Fundamentals of Soil Science -(SAC-111) UNIT 4 - Weathering of Rocks and Minerals: Physical, Chemical and biological weathering ; Soil Forming Factors: Active factors , Passive factors , Soil Forming Processes: fundamental ( addition, losses, translocation, transformation) and specific soil forming processes

1. UNIT 4
Weathering , Processes and factors of Soil
Formation
Dr. Khambalkar Priyadadarshani
Soil Science
RVSKVV, CoA, Gwalior
Class- B.Sc. Agriculture
Year – I Semester-I

2. Steps of weathering
• 1)formation of regolith by the breakdown of bed rocks
• 2)addition of organic matter resulting from decomposition of
plant and animal residues and reorganizing of these
components by soil forming processes like eluviation and
llluviation.
• Definition of weathering : :is natural process of breakdown
and transformation of rocks and minerals into unconsolidated
residues (regolith),” laying on surface of earth, with varying
depth.

3. • Two basic processes of weathering:
• 1) Physical or Mechanical: Disintegration
• 2) Chemical : Decomposition
• Beside these the biological weathering is responsible for both decomposition and disintegration of
rocks and minerals that are carried out by different biological or living agents like fungi, bacteria,
actinomycetes, plants, man, animals etc.
• Joffe states , “strictly speaking there is no biological weathering. Essentially , it is physical and
chemical weathering by biological agents.”
• Different biological agents as state above accelerate chemical weathering by production of CO2
during resperition and by root exudates.
• The end product of weathering is parent material.
Kind / Types of Weathering Processes

4. Weathering Agents
Physical/Mechanical
(Disintegration)
Chemical
(Decomposition)
Biological
(Disintegration and
decomposition)
(a) Physical condition of rocks
(b) Change in temperatrure
(c) Action of water-fragmentation
and transportation, action of
freezing, alternate wetting and
drying, action of glaciers
(d) Action of wind
(e) Atmospheric electric
phenomena
(a) Solution
(b) Hydration
(c) Oxidation and
Reduction
(d) Hydrolysis
(e) Carbonation and
acidic processes
(a) Man and Animals
(b) Higher Plants and their
roots
(c) Micro-organisms and
other living organisms

5. Physical or Mechanical weathering

6. ..
.
.
Chemical Weathering
Chemical weathering is defined as : transformation of original rocks and minerals into new
compounds having different chemical composition and Physical properties.
No chemical weathering is possible without presence of water.
Chemical weathering is more effective when surface area of rock increase.
Chemical weathering is closely related to mineralogical composition of rocks.
The plants, animals and other organisms also contribute directly or indirectly to chemical
weathering they profuce CO2, O2 and other acids that reacts with rocks .
AGENS of Chemical weathering :
(a) Solution
(b) Hydrolysis
(c) Oxidation and Reduction
(d) Hydration
(e) Carbonation and acidic process

7. • (a) Solution: dissolving of solids in a liquid, separate solid materials into
independent soluble ions, each surrounded by the liquid molecules.
Water is universal solvent.
NaCl + H2O …………………> NA+, Cl-, H2O
(A Soluble salts) (water) (Dissolved ions surrounded by water molecules)
• (b) Hydrolysis: Splitting of water in to H+ ions and OH- ions.
It is reaction of substances with water to form hydroxide and other new
substances that are usually soluble than original material.
Feldspar + Water = Clay mineral + Soluble cations and anions
Hydrolysis is double decomposition processes
Chemical Weathering

8. • (C) Hydrolysis:
KAlSi3O8 + HOH……………………….> HAlSi3O8 + KOH
(Orthoclase : (water) (Acid silicateclay: (very soluble)
Slowly soluble) More Soluble)
2HAlSi3O8 + 8HOH……………………..> Al2O3.3H2O + 6H2SiO3
(Recombination) (Bauxite) (Silicicacid)
• (D) Oxidation- Reduction:
Oxidation: chemical combination of oxygen with a compound, loss of electron (change in oxidation
number) positive charge increased
Reduction: chemical reduction of oxygen , gain of electron, negative charge increased.
Oxydation
4FeO + O2 Reduction 2Fe2O3
(ferrous oxide: valance of Fe is 2+) (ferricoxide: valance of Fe is 3+)
Chemical Weathering

9. • (e) Hydration: it is chemical combination of Solid substances viz. mineral or
salt, with water.
• Hydration water combining within the mineral, changes mineral structure,
increasing its volume by swelling making it more softer and easily break and
decomposed
• 1. 2FeO3 + 3H2O ……………………….> 2FeO3.3H2O
• (hematite) (water) Limonite (yellow)
• Al2O3 + 3H2O…………………………> Al2O3.3H2O
(bauxite)
• CaSO4 + 2H2O…………………………> CaSO4.2H2O
• (anhydrite) (gypsum)
Chemical Weathering

10. • Carbonation and other acidic processes: Carbonation is the combination of
carbon dioxide (CO2) with any base; and produce carbonates and bicarbonates:
• CO2 + KOH …………………………………….> K2CO3 + H2O
• K2CO3 + H2O + CO2 ……………………………………..> 2KHCO3
• Carbonic acids dissolved minerals more readily than water alone and forms stable bicarbonates as:
• CO2 + H2O…………………………….…> H+ + HCO3
• CaCO3 + H + HCO3…………………….> Ca( HCO3)2
• (slightly soluble ) (readily soluble)
• The presence of H+ ions in percolating waters and other inorganic acids like HNO3, H2SO4
and some organic acids accelerate decomposition of minerals through chemical weathering.
Chemical Weathering

11. Different biological or living agents which are responsible for carrying out
disintegration and decomposition of rocks and minerals .
Man and animal: Man cuts rocks to build dams, channels, roads and buildings etc. In
Animals, Large number of birds, insects, worms etc. live in rocks and by their activities,
they make hole into rocks and thus hlp in weathering.
Roots of Higher Plants: are penetrate into the joints and crevices of rocks. During
growth and development of trees, roots exert a tremendous pressure and the massive
and hard rock may be broken into smaller ones.
During root respiration CO2 produces and that also take part in weathering process.
Micro-organisms: the micro form of plant and animals like mosses, bacteria, fungi,
actinomycetes etc. these organisms extract nutrients from the massive solid rocks,
nitrogen from air and live with very small amount of moisture.
These organisms act upon decay of plants and animal remains and produce various
complex organic compounds which help in decomposition of minerals.
Biological Weathering

12. Biological Weathering Agents

13. • Climatic condition:
• Physical characteristics: composition of rocks, size of minerals, hardness and cementation
• Chemical and structural characteristics
Factors Affecting Weathering of Minerals
Weathering Sequence- Goldich 1938
Quartz (most resistant) > muscovite, K-feildspars > Na and Ca- feldspars> biotite, hornblend, augite >
Olivine > dolomite and calcite > gypsum

14. • Soil formation is a processes of two phases
(i) Weathering of rocks and minerals i.e. disintegration and decomposition
(ii)Formation of true soil by soil forming factors and process.
FACTORS OF SOIL FORMATION:
Transformation of parent material by various factor is first put forwarded by Dokuchaive
(1889)
Equation: S = f (cl, o, r, p, t…..)
where, S = soil formation, f = function, o = organisms, p = parent materials,
cl = climate, r = relief, t = time
Jenny (1941) then emphasized that soil property is determined by relative influence of all
these factors.
Equation: S = f ( cl, b, r, p, t …..)
Active factors Passive factors
Where, b = biosphere (vegetation, organisms and man) and other same as Dokuchaive’s equation.
Soil forming Factors

15. Soil forming Factors: cl, b, r, p

16. Soil forming Factors: t (time)

17. • Soil forming process or paedogenic processes are
• A. Basic or Fundamental i) addition (of water, organic and mineral matter to the
soil, (ii) losses (of these material) iii) translocation or movement (of material
within soil profile), and iv) transformation of mineral and organic material.
• B. Specific pedogenic
Soil forming Processes
Basic or fundamental Specific paedogenic processes
i) Humification: accumulation of humus Calcification, Gypsification, Decalcification,
Silication, Desilication
ii)Eluviation: washing out, leaching of soil material in A horizon Pdzolization, Laterization, Salinization,
Desalization, Alkalization (Solonization).
Dealkalization (solodization)
iii) Illuviation: washing in, accumulation of leached material from
A horizon to B horizon
Pedoturbation, Argillation
iv) Horizonation: differentiation of soil in different horizons along
the depth of soil body

18. Thank you