Soil formation lectue ers iii

Apr 11, 2023SAASN@Soil Sci/Chem 1

Soil Formation & Soil Formation & other Factorsother Factors


Soil Formation / DevelopmentSoil Formation / Development

Soil Weathering

Weathering Processes

Physical Weathering

Chemical Weathering

Soil Structure

Soil Degradation

Soil Forming Factors

Apr 11, 2023 3SAASN@Soil Sci/Chem

IntroductionIntroduction

Soil weathering Soil 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 WeatheringFollowing are the types of Weathering

1.Physical Weathering2.Chemical Weathering3.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 CoolingSoils undergo contraction and expansion in heating and cooling processes

respectively. Temperature changes on soil become significant over larger period of time.

Wetting and DryingShrinkage and Swelling is the common phenomenon in the clay minerals when they

are cooled or dried.

Grinding and RubbingAbrasion and grinding cause the disintegration, which leads to the break down of

soil particle.

OrganismsWide 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. EluviationSoil 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 Agriculture Patterns on Weathered

soilssoils


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 Factors Soil 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. Climate3. 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.


Soil Forming Factors Soil Forming Factors 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

Soil Forming Factors Soil Forming Factors

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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