect how the climatic processes

impact it. Soils at the bottom of a hill will get more water than soils on the slopes and soils on

the slopes that directly face the sun will be drier than soils on slopes that do not. Also mineral

accumulations plant nutrients type of vegetation vegetation growth erosion and water

drainage are dependent on topographic relief.
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%. Time: All of the above factors assert themselves over time often hundreds or thousands of

years. Soil profiles continually change from wealy developed to well developed over time.

Differences in soil forming factors from one location to another influence the

rocess of soil formation

!arent Materials

Soil forms from different parent materials7 one such parent material is bedroc. As rocs

become e!posed at 0arth&s surface they erode and become chemically altered. The type of soilthat forms depends on the type of rocs available themineralsin rocs and how minerals

react to temperature pressure and erosive forces. Temperatures inside the 0arth are very hot

and melt roc (lithosphere) that moves by tectonic forcesbelow 0arth&s surface. 8elted roc

flows away from the source of heat and eventually cools and hardens. 9uring the cooling

process minerals crystalli#e and new roc types are formed. These types of rocs are called

igneous rocs the original parent material rocs formed on 0arth.

1gneous rocs under the right environmental conditions can change into sedimentary and

metamorphic rocs. olcanoes produce igneous rocs such as granite pumice and obsidian.

Sedimentary rocsare formed when older rocs are broen apart by plant roots ice wedgesand earth movements and become transportedby glaciers waves currents and wind. The

transported particles then become bound together (cemented) as secondary minerals grow in

the spaces between the loose particles and create a new solid sedimentary roc. Sandstone

limestone and shale are types of sedimentary rocs that contain "uart# sand lime and clay

respectively.

8etamorphic;'rystalline rocsform when pressure and temperature below 0arth&s surface

are great enough to change the chemical composition of sedimentary and igneous rocs.

8etamorphic rocs such as "uart#ite marble and slate form under intense temperature and

pressure but were originally "uart# sandstone limestone and shale.
http://soil.gsfc.nasa.gov/soilform/minerals.htmhttp://soil.gsfc.nasa.gov/soilform/minerals.htmhttp://denali.gsfc.nasa.gov/research/lowman/lowman.htmlhttp://geollab.jmu.edu/Fichter/IgnRx/Introigrx.htmlhttp://www.gpc.peachnet.edu/~pgore/geology/geo101/sedrx.htmhttp://soil.gsfc.nasa.gov/soilform/deposits.htm#transport%20agentshttp://www.gpc.peachnet.edu/~pgore/geology/geo101/meta.htmhttp://soil.gsfc.nasa.gov/soilform/minerals.htmhttp://denali.gsfc.nasa.gov/research/lowman/lowman.htmlhttp://geollab.jmu.edu/Fichter/IgnRx/Introigrx.htmlhttp://www.gpc.peachnet.edu/~pgore/geology/geo101/sedrx.htmhttp://soil.gsfc.nasa.gov/soilform/deposits.htm#transport%20agentshttp://www.gpc.peachnet.edu/~pgore/geology/geo101/meta.htm


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A"O#T MINERALS

A mineral is defined as being a naturally occuring element or compound that is formed by

inorganic processes and contains a crystalline structure.+edologists are primarily concernedwith minerals in soil because minerals form the basic framewor of soil. 8inerals originally

form when once-heated 0arth material magma (molten roc) cools and forms solid igneous

roc.9uring the cooling process of magma ions (an atom a group of atoms or compound

that is electrically charged when the loss or gain of electrons occurs) become bonded together

due to electrical attraction. The attracted bonded ions remain fi!ed in position and produce

solid crystalline minerals within igneous roc.The 0arth&s crust formed and continues to form

in this manner.

0arth&s crust contains a combination of naturally occurring elements of which eight elements

are predominant: o!ygen () silicon (Si) aluminum (Al) iron (e) calcium ('a) Sodium

(/a) potassium (


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NAME C$EMICAL FORM#LA

Calcite CaCo)

Dolomite CaMg.CO)/'

G5sum CaSO4

+'$'

-Aatite Ca6.!O4/) + .Cl2 F/

Limonite Fe'+O)+)$'-

$ematite Fe'O)

Gi00site Al'+O)+)$'O

Cla5 Minerals Al silicates


A"O#T 7EAT$ERING

Chemical 8eathering rocesses

'hemical weathering occurs as minerals in rocsare chemically altered and

subse"uently decompose and decay. 1ncreasing precipitation (rain) speeds up the

chemical weathering of minerals in rocs as seen on tombstones and monuments

made of limestone and marble. 1n fact 8ater is an essential factor of chemical

8eathering. 1ncreasing temperature also accelerates the chemical reaction that causes

mineralsto degrade. This is why humid tropical climates have highly weathered

landforms soils and buildings.

Car0onation an1 Solution9this weathering process occurs when precipitation

($*) combines with carbon dio!ide ('$) to form carbonic acid ($'5). 4hen

carbonic acid comes in contact with rocs that contain lime soda and potash the

minerals calcium magenesium and potassium in these rocs chemically change into

carbonates and dissolve in rain water.

$51rol5sis: this chemical weathering process occurs when water ($*) usually

in the form of precipitation disrupts the chemical composition and si#e of a mineraland creates less stable minerals thus less stable rocs that weather more readily.
http://soil.gsfc.nasa.gov/soilform/parmat.htmhttp://soil.gsfc.nasa.gov/soilform/minerals.htmhttp://soil.gsfc.nasa.gov/soilform/minerals.htmhttp://soil.gsfc.nasa.gov/soilform/deposits.htm#depositshttp://soil.gsfc.nasa.gov/soilform/parmat.htmhttp://soil.gsfc.nasa.gov/soilform/minerals.htmhttp://soil.gsfc.nasa.gov/soilform/deposits.htm#deposits


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$51ration9water ($*) combines with compounds in rocs causing a chemical

change in a mineral&s structure but more liely will physically alter a mineral&s grain

surface and edges. A good e!ample of this is the mineral Anhydrite ('aS6).

Anhydrite chemically changes to >ypsum ('aS6-$$*) when water is added.

>ypsum is used in the construction industry to build buildings and houses.

O:i1ation9this process occurs when o!ygen combines with compound elements

in rocs to form o!ides. 4hen an object is chemically altered in this manner it is

weaened and appears as ?o!idi#ed? . A good e!ample of this is a ?rusting? sign post .

The iron in the metal post is o!idi#ing. 1ncreased temperatures and the presence of

precipitation will accelerate the o!idation process.

!h5sical 7eathering !rocesses

=ocs that are broen and degrade by processes other than chemical alteration are

physically or mechanically weathered. A roc broen in to smaller pieces e!poses

more surface area of the original roc. 1ncreasing the e!posed surface area of a roc

will increase its weathering potential.

Animals an1 !lants9Animals burrow into 0arth&s substrate and move roc

fragments and sediment on 0arth&s surface thereby aiding in the disintegration of

rocs and roc fragments. ungi and @ichens are acid-producing microorganisms thatlive on rocs and dissolve nutrients (phosphorus calcium) within rocs. These

microorganisms assist in the breadown and weathering of rocs.

Cr5stalli&ation9As water evaporates moisture from rocs located in arid climates

mineral salts develop from mineral crystals. The crystals grow spreading apart

mineral grains in the process and eventually brea apart rocs.

Temerature ;ariation9minerals in rocs e!pand and contract in climates wheretemperature ranges are e!treme lie in glacial regions of the world or when e!posed

to e!treme heat lie during a forest fire. 'rystal structures of minerals become

stressed during contraction and e!pansion and the mineral crystals separate. or

instance repeated cycles of free#ing and thawing (nown as ree#e-Thaw) of water in

roc cracs further widens cracs and splits rocs apart. rost-wedging forces portions

of roc to split apart.

=ocs that are formed under intense temperature and pressure but cool more slowly and later

in the volcanic magmacooling process are more stable when e!posed to the low temperatures

and pressures at 0arth&s surface. onds holding atoms together determine mineral hardness.

=ocs that have cooled more slowly have time to build stronger bonds so they are more

resistant to the forces of weathering.
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riedrich 8ohs an Austrian mineralogist devised a scale of mineral hardness in 3B3$. e

used ten minerals listed below as standards by which to determine the hardness of minerals

and other objects. These ten minerals were arranged on a scale of increasing hardness. or

instance gypsum can scratch talc and apatite can scratch fluorite calcite gypsum and

talc.Cour fingernail has a general hardness of $.% so you can scratch gypsum and talc2

9iamonds are the hardest mineral in e!istence and are used as cutting instruments.

Mohypsum 'alcite lourite Apatite rthoclase Duart# Topa# 'orundum 9iamond

3 $ 5 6 % E F B G 3*

Since some minerals weather more rapidly than others and weathering processes vary in

intensity and combination weathering products contain different mineral combinations.

+edologistsor soil scientists classify these weathered mineral products as soil separates. Soil

separates range in si#e and are nown as sand silt and clay.

SOIL FORMATION AND CLASSIFICATION

There are more than $** soil types found in 8alaysia. 8ost soils are given a name which

generally comes from the locale where the soil was first mapped. /amed soils are referred to

as soil series.
http://soil.gsfc.nasa.gov/soilform/minerals.htm#Pedologistshttp://soil.gsfc.nasa.gov/soilform/minerals.htm#Pedologistshttp://soil.gsfc.nasa.gov/soilform/deposits.htm#sandhttp://soil.gsfc.nasa.gov/soilform/deposits.htm#silthttp://soil.gsfc.nasa.gov/soilform/deposits.htm#clayhttp://soil.gsfc.nasa.gov/soilform/minerals.htm#Pedologistshttp://soil.gsfc.nasa.gov/soilform/deposits.htm#sandhttp://soil.gsfc.nasa.gov/soilform/deposits.htm#silthttp://soil.gsfc.nasa.gov/soilform/deposits.htm#clay


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Soil survey reports include the soil survey maps and the names and descriptions of the soils in

a report area.

Soils are named and classified on the basis of physical and chemical properties in their

hori#ons (layers). ?Soil Ta!onomy? uses color te!ture structure and other properties of the

surface two meters deep to ey the soil into a classification system to help people use soilinformation. This system also provides a common language for scientists.

Soils and their hori#ons differ from one another depending on how and when they formed.

Soil scientists use five soil factors to e!plain how soils form and to help them predict where

different soils may occur. The scientists also allow for additions and removal of soil material

and for activities and changes within the soil that continue each day.

The soil forming factors are parent material climate topography biological factors and time.

Soil Formation1n the boo by Henny (3G63) &actors of Soil ormation& it was presented an hypothesis that

drew together many of the current ideas on soil formation the inspiration for which was

owned much to the earlier studies of 9ouchaev and the =ussian school. The hypothesis was

that soil is formed as a result of the interaction of many factors the most important of which

are:

'limate (cl)

rganisms (o)

=elief (r)

+arent 8aterial (p)

Time (t)

Henny&s approach was to consider these soil forming factors as control variables independent

of the soil as it evolved and also independent but not necessarily so of each other. e then

attempted to define the relationship between any soil property &s& and the most important soil

forming factors by a function of the form:

sI f (cl o r p t ......) (3)

The dots indicate that factors of lesser importance such as mineral accession from the

atmosphere or fire might need to be taen into account. 0"uation 3 assumes that there is a

causal relationship between sand the soil forming factors. Henny (3GB*) redefined the soil

forming factors as &state& variables and included ecosystem properties vegetation and animal

properties as well as soil properties. +arent material and relief define the initial state for soil

development climate and organisms determine the rate at which chemical and biological

reactions occur in the soil (the pedogenic processes) and time measures the e!tent to which a

reaction will have proceeded. There is a logical progression: of environment (i.e. the soil

forming factors) -J processes -J soil properties underlying the soil formation.


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igure 3. =elationship between soil forming factors processes and soil properties.

To simplify the application of e"uation 3 it has been practice to solve it for changes in a soil

property swhen only one of the control variable (e.g. climate) varies the others beingconstant or nearly so. The relationship is then called a climo-function (climate I control

variable):

sI f (cl) o r p t ... ($)

and the range of soils formed is called a climose"uence. iose"uences topose"uences

lithose"uences and chronose"uences of soils have been recogni#ed in various parts of the

world. The term topo-se"uence is synonymous with 8ilne&s catena concept (8ilne 3G5%).

1ndeed the main virtue of Henny&s attempt to "uantify the relationship between soil propertiesand soil forming factors lies not in the prediction of e!act values of s at a particular site but

rather in identifying trends in properties and soil groups that are associated with readily

observable changes in climate parent material etc.

References9

Henny . 3G63. actors of Soil ormation. 8c>raw-ill /ew Cor.

Henny . 3GB*. The Soil =esource rigin and ehaviour Springer-erlag /ew Cor.

bac to: [Home Page] [Natural Resources]

Climate'limate involves both local (microclimatic) and global (macroclimatic) considerations. The

ey components of climate in soil formation are moisture and temperature. Soil moisture

depends on several factors:

The form and intensity of precipitation (water snow sleet)

1ts seasonal variability

The transpiration and evaporation rate

Slope
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Aspect

9epth of soil profile

Soil te!ture ; permeability of the parent material

A method of defining the soil moisture regime of the soil is using water balance calculations.

Such a calculation is based on the measurement of rainfall distribution a calculation of the

potential evapotranspiration and an assessment of surface runoff and infiltration :

4ater balance e"uation:

1nflow I utflow K;- storage within the system

+ I 0T K S= K 1 K;- S

(5)

where:

P: Precipitation (mm)

ET: Evapotranspiration (mm)

SR: Surface runoff (mm) (may include interflow)

I: Infiltration (mm)

S: Soil moisture storage (mm)

1n the ,.S. precipitation amounts (daily hourly) and intensities (3%-minute data) are collected

at about F*** weather stations. Thepotential evapotranspirationcan be calculated by

empirical e"uations (e.g. Thornthwaite) or by physically-based e"uations (e.g. +enman

8onteith) (8aidment 3GG$). The empirical Thornthwaite e"uation calculates the

evapotranspiration in dependence of air temperature whereas the +enman-8onteith e"uationis the currently most advanced resistance-based model of evapotranspiration. This e"uation

allows the calculation of evapotranspiration form meteorological variables and resistances

which are related to the stomatal and aerodynamic characteristics of the crop. 1nfiltration and

surface runoff can be calculated by empirical e"uations such as the 'urve /umber 8ethod

(Soil 'onservation Service - ,S9A 3GB%). This is a simple method which calculates

infiltration and surface runoff using land use and hydrologic soil groups to derive 'urve

/umbers precipitation amount initial abstraction values and potential ma!imum retention of

a soil. There are many other comple! simulation models such as S4AT (Soil and 4ater

Assessment Tool) (Arnold et al. 3GG5) 40++ (4ater 0rosion +rediction +roject) (,S9A-

A=S 3GG%) or +,S (Smith 3GG$) which calculate infiltration surface runoff and soil

moisture.

The primary topographic attributesslopeand aspecthave a major impact on soil moisture.

This was first e!pressed by even et al. (3GFG) in form of the compound topographic inde!

('T1) (or wetness inde!). 1t describes the effect of topography (slope and aspect) on the

location and si#e of areas of water accumulation in soils. The wetness inde! is calculated by:

wTI ln (A ; tan b) (6)

where:


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wT: Wetness index

: Specific catc!ment area (upslope area draining t!roug! a point(per

unit contour lengt!)

": Slope angle at t!e point

ydrologically the specific catchment area (A) is a measure of surface or shallow subsurface

runoff at a given point on the landscape and it integrates the effects of upslope contributingarea and catchment convergence and divergence on runoff. The wetness inde! reflects the

tendency of water to accumulate at any point in the catchment (in terms of A) and the

tendency for gravitational forces to move that water downslope (e!pressed in terms of tan b as

an appropriate hydraulic gradient). A >1S (geographic information system) can be used to

calculate the wetness inde! based on 908 (digital elevation model) data to derive

information about soil moisture. urthermore the wetness inde! can be used to derive the

#ones of saturation or variable source areas in a study area (catchment) and using a threshold

wetness inde! saturation overland flow is derived . 1n the simulation model T+890@ the

'T1 concept is combined with a storage model (storage for interception infiltration and the

saturated #one) (even et al. 3GB6) . 'are must be taen in applying a static wetness inde! to

predict the distribution of a dynamic process lie soil water content. A dynamic wetness inde!

was developed by arling et al. (3GG6) considering the change of soil moisture (i.e. wetness

inde!) across time.

The depth of soil profilesalso influences the soil moisture content. Thic soil profiles are able

to store large amounts of water. The shallower a soil profile the less water can be stored . Such

soils are prone to low soil moisture contents.

Soil textureinfluences the soil moisture content where assumed the same climatic conditions

sandy soils have the tendency for low soil moisture content silty soils for average soil

moisture contents and clayey soils for high soil moisture contents . This is due to the differentpore space distribution in coarse and fine te!tured soils. Sandy soils have a high amount of

macropores (pore diameter J 3* micrometer ) silty soils a high amount of medium-si#ed

pores (pore diameter *.$ - 3* micrometer) and clayey soils a high amount of fine pores (pore

diameter L *.$ micrometer) .

The termsoil moisturerefers to the presence or absence either of ground water or of water

held at a tension of less than 3%** +a in the soil or in a specific hori#on by periods of the

year. 4ater held at a tension of 3%** +a or more is not available to eep most plants alive. 1n

Table 3. the soil moisture classes as defined in Soil Ta!onomy are listed.

Table 3. A classification of soil moisture regimes.

Soil moisture

regimeCharacteristics

9rySoil moisture content less than the amount retained at 3% atmospheres of tension

(3%** +a - permanent wilting point). &1n most years& - E out of 3* yearsMeric Soils of temperate areas that e!perience moist winters and dry summers (i.e.


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mediterranean climates)

Aridic;Torric Soils are dry more than half the time (in arid climatic #one)

+erudic 1n most years precipitation e!ceeds evapotranspiration every month of the year

,dic 1n most years soils are not dry more than G* consecutive days

,stic

1n most years soils are dry for G* consecutive days and moist in some part for

half the days the soil temperature is above %N' (i.e. during potential growing

season)

A"uicSoils that are sufficiently saturated reducing conditions occur. They usually

have low chroma mottles or have gleyed subsoils

4hen soil moisture is high as in wet or humid climates there is a net downward movement

of water in the soil for most of the year which usually results in greater leaching of soluble

materials sometimes out of the soil entirely and the translocation of clay particles from upper

to lower hori#ons. 1n arid climates there is net upward movement of water in the soil due to

high evapotranspiration rates which results in upward movement of soluble materials (e.g.salts). These accumulated materials can become cemented (-J pans) which are impenetrable

to roots and lower infiltration tremendously.

Temerature

Temperature varies with latitude and altitude and the e!tent of absorption and reflection of

solar radiationby the atmosphere. Solar radiation (direct radiation and diffuse radiation)

increases with elevation differs seasonally and is influenced by cloud cover or other

atmospheric disturbance (e.g. air pollution). The absorption of the solar radiation at the soil

surface is affected by many variables such assoil color vegetation cover,andaspect. 1n

general the darer the soil color the more radiation is absorbed and the lower the albedo. Theeffect of vegetative cover on absorption varies with density height and color of the

vegetation. ence the absorption differs in areas with decidious trees (soil surface is shaded

by trees most of the year) and arable land (soil surface is not shaded throughout the year).

@ight or whitish-colored soil surfaces tend to reflect more radiation. 4hen incoming solar

radiation is reflected there is less net radiation to be absorbed and heat the soil. Snow is

especially effective in reflecting the incoming solar radiation. Soil moisture controls also the

heating up or cooling down of soils. 4ater has a high specific heat capacity (3 cal g-3')

whereas dry soils have a specific heat capacity of about *.$ cal g -3'. This means that sandy

soils cools and heats more rapidly than soils high in silt or clay. nce a wet soil is warmed it

taes longer to cool than a dry soil. 1n the /orthern emisphere south-facing slopes tend to

be warmer and thus more droughty than north-facing slopes.Temperature affects the rate of mineral weathering and synthesis and the biological processes

of growth and decomposition. 4eathering is intensified by high temperatures hence

weathering is stronger in the tropics than in humid regions. Temperature also influences the

degree of thawing and free#ing (physical weathering) in cold regions. iological processes are

intensified by rising temperatures. =eaction rates are roughly doubled for each 3*N' rise in

temperature although en#yme-cataly#ed reactions are sensitive to high temperatures and

usually attain a ma!imum between 5* and 5% N'.

rom 9ouchaev on (about 3BF*) many pedologist in 0urope and /orth America regarded

climate as predominant in soil formation. The relationship between climatic #ones and broad

belts of similar soils that stretched roughly east-west across =ussia inspired thezonal conceptof soils.Oonal soils are those in which the climatic factor acting on the soil for a sufficient


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length of time is so strong as to override the influence of any other factor.Intrazonalsoils are

those in which some local anomaly of relief parent material or vegetation is sufficiently

strong to modify the influence of the regional climate.zonal or immature soils have poorly

differentiated profiles either because of their youth or because some factor of the parent

material or environment has arrested their development. 1n the ,.S. the #onal concept was

used in soil classification as published in the ,S9A Cearboo of Agriculture (aldwin et al.3G5B).

Table $. Soil classification in 3G5B ,S9A Cearboo of Agriculture (highest two categories

only)

Categor5 3 +

Or1er Categor5 6 + Su0or1er

!onal soils

+edocals (soils with

calcium carbonate

accumulation)

+edalfers (soils with

iron and aluminum

accumulation)

Soils of the cold #one:

3. @ight-colored soils or arid regions

$. 9ar-colored soils of the semiarid arid subhumid

and humid grasslands

5. Soils of forest-grassland transitions

6. @ight -colored pod#oli#ed soils of the timbered

regions

%. @ateritic soils of forested warm-temperate and

tropical regions

Intrazonal soils

3. alomorphic (saline and alali) soils of imperfectly

drained arid regions and littoral deposits

$. ydromorphic soils of marshes swamps see areas

and flats5. 'alomorphic

zonal /o suborders

The concept of soil #onality is not very helpful when applied to soils of the subtropics and

tropics. There were land surfaces are generally much older than in 0urope and have

conse"uently undergone many cycles of erosion and deposition associated with climatic

change the age of the soil and its topographical relation to other soils in the landscape are

factors of major importance. The #onal concept is also of little use in regions such as

Scandinavia or the /orthern ,.S. where much of the parent material of present-day soils is

young (+leistocene deposits) and relief plays a powerful role in soil formation.

Table 5. 9efinitions and features of soil temperature regimes.

Temerature

regime

Mean annual

temerature in

root &one =1egree

C>

Characteristics an1 some locations

Pergelic " # +ermafrost (i.e. the depth of free#ing in winter e!ceed thedepth of thawing in summer as a conse"uence a layer of


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permanently fro#en soil of grounds develop) and ice

edges common. Tundra of northern Alasa and 'anada

and high elevations of the =ocy 8ountains.

$r%ic # &''ool to cold soils of the /orthern >reat +lains of the

,.S. forested regions of eastern 'anada.

(rigid " '

A soil with a frigid regime is warmer in summer than a

soil with cryic regime. The difference between mean

summer and mean winter soil temperatures is more that %o

'.

)esic

' & *+

8idwestern and >reat +lains regions where corn and

winter wheat are common crops.

hermic

*+ & --

'oastal +lain of southeastern ,.S. where temperatures are

warm enough for cotton.

H%pothermic

--

'itrus areas of lorida peninsula southern 'alifornia.

Tropical climates and crops.

1f the name of a soil temperature regime has the prefi! iso the mean summer and mean winter

soil temperatures for Hune Huly and August and for 9ecember Hanuary and ebruary differ

less than %o' at a depth of %* cm.

Temerature regime Mean annual soil temerature =1egree C>

Isofrigid L B

Isomesic JI B - 3%

Isothermic JI 3% - $$Isoh%perthermic JI $$

References9

Arnold H.>. Allen +.8. ernhardt >. 3GG5. A 'omprehensive Surface->roundwater low

8odel. Hournal of ydrology 36$: 6F-EG.

aldwin 8. ovt. +rinting ffice 4ashington 9': GFG-3**3.

arling =.9. 8oore 1.9. and >rayson =.. 3GG6. A Duasi-dynamic 4etness 1nde! for'haracteri#ing the Spatial 9istribution of Oones of Surface Saturation and Soil 4ater

'ontent. 4ater =esources =esearch 5* (6): 3*$G-3*66.

even raw-ill 1nc. /ew Cor.


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Smith =.0. 3GG$. +,S: An 1ntegrated Simulation 8odel for Transport of /onpoint-Source

pollutants at the ield Scale. ol 1. 9ocumentation ,S9A-A=S-GB.

Soil 'onservation Service - ,.S. 9epartment of Agriculture. 3GB%. /ational 0ngineering

andboo sec. 6 ydrology ,.S. >overnment +rinting ffice 4ashington 9.'.

,S9A-A=S. 3GG%. 4ater 0rosion +rediction +roject (40++). /S0=@ =eport /o. 33.


Organisms

The soil and the organisms living on and in it comprise an ecosystem. The active components

of the soil ecosystem are the vegetation fauna including microorganisms and man.

/egetation: The primary succession of plants that coloni#e a weathering roc culminates inthe development of a clima! community the species composition of which depends on the

climate and parent material but which in turn has a profound influence on the soil that is

formed. or e!ample in the 8id-4est of the ,.S. deciduous forest seems to accelerate soil

formation compared to grassland on the same parent material under similar climatic

conditions. 9ifferences in the chemical composition of leaf leachates can partly account for a

divergent pattern of soil formation. or e!ample acid litter of pines or heather favors the

development of acid soils with poor soil structure whereas litter of decidious trees favors the

development of well structured soils.

)eso&0)acrofauna: 0arthworms are the most important of the soil forming fauna in

temperate regions being supported to a variable e!tent by the small arthropods and the largerburrowing animals (rabbits moles). 0arthworms are also important in tropical soils but in

general the activities of termites ants and beetles are of greater significance particularly in

the subhumid to semiarid savanna of Africa and Asia. 0arthworms build up a stone-free layer

at the soil surface as well as intimately mi!ing the litter with fine mineral particles they have

ingested. The surface area of the organic matter that is accessible to microbial attac is then

much greater. Types of the mesofauna comprise arthropods (e.g. mites collembola) and

annelids (e.g earthworms enchytraeids).

Table 6. 0arthworm biomass in soils under different land use (4hite 3GBF)

Earth8orm 0iomass =?g@ha>

ardwood and mi!ed woodland: 5F* - EB*

'oniferous forest: %* - 3F*

+asture: %** - 3%**

Arable land: 3E - FE*
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)icroorganisms:The organic matter of the soil is coloni#ed by a variety of soil organisms

most importantly the microorganisms which derive energy for growth from the o!idative

decomposition of comple! organic molecules. 9uring decomposition essential elements are

converted form organic combination to simple inorganic forms (minerali#ation). 8ost of the

microorganisms are concentrated in the top 3% - $% cm of the soil because ' substrates are

more plentiful there. 0stimates of microbial biomass ' range from %** to $*** g ;ha to 3%-cm depth (4hite 3GBF). Types of microorganisms comprise bacteria actinomycetes fungi

algae proto#oa and soil en#ymes.

)an1 )an influences soil formation through his impact to the natural vegetation, i.e., his

agricultural practices, ur2an and industrial development. Heav% machiner% compacts soils

and decreases the rate of 3ater infiltration into the soil, there2% increasing surface runoff

and erosion. 4and use and site specific management 5e.g. application of fertilizer, lime6 also

act on soil development.

Reference

4hite =.0. 3GBF. 1ntroduction to the +rinciples and +ractices of Soil Science. lacwellScientific +ubl. +alo Alto 'A.


Relief8ajor topographical features are easily recogni#ed in the field (e.g. mountains valleys

ridges crests sins plateau floodplains). or detailed description of topography7igital

8levation )odels(908s) are used. 1n a 908 each pi!el of a landscape is described by a data

triplet consisting of 0asting /orthing and the elevation.

908s are available in different "uality. 0!amples for 908s are given in the following:

,.S. >eological Survey (,S>S) topographic F.% ! F.%-min blocs 908 (e"uivalent to a scale

3:$6***): ori#ontal resolution I 5* m root mean s"uare error is generally I;- F m.

,.S. >eological Survey 908: 3o ! 3oblocs representing one-half of the standard 3:$%****

scale 3o! $o"uadrangle maps: ori#ontal resolution I G* m in the north-south direction and

about E* m in the east-west direction accuracy for flat terrain K;- 3% m and for steep terrain

E* m.

igh "uality 908s (e.g. hori#ontal resolutions of % - 3* m). An e!ample for a high "uality

908 is shown in igure $.

igure $. 908 for an e!perimental site at Arlington Agricultural =esearch Station in

Southern 4isconsin.
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18/21

igure 6. 9ifferent shapes of a hillslope.

ased on topographic attributes landform elements can be delineated. 0!amples are given by

uggett (3GF%) +ennoc et al. (3GBF) and 1rvin (3GGE). They related landform elements to

soil properties and hydrologic characteristics which also influence soil genesis. uggett

combined vertical and hori#ontal slope curvatures (slope shapes) to predict soil drainage

classes. e states that in general hydraulic conductivity decreases with depth. Thus material

and soil solution throughflow vary with both profile (downslope) and plan (across-slope)

curvature. The water flu! contains dissolved and suspended materials which it moves fromthe upper reaches of the valley basin to lower parts. This movement may result in eluviation

in the upper #one of the basin and illuviation along the lower reaches. +ennoc et al. (3GBF)

used combinations of gradient plan and profile curvature to define distinct landform

elements which were related to soil moisture. The result of their study indicate that moisture

content relates to elements in the se"uence shoulder L bacslopes L footslopes and divergent

elements L convergent elements. 1rvin (3GGE) related landform elements to soil properties

(e.g. silt depth) in the 9riftless Area of Southern 4isconsin.

>enerally an increase in slope is associated with a reduction in:

@eaching

rganic matter content

'lay translocation

8ineral weathering

ori#on differentiation

Solum thicness

Topographic attributes and vegetation cover affect soil moisture by governing the proportions

of surface runoff to infiltration. Soil with impermeable sub-soils and those developing onslopes may show appreciable lateral subsurface flow. Thus at the top of the slope the soils

tend to be freely drained with the water table at considerable depth whereas the soils at the

bacslopes and footslopes are poorly drained with the water table near or at the soil surface.

The succession of soils forming under different drainage conditions on relatively uniform

parent material comprises a hydrological se"uence an e!ample of which is shown in igure %.

As the drainability deteriorates the o!idi#ed soil profile with its orange-red colors due to

ferric o!ides is transformed into the mottled and gleyed profile of a reduced soil (soil color:

gray green).

The importance of relief was highlighted by 8ilne (3G5%) who recogni#ed a recurring

se"uence of soil forming on slopes in a generally undulating landscape. e introduced the
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term catena (@at. &chain&) to describe a se"uence of contiguous soils e!tending from hill to top

of a hillslope.

igure %. ydrological se"uence of soils formed under major influence of relief: Soil profile

P3 is well drained (summit) P$ moderately well drained (bacslope) P5 poorly drained

(footslope) and P6 very poorly drained (toeslope).

0ach hillslope with a slope gradient is subdued to transport of soil particles. 0rosion tends to

be higher on conve! sites with steep slopes compared to concave sites with low gradient. Thesoils at shoulders tend to be more shallow due to erosion whereas the soils on footslope and

toeslope areas tend to be thicer due to deposition. As erosion reduces the thicness of the A

hori#on the upper part of the hori#on becomes incorporated into the lower part of the A

hori#on and the upper part of the ' hori#on becomes incorporated into the lower part of the

hori#on. The sediment transport is different for each soil particle si#e. The transport of coarse-

si#ed particles (sand) is lowest whereas the transport of fine soil particles (clay) and medium-

si#ed particles (silt) is higher. 'lay particles form aggregates with organic material and iron

and aluminum o!ides hence those aggregates are very stable and are less susceptible to

sediment detachment. 'oarse-si#ed particles are heavy and therefore also difficult to detach.

8edium-si#ed particles (silt) are prone to erosion. 1f erosion occurs on a hillslope the silt

content often is higher in the bottom soils compared to soils on the hillslope shoulder.

1ncreasing the slope length allows water which ran off the upper part of the slope to infiltrate

in the lower part of the slope and to deposit eroded material carried in suspension.

=elief has also an important influence on the local climate and the vegetation. 'hanges in

elevation affect the temperature (a decrease of appro!imately *.% degree ' per 3** m increase

in height) the amount and form of the precipitation and the intensity of storm events. thus

affecting soil moisture relations. These factors interact to influence the type of vegetation.

References

>allant H.'. and H.+. 4ilson. 3GGE. TA+0S->: A >rid-based Terrain Analysis +rogram for the0nvironemental Sciences. 'omputers Q >eosciences $$ (F): F35-F$$.

uggett =.H. 3GF%. Soil @andscape Systems: A 8odel of Soil >enesis. >eoderma 35: 3-$$.

1rvin .H. 3GGE: Spatial 1nformation Tools for 9elineating @andform 0lements to Support

Soil;@andscape Analysis. +h9 Thesis ,niversity of 4isconsin-8adison.

8ilne >. 3G5%. Some Suggested ,nits of 'lassification and 8apping +articularly for 0ast

African Soils. Soil =esearch 6: 3B5-3GB.

+ennoc 9.H. .H. Oebarth and 0. de Hong. 3GBF. @andform 'lassification and Soil9istribution in ummocy Terrain Sasatchewan 'anada. >eoderma 6*: $GF-53%.
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!arent Material

The nature of the parent material has a decisive effect on the properties of soils. +roperties ofthe parent material that e!ert a profound influence on soil development include te!ture

mineralogical composition and degree of stratification. Soil may form directly by the

weathering of consolidated roc in situ(a residual soil) saprolite (weathered roc) or it may

develop on superficial deposits which may have been transported by ice water wind or

gravity. These deposits originated ultimately from the denudation and geologic erosion of

consolidated roc. 'onsolidated material is not strictly parent material but serves as a source

of parent material after some physical and ;or chemical 3eatheringhas taen place. Soils may

form also on organic sediments (peat muc) or salts (evaporites). The chemical and

mineralogical compositions of parent material determine the effectiveness of the weathering

forces. 9uring the early stages of soil formation roc disintegration may limit the rate and

depth of soil development. The downward movement of water is controlled largely by thete!ture of the parent material. urthermore parent material has a mared influence on the

type of clay minerals in the soil profile.

igure E. edroc geology of 4isconsin.


TimeTime acts on soil formation in two ways:

The value of a soil forming factor may change with time (e.g. climatic change new parent

material).

The e!tent of a pedogenetic reaction depends on the time for which it has operated.

)onogenetic soils are those that have formed under one set of factor values for a certain

period of time. Soil that have formed under more than one set of factor values are called

pol%genetic.

ery old soils are formed on weathered consolidated rocs (e.g. granite basalt) where the

rocs were formed more than %** million years ago (+aleo#oium). 1n Africa or Australia

such old soils may be found.

The climate has changed over geological time the most recent large changes were associated

with alternating glacial and interglacial periods of the +leistocene. 0urope and /orth America

sustained four distinct ice invasions whereas each glacial period was separated by long

interglacial ice-free intervals. Those were times of warm or semitropical climate. The totallength of the +leistocene ice age is estimated 3 - 3.% million years. The glaciers disappeared
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from /orthern America appro!imately 3$*** years ago. As the glacial ice was pushed

forward soil was swept away hills were rounded valleys filled and the underlying rocs were

severely ground and gouged. inally when the ice melted a mantle of glacial drift remained a

new regolith and fresh parent material for soil formation. The influence of parent material is

much more apparent in the soils of glaciated regions where insufficient time has elapsed sine

the ice retreated to permit the full development of soils.

ne of our youngest soils are formed on alluvial or lacustrine materials generally have not

had as much time to develop as the surrounding upland soils. Coung in age are also colluvial

soils where sediment transport occurred recently.

1n igure F a hypothetical soil development across time is shown. The parent material might

be relatively unweathered bedroc. After weathering of bedroc and the accumulation of

organic matter at the soil surface there occurs the development of an A hori#on due to

processes such as decomposition and minerali#ation. After an A hori#on is formed slowly a

ori#on is developed due to the formation of clay minerals (denoted by a lower case & t&). 1n a

humid environment such as the central ,nited States material from the upper part of the soilprofile is leached downwards (e.g. clays organic material) and an eluvial hori#on (0 hori#on)

is formed. The accumulated material is precipitated in a hori#on below the 0 the so-called

illuvial hori#on in this case the t hori#on.

igure F. Stages of development of soils across time for a soil in the central ,nited States

under forest.
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Documents

Definitions of Soil