Information about Soil

1. UNIT 6: SOIL
Mr. T.B Mbele

2. 1.INTRODUCTION
. The study of soil is called Pedology
. Definition- Soil is the thin layer of loose mixture of small
particles and rotting organic matter that covers much of the
worlds land surface
. It is the medium in which plants grow and thus it supports
life on earth.

3. 2. WHAT MAKES UP SOIL?
• Soil is made up of mineral grains.
• Water is held between the grains
in the pores spaces
• 25% of the soil is air. Oxygen is
essential
• Organic matter is both coarse
and fine

4. 3. SOIL FORMATION
• Soil formation vary depending on what type of soil is
forming – clay, sand, or silt.
• Three stages of soil formation:
A. Erode – Elements of weather cause the rocks, soil, and
plant matter in an area to erode and become sediment that
contains various minerals and nutrients that were in the
previous materials.

5. B. Harden – The minerals harden over time to form
deposits within the ground – these will later combine with
other elements to create the final soil.
C. Combine – After humans, animals, or environmental
events cause the mineral deposits in the ground to become
loose, they combine with organic matter in upper layers of
the Earth and build until the soil forms.

6. 4. FIVE FACTORS THAT AFFECT SOIL
FORMATION
. Climate- Temperature, precipitation and how they are
distributed across seasons
• Biotic factors- Plants, animals fungi, bacteria and other
micro-organisms
• Topography- Slope position, Aspect and shape
• Parent material- Rock, alluvium (Wind or water deposited
material
• Time- How long the soil has been forming.

7. 5. SOIL HORIZON
• Soil has three
main horizon
which are A, B
and C. However
also O, E and R
are considered

8. 6. SOIL CLASSIFICATION
1. Alfisol
2. Mollisol
3. Ultisol
4. Spodosol
5. Aridisol

9. A. Alfisol
Alfisols are arable soil with water content adequate for at
least three consecutive months.
Alfisols are moderately leached soils that have relatively high
native fertility.

10. B. Mollisol
Soil of an order comprising temperate grassland soils with
a dark, humus-rich surface layer containing high
concentrations of calcium and magnesium.
C. Ultisol
A highly weathered leached red or reddish-yellow acid soil
with a clay-rich B horizon (subsoil), occurring in warm,
humid climates.

11. D. Spodosol
• a soil of an order rich in aluminum oxide and organic
matter, typically characterized by low fertility, and
including most podzols.
E. Aridisol
a soil of an order comprising typically saline or alkaline
soils with very little organic matter, characteristic of arid
regions.

12. 8. Soil colour
• The main pigmenting agents in soils are organic matter,
iron and to a lesser extent, manganese
• When these agents are not covering the mineral grains,
the natural colour of the grains is visible
9. Soil naming
• Soils are named and classified on the basis of physical
and chemical processes in their horizons (layers)
• Soil taxonomy uses colour, texture, structure and other
properties of the soil8

13. 7. SOIL AIR
• The volume of space that is not occupied by air particles
is known as “pore space” which means soil air
• The pore space is usually occupied by water and air
• The air filled pores constitute the gaseous phase of the
soil system
• Oxygen is essential for all biological reactions occurring in
soil
• Soil air diffuses from atmospheric air by having a higher
carbon dioxide and lower oxygen content

14. 8. Soil PH
• An acidity or alkalinity of a substance is
measured in pH units, a scale running
from 0 to 14. A pH of 7 is neutral.
• As numbers decrease from 7, the
acidity gets higher
• As numbers increase from 7 so does
alkalinity
• Soil pH generally range from an
extremely acidic pH of 3 to a very
alkaline pH of 10.
• Most cultivated plants enjoy slightly
acidic conditions with a pH of about 6.5.

15. 9. Physicochemical reactions of Soil:
A. physical properties
Structure of soil:
• It refers to the arrangement of soil separates into units
called soil aggregates
• An aggregates possesses solids and pore spaces
• The aggregate acts like a larger silt or sand particle
depending upon its size.

16. Soil density
• Soil density is expressed in two concepts as particle density and
bulk density.
Particle density:
• The weight per unit volume of the solid portion is called particle
density
• Particle density can be expressed in terms of grams per cubic
meter (g/m3)
• Generally particle density of normal soils is 2.65 grams per cubic
meter
• Particle density is also termed as true density

17. Textural classes Particle density (g/cm3)
Coarse sand 2,655
Fine sand 2,659
Silt 2,798
Clay 2,837
Particle density of different soil textural classes

18. Bulk density:
• The oven dry weight of a unit volume of soil inclusive of pores is called
bulk density
• The bulk density of a soil is always smaller than its particle density
• Bulk density normally decreases, as mineral soils become finer in texture
• The bulk density varies indirectly with the total pore space present in the
soil and gives a good estimate of the porosity of the soil

19. Soil porosity
• A soil’s porosity and pore-size distribution characterize its pore space, that
portion of the soil’s volume that is not occupied by solid material.
• The basic character of the pore space governs critical aspects of almost
everything that occurs in the soil, the movement of water, air, and other
fluids; the transport and the reaction of chemicals; and the residence of
roots and other biota.
Temperature of soil:
• Soil temperature is required for calculating most below ground
ecosystem processes, including root growth and respiration,
decomposition and nitrogen mineralization
• Mean annual soil temperature are 2-4 degrees celcious warmer
than mean annual air temparatures .

20. Soil plasticity
• Soil plasticity is the property of soil by which it undergoes
deformation without cracking or fracturing.
• The absorbed water contributes to the plasticity of the soil since it
allows the clay particles to slip over another
Soil compressibility
• Soil compressibility refers to the tendency of soil to consolidate or
decrease in volume
• The compressibility is partly a function of elastic nature of soil
particles and is directly related to settlement of structures
• Coarse materials such as gravels and sands have low
compressibility and the settlement is considerable less in these
materials as compared highly compressible fine grained organic
soil

21. Soil erodibility
• It refers to the ease with which soil
materials can be removed by wind
or water.
• Easily eroded materials include
unprotected silt, sand and other
loosely consolidated materials,
cohesive soils (with more than
20% clay) and naturally cemented
soils are not easily removed from
its place by wind or water and,
therefore have a low erosion
factor.

22. 10. Chemical properties
A. Inorganic matters of Soil
• Compounds of aluminum, silicon, calcium, magnesium, iron,
potassium and sodium are chief inorganic constituents of soils.
• Chemical composition of soil of one horizon differs greately from
composition of soil in the other horizon.
B. Organic matter of soil
Organic component of the soil consists of organic origin; living ad
dead
In sand soil of arid zone, it is found in every poor quantity (one or
less than one per cent) but in peaty soil, it may be as high as 90%

23. C. Buffer action:
• Buffer action of soil chemistry refers to the resistance to change
in pH of a system
• Property of soil to resist a change in pH is called “buffer action”.
• Buffer solutions are usually formed of a mixture of salt of weak
acid and acid itself
D. Buffering in soil:
• Soils should have good buffering capacity. Therefore, it is
necessary to add considerably large amount of acids or alkalis in
order to bring about any change in the original pH of soil.
• Phosphates carbonates , bicarbonates and other slats of weak
organic acids are important buffering agents in the soils

24. 11. Soil Chemistry in relation with Environment
• Soil chemistry has traditionally focused on the chemical reactions
in soils that affect plant growth and plant nutrition.
• However, beginning in the 1970s and certainly in the 1990s, as
concerns increased about inorganic and organic contaminants in
water and soil and their impact on plan, animal, and human
health the emphasis of soil chemistry is now o environmental soil
chemistry
• Environmental soil chemistry is the study of chemical reactions
between soils and environmentally important plant nutrients,
radionuclides, metals, metalloids and organic chemicals