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Section A3 : Soils - 6 major soil forming / pedogenic
processes
Ref : 99#1, 98#2, 96#1, 95#2, 92#3, 90#3
I.
Savanna : Laterization / Plinthization → ferruginous soils / latosols (Zonal Soil)
Oxisols
TRF : Ferrallitisation ferrallitic soils (Zonal soil) Processes A + B only + human impact C
A.
Intense chemical weathering
1.
TRF : hot and wet throughout the year  intense chemical weathering
2.
E.g. Hydrolysis : breakdown of feldspar, a mineral found in igneous rocks such as granite, into kaolinite
/ kaolin
3.
Mica, a mineral found in granite, + acid  Al and Fe are released  reddish brown soil
B.
Desilication
4.
Besides, chemical weathering produces clay  break down to form silica
5.
Released silica is often leached by heavy rainfall, i.e. desilication.
C.
Plinthization
6.
High concentration of sesquioxides of Fe and Al  plinthite / oxic layer (≥ 30cm thick) underneath
the surface
7.
Savanna  alternate dry and wet seasons  alternate reduction + oxidation
Wet season : reduction : plinthite layer remains soft, sticky and plastic , i.e. latosol
Dry season : oxidation : plinthite layer hardens  hard pan called “laterite” is formed by the process
known as plinthization / laterization.
8.
Mekong Project in Vietnam
a.
 deforestation  erosion + exposed to air  accelerates plinthization
b.  construction of dams  control flooding in the lower course of rivers  soil hardens  hard
pans are formed in the lower course
Examples :
Leizhou Peninsula of Guangdong, SW Taiwan
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Characteristics of Oxisol
General characteristics
Oxisols (Oxic horizon, highly weathered)
1.
highly weathered and thick (16 – 80m)
They are most highly weathered soils.
2.
formed in tropical areas, either TRF or savanna
They form in hot climates with nearly year round moist conditions; hence, the native vegetation is
generally thought to be tropical rain forest.
3.
oxic subsurface horizon
They are characterized with a deep oxic subsurface horizon. This horizon is generally very high in
clay-size particles with large part of silica removed the process of desilication.
4.
high clay content but good drainage and good aeration; easy to plough
Clay content is high but the clays are of low activity, nonsticky type, e.g. kaolinite 高嶺土.
The
low activity clays have low capacity to hold nutrients, cations such as Ca Mg and K, so the base
content is low and hence infertile and moderately acid.
5.
absence of distinctive horizons
Usually the boundaries between subsurface horizons are indistinct, giving the subsoil a relatively
uniform appearance with depth.
6.
red in colour
Oxisol is deep red in colour due to the presence of iron oxides.
Distribution and use
1.
Oxisols are found on about 8% of the world's land.
2.
They occupy old land surfaces and occur mostly in the tropics.
3.
Large areas of Oxisols occur in South America and central Africa.
4.
Millions of people in the tropics depend on them for food and fiber production. However, because
of their low natural fertility, most Oxisols have been left under forest vegetation or are farmed by
shifting cultivation methods.
Conclusion
Oxisols develop in tropical and subtropical latitudes that experience an environment with high
precipitation and temperature. The profiles of Oxisols contain mixtures of quartz, kaolin clay/ kaolinite,
iron and aluminum oxides, and organic matter. For the most part they have a nearly featureless soil
profile without clearly marked horizons. The abundance of iron and aluminum oxides found in these soils
results from strong chemical weathering and heavy leaching. Many Oxisols contain laterite layers
because of a seasonally fluctuating water table, e.g. Mekong Project : dam → control flooding in the
lower course → soil hardens and hard pan forms.
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II.
1.
Podzolization  Podzol in coniferous forest (Zonal soil)
Coniferous forest : cool humid climate (ppt  evapotranspiration) in temperate (middle and high)
latitudes
2.
well-drained soil (sandy)
3.
Acidic vegetation : coniferous trees with pine needles undergo slow decomposition in cool condition
 mor humus (extremely acidic with pH < 4.5) which is rich in organic acids and polyphenols
4.
Bases (Ca, Mg, Na, K) and sesquioxides of Fe and Al are dissolved and leached downward, leaving
behind silica, which tends to accumulate in the zone of eluviation.
5.
2 distinct horizons
 Leaching drains coloured minerals away, and silica ( whitish-grey in colour) remains  bleached A
horizon
Sometimes, bases, colloids and sesquioxides of Fe and Al leached from the upper soil are deposited
in the illuviation zone  reddish-brown (dark coloured) B horizon
Example : Da Hinggan Mountain in China
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III.
Calcification  calcareous / calcimorphic soils (Intrazonal soils) ― a kind of aridisol
1.
Continental climate where evaporation  low annual precipitation , e.g. savanna
2.
Parent materials contain calcium carbonate, e.g. limestone
3.
Weathering  release of Ca
4.
Insufficient rainfall (250-1000mm annually) to leach out all the bases
 high concentration of Ca, Mg, Fe and Al
5.
In dry period,  evaporation and capillary action  Ca and Mg are brought upward and precipitate
as a stony layer, called caliche / calcic layer in the B horizon
6.
In rainy seasons, Ca and Mg are eluviated to form a soft calcic layer
Example : a calcic layer is also found below the humus horizon of chernozem under the meadow-steppe
vegetation in the semi-humid region of the temperate zone, e.g. The West of Da Hinggan Mountain
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IV.
Desert : salinization  Desert soil (Zonal soil) ― a kind of aridisol
1.
Desert climate : annual ppt  250 mm; potential evapotranspiration > ppt
2.
High water table/ Water table close to the ground surface
3.
Poor drainage, e.g. valley floors, flat basins in the continental interior, e.g. Qinghai, Xinjiang 
water is available for intense evaporation
4.
Release of soluble salts, e.g. Ca and K, by weathering, or extinct lakes
5.
Rainfall is too low for leaching
6.
Intense insolation +  T゜  evaporation + capillary action  salt solution is drawn upward to
form hard pan, or whitish salt crust.
7.
Soil is affected by desiccation (as infiltration is prevented)   biological productivity 
desertification
8.
The problem is enhanced by overgrazing, over-cultivation, devegetation for firewood as fuel,
excessive irrigation with salt-laden ground water and the construction of reservoir dam in
semi-arid lands, e.g. Tarim Basin, and salt water intrusion in coastal areas, e.g. Shanghai (overdrawn
of under-groundwater for domestic use)
Example : Qinghai, Xinjiang
Aridsols are soils that develop in very dry environments. The main characteristic of this soil is poor and
shallow soil horizon development. Aridsols also tend to be light colored because of limited humus additions
from vegetation. The hot climate under which these soils develop tends to restrict vegetation growth.
Because of limited rain and high temperatures soil water tends to migrate in these soils in an upward
direction. This condition causes the deposition of salts carried by the water at or near the ground surface
because of evaporation. This soil process is of course called salinization.
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Characteristics of Aridisols (Dry soils)
General characteristics
1.
low rate of weathering and thin (mostly < 3m)
Water deficiency is a major characteristic of these soils. The soil moisture level is sufficiently high
to support plant growth for no longer than 90 consecutive days.
2.
formed in arid environments
The natural vegetation consists mainly of scattered desert shrubs and short bunchgrasses.
3.
limited leaching + accumulation of soluble minerals
Aridity restricts leaching of soluble materials. So, they often accumulate in the profile.
These soils
may have a horizon of accumulation of calcium carbonate, gypsum, soluble salts, or exchangeable
sodium.
During the dry seasons, the soluble soil solution may be drawn upward by capillary action.
So, hard pans, e.g. cacic/ caliche / calcic layer, gypsic, salic / salt crust or natric may be formed.
These hard layers hinder root penetration and also greatly increase the cost of excavations for
buildings.
4.
low organic matter content
Aridity causes scanty vegetation and hence low O.M. and humus content.
5.
light in colour
Aridisols are light in color due to the precipitation of soluble salts as hard crust
6.
for drought resistant crops except with irrigation
Except where there is groundwater or irrigation, the soil are only moist for short periods during the
year.
These short moist periods may be sufficient for drought-adapted desert shrubs and annual
plants, but not for conventional crop production.
If groundwater is present near the soil surface,
soluble salts may accumulate in the upper horizons to levels that most crop plants cannot tolerate.
Distribution and use
1.
Aridisols occur on nearly 12% of the land area worldwide.
2.
Most Aridisols are present in the Sahara desert in Africa, the Gobi and Taklamakan deserts in China,
and the Turkestan desert of the former Soviet Union. Most of the soils of S and central Australia
are Aridisols, as are those of S. Argentina, SW Africa, Pakistan, and the Middle East countries.
3.
highly productive if irrigation is available
Without irrigation, Aridisols are not suitable for growing cultivated crops.
Some areas are used for
low-intensity grazing, especially with sheep or goats, but the production per unit area is low.
Yet,
when overgrazing occurs, the once-grassy areas become increasingly bare, and the soils will be prone
to wing erosion and rainsplash effect.
Desertification of sub-Saharan Africa and the western United
States is evidence of such degradation.
Some xerophytic plants, e.g., jojoba, have been cultivated on Aridisols to produce various industrial
feedstocks, such as oil and rubber.
Where irrigation water and fertilizers are available, some Aridisols can be made highly productive.
Irrigated valleys in arid areas are among the most productive in the world.
carefully managed to prevent the accumulation of soluble salts.
However, they must be
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V.
Gleying  gley soil (intrazonal soil)
1.
formed in cool or cold environment, e.g.subarctic lands / tundra OR
2.
poorly drained soil / output of water is restricted, e.g. by a permafrost / very gentle flat land /
depressions / topographic hollows with impermeable underlying rock
3.
pore spaces are filled with stagnant water  waterlogging / anaerobic condition  reduction of
ferric iron (reddish coloured) to ferrous iron (grey-blue)
4.
low temp.  heavy accumulation of organic matter  peaty materials on surface layer
5. Beneath the peat layer, it is the glei horizon where Fe is chemically reduced.
Sometimes, iron II may be re-oxygenise to form red mottles.
6. Gleying is common in paddy and bog soils, with a seasonal pattern
Examples :
Three Gorges Dam in China ↑ water table  risk of gleying
In H.K., gleying occurs in mudflats colonized by mangroves in Deep Bay and in the nearby gei wai ponds in
Mai Po
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VI. Tundra : involves the process of gleying in summer  Tundra Soil (Zonal soil)
1. Tundra : extremely cold, temp. drops below freezing point for most of the year (around 8 months)
2.
Cold and dry
 limited plant growth and few soil biota
 slow decomposition of organic matter
 thin black / dark brown peat layer (very acidic mor humus)
3. Late spring : snow melting of active layer  humic acid releases iron
4. Permafrost (impermeable layer) restricts penetration of water  through waterlogging and gleying in
spring and early summer occurs in active layer  reduction of iron  blue-grey colour
5. No distinct horizons because the shattered angular fragments are raised to the surface by frost
heave / frost heaving.
Factors influencing soil formation
http://www.fed.cuhk.edu.hk/geo/AL/core/biotic/soilFormation.html
Soil classification
http://www.geog.ouc.bc.ca/physgeog/contents/11f.html