Option Unit Geo Ecology

Soils

Learning Objectives1. Describe what soil is made of (soil composition)

2. Explain how soil is formed.

3. Describe a typical soil profile.

4. 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 CompositionMINERAL 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 LEAVESAND 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 Characteristics1. Colour2. Structure – how the soil grains are stuck

together3. Texture – how the soil feels to touch4. Humus 5. Soil PH6. 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 THEPROCESSES THAT HAVE OCCURRED SUCH AS LEACHING.

Soil Characteristcs2. 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=ob2AMqZMLiw

• 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 VISIBLETO 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 CLAY- USUALLY MIXTURE OF THE THREE.

LOAM CONTAIN ROUGHLY EQUAL AMOUNTS OFEACH. 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 Characteristics4. 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. Weathering2. Humification3. Leaching4. Podzolisation5. Gleying6. Laterisation7. Salinisation8. Calcification

The factors of climate, relief, rock type, living things and time all influence which one of these processes take place.

WeatheringPhysical 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 rainRaindrops 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 windWind 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 wind2. 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 topsoil2. 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. TerracesLarge 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 bundsThese 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 weatherReduces 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 thecharacteristics of soil.

Soil characteristics are affected by their immediate environment and bycombination 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)