The Chinese University of Hong Kong
Department of Geography and Resource Management
Soil Formation Factors with
Special Reference to Oxisols
and Aridisols
Chau Kwai Cheong
<kwaicchau@cuhk.edu.hk>
29 September 2001
Department of Geography and Resource Management 2001
Outline of Presentation
1.
2.
3.
4.
5.
6.
What is soil?
What is a true soil?
How are soils formed?
How are soils distributed spatially?
What are oxisols?
What are aridisols?
Department of Geography and Resource Management 2001
What is Soil?
The most basic natural resource of the world
• Recycling system for nutrients and
organic wastes
• Habitat for soil organisms
• Engineering medium
• Medium for plant growth
• System for water supply and purification
Source: Brady N. C & Weil R. R. The Nature and Properties of Soils. 1999. p . 3.
Department of Geography and Resource Management 2001
Regolith, soil and bedrock
What is Soil? (cont’d.)
• Regolith:
The unconsolidated mantle of weathered rock and soil
material on the earth’s surface; loose earth materials above
solid rock.
• Soil:
(1) A dynamic natural body composed of mineral and
organic materials and living forms in which plants grow.
(2) The collection of natural bodies occupying parts of the
earth’s surface that support plants and that have properties
due to the integrated effect of climate and living matter
acting upon parent materials, as conditioned by relief, over
periods of time.
• Bedrock:
The solid rock underlying soils and the regolith in depths
ranging from zero (where exposed by erosion) to several
hundreds feet.
Source: Brady N. C & Weil R. R. The Nature and Properties of Soils. 1999. p . 827-862.
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What is Soil? (cont’d.)
Volume composition
Source: Brady N. C & Weil R. R. The Nature and Properties of Soils. 1999. p . 15.
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What is a true soil?
Uniform weathered parent
materials  True Soils
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What is a true soil? (Cont’d)
A true soil has layers or horizons:
- Accumulation of organic matter
- Incorporation of organic matter
(humus) into upper soil by
animal, water and gravity
- Downward movement of soluble
ions (e.g. Ca2+, SO42-)
- Formation of clay (secondary
aluminosilicate)
- Further leaching of soluble ions
and translocation of clay
downwards
- Structural formation
Source: Brady N. C & Weil R. R. The Nature and Properties
of Soils. 1999. p . 3.
- Formation of eluvial and illuvial
layers
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How are soils formed?
 Weathering (physical, chemical
and biological)
 Soil genesis (additions, losses,
transformation, translocation)
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Weathering
1. Physical weathering exposes rock surfaces to air,
water and carbon dioxide
2. Chemical weathering of solution, oxidation,
hydration, hydrolysis and carbonation proceed
jointly
Solution
NaCl(s) + H2O  Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq)
Oxidation
4Fe2+ + 3O2

2Fe2O3(s)
Hydration
Fe2O3(s) + H2O  2FeO.OH(s)
Haematite (red)
goethite (brown)
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Weathering (cont’d)
Carbonation
CaCO3(s) + H2O + CO2  Ca2+(aq) + 2HCO3-(aq)
limestone
calcium bicarbonate
K2O.Al2O3.6SiO2(s) + 2H2O + CO2  Al2O3.2SiO2.2H2O(s) + K2CO3(aq) + 4SiO2(s)
Orthoclase feldspar
kaolinite
potassium
carbonate
Hydrolysis
K2O.Al2O3.6SiO2(s) + 11H2O + CO2  Al2O3.2SiO2.2H2O(s) + 4H4SiO4(aq) + 2K+(aq) + 2OH-(aq)
Orthoclase feldspar
kaolinite
silicic acid
In the presence of acid rain (e.g. H2SO4; HNO3; H2CO3) generally:
Aluminosilicates(s) + H2O + H2SO4  clay mineral(s) + (Ca2+, K+, Na+)(aq) +
OH-(aq) + H4SiO4(aq) + SO42-(aq)
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Weathering (cont’d)
3. Biological weathering (Root growth
and exudates)
- Root pressure
- Carbonation
- Organic acids
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Soil genesis
1. Additions ---- Organic matter, gases
2. Losses ---- salts, carbonate, bicarbonate,
ammonia
3. Transformation ---- organic matter to humus,
primary mineral to secondary mineral
4. Translocation ---- humus, clay, sesquioxides
Department of Geography and Resource Management 2001
Soil genesis (Cont’d.)
Source: Brady N. C & Weil R. R. The Nature and Properties of Soils. 1999. p . 11 & 63.
How are soils distributed spatially?
Source: Brady N. C & Weil R. R. The Nature and Properties of Soils. 1999. p . 82.
Conclusion
(1) “Soil formation is stimulated by climate and living
organisms acting on parent materials over periods
of time and under the modifying influence of
topography” (Brady and Weil, 1996)
Soil Formation = f ( C, R, P, O, T )
(Jenny 1941)
Where, S = Soil formation / Soil properties
C = Climate
R = Relief
P = Parent material
O = Organism
T = Time
(2) “The great diversity of soils in the world results not
from
the operation of many different processes, but rather
from variations in the intensity and length of time the
processes have operated” Department
(Bradyof1974)
Geography and Resource Management 2001
Oxisols
Properties and Distribution
 Most highly weathered soils found mostly in tropical areas (16-80m
thick)
 Occupy nearly 9% of the world’s land
 Characterized by a deep oxic subsurface horizon
 The oxic horizon is dominated by clay-size particles of the hydrous
oxides of iron (Fe2O3) and aluminum (Al2O3.3H2O), with a low silicasesquioxide ratio of around 1.5.
 Deep red in color due to the presence of iron oxide
 High clay content, but the clays are of low-activity, nonsticky type
(e.g. kaolinite)
 Good physical property, easy to till and drains extremely well
 Acidic in reaction, low levels of humus and base content, infertile
 Formed under rainforest vegetation in the tropics, hence most of the
nutrients are stored in the overstorey layer
Department of Geography and Resource Management 2001
Oxisols (cont’d.)
Source: Brady N. C & Weil R. R. The Nature and Properties of Soils. 1999. p . 107.
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Oxisols (cont’d.)
Source: Mohr, van Baren & van Schuylenborgh.
1972. Tropical
Soils. p.
216Resource
& 217. Management 2001
Department
of Geography
and
Formation
 Intense weathering of rock minerals results in the release of
silica (desilication), alkali and alkaline earths from the primary
aluminosilicates.
K2O.Al2O3.6SiO2(s) +11H2O + CO2 Al2O3.2SiO2.2H2O(s) + 4H4SiO4(aq)+ 2K+(aq) + 2OH-(aq)
Orthoclase feldspar
kaolinite
silicic acid
 Alkali and alkaline earths are completely leached out of the
profile.
 The released silica (SiO2) either combines with alumina
(Al2O3) to form kaolinite (Al2O3.2SiO2.2H2O) or is leached by
the percolating water. The soil is thus impoverished in silica,
but enriched with iron oxides (Fe2O3.nH2O) and hydroxides
(FeO.OH).
Department of Geography and Resource Management 2001
Formation (cont’d.)
 Alternatively, the decrease in silica can also be
relative, and caused by the deposition of
sesquioxides (R2O3) transported laterally with
groundwater from the higher surroundings.
 Desilication results in the formation of the oxic
mineral horizon underneath the surface. It consists
of weathered mixtures of sesquioxides, clay and
quartz sand. It is poor in humus and is at least 30
cm thick.
 Under high rainfall conditions and where there is no
erosion of the land, this oxic horizon remains soft
all the time. Conversely, it hardens irreversibly to
form laterite when exposed to air.
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Use and Problems
 Supports 300-400 million shifting cultivators in the humid
tropics
 Under undisturbed conditions, deep-rooted trees can pump
nutrients from the subsurface layers and recycle them
effectively forming a tight nutrient cycle.
 Deforestation (excessive logging, conversion to ranches and
tropical plantations, fire) disrupts the nutrient cycling
process and upsets the ecological balance
 Phosphorus deficiency due to fixation by iron and aluminum
oxides
 Intensive leaching of soluble ions and plant nutrients under
high rainfall conditions
 When the oxic horizon is exposed to air as a result of
deforestation and erosion of the surface soil, it hardens
irreversibly to form the so-called laterite (Mekong Project)
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Management
 Avoid removal of the natural vegetation to protect the
soil and preserve the nutrients
 Shorten the period of farming after slash and burn
(sustainable shifting cultivation)
 Growth of perennial tree crops, such as rubber,
coconut and pepper to restore the nutrient cycling
system
 Inclusion of legumes in crop rotation (e.g. peanut) to
provide additional nitrogen
 Restore degraded soils (abandoned pastures, mines)
with native tree species
Department of Geography and Resource Management 2001
Aridisols (Dry Soils)
Properties and Distribution
 Dry soils in arid region
 Occupy 12% of all global soils
 Soil moisture supports plant growth for no longer
than 90 consecutive days
 Associated with scattered desert shrubs and short
bunchgrasses
 Horizon of accumulation of calcium carbonate
(calcic), gypsum (gypsic), soluble salts (salic) or
sodium (natric)
 Erosion of fine particles leaves behind windrounded pebbles known as desert pavement or
Gobi desert
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Aridisols (cont’d.)
(Sahara Desert, Gobi and Taklamakan Deserts in China,
Turkestan Desert, southern and central Australia, SW Africa
and southern Argentina)
Source: Brady N. C & Weil R. R. The Nature and Properties of Soils. 1999. p . 94.
Department of Geography and Resource Management 2001
Annual Precipitation of China
Aridisols (cont’d.)
Source: The National Physical Atlas of China. China Cartographic Publishing House. p . 83.
Aridisols (cont’d.)
Source: Mohr, van Baren & van Schuylenborgh. 1972. Tropical Soils. p. 175.
Department of Geography and Resource Management 2001
Aridisols (cont’d.)
Source: Mohr, van Baren & van Schuylenborgh. 1972. Tropical Soils. p. 187.
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Formation
 Occur in poorly drained valley floors, flats and basins
in the continental interior
 Weathering of parent materials or deposit of
secondary aluminosilicates
 Surface runoff evaporates, leaving behind large
amounts of carbonates (CO32-), bicarbonates (HCO3-),
sulfates (SO42-) and chlorides (Cl-) of sodium, calcium,
magnesium and potassium
 Sources of the salts
- weathering of rocks and minerals
- brought to the surface through rainfall, irrigation,
capillarity
- salt deposits during geological time in the bottom of
now extinct lakes or oceans
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Formation (cont’d.)
 Most abundant in areas with an evaporation-toprecipitation ratio of >2.0
Region
Humid
Sub-humid
Semi-arid
Arid
Aridity index (k)*
<1.0
1.0-1.5
1.5-2.0
2.0-4.0
>4.0
Vegetation
Forest
Forest-steppe
Steppe
Desert-steppe
Desert
% total
32.2
14.5
21.7
31.6
Source: Chinese Academy of Science (1958)
* Evaporation-to-precipitation ratio
 Depth and thickness of salt accumulation depend on
climate and seasonal distribution of rainfall
(Salinization alternates with desalinization)
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Use and Problems
 Salt accumulation (up to 10% in NW China) decreases the
osmotic water potential which, in turn, reduces the rate of
water uptake by roots and germinating seeds. Plants die off as
a result of wilting.
 Water deficiency
 Where salts accumulate as crust on the surface, the soil is
vulnerable to dessication and wind erosion, resulting in the
decline of biological productivity.
 Extreme temperatures and strong winds detrimental to plant
growth
 Coarse texture/stoniness not favourable to crop growth
 Phosphorus deficiency due to fixation by calcium
 High sodium content destabilizes soil structure
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Management
 Minimize irrigation with salt-laden
water and practise drip irrigation
 Avoid over-grazing, overcultivation and excessive cutting
of vegetation
 Avoid construction of reservoir
dam upstream of inland rivers
(e.g. Tarim River)
 Recycle organic matter
 Reclamation with salt-tolerant grasses,
shrubs and trees
 Increase surface roughness to halt the
advance of moving sand dunes
(Phragmites spp.)
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Thank you !
Department of Geography and Resource Management 2001