Soil
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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
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.
All of these functions, in their turn, modify
the soil and its properties.
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.
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]
Functions
Soil functions as 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, with effects ranging from ozone
depletion and global warming to rainforest
destruction and water pollution. With
respect to Earth's carbon cycle, soil acts
as an important carbon reservoir, and it is
potentially one of the most reactive to
human disturbance[15] and climate
change.[16] As the planet warms, it has
been predicted that soils will add carbon
dioxide to the atmosphere due to
increased biological activity at higher
temperatures, a positive feedback
(amplification).[17] This prediction has,
however, been questioned on
consideration of more recent knowledge
on soil carbon turnover.[18]
Soil acts as an engineering medium, a
habitat for 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.[19] 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.[20][21] Soil has
a mean prokaryotic density of roughly 108
organisms per gram,[22] whereas the
ocean has no more than 107 prokaryotic
organisms per milliliter (gram) of
seawater.[23] Organic carbon held in soil is
eventually returned to the atmosphere
through the process 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.[24] 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.[25]
Soils can effectively remove impurities,[26]
kill disease agents,[27] and degrade
contaminants, this latter property being
called natural attenuation.[28] Typically,
soils maintain a net absorption of oxygen
and methane and undergo a net release of
carbon dioxide and nitrous oxide.[29] Soils
offer plants physical support, air, water,
temperature moderation, nutrients, and
protection from toxins.[30] Soils provide
readily available nutrients to plants and
animals by converting dead organic matter
into various nutrient forms.[31]
Composition
Soil profile: Darkened topsoil and reddish subsoil
layers are typical in of humid subtropical climate
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.[32] 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.[33]
The pore space allows for the infiltration
and movement of air and water, both of
which are critical for life existing in soil.[34]
Compaction, a common problem with
soils, reduces this space, preventing air
and water from reaching plant roots and
soil organisms.[35]
Given sufficient time, an undifferentiated
soil will evolve a soil profile which consists
of two or more layers, referred to as soil
horizons. These differ in one or more
properties such as in their texture,
structure, density, porosity, consistency,
temperature, color, and reactivity.[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 soilforming 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.[36]
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.[37] 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.[38] 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.[39]
Soils supply plants with nutrients, most of
which are held in place by particles of clay
and organic matter (colloids)[40] 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.[41][42]
Plant nutrient availability is affected by soil
pH, which is a measure of the hydrogen
ion activity in the soil solution. Soil pH is a
function of many soil forming factors, and
is generally lower (more acid) where
weathering is more advanced.[43]
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,[44] 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.[45]
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
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[46] and analogous conditions in
planet Earth deserts.[47]
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 mineralweathering mycorrhizal fungi[48] 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,[49]
inselbergs,[50] and glacial moraines.[51]
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.[52] When
reordered to climate, relief, organisms,
parent material, and time, they form the
acronym CROPT.[53]
Physical properties
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.[54] 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.[55] Soil bulk density, when
determined at standardized moisture
conditions, is an estimate of soil
compaction.[56] 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.[57] 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.[58]
Soil moisture
It has been suggested that this section be split
out into another article titled Soil moisture.
Learn more
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.[59] 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.[60]
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.[61]
In addition, water alters the soil profile by
dissolving and re-depositing minerals,
often at lower levels.[62] In a loam soil,
solids constitute half the volume, gas onequarter of the volume, and water onequarter of the volume[32] of which only half
will be available to most plants, with a
strong variation according to matric
potential.[63]
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.[64] At
that point, plants must apply suction[64][65]
to draw water from a soil. The water that
plants may draw from the soil is called the
available water.[64][66] 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,[67][64][68]
plants begin to wilt and then die. Water
moves in soil under the influence of
gravity, osmosis and capillarity.[69] 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.[70]
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.[71] When
saturated, the soil may lose nutrients as
the water drains.[72] 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.[73] 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.[74][75] Plant
roots must seek out water and grow
preferentially in moister soil microsites,[76]
but some parts of the root system are also
able to remoisten dry parts of the soil.[77]
Insufficient water will damage the yield of
a crop.[78] Most of the available water is
used in transpiration to pull nutrients into
the plant.[79]
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).[80]
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.[81]
Water retention
…
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.[82] 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.[83] The total amount of water held
when field capacity is reached is a
function of the specific surface area of the
soil particles.[84] As a result, high clay and
high organic soils have higher field
capacities.[85] 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.[86] 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).[87] 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.[88] 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.[89]
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.[71]
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.[85] 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.
Wilting point, field capacity, and available water of various soil textures (unit: % by volume)[90]
Soil Texture
Wilting Point
Field
Available
Capacity
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.[91][92]
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
. Soil temperature. Warm soils take in
water faster while frozen soils may
not be able to absorb depending on
the type of freezing.[93]
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.[94] Water flows
through the ground unevenly, in the form
of so-called "gravity fingers", because of
the surface tension between water
particles.[95][96]
Tree roots, whether living or dead, create
preferential channels for rainwater flow
through soil,[97] magnifying infiltration
rates of water up to 27 times.[98]
Flooding temporarily increases soil
permeability in river beds, helping to
recharge aquifers.[99]
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.[100][101] 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.[102][103] 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.[104]
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.[105]
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[106] 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.[107]
Interestingly, this equation attributed to
Richards was originally published by
Richardson in 1922.[108] The Soil Moisture
Velocity Equation,[109] which can be solved
using the finite water-content vadose zone
flow method,[110][111] 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,[112] 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.[113][114] 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.[115]
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 cohesiontension theory.[116] The upward movement
of water and solutes (hydraulic lift) is
regulated in the roots by the
endodermis[117] and in the plant foliage by
stomatal conductance,[118] and can be
interrupted in root and shoot xylem
vessels by cavitation, also called xylem
embolism.[119] In addition, the high
concentration of salts within plant roots
creates an osmotic pressure gradient that
pushes soil water into the roots.[120]
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.[121][122]
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.[123]
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.[124] 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.[125]
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.[126]
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%.[127]
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.[128] 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.[127][129]
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.[130] 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.[131]
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.[132] Calcareous soils
regulate CO2 concentration by carbonate
buffering, contrary to acid soils in which all
CO2 respired accumulates in the soil pore
system.[133] At extreme levels CO2 is
toxic.[134] This suggests a possible
negative feedback control of soil CO2
concentration through its inhibitory effects
on root and microbial respiration (also
called 'soil respiration').[135] In addition, the
soil voids are saturated with water vapour,
at least until the point of maximal
hygroscopicity, beyond which a vapourpressure deficit occurs in the soil pore
space.[34] 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.[136] 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.[137] 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.[138][137] 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.[139] Aerated soil is also a
net sink of methane CH4[140] but a net
producer of methane (a strong heatabsorbing greenhouse gas) when soils are
depleted of oxygen and subject to elevated
temperatures.[141]
Soil atmosphere is also the seat of
emissions of volatiles other than carbon
and nitrogen oxides from various soil
organisms, e.g. roots,[142] bacteria,[143]
fungi,[144] animals.[145] These volatiles are
used as chemical cues, making soil
atmosphere the seat of interaction
networks[146][147] playing a decisive role in
the stability, dynamics and evolution of
soil ecosystems.[148] 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.[149]
We humans can get some idea of the soil
atmosphere through the well-known 'afterthe-rain' scent, when infiltering rainwater
flushes out the whole soil atmosphere
after a drought period, or when soil is
excavated,[150] a bulk property attributed in
a reductionist manner to particular
biochemical compounds such as petrichor
or geosmin.
Solid phase (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,[151] but the mineralogy
of those particles can strongly modify
those properties. The mineralogy of the
finest soil particles, clay, is especially
important.[152]
Chemistry
It has been suggested that this section be split
out into another article titled ChemistryLearn
of soil.
more
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.[153] 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.[154] 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. Cationexchange 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;
cmolc/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
…
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.[155]
1. Isomorphous substitution occurs in
clay during its formation, when lowervalence cations substitute for highervalence cations in the crystal
structure.[156] 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.[157]
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).[158]
4. Hydrogens of humus hydroxyl groups
may also be ionised into solution,
leaving, similarly to clay, an oxygen
with a negative charge.[159]
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.[160][161]
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+[162]
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).[163]
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.[164] Unlike permanent
charges developed by isomorphous
substitution, pH-dependent charges are
variable and increase with increasing
pH.[42] Freed cations can be made
available to plants but are also prone to be
leached from the soil, possibly making the
soil less fertile.[165] 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.[166]
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.[167] 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.[168]
Live plant roots also have some CEC,
linked to their specific surface area.[169]
Cation exchange capacity for soils; soil textures; soil colloids[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,[171] 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.[172] Phosphates
tend to be held at anion exchange
sites.[173]
Iron and aluminum hydroxide clays are
able to exchange their hydroxide anions
(OH−) for other anions.[174] 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.[170] 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).[175]
Reactivity (pH)
…
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.[176]
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.[177]
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.[178] As a result of a trade-off
between toxicity and requirement most
nutrients are better available to plants at
moderate pH,[179] 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.[180] 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,[181][182]
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.[183]
In high rainfall areas, soils tend to acidify
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.[184] 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.[185] In areas of extreme
rainfall and high temperatures, the clay
and humus may be washed out, further
reducing the buffering capacity of the
soil.[186] In low rainfall areas, unleached
calcium pushes pH to 8.5 and with the
addition of exchangeable sodium, soils
may reach pH 10.[187] Beyond a pH of 9,
plant growth is reduced.[188] High pH
results in low micro-nutrient mobility, but
water-soluble chelates of those nutrients
can correct the deficit.[189] 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.[190][191]
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 baseforming cations is called base saturation.
If a soil has a CEC of 20 meq and 5 meq
are aluminium and hydrogen cations (acidforming), 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 acidforming cations or protons). Base
saturation is almost in direct proportion to
pH (it increases with increasing pH).[192] 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).[193] 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.[194]
Buffering
…
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.[195] 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.[196] More generally,
according to pH levels, several buffer
systems take precedence over each other,
from calcium carbonate buffer range to
iron buffer range.[197]
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.[198]
Nutrients
Plant nutrients, their chemical symbols, and the ionic forms common in soils and available for
plant uptake[199]
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 − (chloride)
Cl
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 nonessential. 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]
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.[210]
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.[211][212] 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.[213] 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.[214]
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.[215] 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.[216]
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.[217]
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 rockygravel soils, have little or no organic
matter. Soils that are all organic matter,
such as peat (histosols), are infertile.[218]
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.[219]
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.[220] Humus also feeds
arthropods, termites and earthworms
which further improve the soil.[221] The end
product, humus, is suspended in colloidal
form in the soil solution and forms a weak
acid that can attack silicate minerals.[222]
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.[223]
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.[224] 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.[216] 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.[223] 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.[225]
Lignin is resistant to breakdown and
accumulates within the soil. It also reacts
with proteins,[226] which further increases
its resistance to decomposition, including
enzymatic decomposition by
microbes.[227] 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.[228]
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.[229] Proteins normally decompose
readily, to the exception of scleroproteins,
but when bound to clay particles they
become more resistant to
decomposition.[230] As for other proteins
clay particles absorb the enzymes exuded
by microbes, decreasing enzyme activity
while protecting extracellular enzymes
from degradation.[231] 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.[232] High soil tannin content can
cause nitrogen to be sequestered as
resistant tannin-protein complexes.[233][234]
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.[235] 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.[218] Humus also absorbs
water, and expands and shrinks between
dry and wet states to a higher extent than
clay, increasing soil porosity.[236] 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.[237]
Charcoal is a source of highly stable
humus, called black carbon,[238] 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.[239]
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[240] or excess
moisture which results in anaerobic
conditions.[241] 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.[242] 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.[243]
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.[244] 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.[244]
Horizons
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.[245]
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,[246] moraines,[247] volcanic
cones[248] sand dunes or alluvial
terraces.[249] 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.[250] 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.[251] 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.[252] 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.[253] Fossil
soil horizons from paleosols can be found
within sedimentary rock sequences,
allowing the study of past
environments.[254]
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.[255]
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.[256] 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.[257] 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[258] and in the
juxtaposition of humified organic matter
and mineral particles, without intimate
mixing, in the underlying mineral
horizons.[259]
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.[260] 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)[261] aims to
establish an international reference base
for soil classification.
Uses
Soil is used in agriculture, where it serves
as the anchor and primary nutrient base
for plants. The types of soil and available
moisture determine the species of plants
that can be cultivated. Agricultural soil
science was the primeval domain of soil
knowledge, long time before the advent of
pedology in the 19th century. However, as
demonstrated by aeroponics, aquaponics
and hydroponics, soil material is not an
absolute essential for agriculture, and
soilless cropping systems have been
claimed as the future of agriculture for an
endless growing mankind.[262]
Soil material is also a critical component
in the mining, construction and landscape
development industries.[263] 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. Loss of soil through urbanization is
growing at a high rate in many areas and
can be critical for the maintenance of
subsistence agriculture.[264]
Soil resources are critical to the
environment, as well as to food and fibre
production, producing 98.8% of food
consumed by humans.[265] Soil provides
minerals and water to plants according to
several processes involved in plant
nutrition. Soil absorbs rainwater and
releases it later, thus preventing floods and
drought, flood regulation being one of the
major ecosystem services provided by
soil.[266] Soil cleans water as it percolates
through it.[267] 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.[268] Above-ground
and below-ground biodiversities are tightly
interconnected,[235][269] 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 in deserts, cyanobacteria, lichens and
mosses form biological soil crusts which
capture and sequester a significant
amount of carbon by photosynthesis. Poor
farming and grazing methods have
degraded soils and released much of this
sequestered carbon to the atmosphere.
Restoring the world's soils could offset the
effect of increases in greenhouse gas
emissions and slow global warming, while
improving crop yields and reducing water
needs.[270][271][272]
Waste management often has a soil
component. Septic drain fields treat septic
tank effluent using aerobic soil processes.
Land application of waste water relies on
soil biology to aerobically treat BOD.
Alternatively, Landfills use soil for daily
cover, isolating waste deposits from the
atmosphere and preventing unpleasant
smells. Composting is now widely used to
treat aerobically solid domestic waste and
dried effluents of settling basins. Although
compost is not soil, biological processes
taking place during composting are similar
to those occurring during decomposition
and humification of soil organic
matter.[273]
Organic soils, especially peat, serve as a
significant fuel and horticultural resource.
Peat soils are also commonly used for the
sake of agriculture in nordic countries,
because peatland sites, when drained,
provide fertile soils for food
production.[274] However, wide areas of
peat production, such as rain-fed
sphagnum bogs, also called blanket bogs
or raised bogs, are now protected because
of their patrimonial interest. As an
example, Flow Country, covering 4,000
square kilometres of rolling expanse of
blanket bogs in Scotland, is now candidate
for being included in the World Heritage
List. Under present-day global warming
peat soils are thought to be involved in a
self-reinforcing (positive feedback)
process of increased emission of
greenhouse gases (methane and carbon
dioxide) and increased temperature,[275] a
contention which is still under debate
when replaced at field scale and including
stimulated plant growth.[276].
Geophagy is the practice of eating soil-like
substances. Both animals and humans
occasionally consume soil for medicinal,
recreational, or religious purposes.[277] It
has been shown that some monkeys
consume soil, together with their preferred
food (tree foliage and fruits), in order to
alleviate tannin toxicity.[278]
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.[279] Soil organisms metabolise
them or immobilise them in their biomass
and necromass,[280] thereby incorporating
them into stable humus.[281] The physical
integrity of soil is also a prerequisite for
avoiding landslides in rugged
landscapes.[282]
Degradation
Land degradation refers to a humaninduced or natural process which impairs
the capacity of land to function.[283] Soil
degradation involves acidification,
contamination, desertification, erosion or
salination.[284]
Soil acidification is beneficial in the case
of alkaline soils, but it degrades land when
it lowers crop productivity, soil biological
activity and increases soil vulnerability to
contamination and erosion. Soils are
initially acid and remain such when their
parent materials are low in basic cations
(calcium, magnesium, potassium and
sodium). On parent materials richer in
weatherable minerals acidification occurs
when basic cations are leached from the
soil profile by rainfall or exported by the
harvesting of forest or agricultural crops.
Soil acidification is accelerated by the use
of acid-forming nitrogenous fertilizers and
by the effects of acid precipitation.
Deforestation is another cause of soil
acidification, mediated by increased
leaching of soil nutrients in the absence of
tree canopies.[285]
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, mainly
through microbial enzymatic activity.[286]
Soil organic matter and soil minerals can
adsorb the waste material and decrease
its toxicity,[287] although when in colloidal
form they may transport the adsorbed
contaminants to subsurface
environments.[288] Many waste treatment
processes rely on this natural
bioremediation capacity. Exceeding
treatment capacity can damage soil biota
and limit soil function. Derelict soils occur
where industrial contamination or other
development activity damages the soil to
such a degree that the land cannot be
used safely or productively. Remediation
of derelict soil uses principles of geology,
physics, chemistry and biology to degrade,
attenuate, isolate or remove soil
contaminants to restore soil functions and
values. Techniques include leaching, air
sparging, soil conditioners,
phytoremediation, bioremediation and
Monitored Natural Attenuation (MNA). An
example of diffuse pollution with
contaminants is copper accumulation in
vineyards and orchards to which
fungicides are repeatedly applied, even in
organic farming.[289]
Desertification
Desertification is an environmental
process of ecosystem degradation in arid
and semi-arid regions, often caused by
badly adapted human activities such as
overgrazing or excess harvesting of
firewood. It is a common misconception
that drought causes desertification.[290]
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.[291]
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.[292] It is now
questioned whether present-day climate
warming will favour or disfavour
desertification, with contradictory reports
about predicted rainfall trends associated
with increased temperature, and strong
discrepancies among regions, even in the
same country.[293]
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 unsuitable land use
practices.[294] 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 and in
erosion-prone places such as steep
slopes.[295] Historically, one of the best
examples of large-scale soil erosion due to
unsuitable land-use practices is wind
erosion (the so-called dust bowl) which
ruined American and Canadian prairies
during the 1930s, when immigrant
farmers, encouraged by the federal
government of both countries, settled and
converted the original shortgrass prairie to
agricultural crops and cattle ranching.
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.[296]
Soil piping is a particular form of soil
erosion that occurs below the soil
surface.[297] 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.[298] The
term sand boil is used to describe the
appearance of the discharging end of an
active soil pipe.[299]
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.[300] 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.[301][302]
Reclamation
Soils which contain high levels of
particular clays with high swelling
properties, such as smectites, are often
very fertile. For example, the smectite-rich
paddy soils of Thailand's Central Plains are
among the most productive in the world.
However, the overuse of mineral nitrogen
fertilizers and pesticides in irrigated
intensive rice production has endangered
these soils, forcing farmers to implement
integrated practices based on Cost
Reduction Operating Principles
(CROP).[303]
Many farmers in tropical areas, however,
struggle to retain organic matter and clay
in the soils they work. In recent years, for
example, productivity has declined and
soil erosion has increased in the low-clay
soils of northern Thailand, following the
abandonment of shifting cultivation for a
more permanent land use.[304] Farmers
initially responded by adding organic
matter and clay from termite mound
material, but this was unsustainable in the
long-term because of rarefaction of
termite mounds.[305] 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.[306]
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.[307]
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.[308]
Studies of soil fertility
…
The Greek historian Xenophon (450–355
BCE) is credited with being the first to
expound upon the merits of greenmanuring crops: "But then whatever weeds
are upon the ground, being turned into
earth, enrich the soil as much as dung."[309]
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.[309][310] 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,[311] 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.[312] 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[313] 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.[314]
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.[315] 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.[316][317] 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.[316][318]
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.[319] 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.[320] 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.[321] 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.[322]
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.[323] 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.[324] 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,[325] and twenty
years later Robert Warington proved that
this transformation was done by living
organisms.[326] In 1890 Sergei
Winogradsky announced he had found the
bacteria responsible for this
transformation.[327]
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.[323]
Crop rotation, mechanisation, chemical
and natural fertilisers led to a doubling of
wheat yields in western Europe between
1800 and 1900.[328]
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.[329] 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.[330] 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.[331]
Curtis F. Marbut, influenced by the work of
the Russian team, translated Glinka's
publication into English,[332] and as he was
placed in charge of the U.S. National
Cooperative Soil Survey, applied it to a
national soil classification system.[316]
See also
Wikimedia Commons has media
related to Soils.
Wikiquote has quotations related to:
Soil
Acid sulfate soil
Agrophysics
Crust
Agricultural science
Factors affecting permeability of soils
Index of soil-related articles
Mycorrhizal fungi and soil carbon
storage
Shrink–swell capacity
Soil moisture velocity equation
Soil zoology
World Soil Museum
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Growth ". In Stefferud (1957).
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Further reading
Soil-Net.com A free schools-age educational
site teaching about soil and its importance.
Adams, J.A. 1986. Dirt. College Station,
Texas: Texas A&M University Press ISBN 0-
89096-301-0
Certini, G., Scalenghe, R. 2006. Soils: Basic
concepts and future challenges. Cambridge
Univ Press, Cambridge.
David R. Montgomery, Dirt: The Erosion of
Civilizations, ISBN 978-0-520-25806-8
Faulkner, Edward H. 1943. Plowman's Folly.
New York, Grosset & Dunlap. ISBN 0-93328051-3
LandIS Free Soilscapes Viewer Free
interactive viewer for the Soils of England
and Wales
Jenny, Hans. 1941. Factors of Soil Formation:
A System of Quantitative Pedology
Logan, W.B. 1995. Dirt: The ecstatic skin of
the earth. ISBN 1-57322-004-3
Mann, Charles C. September 2008. " Our
good earth" National Geographic Magazine
"97 Flood" . USGS. Archived from the
original on 24 June 2008. Retrieved 8 July
2008. Photographs of sand boils.
Soil Survey Division Staff. 1999. Soil survey
manual. Soil Conservation Service. U.S.
Department of Agriculture Handbook 18.
Soil Survey Staff. 1975. Soil Taxonomy: A
basic system of soil classification for making
and interpreting soil surveys. USDA-SCS Agric.
Handb. 436. United States Government
Printing Office, Washington, DC.
Soils (Matching suitable forage species to
soil type) , Oregon State University
Gardiner, Duane T. "Lecture 1 Chapter 1 Why
Study Soils?" . ENV320: Soil Science Lecture
Notes. Texas A&M University-Kingsville.
Archived from the original on 9 February
2018. Retrieved 7 January 2019.
Janick, Jules. 2002. Soil notes , Purdue
University
LandIS Soils Data for England and Wales a
pay source for GIS data on the soils of
England and Wales and soils data source;
they charge a handling fee to researchers.
External links
Wikiversity has learning resources
about Soil Formation
The Wikibook Historical Geology has a
page on the topic of: Soils and
paleosols
Short video explaining soil basics
The Soil Water Compendium (soil water
content sensors explained)
Global Soil Partnership
FAO Soils Portal
World Reference Base for Soil
Resources
ISRIC – World Soil Information (ICSU
World Data Centre for Soils)
World Soil Library and Maps
Wossac the world soil survey archive
and catalogue
Canadian Society of Soil Science
Soil Science Society of America
USDA-NRCS Web Soil Survey
European Soil Portal (wiki)
National Soil Resources Institute UK
Plant and Soil Sciences eLibrary
Copies of the reference 'Soil: The
Yearbook of Agriculture 1957' in multiple
formats
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