Soil formation lectue ers iii

Aug 3, 2013SAASN@Soil Sci/Chem 1

Soil Formation & otherSoil Formation & other
FactorsFactors

Soil Formation / DevelopmentSoil Formation / Development
 Soil Weathering
 Weathering Processes
 Physical Weathering
 Chemical Weathering
 Soil Structure
 Soil Degradation
 Soil Forming Factors
Aug 3, 2013 3SAASN@Soil Sci/Chem
IntroductionIntroduction

Soil weatheringSoil weathering
Weathering of soil as Green Technology

Soil WeatheringSoil Weathering
Soil Formation
 Soil formation and development is a dynamic rather than static process.
 Describes the means by which soil, rocks and minerals are changed by physical
and chemical processes into other soil components. weathering may proceed
rapidly over a decade or slowly over millions of years.
Soil Parent Material
 All soils initially come from rocks so its termed as “Parent Material”. The
processes of soil formation is called “Pedogenesis”.

Soil TransportationSoil Transportation
 Soil Transportation is possible by the various means:
1. Glaciers
 Melt down then pushed away by drift and till the soil is shifted miles away from
where it was present.
2. Water
 Water also runs away the soils when rivers flow, the soils are transported from one
place to an other, these formed soils are called “Alluvial Soils”.
 Rainfall also transport soils even gentle rains may wash the exposed lands, this
moved soil is called as “Sheet Erosion”. When there is lot of rain there is formation
of small gulley termed as “Gulley Erosion”
 Farmers must plough in lines across the sloppy areas instead of ploughing
downwards to avoid this type of erosion.
3. Wind
 It moves surprisingly large quantities of soil, e.g. in south England soil deposits can
be seen that have been blown from the North African deserts.

Soil WeatheringSoil Weathering
Following are the types of Weathering
1.Physical Weathering
2.Chemical Weathering
3.Biological Weathering
4.Mechanical Weathering
Physical Weathering
In physical weathering the soil just brake down but nature remains the same as it
was initially present in the parent material.
 Freezing and Thawing
Water on freezing expands, resulting in the breakdown of soil structures. Similarly
the melting of ice also causes the grinding down of the mountains over time.

Physical WeatheringPhysical Weathering

Freezing & ThawingFreezing & Thawing

Soil Physical WeatheringSoil Physical Weathering
 Heating and Cooling
Soils undergo contraction and expansion in heating and cooling processes
respectively. Temperature changes on soil become significant over larger period
of time.
 Wetting and Drying
Shrinkage and Swelling is the common phenomenon in the clay minerals when they
are cooled or dried.
 Grinding and Rubbing
Abrasion and grinding cause the disintegration, which leads to the break down of
soil particle.
 Organisms
Wide range of organisms present in soil cause its mixing, churning, aerating and
making their ways through soil cause slow soil weathering.

Expansion % ageExpansion % age

Physical WeatheringPhysical Weathering

Soil Phy/Chem Weathering…Soil Phy/Chem Weathering…
 Unloading
Large Glaciers when melt down soil become unloaded compacted structure gets
expanded that results in the soil weathering.
 Chemical Weathering
This process involves the brake down followed by the change in its parent material.
For instance the hard material may change to the soft.
 Hydrolysis
This process involves the reaction of water to other compounds
2KAlSi3O8 + 3H20 Al2Si2O5(OH)4 + 4SiO2 + 2K(OH)
potassium feldspar in acidic water hydrolyses to kaolinite + quartz + potassium
hydroxide
 Carbonation
Certain reactions take place resulting in the formation of Carbonic Acid.

Chemical WeatheringChemical Weathering
 Hydration
Water can act to many other compounds to break down soil.
H2O + CO2 + CaCO3 Ca+2
+ 2HCO3
 Oxidation
Addition of oxygen to other compounds can led to the structural changes in soil.
4Fe+2
+3O2 2Fe2O3
 Reduction
The removal of Oxygen and addition of Hydrogen also causes the changes in soil
structure.

Soil Chemical WeatheringSoil Chemical Weathering

Soil FormationSoil Formation
 Soil Formation Processes
1. Leaching
Where soluble material is removed in solution
2. Podsolisation
When strong acid soil solutions cause the leaching of the complexes present in soils.

3. Eluviation
Soil particles held in suspensions and clays are removed
4. Illuviation
Soil particles held in suspension, such as clay, are accumulated (eg. deposited).
5. Gleying
Gleying occurs in waterlogged, anaerobic conditions when iron compounds are
reduced and either removed from the soil, or segregated out as mottles or
concretions in the soil. Marshy wetlands often contain gleyed soils.
Soil structure
How fine or course the mineral matter is in the soil that is dependent on the
amount of sand, silt and clay particles in the soil.

Agriculture Patterns on Weathered soilsAgriculture Patterns on Weathered soils

Soil Texture / StructureSoil Texture / Structure

Soil HorrizonsSoil Horrizons

Degradation of Soil StructuresDegradation of Soil Structures
 Slaking
 Slaking is the breakdown of large, air-dry soil aggregates (>2-5 mm) into smaller
sized micro aggregates (<0.25 mm) when they are suddenly immersed in water.
 Slaking indicates the stability of soil aggregates, resistance to erosion and suggests
how well soil can maintain its structure to provide water and air for plants and soil
when it is rapidly wetted.
Cementation
 In certain cases subsoil iron ‘pan’ or hard ferric layer can form, that prevents
the roots penetration. It may be result of heavy loading on soil surface.
Sodium Leaching
 When saline soils are heavy irrigated it causes the deflocculating of the sodium
clays, that results in the structural collapse

Degradation of Soil StructuresDegradation of Soil Structures
Cultivation
 The movement of heavy machinery on the topsoil compresses the layers
beneath the plough layer of soil.
 So called ‘plough pans’ may also be formed by the set depth time and again year
over year plough.
 The formation of this layer impedes drainage and adversely affects crop yield.

Soil Formation StagesSoil Formation Stages
 Four stages

Soil Forming FactorsSoil Forming Factors
Soil Formation Depends:
 Five soil-forming factors have been identified that influence the development of
a specific soil. if one or more of the factors differ, the soils will be different. The
factors are:
1. Parent material
2. Climate
3. Living organisms
4. Topography
5. Time
 Parent material is the material from which a soil forms. Few soils
weather directly from the underlying rock . These "residual" soils have
the same general chemistry as the original rocks. Fast-moving water
leaves gravel, rocks, and sand. Slow-moving water and lakes leave fine
textured material (clay and silt) when sediments in the water settle out.

 Climate determines the nature (physical, chemical or biological) and rate of
weathering (that acts on parent material to form soil). The most important
elements of climate for soil formation are precipitation and temperature.
Seasonal and daily changes in temperature affect moisture effectiveness,
biological activity, rates of chemical reactions, and kinds of vegetation.
 Topography There is a strong interaction between topography and
vegetation and their influence on soil formation.
Slope influences
1) the relative rate of water infiltration into the soil,
2) surface runoff and its associated soil erosion, and
3) distribution of vegetation.

Topography

 Biota Living organisms, including plants, microbes, soil animals, and
humans, are collectively referred to as biota. Soil development is affected by
both the type and number of organisms that live in and on the soil. Plants
influence the amount of organic matter buildup in the soil. Micro-organisms
affect chemical exchanges between roots and soil.
 Time for all these factors to interact with the soil is also a factor. Over time,
soils exhibit features that reflect the other forming factors. Soil formation is a
slow process that takes hundreds or even thousands of years. A younger soil
will reflect characteristics of the parent material better than an older soil.

Soil formation lectue ers iii

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