Download SOIL FORMING PROCESSES

Lecture 5
SOIL FORMING
PROCESSES
Soil horizons

Horizons in the
soil profile are the
results of soil
formation
processes
PROCESS OF SOIL FORMATION
The layers in soil, called soil horizons, develop their
characteristics not only because of the geological materials
at a given site, but also because of soil forming processes
at work, over time, in that zone of the soil.
The soil forming processes fall into the following
categories:
1. Addition
2. Removal
3
Mixing
4. Translocation
5. Transformation
1. Additions
• water (from the surface, and by ground
water discharge)
• suspended and dissolved materials
carried by water
• solids transported by wind
• gases from the air
• energy from the sun
• organic carbon by plants in form of
roots and root-derived material
• organic carbon by photoautotrophic
bacteria
• organic nitrogen by nitrogen-fixing
bacteria
• plant and animal remains on and in the
soil.
2. Removal
•
materials removed by erosion ; wind, water, ice, gravity
• dissolved and suspended material may be
leached out from the bottom of a soil profile
• uptake of nutrients from the soil by plants
• carbon dioxide gas produced by plant root,
microbial and faunal respiration
• other gases such as nitric oxide, nitrous
oxide and nitrogen produced by
denitrifying bacteria
•
other gases such as methane which are
produced under anaerobic conditions
3. Mixing

By animals ; earthworm, insects, rats etc
4. Translocation
•Translocation of materials within the
soil profile is primarily due to gradients
in water potential and chemical
concentrations within the soil pores.
• Soluble minerals (calcium carbonate),
colloidal material(Clays) , organic
compounds, and iron may move up or
down the profile, between horizons,
with water movement.
• Biological activity may cause gradient
in the chemical composition of the water
and air-filled pores of the soil.
Translocation Processes:
i.
E.g.
Clays= lessivage
ii. Fe Oxide + humus= podzolization
iii. Calcium carbonate= calcification
iv. Laterization = Fe, Si
i)
(Clay
movement)
3. Change in chemical
composition of soil
- Flocculation of clay
Clays -> lessivage =formation of Bt horizon
ii) Podzolization
Podzolization develop in cool and
moist climates under pine forests.
They are typical of the colder portions
of the humid continental and
Subarctic climates. The E horizon is
heavily leached and basically
composed a of light colored layer of
sand.
Typical ashy, gray layer left from
leaching of sesquioxides is apparent in
this podzolized soil. The upper portion
of the B horizon is stained reddish
color from the accumulation of
sesquioxides. The profile gets lighter in
color as depth increases.
Albic and Spodic Horizon
i)
(Albic)
ii) Spodic horizon

Accumulation B horizon
( Bh, Bir, Bhir )
iii) Calcification
•e.g. Mollisol soil enriched with calcium
carbonate. Calcification occurs in warm,
semi-arid environments, usually under
grassland vegetation. Soil tends to be
rich in organic matter and high in
soluble bases. The B horizon of the soil
is enriched in calcium carbonate
precipitated
from
water
moving downward through the soil or
upward
movement
of
capillary water from below.
iv) Laterization
The deep red to bright orange-red soils of the
tropics are a product of laterization.(movement of
Fe2+ and oxidation – Fe oxides. Laterization occurs
in the hot, rainy tropics where chemical
weathering proceeds at a rapid rate.
This ultisol displays the typical features of a soil
having undergone laterization
Soils subject to laterization end to be
acidic and lack much organic matter
as decomposition and leaching is
extreme.
Exposure of the soil to the hot tropic
sun by deforestation bakes the soil
dry, reducing infiltration, increasing
runoff, and reducing fertility.
Transformation
• Soil components are transformed
by chemical and biological reaction.
• Organic compounds decay, some
minerals dissolve, other minerals
precipitate.
• These transformation result in the
development of soil structure, and
in changes in colour, relative to the
parent material.
Secaondary minerals
(Fe, Mn, Al)
(hematite, geotite)