eluviation and illuviation soil (2).ppt

Eluviation and illuviation ppt for geographers

1. Soil forming factors and
processes

2. objectives
•By the end of this section you should be able to:
–Distinguish between soil-forming processes and factors
–Discuss the parameters influencing weathering
–Differentiate between eluviation and illuviation
–Describe four composite soil forming processes
–Compare processes occurring in soils in humid and dry
climates
–List Jenny’s five soil forming factors
–Give examples of soil formation as affected by parent
material, climate, relief, time and biological activity
–Explain what a catena is.

3. Processes in soil formation
•The processes of soil formation are those which alter
the regolith and give it the acquired characteristics that
differentiate the soil from its original parent material.
•Processes, thus, result in horizon formation that is
typical for that particular process.
•There are many different types of processes.
•There may occur together or separately.
•Most different soil types are produced from a
combination of processes of weathering, addition of
organic matter, transportation of soil constituents and
accumulation of soil constituents.

4. weathering
•Weathering proceeds first with physical
disruption of rocks/particles and this
breakdown then facilitates chemical
weathering.
•Weathering results in reduction in particle size
and changes in mineralogy.
•Weathering is influenced by many parameter.

5. Parameters of weathering
•Mineralogy: some minerals are more stable and
resistant to weathering while others break down
quickly.
–In the fine earth fraction (<2mm) the minerals
undergo stages in weathering, which involves a
change of chemical composition (Figure 3.1)
–The end products in highly weathered soils are inert
clays such as kaolinite, gibbsite, hermatite and
geothite.
–These are the most stable minerals that remain after
the other less stable ones are transformed or broken
down.

6. •Water: the presence of water enhances soil formation,
through its effect on mineral weathering, production
of organic matter and the movement of soil particles
and chemicals within the soil
–Water dissolves products and causes hydrolysis, hydration
and dissolution reactions.
•Temperature: mechanical breakdown of rocks
(freezing- thawing, or expansion- contraction changes
induced by temperature variations)
–Its effect of speeding up chemical reactions
–Increase in temperature increases chemical reactions thus
promote decomposition of minerals.

7. •Oxidation and reduction: weathering is also
affected by the amount of oxygen in the zone
and the type of minerals present.
•Oxidation or reduction reactions are
particularly manifest in soils rich in iron as iron
is an element that easily undergoes change in
oxidation status.
–If ferrous iron is (Fe2+) is oxidized to ferric state,
this results in a less stable mineral, which is then
more prone to weathering

8. •In conclusion: weathering produces a soil
which consists of a mixture of minerals at
various stages of decomposition.
•Weathering depends on the original mineral
compositions, their resistances to weathering,
the amount of water present, the
temperature and the oxidation or reducing
conditions.

9. Leaching
•Leaching is a process by which soluble constituents are
removed from the soil. When excess water moves through
the soil, it will carry salts with it.
•The major effect of leaching is to make soil more acidic and
to reduce the amount of bases.
•The bases are primarily Ca, Mg, K and Na.
•After prolonged leaching, only quartz, kaolinite, iron (III)
oxides and aluminium oxides remain in the soil.
•Plants may reverse the effects of leaching as they take up
water, thereby reducing deep drainage.
•Plants also tend to take up nutrients (salts) from the
surface and these may then be returned to the surface as
plant litter.

10. Eluviation
•Eluviation refers to the loss in colloidal
suspension, of mineral from the upper soil
horizons.
•This includes clays, sesquioxides (iron/aluminium
oxides) and colloidal humus.
•Elluviation is a mechanical loss as opposed to a
chemical loss that occurs in leaching (Figure 3.2)
•If this occurs to a larger extent, an E horizon
(albic or eluvial) might be produced.
•An E horizon is typically sandy, pale coloured and
very acidic.

11. illuviation
•Illuviation is the deposition of eluviated
materials, in the B horizon.
•The deposited clay may form clay skins around
particles/ agrregates
•Illuviation may lead to formation of clay rich
zones called argillic (Bt) horizons.

12. Organic matter accumulation
•When OM falls on to the soil, it can be broken
down to form humus (humification) or it can
accumulate to form litter layer.
•If the organic matter is acidic, it can promote
podzolisation.
•OM darkens the soil colour, especially of the
topsoil (horizon). In conditions of poor
drainage, peat formation is encouraged.

13. Gleying
•The presence of water for long periods in the soil, brings about
anaerobic conditions (hydromophy)
•Under these conditions of low oxygen content, bacteria in the soil
reduce various compounds including iron.
•The iron is changed from the iron (III) Fe3+ (responsible for
formation of red coloured compounds) to iron (II) Fe2+.
•The iron is much more soluble in this form and may be removed by
draining water, leaving the soil grey coloured or gleyed
•If there is a fluctuating water table, the iron (II) compounds may be
re-oxidised when the water table drops, forming mottles of rust
coloured iron (III) compounds.
•Gleyed soils are very common in vlei soils and dambos
•If the gley soil has a prolonged exposure to air (dries out) then the
grey layer with mottles hardens irreversibly into a layer called
laterite (indurated ironstone)

14. Ferralization
•This process is characteristic of hot, tropical
climates and only occurs after extreme
weathering and leaching have occurred in soils.
•This process in the past was referred to as
laterization or kaolinization.
•It involves the relative accumulation of iron and
aluminium oxides and hydroxides (sesquioxides)
with the loss of silica. The soils produced are
acidic with a characteristic red colour due their
high sesquioxide content.

15. Podzolisation
•Podzolisation is a combination of soil processes that
occur in cool humid parts of the world such as Russia
and Canada, often where siliceous parent material
(lots of silica/quartz) is covered by coniferous forests.
•The conifers produce a very acidic leaf litter.
•As rain falls on the litter and then moves into the soil,
it facilitates breakdown of compounds.
•OM and iron are mobilized and removed from the
surface soils.
•The removal of iron is enhanced by the formation of
soluble organo- metal complexes called chelates

16. •After some time, mainly silica (quartz) remains, giving a
charateristic bleached, acidic E (albic) horizon, below the
surface.
•The organic substances and iron, may be deposited lower
in the illuvial horizon (Bh) as water movement slows down.
•The soil is strongly acidic and there is little earthworm or
soil fauna activity. As a result the horizons are not mixed
and stay very distinctly separate.
•This profile that develops from these processes, is called a
podzol, which consists of a humic layer overlying an albic
horizon which further overlies a Bs horizon

17. salinization
•This is a combination of processes where
there is accumulation of salts in soils usually
due to weathering, low leaching, and high
evaporation rates.
•High groundwaters can also give rise to saline
soils.
•Saline soils often have white crusts or white
concretions in the soil due to the presence of
these salts.

18. Overview of factors affecting soil
formation
•In 1941 H. Jenny, in the USA, produced a book which
hypothesized that soils are formed as a result of the
interaction of many factors, mainly climate (cl), parent
material (p), time (t), relief or topography (r), and
organisms (o).
•Soil (s) is therefore a function of the above factors or: s= f
(cl, p, t, r, o) also referred to as Jenney’s five soil forming
factors.
•The factors can be studies by holding 4 factors constant
and varying only one. E.g. if climate alone is varied, then
the range of soils formed is a climosequence.
•Similarly, one can define a toposequence (vary r) or
chronosequence (vary t).

19. climate
•Climate is one of the many influential factors
in soil formation because it determines the
many of the processes in soils.
•The key components of climate that affect soil
formation are moisture and temperature
(white 1989)
•These components influence on the type of
vegetation at any given site and this in turn
influences the organic status of the soil.

20. Effect of moisture
•The actual amount entering and remaining within the
soil depends on:
–The form and intensity of precipitation (snow, rain,hail)
–The seasonal variability of precipitation
–The evaporation rate from the soil and plants
–The amount of runoff
–The permeability of the soil/ parent material
•In arid regions, any water that enters the soil is quickly
lost by evaporation.
•Some weathering may occur but the soil is too dry
much of the time for weathering to take place.

21. •The limited water movement in the soil also prohibits
the removal of soluble products from the soil, through
the process of leaching
•The result is that soils may tend to be shallow due to
limited weathering
•They may also contain large amounts of salts and are
therefore saline.
•Soils of humid regions may have enough water present
all the time for weathering and leaching to take place.
•In general, an increase in rainfall is associated with an
increase in weathering and leaching.

22. •In hot, humid regions, soils are often very deep due to
intensive weathering and leaching.
•As weathering progresses, the primary minerals break
down and clays (secondary minerals) are synthesised from
the breakdown products.
•Moisture status also has influence on vegetation.
•Higher rainfall encourages greater plant growth and
favours accumulation of OM in soils
•In contrast, lower rainfall areas are likely to have less OM
due to poor vegetation growth.
•Temperature also affects the amount of OM in the soil.
Higher temperatures tend to favour more production of
vegetation which adds OM to the soil.

23. •However, increased temperatures also increase rate of
decomposition, which breaks down the OM more
quickly.
•On balance though, many humid tropical soils have
greater OM contents than humid temperate soils, as the
annual production of OM is so great in the tropics.
•Interestingly, OM content of tundra soils are often high.
•Under tundra conditions, precipitation and temperatures
are low with meagre biomass production.
•However, decomposition is so slow that OM
accumulates over a long time in these soils.

24. Parent material
•PM is the building block on which the other factors act.
•PM may consist of consolidated material (rocks) or unconsolidated
material such as alluvium or fresh volcanic ash.
•Usually physical weathering takes place before chemical
weathering occurs
•Weathering can occur in situ which means that the weathering and
soil formation occurs in and on top of the PM.
• However soils can also be formed from transported material,
meaning that they are weathered at one site then transported and
deposited on another site.
•They, therefore overlay PM that is not necessarily the PM from
which they were originally derived
•PM ranges from igneous, metamorphic, and sedimentary rocks to
unconsollidated deposits formed by wind, water, glaciers or gravity.

25. Types of PM
•Igneous rocks: these are formed by solidification of
magma in the earth’s crust.
–They are divided broadly into light coloured, acidic,
rocks (with relatively high quartz contents e.g. granite)
and darker coloured, basic, rocks (which are lower I
quartz but rich in ferromagnesian minerals e.g basalt).
–Granit rocks which are rich in quartz tend to weather to
produce soils which are sandy and high in quartz, as
quartz is resistant to weathering.
–Granite soils cover most of the central and south-
western Zim

26. •In contrast, basic rocks such as basalt, which
contain more calcium and magnesium, produce
clayey soils with very low sand contents.
•These can typically be seen in south-eastern Zim
(Chisumbanje) where basalts have produced
black clay soils.
•Ultrabasic rocks such as serpentines, which
contain large quantities of ferromagnesium
minerals, may weather to produce magnesium-
rich, clay soils such as those found along Great
Dyke in Zim

27. •Sedimentary rocks: these are composed of the
weathering products of igneous and metamorphic
rocks and are formed after deposition from wind and
water.
–Usually the sediments are deposited in layers and are the
subjected to consolidation and hardening. Which leads to
the formation of the sedimentary rocks.
–These rocks can also be subjected to folding, faulting and
tilting.
–The size of fragments making up sedimentary rocks
decrease in the order from conglomerates (very coarse),
sandstones (coarse), siltstones (medium) to mudstone
(fine).

28. •These rocks give rise to different soils depending on
their mineralogy and fragment sizes. Sandstones
generally give rise to sandy soils while mudstone
usually produce clay soils.
•Other sedimentary rocks are formed from precipitation
of calcium compounds which give rise to limestone
and chalks. These weather to produce calcium-rich
(calcareous) soils.
•Extensive areas of sedimentary rocks occur in the
Zambezi Valley and Escarpment area in the nortwest of
the country ranging from grits, sandstones and
siltstones to mudstones.

29. •Metamorphic rocks: when igneous or
sedimentary rocks are subjected to intense heat
and great pressure, they are transformed into
metamorphic rocks.
–Examples are gneiss, slate and schist. The minerals
usually undergo transformation during the heating
and pressurization and generally change into stable
compounds that may be resistant to weathering.
–The type of minerals and their size in the rock again
influences the soils developed from the rock. So
gneiss weathers to produce sandy soils while slates
weather to produce clay soils.

30. •Water deposited material: transport of particles
by water followed by deposition, produces
alluvial soils.
–Example of these soils can be seen on flood plains of
rivers.
–During transport, the particles may be abraded and
sorted by density.
–Typically, alluvial soils smooth rounded particles.
–These soils frequently have distinct layers where
particles have been sorted due to speed of water flow
or deposited during successive floods.

31. •Glacially deposited material: deposits of materials are
formed as glaciers move and melt. These deposits are
common in Asia, North America and Europe and are called
morraines of tills.
•Wind deposited material: wind moves particles by rolling,
saltation and suspension processes. These wind blown
particles may then be deposited when the wind velocity is
slowed and they form aeolian deposits. Eg Kalahari sands in
the west of the country.
•Gravity deposited material: loose rocks and stones formed
by weathering of exposed rocks will move downslope
under the influence of gravity. These are called colluvial
deposits.

32. Time
•The length of time during which materials have been subjected to
weathering plays a significant role in soil formation.
•If the PM is hard and resistant (e.g. Granite), then the soil may take
many thousand of years to develop.
•Young soils retain many of the features of the PM. As they become
older, they acquire other features such as OM and increasing
development and distinctness of horizons.
•a chronosequence of soil development is shown in Figure 3.4
•In general, soils take many thousand of years to develop. The work
of Owens and Watson (1979) on rates of soil formation in
Zimbabwe, indicated that 11.0mm and 4.1 mm of soil were
produced per thousand years, under moderate temperature and
rainfall conditions

33. Topography or relief
•Topography modifies soil profile development in three ways :
–By influencing rainfall absorbed, and therefore affecting the
amount of moisture in the soil.
–By influencing the rate of soil erosion
–By influencing the amount of subsurface water movement
which will transport soil materials from one place to another.
•The main influence is that on the movement and distribution of
water within and over the soils
•As steepness of slope increases, there is greater runoff and
erosion and less water enters the soil to be available for
chemical and biological activity, or weathering.

34. •Therefore soils on steep slopes tend to be shallow and poorly formed.
•The role of topography is closely linked with catena concept.
•The concept of the catena was first proposed by Milne (1935) to
describe a toposequence in East Africa.
•Catena is defined as a sequence of soils derived from the same PM but
differing in properties because they occupy different topographic
positions.
•The difference in soil properties arise because of:
–The differences in soil moisture regimes that exists between soils in
the sequence, and
–The degree and nature of lateral water movement at each position.

35. •A simple catena, therefore, shows a repetitive soil pattern
linked to changes in topography (Figure 3.5)
•Taking an example of UZ (Harare) catena. The catena
developed on basic (mafic) PM (figure 3.6)
•In the upland position, just below the crest, sufficient rainfall
penetrates the soil to allow for weathering and development
of deep profile.
•This site is well drained (above water table) and water moves
internally and carries bases down slope.
•The soil is mostly composed of stable iron oxides and hence
has a red clour.

36. •Lower down the slope, the water table is closer to the surface
and influences the soil morphology.
•The soil experiences seasonal waterlogging which induces
formation of mottles in the subsoil.
•The soil is brown colour due to the iron oxides being reduced.
•Still further down the slope, the soils become permanently
waterlogged and under these reducing conditions all iron is
reduced to the ferous state.
•There is little stream flow and poor drainage encourages
accumulation of bass, OM and presence of active swelling
clays.

37. •The clays combine with OM to produce black
soil above the waterline.
•However, the soils below the waterline which
are permanently wet, exhibit gley colours.
•Other examples on catena is the Great Dyke
(Nyamapfene, 1991; 1992)

38. organisms
•Biological activity plays an important role in soil development.
•Biological agents (organisms) include vegetation, soil fauna,
soil micro-organisms and man.
•Plants affect soil genesis through addition of OM, cycling of
nutrients and water, and through root activity
•Plants such as pines and spruces produce acidic leaf litter
which may lead to creation of a specific soil type called
podzol.
•Organisms also affect soil genesis as consumers and
decomposers of OM and through their earth moving
activities.

39. •Of particular importance in Zimbabwe is the action of termites and ants.
•Many different types of termites mounds (termitaria) and anthills are found
here.
•The soils in these termataria/ anthills have distinctly different properties
compared to the surrounding soils, including more bases and higher clay
contents
•Termites may bring fine soil up to the surface leaving stones deeper down.
•Earthworms have a major effect on soil properties in temperate regions,
promoting well aerated and aggregated soils.
•Man influences soil formation through his management ( or often
mismanagement) of vegetation, through farming and keeping of domestic
animals, and by urban and industrial development.
•Many of these changes induced by man lead to degradation/ pollution of
soils.

40. summary
•Factors are external conditions which cause various processes
to occur in soils.
•Weathering is most affected by temperature and
precipitation.
•Soil forming processes such as leaching, eluviation, illuviation
cause movement and redistribution of salts, clay, OM and
sesquioxides in soils
•There are five soil forming factors (climate, PM, time,
topography and organisms) which give rise to different types
of soils.

41. •PM include consolidated igneous, sedimentary
and metamorphic rocks and unconsolidated
material such as alluvium, colluvium, aeolian
and glacial deposits.
•Topography affects soil formation primarily
through its effect on water movement and
distribution down the slope. Catenas are a
sequence of soils developed at different
positions on the slope.