This document discusses the composition, formation, types, and characteristics of soils. It begins by explaining that soils are composed of 45% mineral matter, 25% each of water and air, and 5% organic material. Soils are formed through the processes of weathering and decomposition of rocks and organic materials. Soil types are classified based on soil profiles, which describe the vertical layers or horizons of a soil. Key soil characteristics that influence fertility include color, structure, texture, organic content, pH, and water content. The document provides details on various soil textures including sandy, clay, silty, and loam soils and explains how these textures impact drainage and nutrient retention.

1. Soils
Composition, Formation, Types and Characteristics
Course : B.Sc. Microbiology/ Bio-Technology/ Bio-Chemistry
Sem I
Sub: Environmental Science
Unit 3.2

2. Soil Composition
• What is soil made up of?

3. Soil Composition
45%
25%
25%
5%
Mineral Mater
Water
Air
Organic Material

4. Soil Composition
1. Mineral Matter: Rock particles from the bedrock
and weathered rock.
2. Air: found in the pore spaces between rock
grains.
3. Water: Also found in pore spaces.
4. Organic Material:
– Humus – a black sticky gel produced from decaying
leaves and animals.
– Living organisms – earthworms, beetles, fungi, bacteria.

5. How are Soils Formed?
1. Solid rock particles
break down from
mechanical
weathering.
2. Chemical weathering
releases important
nutrients from the rock
grains.
3. Seeds are blown or
carried onto the soil
grains and may grow
into plants that enrich
the soil when they die.
4. Micro-organisms
decompose the
remains of plants to
form humus which
further enriches the
soil.
5. The cycle continues
until soils reaches
maximum fertility given
the climate it is in.

6. Soil Formation
1

7. How are Soils Formed?
1. Decomposition
2. Weathering (not
moving)– rain and
wind break rocks
down into soil
2

8. How are Soils Formed?
1. Decomposition
2. Weathering
3. Erosion (moving)–
when the rocks
break and fall apart
in the wind or rain
3

9. Soil Profiles (Types)
• Show the type of soil in the region.
• Defined as a vertical section of soil from the ground
surface downwards to where soil meets the
underlying rock.
• Composed of a number of distinctive layers called
horizons.
• The key to understanding the processes involved in
soil development.

10. Soil Profiles
O = Organic horizon
• This has a high
percentage of organic
matter – leaves, plants
and dead animals.
• Humus is formed as
the dead organic
matter decays.
• Also called the litter
layer.
4

11. Soil Profiles
A Horizon
• Known as topsoil.
• A dark organic
material mixed with
mineral soil grains.
• It is a source of plant
nutrients and contains
the majority of plant
roots.
4

12. Soil Profiles
B Horizon
• Also called the subsoil.
• The B horizon is the
most widely used to
identify soil type.
• May result purely from
the weathering of
underlying rock or from
movement downwards
of materials from
horizons above.
4

13. Soil Profiles
C Horizon
• Lies just above the
bedrock.
• The overlying soil
horizons often
develop from the C
horizon.
4

14. Soil Characteristics
• The characteristics of a soil influence the soil’s
fertility.
• Characteristics:
1. Colour
2. Soil Structure
3. Texture
4. Organic Content
5. PH Value
6. Water Content

15. Soil Characteristics
1. Colour
• The colour of a soil indicates the amount of organic
material in it.
• Humus-rich soils = black / dark drown.
• Leached soils = white.
• Dark soils absorb more sunlight and warm up more
quickly, which aids seed germination.
• Pale coloured soils reflect sunlight and are slower to
warm up.

16. Soil Characteristics:
2. Soil Structure
• This refers to the shape of the soil grains or
particles.
• Soil grains or particles cluster together to form
small lumps called peds.
• The shape of these peds indicates the structure
of the soil.
• Common soils structures:
1. Crumb/ granular
2. Blocky
3. Platy

17. Crumb / Granular
• Loam soils have this structure.
• The peds are small rounded clumps of soil
particles similar to breadcrumbs in size.
• This structure is good for holding air and
water.

18. Blocky
• Sandy soils have this structure.
• Peds are closely packed angular blocks.
• Well drained but can be compacted easily -
plants have difficulty growing when this
occurs.
• Adding humus to this soil can improve
structure for plant growth.

19. Platy
• Forms in clay soils.
• Soil peds are arranged in thin layers.
• Prevents good drainage of water through
the soil.
• Plants have difficulty growing in this soil
type.

20. Soil Characteristics 3.Texture
• How a soil feels when you touch it.
• Determines whether a soil can support plant
growth.
• There are 4 main textures:
1. Sandy soils
2. Clay soils
3. Silty soils
4. Loam soils

21. Sandy soils
• Have gritty texture (85 – 100% sand).
• Formed from weathered rocks such as
limestone and granite.
• Waterlogging is rare in sandy soils – they are
very free draining.
• Watering and feeding of plants (fertilising) is
needed regularly because the nutrients drain
away easily.
• Tend not to stick together when wet.

22. Clay soils
• Contain 40-100% clay.
• Very sticky and lumpy when wet.
• Rock hard when dry.
• Made up of very fine particles with few air spaces.
• Hard to work and often waterlogged.
• Rich in nutrients so plants will grow well if enough
drainage.

23. Silty soils
• Contain 40-100% silt.
• Composed of minerals (mainly quartz) and
fine organic particles.
• Have more nutrients than sandy soils and do
not tend to become waterlogged.
• Have a smooth and powdery texture when
dry.

24. Loam soils
• Contain roughly equal amounts of sand, silt and
clay.
• Usually drain well and retain enough moisture to
support roots.
• Nutrient rich.
• Light and easy to dig.
• Most plants will grow in loam soils.
• Feels crumbly in texture.

25. 5

26. Soil Characteristics
4. Organic / Humus Content
• This refers to the amount of dead leaves, roots,
plants and animals that have rotted away to form
humus.
• Humus binds the soil together limiting soil erosion.
• Humus content affects the colour, texture and
structure of soils.
• Soils rich in humus tend to be dark with a good
crumb structure and loam texture.

27. Soil Characteristics
5. pH Value
• pH value = a measure of a soil’s acidity or
alkalinity.
• pH of a soil is affected by the parent material
(rock it developed on).
• Most plants prefer a slightly acidic soil with a
pH value of 6.5.
• Very acidic soil discourages the presence of
living things which reduces humus content of
soils.

28. Soil Characteristics
6. Water content
• Water is important for soils because it:
– Enables plants to absorb nutrients dissolved from
the mineral grains.
– Enables the survival of micro-organisms responsible
for humus formation.
– Reduces soil erosion by wind.
– Enables formation of soil horizons.
– Binds soil particles together.

29. Soil Characteristics
6. Water content
• Some soils allow percolation and leaching to
occur easily, e.g. sandy soils.
• Other soils, like clays, limit these processes
and become waterlogged.

30. References
1. http://image.slidesharecdn.com/soil-131118202444-
phpapp01/95/soil-5-638.jpg?cb=1384828112
2. https://lh3.ggpht.com/7Nbbk1F2Bqehl6vAR2PETUPVe9U
8Qo98MFEbjl2SabmWeKysAdlJ8kvN0OCCPL1EbtWlZw=s1
14
3. https://lh4.ggpht.com/CPnWZk4wu_j0hHRP-
YhFc0Ldb9Dz27BLkwCx1WDOLUkK56K8YSL8DyKJYKYknW
YY7SxZiA=s119
4. http://warehouse.olc.edu/~jtinant/webfolder/JTaulman/fi
eld_ecology2_files/image002.jpg
5. https://lh5.ggpht.com/43L_dzGVIFEYffbge10jHhXKCJRcC
OFqRy4Xs4SnLZAHL6fc_2zmMd9wjxMURF2kYuFF=s89

31. Books:
1. Environmental studies by R.Rajagopalan
2. Environmental Science by Richard T Wright & Bernard J
Nebel
References