Option Unit
Geo Ecology
Soils
Learning Objectives
1.
2.
3.
4.
Describe what soil is made of (soil composition)
Explain how soil is formed.
Describe a typical soil profile.
Name and describe the main characteristics of
soil.
5. Name and explain processes that affect soil
formation.
6. Describe the factors affecting soil formation.
7. Explain how these factors and processes
influcence soil characteristics.
Soil composition
(what soil is made of)
Soil Composition
MINERAL MATTER (45%):
ROCK PARTICLES FOUND ON THE BEDROCK AND WEATHERED
ROCK
AIR (25%):
FOUND IN THE PORE SPACES BETWEEN THE ROCK GRAINS.
WATER (25%):
ALSO FOUND IN PORE SPACES. IN DRY WEATHER, WATER
FORMS A THIN FILM AROUND THE GRAINS. IN WET WEATHER, IT
FILLS THE PORES.
HUMUS (4%):
PRODUCED FROM DECAYING VEGETATION SUCH AS LEAVES
AND THE REMAINS OF DEAD ANIMALS.
LIVING ORGANISMS (1%):
EARTHWORMS, BEETLES, FUNGI, BACTERIA
How Is Soil Formed?
• STEP ONE
Solid rock particles are broken down by
mechanical weathering. This forms small
soil grains
• STEP TWO
Chemical weathering releases important
nutrients from the rock e.g. calcium,
potassium
• STEP THREE
Seeds are blown or carried onto the soil grains
and may grow into plants that enrich the soil
when they die. Eg mosses and lichens
• STEP FOUR
Micro- organisms break down the remains of
plants and form humus. This helps bind the soil
together and improve fertility of soil.
• STEP FIVE
The cycle continues until the soil reaches max
fertility.
Soil Profile
Soil Profile
O – ORGANIC
HORIZON. Leaves, dead
animals, dead plants.
Humus is formed as the
dead organic matter
begins to decay.
A HORIZON –
Topsoil. Dark
Coloured.
B HORIZON –
Zone of mineral
accumulation
where rainwater
percolates
through the soil.
C – HORIZON
Soil Characteristics
6 Soil Characteristics
1. Colour
2. Structure – how the soil grains are stuck
together
3. Texture – how the soil feels to touch
4. Humus
5. Soil PH
6. Water Content
Soil Characteristics
1. COLOUR:
SOILS HAVE A RANGE OF COLOURS.
EG: PEAT IS DARK BLACK, OTHER SOIL
MAY BE BRIGHT RED OR YELLOW.
THE COLOUR OF A SOIL DEPENDS ON
ITS PARENT MATERIAL OR ON THE
PROCESSES THAT HAVE OCCURRED
SUCH AS LEACHING.
Soil Characteristcs
2. STRUCTURE:
•
THIS DESCRIBES THE WAY IN WHICH SOIL GRAINS ARE LUMPED
TOGETHER BY HUMUS AND CLAY PARTICLES.
•
IF YOU PULL A PLANT AWAY FROM THE GROUND, ITS ROOTS WILL HOLD
ONTO THE SOIL GRAINS. LOOK CLOSELY AND YOU WILL SEE THAT THE
GRAINS ARE IN SMALL LUMPS KNOWN AS PEDS.
•
THE SHAPE OF THESE PEDS INDICATES THE STRUCTURE OF THE SOIL.
•
THE SPACES BETWEEN THE PEDS HOLD AIR AND WATER AND ARE
IMPORTANT FOR PLANTS TO ACCESS AIR AND WATER IN SOIL.
•
OVERCROPPING AND OVERGRAZING DAMAGE THE STRUCTURE OF THE
SOIL, REDUCING ITS ABILITY TO SUPPORT PLANT GROWTH.
Crumb – small rounded clumps similar to breadcrumbs in
size. Loam soils have this structure and it is very good for
holding air and water. It is well drained and plants grow
well.
Blocky Structure – peds are closely packed angular
blocks. Well drained but can be compacted easily and
plants have difficulty growing in it when this occurs. Sandy
soils have this structure.
Platy Structure – peds are arranged in thin layers. Forms
in clay soils and is poorly drained. Plants have difficulty
growing in soils with this structure.
Soil Characteristics
3. TEXTURE:
http://www.youtube.com/watch?v=ob2AMqZ
MLiw
•
CONTROLLED BY THE AMOUNT OF SAND, SILT, AND CLAY
PARTICLES IN THE SOIL.
•
DIFFERENT SOILS HAVE DIFFERNET AMOUNTS OF PORE
SPACE WHICH AFFECTS HOW MUCH AIR CAN BE HELD
BETWEEN PARTICLES AND ALSO THE WATER CONTENT
AND DRAINAGE CAPACITY OF THE SOIL.
Texture
• THERE ARE THREE MAIN TEXTURES:
CLAY (PARTICLE DIAMETER 0.0002mm – NOT VISIBLE
TO NAKED EYE)
SILT (PARTICLE DIAMETER BETWEEN 0.0002 AND
0.05mm – BARELY VISIBLE TO NAKED EYE)
SAND (PARTICLE DIAMETER 0.05mm TO 2mm –
VISIBLE TO NAKED EYE).
• WHEN DESCRIBING SOIL TEXTURES, THE TERMS SILT, CLAY
AND SAND ARE IMPORTANT BECAUSE THEY INDICATE THE
SIZE OF THE GRAINS IN THE SOIL.
• CLAY SOILS:
SMALLEST SIZED PARTICLES.
STICKY SOIL WHEN WET.
CONTAIN APPROX 0-45% SAND, 0-40% SILT AND 40-100%CLAY.
NATUARALLY HIGH IN NUTRIENTS SO PLANTS GROW WELL.
OFTEN BAKED DRY IN SUMMER WITH VISIBLE SURFACE
CRACKS.
OFTEN CONSTANTLY WET AND WATERLOGGED IN WINTER.
CAN BE DIFFICULT TO DIG.
HOLDS TOGETHER LIKE PLAYDOUGH.
• SILTY SOILS:
CONTAINS PARTICLES SMALLER THAN SAND BUT
LARGER THAN CLAY SOILS.
FEELS POWDERY.
STICKS TOGETHER WHEN WET BUT OFTEN WILL NOT
HOLD ITS SHAPE WHEN DRY.
CAN BE BADLY DRAINED.
• SANDY SOILS:
CONTAINS PARTICLES VISIBLE TO NAKED EYE.
FEEL GRITTY – TEND NOT TO STICK TOGETHER WHEN WET.
GENERALLY CONTAIN 85-100%SAND,0-15%SILT-0-10%CLAY.
WATERLOGGING RARE AS THEY ARE VERY FREE DRAINING.
WATERING AND FEEDING OF PLANTS OFTEN NECESSARY AS
NUTRIENTS CAN DRAIN AWAY EASILY.
QUICK TO WARM IN SPRING SO PLANTING CAN BE DONE
EARLIER IN SANDY SOILS THAN OTHER TYPES.
• LOAM SOILS:
RARELY COMPOSED OF JUST SAND, SILT OR CLAYUSUALLY MIXTURE OF THE THREE.
LOAM CONTAIN ROUGHLY EQUAL AMOUNTS OF
EACH.
MOST PLANTS WILL GROW IN LOAM SOILS.
BROWN AND CRUMBLY IN TEXTURE.
RARELY WATERLOGGED IN WINTER OR OVERDRY IN
SUMMER.
LIGHT AND EASY TO DIG.
NATURALLY HIGH IN NUTRIENTS.
Soil Characteristics
4. HUMUS CONTENT:
•
IMPORTANT DUE TO ITS ABILITY TO SUPPORT PLANT GROWTH.
•
SOLS RICH IN HUMUS TEND TO BE DARK IN COLOUR. HUMUS IS A
DARK BLACK GEL FORMED BY ROTTING PLANTS AND VEGETABLE
MATERIAL IN SOIL.
•
THIS GEL HELPS TO BIND SOIL GRAINS TOGETHER. AS IT IS
WASHED INTO THE GROUND BY RAIN, IT ADDS NUTRIENTS TO
THE SOIL. LIVING THINGS ALSO ADD HUMUS TO SOIL BY
EXCRETION.
•
EARTHWORMS, BEETLES AND LARVAE BURROW THROUGH SOIL
AND CREATE AIR SPACES FOR PLANT ROOTS.
•
WHEN THESE CREATURES DIE, THEIR BODIES DECOMPOSE AND
FURTHER ENHANCE THE NUTRIENT QUANTITY IN THE HUMUS.
Soil Characteristics
5. ACIDITY/ALKALINITY OF THE SOIL:
•
THE PH OF THE SOIL IS HOW ACIDIC OR
ALKALINE IT IS (LITMUS TEST – JC SCIENCE).
MOST PLANTS PREFER A SLIGHTLY ACIDIC SOIL
WITH A PH OF 6.5
•
PEAT SOILS ARE EXTREMELY ACIDIC AND
CONTAIN FEW LIVING PLANTS.
•
NEUTRAL SOILS ARE MOST SUITED TO BACTERIA
WHICH HELP IN TURN TO RELEASE NUTRIENTS
SUCH AS NITROGEN INTO THE SOIL.
Soil Characteristics
6. WATER CONTENT/ RETENTION:
•
THE AMOUNT OF WATER A SOIL CAN HOLD DEPENDS ON
THE HUMUS CONTENT, TEXTURE AND STRUCTURE.
•
SOILS RICH IN HUMUS CAN HOLD MORE MOISTURE THAN
THOSE WHICH LACK HUMUS.
•
SOILS THAT HAVE A SANDY TEXTURE, ARE OFTEN DRY
BECAUSE WATER DRAINS BETWEEN THE SAND GRAINS
VERY QUICKLY.
•
A CLAY SOIL TENDS TO HOLD MORE WATER DUE TO THE
VERY SMALL GRAINS WHICH HAVE LARGE SURFACE AREAS
AND THEREFORE MORE WATER AROUND THEM.
Processes affecting soil
characteristics
You know some of these
already.. Think!!
Processes
1.
2.
3.
4.
5.
6.
7.
8.
Weathering
Humification
Leaching
Podzolisation
Gleying
Laterisation
Salinisation
Calcification
The factors of climate, relief, rock type, living things and
time all influence which one of these processes take place.
Weathering
Physical and chemical breakdown of rocks
into smaller pieces. Soil grains can be
released from rocks by weathering.
The soil grains produced by weathering
keep the characteristics of the parent rock
such as PH, texture and colour.
Carbonation and oxidation can release nutrients
such as phosphorous, calcium and iron from the
mineral grains.
Humification
• Dead organic matter is converted into humus by
fungi and bacteria. Makes the soil fertile as it
releases nutrients into the soil.
• Rain washes the humus into the soil.
• Climate is an important factor affecting the rate
of humification. Hot humid climates –
humification is very fast. The lack of water in
deserts may limit the amount of humus that can
be washed into the soil. In Ireland humification
occurs more slowly in Winter and in cold artic
climates humification may stop.
Leaching
• The removal of nutrients from the soil by
water.
• Sometimes minerals build up in a layer
lower down in the soil (hardpan)
• A certain amount of leaching is needed to
wash humus into soil. However excessive
leaching makes the soil infertile.
Podzolisation
• Type of leaching that occurs where rainwater is more
acidic.
• Podzol soils foirm under coniferous forests as these
forests decompose they add to the acidity of the
rainwater.
• The water seeping through the soil beneath this dead
vegetation becomes acidic due to the absorption of
humic acids from rotten vegetation.
• The top layer of podzolised soils is ash grey in colour.
The layer below is enriched with the dissolved mineral
from above and is darker in appearance.
• Podzols may also contain a layer of reddish iron oxide in
the B horizon. This is called a hard pan or iron pan and
prevent water from draining through the soil.
Gleying
• Process when the soil is waterlogged and lacks
oxygen.
• Water is between the soil peds and prevents
living things from surviving. They lack organic
matter.
• Climate and relief can lead the gleying. Heavy
rain can cause the land to become waterlogged.
If the land dips and hollows water can collect in
the hollows
• Gley soils have patches of grey/blue colouration.
• Common in Ireland’s drumlin belt (Co. Cavan,
Co. Monaghan).
Laterisation
• Type of severe chemical weathering that occurs
in tropical and equatorial regions.
• In areas of heavy rainfall such as the tropics and
equatorial regions all alkaline material is taken
from the soil by carbonation. The PH of the soil
is slightly acidic.
• The oxidation of iron and alluminium gives the
soils a red appearance and they are known as
latosols. If the soil dries out it is turned into a
hard laterite.
Salinisation
• Occurs when mineral salts move up through the
soil towards the surface.
• Can happen in hot desert areas of the world
where percipitation is low. The amount of water
evaporating out of the soil is greater than
percipitation falling.
• Evaporation causes salts in ground water to rise
through the soil and collect in the upper layers.
Salt is deposited on the surface as a hard white
crust. If salt concentration is too high plants are
poisoned and die.
Calcification
• Process by which calcium carbonate is
concentrated near the surface of the soil.
• Occurs in regions of low rainfall.
• The amount of water drawn up through the soil
by plants may be greater than the percipitaiton
falling on the soil. As a result calcium carbonate
builds up in the upper layer of the soil.
• Calcium carbonate is a useful substance for
plants.
Factors Affecting Soil Processes
and Soil Characteristics
• Climate – most important factor in soil formation
• Relief – Can influence the dept and drainage of a
soil
• Parent Material – The type of rock that a soil
develops from can influence PH, colour,water
content and texture of soil.
• Living Things – Influence soil fertility
• Time – The longer the soil forming process the
more developed the soil will become.
Activity
• Read the handout to discover how the
above factors affect the formation of soil.
They do by influencing the processes
(weathering, leaching etc) and
characteristics ( colour, PH, etc)
Soil Classification
3 Main Groups
ZONAL SOILS
•
These form where the influence of climate and
biological activity have been stable for along time.
•
These soils often have distinctive profiles and clear
horizons. They would include:
•
Brown Earths: brown in colour; crumb structure; high
natural fertility/ no distinct horizons. Found in many
parts of Ireland.
•
Podzols: Associated with leaching; poorly drained soil;
two distinct horizons (ash-grey and reddish brown);
need addition of fertilizers. Found in Cork, Kerry,
Wicklow, Mayo and Donegal.
INTRAZONAL SOILS
• These soils form where some local factor
such as parent material (e.g. presence of
limestone) or drainage conditions
predominate. They would include: Gley
and Peat soils.
GLEYS
• Gleys: These are soils that are often
waterlogged due to impermeable underlying
rock. The waterlogged condition stopped the
breaking down of organic matter. These soils are
blue/grey in colour; of poor fertility; mainly suited
to pastoral farming.
• Found in e.g. north west Munster and in the
Drumlin Belt of Cavan/Monaghan.
PEAT SOILS
• Peat Soils: Black in colour and consist of
partially decomposed vegetative matter. Heavily
leached and low in fertility.
• Blanket peat is found in the uplands, especially
on acid underlying rock such as the granite of
the Wicklow Mountains. Raised peat found in
waterlogged areas of the Central Plain.
AZONAL SOILS
1. Azonal Soils - These are immature soils
where the soil farming processes have not had
sufficient time to complete their task. These
soils do not have a well developed profile.
•
They would include:
Lithosols. Associated with recently weathered
bed rock where mass movement and erosion
prevent the development of a soil profile.
Infertile, used possibly for forestry; some areas
of Connemara
What to Know….
• Should be aware of the different soil types in
each group.
• Should describe one Irish soil in detail – Brown
Earth soil.
• Should describe the aridsols of desert climates
(we did this when studying the desert biome!)
*Via syllabus you should be able to compare two
soil types*
Irish Brown Earth Soils
Read the handout on brown earth soils.
It is broken up into the factors that affect its
formation and the characteristics of brown
earth soils and the processes affecting the
formation of brown earth soils
Human Interaction
Question
Soil Erosion and Conservation
Learning Objectives
1. Name and explain the causes of soil
erosion.
2. Describe in detail how human activities
can cause soil erosion.
3. Name and explain the primary methods
of soil conservation.
Soil Erosion
Most soil erosion is caused by natural
processes such as flowing water and by
wind. Human activities such as overcropping
and overgrazing land and deforestation also
cause soil erosion.
Natural cause of soil erosion
Soil erosion by rain
Raindrops on average fall at a speed of
32.19km/hr. The force of impact of raindrops break
apart the soil grains. During heavy rain water can
no longer seep into the soil and begins to flow over
the surface (runoff). The water makes channels
called rills and gullies in the soil. Millions of tonnes
of soil are removed from farmland by rainwater in
this way.
Soil erosion by wind
Wind is effective at blowing dry exposed soil
away. Wind removes soil by
• Saltation – fine and medium sand sized particles
are lifted a short distance into the air
• Suspension – very fine soil particles are lifted
from the surface, carried high into the air, it
remains suspended in the air for long periods.
• Surface Creep – the movement of large soil
particles along the surface of the soil.
The amount of soil erosion by wind and rain
that occurs depends on
1. The quantity of water moving downhill
2. The speed of the water
3. The strength of the wind
4. The steepness of slope.
5. The condition of the soil surface and the
type of soil.
Human causes of soil erosion
•
Can trigger soil erosion due to poor farming methods and
deforestation.
•
Farming and deforestation can change soil characteristics and
damage soil structure.
•
Human activities such as overgrazing, overcropping and
deforestation have led to desertification, soil erosion and famine.
•
Tourism, outdoor activities such as quad biking damage fragile soil
structure.
The amount of soil erosion caused by human activities depends on
1. The type of cultivation – may leave soil exposed to the wind
2. The amount of vegetation cover removed – deforestation can lead
to landslides.
3. The intensity of land use – heavily used soils are more easily
eroded.
4. The length of time land left rested – overused soils are often dry.
Problems caused by soil erosion
1. Loss of valuable topsoil
2. Poor soil washed downhill can bury
valuable fertile soil on the lowland below.
3. Damage to fields because gully erosion
reduces the field size and takes land out
of production.
4. Erosion causes a steady but slow plant
productivity decline.
5. Desertification
Case Study
• Soil erosion and desertification in the
Sahel region in Africa.
• Read the photocopied handout on the
above case study.
*Human interaction answer*
Methods for soil conservation
1. Windbreaks
Barriers formed by trees and other plants with
many leaves. They are planted around the edges
of fields. They stop the wind from blowing soil
away and from destroying or damaging crops.
They work best when they allow a little wind to
pass through to avoid violent gusts of wind forming
close to the ground that would carry away soil.
2. Contour ploughing/ strip ploughing
The tractor operator follows the contours of
a hillside. They go around the sides of hills.
The plough creates ‘mini terraces’, slowing
or stopping the flow of rainwater and
encouraging it to seep into the soil.
If ploughing straight up and down the terrace
would create ditches allowing water to flow
down picking up speed and soil.
3. Stubble planting
The old stubble of harvested crops is not ploughed
back into the soil. Any fertilisers and new seed
planted afterwards are inserted into the soil
through small slits cut into the soil. Ie the soil is left
undisturbed.
The stubble will reduce wind and water erosion
while the new crop is growing. The stubble left in
the soil will rot into it eventually.
4. Terraces
Large steps cut into a hillside to make flat land for
agriculture. Work by reducing slope length and steepness
so water looses energy (cant carry soil away as fast).
5. Stone walls or bunds
These low walls are placed along the contour of a
hill and capture water allowing it to filter into the
soil rather than running off downhill.
6. Reduce ploughing in dry/windy weather
Reduces the risk of wind erosion. Soil is lost during the
plouging process. If the following days are windy more soil
will be lost. Rain is needed to dampen and settle the fine
soil particles.
Exam Questions
• Remember the marking scheme…
• 3 or 4 paragraphs in each answer
• Coherance marks for each paragraph.
You will have to adjust your notes to suit the
question you are asked.
Explain how weathering, leaching and podzolisation have impacted on the
characteristics of soil.
Soil characteristics are affected by their immediate environment and by
combination of processes operating in that environment.
Examine any three soil processes that affect soil characteristics.
Examine two of the natural processes which influence soil formation.
Discuss how human activities can accelerate soil erosion.
Examine two ways in which human activities have impacted on soils.
Examine how over-cropping / over-grazing and desertification can affect soils.
Describe and explain the characteristics of any one soil type studied by you.
With reference to one soil type you have studied, examine how parent material,
climate and organic matter influence the soil (composition)