This document discusses soil genesis and the process of soil formation. It explains that soil is formed through the weathering of rocks and minerals over long periods of time, as well as the mixing in of organic matter from dead plants and animals. The key processes involved are physical and chemical weathering of parent materials, accumulation of organic matter, losses due to leaching and erosion, transformation of minerals, and translocation of materials within the soil profile. The main factors that influence soil formation are also outlined as parent material, climate, living organisms, topography, and time.

1. Introduction to Geology, Soil and
Environment
SES-101
Dr Irshad Bibi
Institute of Soil & Environmental Sciences,
University of Agriculture Faisalabad

2. Soil Genesis/Soil Formation
• Soil genesis is the mode of origin of soil. It includes
both:
 Production of parent material
 Soil profile development
• The process of weathering and soil profile
development occur simultaneously.
• Soil formation is a long term process (may take
1000’s of years).

3. Soil Genesis/Soil Formation

4. Soil Genesis/Soil Formation

5. Soil Genesis/Soil Formation
• Soil is formed as a result of weathering of rocks and
minerals
• The surface rocks break down into smaller pieces-
physical weathering.
• The physically weathered particles of rocks mix with
organic matter.
• This creates a thin layer of soil; through the growth
of lower plants soil development is enhanced
• Later on higher plants grow and attract animals.
• The death and decay of plants, animals makes soil
rich.
•This process continues until soil is fully developed.

6. Soil Genesis/Soil Formation

7. Steps in Soil Formation Process
• Weathering of rocks
 Physical weathering (through the growth of plant
roots, freezing/thawing action etc.)
 Chemical weathering (e.g. through dissolution
through the action of H2O, acids, alkalies)
 Biological weathering (e.g. through plant root
exudates or microbial action)
 Mixing with the organic matter added through
death/decay of organism e.g. Plants and animals

8. Steps in Soil Formation Process
• Weathering of rocks and minerals
 Minerals in a rock differ in their resistance to
weathering
 Some minerals are very persistent and weather at
a very slow rate.
 Some other minerals weather at a greater rate.
 Minerals have been classified based on their
resistance to weathering.

9. Weathering Sequence of Minerals
Weathering ability Minerals
Very unstable Halite, gypsum, calcite dolomite
Unstable Olivine, apatite, serpentine
Moderately stable Biotite, chlorite, hornblende
Stable Albite, muscovite, orthoclase
Very stable Quartz, clay minerals, hematite, gibbsite

10. Steps in Soil Formation Process
Weatherability depends on several factors.
 Factors related to mineral itself.
 Factors related to the weathering environment.
 Factors related to mineral are:
fitness of cations into the voids created
by O atoms
presence of Fe(II)
For example the tri-octahedral clay minerals are
less resistant to weathering than di-octahedral clay
minerals.
Biotite- K(Mg,Fe)3AlSi3O10(OH)2 is less resistant
Muscovite- KAl2(AlSi3)O10(OH)2 is more resistant

11. Basic Processes of Soil Formation
Four basic processes that bring about variations in
unconsolidated parent material are referred to as
‘Pedogenic processes’.
 Addition
 Losses
 Transformation
 Translocation

12. Basic Processes of Soil Formation

13. Basic Processes of Soil Formation
Addition/Accumulation: Input of materials to the
developing soil profile from outside sources (e.g.
Accumulation of sediments brought about by flooding
event in a river etc.)
 Fallen plant leaves
 Twigs
 Dust
 Animal dung
 Salts or silica dissolved in groundwater and
deposited at/or near surface on evaporation.

14. Basic Processes of Soil Formation
Losses: The loss or removal of materials from soil
profile are known as losses. Materials are lost by
 Leaching of materials to groundwater-It causes the
loss of water and dissolved substances such as salts or
silica;
 Erosion of surface material (e.g. Wind/water erosion)
 Animal grazing and crop harvest-remove large
amounts of organic matter and nutrient elements (e.g.
N, P, K etc.)

15. Basic Processes of Soil Formation
Transformation: The physical or chemical modification
of soil constituents. It involves both
 Break down and;
 Synthesis
•For example, weathering of primary minerals results
in disintegration of various silicate clays.
•The products of weathering or break down may form
•Additional silicate clays (e.g. Secondary silicate
minerals) and hydroxides of Al, Fe etc.
•residues-humus and organic acids etc.

16. Basic Processes of Soil Formation
Translocation: The movement of material laterally
within a horizon or vertically from one horizon to
another
For example, the movement of
 dispersed clay particles,
 dissolved salts,
 dissolved organic substances.
•Water is the most common translocation agent.
•Soil organisms also play a major role- incorporation
of surface organic matter into A and B horizons by
earthworms;
•Transport of B and C horizons to surface by termites,
rodents etc.

17. Basic Processes of Soil Formation
Translocation:
Eluviation-removal of fine particles from a soil horizon
and
Illivulation-Addition of fine particles into a soil horizon
within a profile.

18. Factors of Soil Formation
1. Parent Material
2. Climate
3. Living Organisms
4. Topography
5. Time

19. Factors of Soil Formation
• Each of these factors affects soil properties.
• These factors may affect soil properties
independently
• or these factors may be dependent on each other.
• For example, the effect of organisms may be
dependent on climate etc.
• Climate and organisms -active factors.
• Parent material, topography and time-passive factors

20. Factors of Soil Formation
Parent Material
The unconsolidated initial material from which soils
develop.
The lower stratus or C horizon below the solum is
considered as parent material.

21. Factors of Soil Formation

22. Factors of Soil Formation
Parent Material
Parent material may be formed by:
weathering of rocks or
Transported by agencies (e.g. ice, water, wind, gravity)
The transported material is named according the
transporting agencies as
Colluvial-transported by gravity
Alluvial-transported by water
Glacial-transported by glacial ice
Eolian-transported by wind ( loess or sand dunes)

23. Factors of Soil Formation
Parent Material
Loess-wind blown silt size particles
For example materials deposited by wind in the
Pothwar plateau region of Pakistan.
Sand dunes-sand size particles blown and deposited by
wind.
For example, the material deposited in the Thar and
Thal deserts of Pakistan.

24. Factors of Soil Formation
Parent Material
Parent material greatly affects the soil characteristics.
The characteristics of parent material that affect soil
properties include
 rate of weathering
 nutrients
 particle size
Soil properties controlled by parent material include:
 Soil texture
 Nature of clay minerals
 Water holding capacity (WHC)
 Fertility and nutrient holding capacity
 Soil permeability

25. Factors of Soil Formation
Parent Material
Effect of Nutrients
Potassium rich parent material-illite dominated soils
Effect of Particle Size
Sandstone-sandy soils
Shales-clayey soils