2. Soil can be defined as the solid material on the Earth’s surface that
results from the interaction of weathering and biological activity on the
parent material or underlying hard rock.
The study of soils as naturally occurring phenomena is called pedology
(from the Greek word pedon, meaning soil or earth).
Pedology takes into account:
•factors and processes of soil formation
•soil characteristics
•distribution of soil types
Soil: some definitions
3. The basic unit of study: Soil Profiles
A soil profile is a vertical cross-
section of a soil. It is divided into a
number of distinct layers, referred to
as horizons.
The horizons are normally
designated by symbols and letters.
The presence or absence of
particular horizons allows pedologists
(soil scientists) to classify the soil.
In addition, the organic or O horizon
can form above the mineral soil-
commonly in forested areas, resulting
from the dead plant and animal
remains.
TOPSOIL,
upper or A
horizon
SUBSOIL,
middle or B
horizon
PARENT
MATERIAL,
lower or C
horizon
This diagram shows
simplified soil horizons
4. Soil Formation
§ The Importance of Soil
• All life depends on the thin top layer of soil
covering the earth’s surface.
• Topsoil provides:
• Support
• Water
• Air
• Nutrients
5. Capillary
action
Translocation
Click over factors for further explanation.
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Leaching
Soils are complex and dynamic systems, in which many processes are
taking place.
Decomposition
and Humification
Weathering
Soil Forming Factors
Soil
PROCESSES
6. Weathering
This refers to the breakdown and decomposition of rocks
and minerals by factors including air, water, sun and
frost.
Physical weathering involves continual breakdown or
rocks into smaller and smaller particles.
Chemical weathering involves alteration of the chemical
composition of rock minerals.
8. Mechanical
§ Geological Activity – abrasion
• often occurs to rocks in rivers, beaches, or desert
areas
§ Glacial Activity
§ Expansion of ice
§ Effects of Temperature
Weathering: Breakdown of Rock near
the Surface
10. Weathering: Breakdown of Rock near
the Surface
Chemical Alteration
§ Carbon dioxide, sulfur dioxide and various
nitrogen compounds from the air form acids
when dissolved in water. These acids may
react with the rock and increase
breakdown.
15. Decomposition and Humification
Decomposition is the breakdown of plant derived material into its simpler
organic constituents. This is accomplished by enzymes, earthworms,
mites and other organisms.
Humification is the breakdown of plant remains- leading to the formation of different
types of humus. It is probably the most important
biological process taking place in soils.
MULL humus develops under deciduous woodland, where base-rich plant remains
are actively broken down by a prolific soil biota.
MODER humus is intermediate between mor and mull.
MOR humus usually develops beneath coniferous woodland or heather moorland,
under cool, wet climatic conditions. Breakdown is slow due to the absence of soil
biota.
16. Capillary action
Where evaporation exceeds
precipitation, moisture moves upwards
within the soil profile by capillary action.
It is therefore in the reverse direction to
leaching.
In Britain precipitation generally
exceeds evaporation. As a result
capillary action in British soils rarely
occurs, apart from in very sandy soils.
17. Leaching
§ Wherever rainfall exceeds evaporation and there
is free downward movement of water through the
soil pore system, soluble minerals are leached or
removed from the soil profile.
Continual leaching tends to impoverish the upper mineral horizons
by removal of basic cations (cations are ions having a a positive
electrical charge e.g. Ca2+).
§ Leaching is most active in sandy soils with high porosity and is
least
in fine-textured soils such as clays
which have restricted pore spaces.
A soil with small soil peds or crumbs and
high porosity leading to free drainage and
active leaching
18. Translocation
The movement of material in solution or
suspension from one horizon to another is
referred to as translocation.
The upper mineral horizon losing the material
is the ELUVIAL or E horizon. This is where
maximum leaching or eluviation takes place.
The E horizon near the surface of a podzol is
a good example of an eluvial horizon.
The lower horizon gaining the material is the
ILLUVIAL horizon (often a subsoil or B
horizon). This is the zone of maximum
accumulation.
19. Soil colour
Generally soil colour is determined by the amount of organic
matter and the state of the iron. Soil colour is also related to soil
drainage, with free draining, well AERATED soils (with pore
space dominated by oxygen) having rich brown colours.
In contrast, poorly draining soils, often referred to as gleys,
develop under ANAEROBIC conditions (the pore space
dominated by water) and have grey or blue-grey colours.
Soils with periodic waterlogging are imperfectly drained and are
often highly mottled with blotches of contrasting colour.
MOTTLES are often rusty in colour and are due to iron
concentration.
Such colours are the result of oxidation-reduction; iron is the
main substance affected by these processes. If the iron is
released in an anaerobic environment, then it stays in the
reduced state giving it the grey blue colour of waterlogged soils.
20. Soil texture
Soil clod on right (above) is dense with a poor texture and leads
to a poor structure if badly managed. Due to continual
compaction it is blue-grey in colour due to low oxygen conditions.
It is a poor environment for root development. Soil clod on left
has been well managed and is relatively loose with ample pore
space for good, healthy root development.
Soil texture is a term used to describe the distribution of the
different sizes of mineral particles in a soil.
Textures range from clay, sand, and silt at the extremes, to a loam which has all
three sized fractions present. The main influence of texture is on permeability which
generally decreases with decreasing particle size.
Soil high in silt and clay
with compact subsoil
lacking in pore spaces
22. What Determines Soil Type
§ Climate
§ Vegetation
§ Drainage
§ Time
§ Parent Material
• Residual - Transported
• Least Important Factor for Mature Soils
26. Soil Horizons and Profiles
Soil Profile
§ Suite of Layers at a Given Locality
Soil Horizons
§ Layers in Soil
§ Not Deposited, they are Zones of Chemical Action
§ Layers (horizons) of mature soils
• O horizon: leaf litter
• A horizon: topsoil
• B horizon: subsoil
• C horizon: parent material, often bedrock
27. Principal Soil
Horizons
§ O - Organic (Humus) Often
Absent
§ A – Leaching
• K, Mg, Na, Clay
Removed
§ B – Accumulation
• Absent in Young Soils
• Distinct in Old Soils
• Al, Fe, Clay (Moist)
• Si, Ca (Arid)
§ C - Parent Material
28. Soil Characteristics
§ Physical
• Texture
• Porosity
• Permeability
• Humus
§ Chemical
• pH
• *You need to
know how to fix
pH problems
• Nitrogen
• Phosphorous
• Potassium
29. § Porosity - the percentage
of interconnected space
in rock and soil that can
contain water
Permeability – the degree to
which the pores in the rock
or soil are connected
together so that water can
move freely
30. Main Soil Textures
Soil Type Texture Permeability Porosity
Sand Gritty High Low
Silt Smooth &
Slippery
Med Med
Clay Sticky Low High
31. Soil Formation vs. Soil Erosion
§ Soil formation
• Takes hundreds of years to form 1 cm (0.4
inches) of soil
§ Soil erosion
• Blown away in weeks or months from plowing
and clearing forests – any time we leave the
topsoil unprotected
32. Topsoil Erosion Is a Serious Problem in
Parts of the World
§ Soil erosion
§ Two major harmful effects of soil erosion
• Loss of soil fertility through depletion of plant
nutrients in topsoil
• Water pollution in nearby surface waters where
eroded soil ends up as sediment
• Kills fish, shellfish
• Clogs irrigation ditches, reservoirs, lakes, and boat
channels
• Eroded soil may also be polluted with pesticides
and fertilizers
36. Transformarion
A process in which one form (organic matter)is change into other
form(inorganic form).
1)Mineralization:
As mineralization is conversion of organic matter of nutrients into
inorganic form.
Nutrients elements are converted from unavailable form to
available form in plants.
37. Example
2)Decomposition
The waste of plants and animals are released into the soil.Micro-
organisim are present in soil.They decompose the dead bodies of
plants and animals.They convert organic form (dead form) into
organic form.
38. Soil components are transformed by chemical and biological
reaction.
Organic compounds decay
some minerals dissolve
other minerals precipitate.
Benefits of Transformation:
These transformation result in the development of soil
structure, and in changes in color, relative to the parent
material.
39. Factors that
effect the soil
formation
Parent material
Organisim
REMEMBER
All of the 5 factor(parent
Material,topography,climate,time and
organisim) are acting at the same rate
at different rate of speed and
efficiency.
40. • Parent Material
t refers to the mineral material or organic material
from which the soil is formed.
The soils will carry the characteristics of its parent
material such as color, texture, structure, mineral
composition.
For example, if soils are formed from an area with
large rocks (parent rocks) of red sandstone, the
soils will also be red and have the same feel as its
parent material.
41. Continue
Soil minerals form the basis of soil. They are produced from
rocks (parent material) through the processes of weathering
and natural erosion(parent material are actually rock).
Water, wind, temperature change, gravity, chemical
interaction, living organisms and pressure differences all help
break down parent material.
The types of parent materials and the conditions under which
they break down will influence the properties of the soil
formed
42. Forexample
Most common parent Material cover the surface of land which is
volcanic ash.
The properties of volcanic ash are:
Light weight
Hold water(material which is transported by H2O calledAlluvium)
Influemce soil genesi
43. Organisim
These organisim include Bacteria,fungi,plants
and animals.These organism effect the soil
formation.
Plants provide lots of vegetative residues that are
added to soils.
Their roots also hold the soils and protect them
from wind and water erosion But root are
incarporated inside soil and soil is broken
down by organisim which present inside soil
They shelter the soils from the sun and other
environmental conditions and block sunlight to
reach on soil.
44. • Continue
Fungi, bacteria, insects, earthworms, and
burrowing animals help with soil aeration.
Worms help break down organic matter and aid
decomposition.
Animal droppings, dead insects, and animals result
in additional decaying organic matter.
Microorganisms also help with mineral and nutrient
cycling and chemical reactions.
Soil which have dark colour show high nutrient
level than light colour.
45. • Continue
Microorganisms are essential to soil formation
because
they control the flux of nutrients to plants (i.e.,
control of carbon, nitrogen, and sulfur cycles,),
promote nitrogen fixation
promote soil detoxification of inorganic
naturally occurring organic pollutants.
48. • Climate
This is probably the most important factor that
can shape the formation of soils. Two important
climatic components, temperature, and
precipitation are key. They determine how
quickly weathering will be and what kind of
organic materials may be available on and inside
of the soils.
49. Function of
precepitation
The variability of Precipitation effects the
composition of soil.
For example, Areas with low rainfall rates and
high evaporation rates led to the accumulation
of salts in soil.
The soil underlying tropical rainforest appear to
nutrient poor due to extensive leaching due to
heavy rainfall.
50. • Function Of
Temperatur
e
Its also plays an important role because
temperature variations cause shrinkage and
swelling, forest action and general soil
Weathering.
51. • Time:
Soils can take many years to form. Younger soils
have some characteristics from their parent
material, but as they age, the addition of organic
matter, exposure to moisture, and other
environmental factors may change its features.
With time, they settle and are buried deeper
below the surface, taking time to transform.
Eventually, they may change from one soil type
to another.
52. Contd......
The formation of soil is not one day process, it
takes several years of formation.
Younger soil have similar characteristics like
parent material.
54. • Topograph
y:
Slope and aspect affect the moisture and temperature of soil.
Steep slopes facing the sun are warmer.
Steep soils may be eroded and lose their topsoil as they form.
Thus, they may be thinner than the more nearly level soils that
receive deposits from areas upslope.
Deeper, darker coloured soils may be expected on the bottom land.
55. • Biological
factors:
Plants, animals, microorganisms, and humans
affect soil formation.
Animals and microorganisms mix soils and
form burrows and pores. Plant roots open
channels in the soils.
Different types of roots have different effects on
soils. Grass roots are fibrous near the soil
surface and easily decompose, adding organic
matter.
56. Contd....
Taproots open pathways through deeper layers.
Microorganisms affect chemical exchanges
between roots and soil. Humans can mix the soil
so extensively that the soil material is again
considered parent material.
The native vegetation depends on climate,
topography, and biological factors, plus many
soil factors such as soil density, depth,
chemistry, temperature, and moisture.
57. Contd....
Leaves from plants fall to the surface and
decompose on the soil. Organisms decompose
these leaves and mix them with the upper part of
the soil. This increase soil fertility.Trees and
shrubs have large roots that may grow to
considerable depths.
