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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 29 September 2001. Department of Geography and Resource Management 2001. - PowerPoint PPT Presentation
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Soil Formation FactorsSoil Formation Factors with with Special Reference to Oxisols Special Reference to Oxisols
and Aridisols and Aridisols
Chau Kwai Cheong<[email protected]>
29 September 2001
The Chinese University of Hong KongDepartment of Geography and Resource Management
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
Outline of PresentationOutline of Presentation
1. What is soil?2. What is a true soil?3. How are soils formed?4. How are soils distributed spatially?5. What are oxisols?6. What are aridisols?
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
What is Soil?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
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
Source: Brady N. C & Weil R. R. The Nature and Properties of Soils. 1999. p . 3.
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
What is Soil? (cont’d.)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.
Regolith, soil and bedrock
Volume composition
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
What is Soil? (cont’d.)What is Soil? (cont’d.)
Source: Brady N. C & Weil R. R. The Nature and Properties of Soils. 1999. p . 15.
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
What is a true soil?What is a true soil?
Uniform weathered parent materials True Soils
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
What is a true soil? (Cont’d)What is a true soil? (Cont’d)
- 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+, SO4
2-)
- Formation of clay (secondary aluminosilicate)
- Further leaching of soluble ions and translocation of clay downwards
- Structural formation
- Formation of eluvial and illuvial layers
A true soil has layers or horizons:
Source: Brady N. C & Weil R. R. The Nature and Properties of Soils. 1999. p . 3.
Weathering (physical, chemical
and biological)
Soil genesis (additions, losses, transformation, translocation)
How are soils formed?How are soils formed?
Department of Geography and Resource Management 2001
WeatheringWeathering1. 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)
Department of Geography and Resource Management 2001
Oxidation
4Fe2+ + 3O2 2Fe2O3(s)
Hydration
Fe2O3(s) + H2O 2FeO.OH(s)
Haematite (red) goethite (brown)
Hydrolysis
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
Carbonation
K2O.Al2O3.6SiO2(s) + 11H2O + CO2 Al2O3.2SiO2.2H2O(s) + 4H4SiO4(aq) + 2K+(aq) + 2OH-
(aq)
Orthoclase feldspar kaolinite silicic acid
Weathering (cont’d)Weathering (cont’d)
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)
Department of Geography and Resource Management 2001
3. Biological weathering (Root growth and exudates)
- Root pressure - Carbonation - Organic acids
Weathering (cont’d)Weathering (cont’d)
Department of Geography and Resource Management 2001
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
Soil genesisSoil genesis
Department of Geography and Resource Management 2001
Soil genesis (Cont’d.)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?How are soils distributed spatially?
Source: Brady N. C & Weil R. R. The Nature and Properties of Soils. 1999. p . 82.
(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 propertiesC = ClimateR = Relief
P = Parent material O = Organism
T = Time
ConclusionConclusion
(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” (Brady 1974)
Department of Geography and Resource Management 2001
OxisolsOxisolsProperties 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 silica- sesquioxide 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 2001Department of Geography and Resource Management 2001
Oxisols (cont’d.)Oxisols (cont’d.)
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
Source: Brady N. C & Weil R. R. The Nature and Properties of Soils. 1999. p . 107.
Oxisols (cont’d.)Oxisols (cont’d.)
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001Source: Mohr, van Baren & van Schuylenborgh. 1972. Tropical Soils. p. 216 & 217.
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).
FormationFormation
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
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.
Formation (cont’d.)Formation (cont’d.)
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
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)
Use and ProblemsUse and Problems
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
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
ManagementManagement
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
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 wind- rounded pebbles known as desert pavement or Gobi desert
Aridisols (Dry Soils)Aridisols (Dry Soils)
Properties and DistributionProperties and Distribution
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Aridisols (cont’d.)Aridisols (cont’d.)
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
(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.
Aridisols (cont’d.)Aridisols (cont’d.)Annual Precipitation of China
Source: The National Physical Atlas of China. China Cartographic Publishing House. p . 83.
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
Aridisols (cont’d.)Aridisols (cont’d.)
Source: Mohr, van Baren & van Schuylenborgh. 1972. Tropical Soils. p. 175.
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
Aridisols (cont’d.)Aridisols (cont’d.)
Source: Mohr, van Baren & van Schuylenborgh. 1972. Tropical Soils. p. 187.
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 (CO3
2-), 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
FormationFormation
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
Most abundant in areas with an evaporation-to- precipitation ratio of >2.0
Formation (cont’d.)Formation (cont’d.)
Region Aridity index (k)* Vegetation % totalHumid <1.0 Forest 32.2Sub-humid 1.0-1.5 Forest-steppe 14.5Semi-arid 1.5-2.0 Steppe 21.7Arid 2.0-4.0
>4.0Desert-steppeDesert
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)
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
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
Use and ProblemsUse and Problems
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
Minimize irrigation with salt-laden water and practise drip irrigation
Avoid over-grazing, over- cultivation 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.)
ManagementManagement
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001
Thank you !Thank you !
Department of Geography and Resource Management 2001Department of Geography and Resource Management 2001