Soils
Pedology… the scientific study of soils. The surface layer of loose
material that covers the earth’s surface containing organic & inorganic
matter.
Soil Composition:
Made up of mineral matter, organic matter, air & water.
Mineral Matter:
Got from parent material such as bedrock (limestone
sandstone) which is broken down by physical & chemical actions.
Also got from glacial, river & wind blown deposits.
Mineral content refers to calcium, potassium,
phosphorus & other such compounds. These are food for plants.
Parent material determines the colour, texture, depth
and pH value of soil.
7.1 SOILS: Composition
Organic Matter
Also know as humus. Formed by decaying plants, animal life & through
actions of bacteria & micro-organisms. Burrowing animals & worms
aerate the soil & mix soil layers.
Humus improves soil texture & binds soil particles together.
Plant roots also bind soil, take up nutrients & return them when they die.
Climate
Distribution of soils coincide with distribution of climates.
Climate, mainly temp & precipitation, influences the type of veg growth
Chemical weathering in hot climates creates great depth of soils.
Large the vegetation the greater the amount of humus.
Climate influences rate of weathering of soil & decay of plant matter.
Heavy rainfall causes leaching. Weathering is the first stage of soil
Slope (topography) & water
Steep slopes result in removal of fine particles by rainwater but better
drained.
Flat slope encourage accumulation of fine particles.
Aspect (way it faces) creates different soils on north & south slopes.
Water moves through the layers & carries nutrients with it…leaching.
This can create a hard pan & impedes drainage.
Air
Vital for oxidation… converts organic matter into oxygen.
Vital for presence of bacteria which require oxygen.
7.1 SOILS – Development Profiles
General
O Organic Layer consists of plant
litter and humus
Not present in cultivated fields
A Topsoil consists of humus and
minerals
The ploughed layer
B Subsoil low in organic material
C Transition Area between soil and
parent material consists of partially
disintegrated bedrock
R Bedrock
7.1 SOILS – Development Profiles
Brown Earth
Source ENCARTA
Tropical Red Soil
Soil Characteristics
Texture, structure, colour, moisture, humus, air, pH
Texture
Refers to proportion of sand silt & clay. Texture determined by mixing
water & soil in a jar
Texture influences soils ability *to retain & transmit moisture
*retain nutrients
*allow roots to penetrate
Sandy soils…. 70% or more sand & have few nutrients…feel gritty
Clay soils… 50% or more clay & rich in nutrients …easily
waterlogged…feel sticky
Silty soils intermediate btw sandy & clay… feel smooth
Loam soils fertile… mixture of various particle sizes…. well aerated
with some moisture & plant food.
7.1 SOILS: Characteristics
– Determining Soil Texture
 Fill a screw topped
clear glass jar two
thirds full with soil
 Add water until jar is
almost full
 Shake the jar for 1
minute approx.
 Leave the jar to settle
for 24 hours
100
90
80
70
60
50
Clay
Silt
Fine
Sand
40
30
20
10
0
Coarse
Sand
Colour
Humus soils are dark brown or black.
Brown soils get colour from decayed leaves & plants.
Dark soils absorb more sunlight & so are warmer than light soils…
helps seed germination & longer growing season.
Structure
Refers to shape of soil particles.
Clusters of particles form in undisturbed soil called peds.
Shape & size of peds determine the pore spaces for air & water &
organisms to pass through.
Crumb structure soils give highest agric. yield as it provides a
balance btw air, water & nutrients.
7.1 SOILS: Characteristics –
Structure Single Grained Structure
 Associated with sandy soils
 Water, air and organisms can
pass through it easily
Single grained
 Roots can spread out into the
soil easily
 Water retention is poor Nutrient retention is poor
 Prone to leaching
 Vulnerable to drought
7.1 SOILS: Characteristics –
Structure
Crumby Structure
 Associated with good
agricultural land – loam soils
Crumb : 1 - 5mm
 Water, air and organisms can
pass through crumby soils
easily
 Roots can spread out into the
soil easily
 Water retention is good as it is
soaked up by the crumbs of
soil – Nutrient retention good
7.1 SOILS: Characteristics –
Structure
Massive Structure
Massive

Usually associated with clay type
soils

Water, air and organisms can not pass
through this type of soil easily

Roots have difficulty penetrating the
large lumps of clay

Soils are often waterlogged after
periods of rain

Soil dries into hard clumps after a dry
period
Moisture Content
Water moves down by percolation & up by capillary attraction.
Carries many mineral compounds…. creates process of solution.
Water content varies from nil (arid) to very high (waterlogged).
Persistent percolation causes leaching (minerals drawn downwards).
Leaching creates podzol soils.
Organic Content
Formed from decaying plants & animals.
Humus gives soil a dark colour…. falling leaves, decaying grass & roots
(organic matter) main input for humus… create black earth soils.
Bacteria & fungi break down organic matter.
Soil organisms will mix plant litter into the A horizon where it will
decompose into humus.
pH value
pH a measure of its acidity or alkalinity.
Low pH is acidic soil… happens where minerals like calcium &
potassium are leached. Neutralised by adding limestone.
Very low acidic soil slows decomposition & creates peat.
High pH means alkaline soil… indicate high lime (calcium) &
potassium content.
Neutral soil most suitable to farming.
7.1 SOILS: Characteristics – pH
value
pH = potential Hydrogen ions
pH value of 7 = neutral
7.1 SOILS: Characteristics Texture
Particle Size
Clay:
Less than 0.002mm
Silt:
0.002 – 0.02mm
Sand:
0.02 – 2.0mm
7.1 SOILS: Characteristics Texture
Sample
Sand 70%
Clay 20%
Silt 10%
7.2 SOILS – Influences
Affected by:
• Immediate Environment
• Processes Including Human
Interference
Soil Formation
Factors that affect soil formation:
Parent Material
Soil formation occurs more rapidly if its composed of boulder clay
deposits rather than bedrock.
Soils inherit characteristics of parent material.
Soils from limestone will have calcium & become alkaline.
Soils from sandstone will be sandy, easily drained & heat quickly.
Climate
Temp & precipitation effects include:
* Heat & moisture encourages growth.
* Hot wet regions have lots of veg. covers… more humus
* Decomposition occurs quickly
* Heavy rainfall causes leaching
* Spells of drought increases minerals like calcium due to evaporation.
Topography
Level surfaces.. deeper soils.
Steep slopes… thin soils, erosion
but well drained. South facing slopes warmer… encourage growth.
Living Organisms
Roots bind soil & absorb moisture & nutrients.
Insects break down & aerate soil.
Time
Soils develop over thousands of years..speed depends on parent material.
Can be washed away in floods or eroded quickly like Dust Bowl In
North America.
Precipitation
Water essential for growth….absence leads plants to die or adapt (desert)
Water retention determined by soil structure & texture.
Clays hold more water than sandy soils.
7.2 SOILS – Global Climate
Types
7.2 SOILS – Global
Patterns
7.2 SOILS – European
Patterns
Factors influencing Soil Characteristics
Processes
Weathering
Physical weathering breaks down the parent rock… soil retains these
characteristics.
Chemical weathering breaks down soil by carbonation, hydration,
hydrolysis & oxidation. Weathering release nutrients into soil.
Humification
Breakdown of plant matter into humus. Faster in hot, humid regions.
Leaching
Heavy rainfall carries minerals to lower horizons. Upper soil becomes
acidic & mineral deficient.
7.2 SOILS – Influences
Processes - Weathering
Podzolisation
Rainwater becomes acidic in coniferous or acid bog soils & removes
minerals & soil becomes acidic. A horizon becomes greyish … drained
of all colouring materials & B horizon reddish….has many of the
dissolved minerals. Hard pan develops causing waterlogging. Infertile
podzol soil.
Laterisation
Intense rainfall & rapid decay means little acid in soil.
Leaching removes minerals except iron & aluminium…..due to large
concentration of these minerals. These build up close to surface creating
bauxite (alum) & red laterite (iron) soils. Occurs in tropics where
rainfall & temp are high.
Salinisation
Ground water rising to surface by capillary action carries salt deposits…
create a hard toxic crust. Occurs where rainfall is low & evaporation high
7.2 SOILS – Influences
Processes - Erosion
Calcification
Calcium remains close to surface in low rainfall areas due to
evaporation & capillary action. Very little leaching due to low
rainfall.
Gleying
Waterlogged & poorly aerated soils where lack of oxygen retards
decomposition…. Drumlin belt.
7.2 SOILS – Influences
Human Interference
Human
• Over-cropping
• Over-grazing
• Desertification
• Conservation
7.2 SOILS – Human
Interference
Clearing Bush-land in Australia
7.2 SOILS – Human
Interference
Erosion caused by raindrops on bare land
7.2 SOILS – Human
Interference
Erosion caused by tilling dry land
7.2 SOILS – Human
Interference
Erosion caused by wind
Source:
Seafriends.org.nz
7.2 SOILS – Human
Interference
Livestock Damage Source:
Seafriends.org.nz
7.2 SOILS – Human
Interference
Source:
Soil Protection
Seafriends.org.nz
7.2 SOILS – Human
Interference
Erosion caused by tilling hill slopes
Source:
Seafriends.org.nz
7.2 SOILS – Human
Interference
Contour ploughing
Strip cropping
Source:
Seafriends.org.nz
Terracing
Paddy (Padi) fields
7.2 SOILS – Human
Interference
Soil Protection
Source:
Seafriends.org.nz
7.2 SOILS – Human
Interference
7.2 SOILS – Human
Interference
Desertification – Europe
Classification of Soils
Three basic groups: Zonal, Intrazonal, Azonal.
Zonal Soils
Classified according to climate zones in which they occur.
They are mature soils with distinctive profiles & mature horizons.
Tundra:
Climatic Zone Tundra Vegetation zone Tundra.
Artic regions of Canada, Russia & Scandanavia.
Vegetation…. Lichens, shrubs & mosses.
Shallow, brown to dark grey A Horizon…. permafrost.
Latosols:
Climatic zone Tropical Vegetation zone Tropical rainforest
Found in tropics & rainforests like Indonesia & Brazil.
High rainfall has leached out most of the minerals except iron &
aluminium…. very little acid left in soil.
Iron oxide builds up in a layer due to leaching… called lateralisation.
Chemical weathering is dominant & decay is rapid.
Iron oxide tints the soil a red colour (as it is exposed to oxygen) forming
laterite while the aluminium tends to form bauxite.
Intrazonal soils
Individual soils that develop within zonal soil regions due to local
factors such as parent material or drainage having more influence
than climate.
Terra Rossa
Climatic zone warm temperate maritime Vegetation zone
Mediterranean.
Mature limestone based soil.
Iron minerals are oxidised by chemical weathering creating a red
soil.
Found in Mezzogiorno, Italy & coffee-growing regions of Brazil.
Azonal Soils
Soils with an immature profile not yet fully developed.
Parent material is weathered rock or glacial deposition.
Location not confined to any one climatic zone.
Regosols:
Derived from volcanic, sand or alluvial deposits.
A Horizon is light in colour and B horizon does not exist.
C horizon a mixture of sand & silt.
Form alluvial deposits on floodplains like the Tigris-Euphrates rivers in
Iraq & the Indus in India.
Profile of an Irish soil
Brown earth: Most common Irish type.
Well drained, medium textured with a uniform dark brown profile.
pH within 5 to 7 range & a crumb structure.
Parent material is boulder clay from ice age.
Highly productive, though quality varies, & easily adapted to pasture &
tillage.
Develop in cool temperate climates covered with deciduous forests.
Heavy leaf fall & rapid decay resulted in thick layers of dark humus….
gives it the brown colour & makes it crumbly.
Tree roots absorb large amounts of nutrients but return them to the soil
when leaves fall off.
Temp suitable for earthworms & organisms & these with
burrowing animals mix up soil…. less distinct boundary between A
& B Horizons. Well drained soil.
3 main types of Brown earth soils found in Ireland.
1. Podzol… Rainwater becomes acidic & removes minerals… on
lowland limestone areas of central & eastern Ireland. Slightly
leached.
2. Acid brown earth… dev. from parent material poor in lime like
granite & sandstone….. acidic soils… found in South East.
3. Shallow brown earth… fertile but shallow & stony with parent
rock too near the surface for cultivation…. Burren & east Galway.
7.3 BIOMES – World
Regions
Taiga
Desert
Deciduous
Forest
Tropical
Grassland
Rain Forest
Tundra
Human Interference with soil characteristics… Desertification
Desertification: The reduction in veg. cover thus exposing the soil to
erosion, making a region unable to provide for its natural wildlife or
human pop.
Desertification does not just refer to moving forward of deserts but to
formation of degraded patches of soil & veg cover.
Two main factors 1. Climatic Change
2. Human Interference.
E.g. Sahel…. narrow band of Africa that lies between the Sahara to
the north & savannah (grass & open forest) & equatorial forest to the
south. Area receives rain during a short active wet season (late June to
Sept.)
It supports a nomadic, pastoral based society. Sahara desert advancing
southwards into Sahel by between 5 & 10km per year.
Causes of Desertification: Climatic Change
1. Decrease in rainfall… fell by 30% in last ten years. Rain arrives late
& wet season shorter… effects wildlife & veg.
2. Increased drought… below average rainfall & periods of total
drought 1968-1973 & 1983-1985.
3. Global warming… higher world temp.-- increased evaporation &
reduced condensation.
Factors resulted in : Less rainfall
wells, rivers & water holes drying up
Water table fell.
Overall result:
Vegetation dies
Lack of veg. cover
Soil erosion
Desertification
Causes of desertification… a) Human interference
Increased animal numbers…cattle ownership = wealth & status -
led to increased herd numbers. …. overgrazing.
Increased number of boreholes dug---- nomads stayed longer -
pressure on grasslands. Boreholes drained water table & promoted soil
salinisation.
Intensive land use led to overcropping & overgrazing….increased
tillage for cash crops like groundnuts so no more fallow year. Reduced
yields led to increased tillage area on marginal lands.
Trees & bushes cut down for firewood….led to increases wind effect
& soil erosion.
Result: Veg. lost by grazing & trampling.
Soil depleted of nutrients.
Moisture evaporated from soil…. erosion & desertification.
7.4 BIOMES – Human
Activities
Intensive Agricultural Practices
Bolivia Source: UNEP
Causes of desertification b) Population Growth:
Higher birth rates… meant more cash crops & higher demand for food
so more overcropping leading to reduced yields & infertile soil….
move to new lands & cleared it of its existing veg.
Wood cut for heating & cooking (90% is wood) & buildings in urban
areas…. deforestation as tress, shrubs & small bushes cut down.
Result: Less humus, infertile soil & lack of wind block by veg.
Soil erosion & desertification.
Soil conservation:
1. Stone lines…. placing small stones on slopes to reduce run-off….
aligned with the contours so they trap water & give it time to
infiltrate into the soil.
Benefit: replenish water table; reduce soil erosion; traps soil & seed so
plants begin to grow. Method takes up 2% of land but can increase
yields by up to 50%.
2. Contour ploughing: Soil is ploughed across the slope. Furrows act
like dams holding water. Traps water & reduces soil erosion by 50%.
On steeper slopes terracing… slope cut into a series of steps & front of
each terrace is edged with stones or mud to trap water.
Crop rotation used to replace nutrients into the soil e.g. alfalfa grasses
replace nitrogen.
Improved farming methods:
Strip farming… planting crops in widely spaced rows, filling the space
with a different type & height crop, that mature at different times….
Allows water to soak into ground, different nutrients required so soil not
exhausted & soil not exposed to wind as a crops protect it.
Shelter belts: trees & shrubs planted to reduce wind erosion & bind soil.
Some are fruit producing trees which provides food.
New animal breeds: smaller better quality cattle that fattened quickly or
produced more milk.
7.4 BIOMES – Human
Activities
Industrial Development – Acid
Rain
Effects of Acid Rain
Source: angelfire.com
New animal breeds: smaller herds, better quality cattle that
fattened quickly or produced more milk…. reduced grazing
needs & increased milk & meat production.
Sheep & goats introduced to areas of poor quality scrub.