soil formation

1.  Geologic definition: Loose surface of the
earth as distinguished from solid bedrock or
Soil
 Material which nourishes and supports
growing plants (includes rocks, water, snow,
air).

2. Soil
 Mixture of inorganic matter (mineral), organic
matter, water, and air.
Organic Matter
original tissue—twigs and leaves on
forest floor
humus—rich brown color in soil 
decomposed material
As organisms die, they are attacked
by microorganisms: fungi, bacteria,
and others.

3. % Soil components varies:
• Type of vegetation
• Amount of mechanical compaction
• Amount of soil water present.

4. Soil Formation

5. Why do we study soil? Because It’s A(n)
Great integrator
Producer and
absorber of
gases (CO2 and
others)
Medium for
plant growth
Medium of crop
production
Home to organisms
(plants, animals and
others)
Waste decomposer
Snapshot of
geologic, climatic,
biological, and
human history
Source material for
construction,
medicine, art, etc.
Filter of water
and wastes
Essential natural resource
Medium of heat and
water storage

6. Well-drained
Poorly drained

7. Weathering forces
• Rocks & minerals break down 
smaller pieces
– Temperature changes
– Water action
– Plant roots
– Ice expansion
– Mechanical grinding

8. Temperature changes—heating and cooling of rocks can cause rock
to crack into smaller pieces.
Water action--Water soluble minerals dissolve when exposed to
water. Some rocks may contain some minerals that are water soluble
and only that part of the rock will dissolve
Plant roots—Roots of plants growing in cracks in rocks can widen
cracks and break rocks apart.
Ice expansion—If a rock has a crack that can fill up with water, when
the water expands when it freezes. This can break rock into smaller
pieces.
Mechanical grinding—wind blowing sand across rocks, or glaciers
grinding rocks against each other. Example—rock tumbler

9. Blocky
Platy
Granular

10. Clay Mineralogy
• Clay is a particle SIZE
• Predominant make-up is minerals

11. 1. Silicate Clays (aluminosilicates)
Composed of tetrahedral and octahedral “sandwiches”
Tetrahedron: central cation (Si+4, Al+3) surrounded by 4
oxygens
Octahedron: central cation (Al+3,Fe+2, Mg+2) surrounded
by 6 oxygens (or hydroxyls)

12. tetrahedron

13. octahedron

14. • 1000s of tetrahedra and octahedra connect in clay
minerals to give:
– Planes of Si, Al, Mg
– Planes of Oxygen, hydroxyl groups
• Sheets combine to form layers
• Layers are separated by interlayer space
– Water, adsorbed cations

16. kaolinite

17. kaolinite
Scanning Electron Microscope image

18. Montmorillonite

19. Illite

21. DENSITY INDEX OR RELATIVE DENSITY
• The density index is defined as,
ID = (emax – e / emax – emin)
Where,
emax = void ratio in the loosest state
emin = void ratio in the densest state
e = natural void ratio of the deposit
• This term is used for cohesion less soils only.
• When the natural state of the cohesionless soil is in
the loosest form,
emax= e.