3. INTRODUCTION
SOIL GENESIS
Pedogenesis from the Greek word Pedon meaning
soil and genesis meaning origin or birth.
Soil genesis is the process of Soil formation as
regulated by the effects of place, environment, and
history.
4. SOIL FORMING PROCESS
Based on the specific physical conditions prevailing
and the physical, chemical or biological activities
involved, the following processes involved in the
process of soil formation.
1. Translocation
2. Organic changes
3. Podzolisation
4. Gleying
5. Desilication
5. 1. Translocation
It involves several kinds of physical movements which are
predominantly in the downward direction. The processes
which can be categorized under translocation include the
following.
a. Leaching
b. Eluviation
c. Illuviation
d. Calcification
e. Salinisation
7. b. Eluviation
It refers to the downwash of
clay and other soluble
material, leaving behind a
deprived horizon.
8. c.Illuviation
It is the reverse of
eluviation’s. illuviation is
said to have occurred
when accumulation or
deposition of material
from the upper layers
leaves behind an
enriched horizon.
9. d. Calcification
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 with calcium
carbonate precipitated
from water moving
downward through the soil,
or upward by capillary
action of water from below
11. 2. Oraganic changes
These changes occur mainly on the surface and follow
a specific sequence. Degrading or break down of the
organic material by algae, fungi, insects and worms
causes humification which leaves behind a dark,
amorphous humus.
Extreme wetness may leave behind a peaty layer. On
further decay, the humus releases nitrogenous
compounds into the soil. This stage is called
mineralisation. The organic changes, thus, refer to the
accumulated effect produced by these processes.
Degrading → Humification → Mineralisation
12. 3. Podzolisation
podsolisation occurs when
strongly acid soil solutions
cause the breakdown of clay
minerals. As a result silica,
aluminium and iron form
complexes with organic
substances in the soil. These
minerals are removed from
the surface zone of the soil
and can accumulate in
distinct dark sub-surface
layers - very evident on
inspection. Upland heaths
and moors often contain
podsols.
13. 4. 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
14. 5. Decilication
It is the process by which
silica, together with many
bases, is removed from a
soil profile by intense
weathering and leaching.
It is characteristic of
humid, tropical areas and
leads to development of
ferralsol soils. Such soils
often have low organic
content because of rapid
decomposition by micro-
organisms.
15. Factors Influencing Soil Formation
There are five elements which control the pace
and direction of soil formation
• Parent rock1.
• Climate2.
• Biotic activity3.
• Topography4.
• Time5.
16. 1. Parent rock
This is the material in
which the soil develops
and may be the
consolidated geology of a
natural or an
anthropogenic deposit.
The parent material
affects the mineral status,
drainage and
susceptibility to erosion
17. 2. Climate
Temperature, rainfall, humidity and
evapotranspiration. These influence the net movement
of particulates and dissolved solids in the soil water.
Temperature and rainfall also influence the rate of
weathering processes.
18. 3. Biotic activity
Soil development is reliant on
biological activity. Plants and
animals provide the organic
component of soil; this is mixed
with the mineral component by
the soil fauna. Micro-organisms
play a crucial role in the decay of
organic matter and in the
formation of soil structure.
Humans also play a key role in
many soil forming processes, as a
vector of erosion and deposition,
through cultivation and land
management and in creating
whole soil profiles
19. 4. Topography
There are two topographic factors in soil formation. The
first is the steepness. All the soils around
the Earth are slowlysinking deeper and deeper as lower
levels of dirt turn into soil. At the same time, wind and
water carry the surface soil away through erosion.
1. Slope and Soil Formation
2. Drainage and Soil Formation
20. 5. Time
The passage of time allows soil features and horizons
to develop. When the soil is largely in equilibrium with
its environment it is considered to be mature. The
time this takes to happen varies according to the
environment and the parent material. Generally soils
develop more quickly on free-draining sandy soils,
than on slowly draining clay soils
