Soil Geography
• Soil geographers focus on the relationships
between soils and landscapes.
– How and when were soils formed in a given area?
– How are the physical properties of soils related to
topography, climate, vegetation and fauna?
– How do soils contribute to ecosystem function/health?
• Pedologists are more concerned with the specific
chemical and biological properties of soils, though
some spatial analysis is still done.
Functions of Soils
Supports growth of higher plants
a. medium
b. nutrient elements
Hydrological regulation
a. supply
b. purification
Nature’s recycling system
a. role in life cycle
b. global climate
Habitat for living organisms
mammals, reptiles, insects, bacteria
Engineering medium
a. building material
b. foundation
Volume composition of a loam surface soil
Soil as a Medium for Plant Growth
Physical support
anchor root system
Ventilation
CO2 &O2 for root respiration
Water
high water-holding capacity
promotes cooling, nutrient transport, turgor & photosynthesis processes)
Temperature Moderation
amplitude of temperature wave decreases with depth
Protection from Toxins
gas ventilation & decomposition or adsorption of organic toxins
Nutrient Element Supply
Dissolved ions: metallic – K, Ca, Fe & Cu; non-metallic – N, S, P & B;
Plants acquire nutrients directly – animals indirectly through plants
Regulation of Water Supplies
Nearly all water in lakes, rivers and aquifers passes through or over soils
Consider the impact of soil removal on pathway and timing of water
delivered to a stream in a mountainous catchment.
Storage in soils, usage by vegetation, seepage to groundwater
Groundwater may take months or years to reach a water body as
baseflow.
Water is purified and cleansed while passing through soils.
Contrast with destructive flash flood of muddy water with shallow soil of
low permeability
Recycler of Raw Materials
Nutrients must be reused to maintain productivity
Environments with poor recycling end up with deep organic layer
The most productive environments have soils that recycle rapidly
(tropical rainforest)
Organic waste is converted to useful, nutrient-rich humus
Mineral nutrients re-converted to forms useful to plants
Carbon returned to atmosphere as CO2, the required gas for
photosynthesis, and an important greenhouse gas
Habitat for soil organisms
A handful of soil may contain billions of organisms belonging to
thousands of species
How is this possible? Range of niches and habitats (anoxic vs.
aerated pores, temperature variation, pH variation etc.)
Organic matter and plant roots (a) promote the growth of microbes
and higher plants.
Soils low in organic matter generally are associated with lower
productivity and biodiversity.
(a)
High organic matter content
(b)
Low organic matter content
Engineering Medium
Designs for roadbeds or buildings
need to account for soil properties
Poor soil management and
population pressure are often cited
as reasons for the downfall of great
civilizations
Is same happening today on a larger
scale?
SOIL HORIZONS
Partially decomposed organic
material dominates
ELUVIATION
E Horizon
may be present
ILLUVIATION
The exposed wall of
a soil pit or road cut is
called the soil profile
Good mix of mineral
and organic particles
(mainly mineral)
Silicate clays, iron
oxides, aluminium oxides,
and calcium carbonates
accumulate (little organic
matter)
Least weathered
part of the soil profile
Regolith (above bedrock)
May be transported (ie.,
distinct from parent material)
http://www.physicalgeography.net
It is not always easy to differentiate
between distinct soil horizons
Taking samples from each level
identified can help
(b)
(a)
Topsoil
•The organically-enriched A horizon at the soil surface
in a cultivated soil
•Most nutrient-rich portion of cultivated soils
•Contains the majority of plant roots
Subsoil
•The soils that underlie the topsoil
•Lower in most nutrients
•Drainage properties important in determining
susceptibility to waterlogging and soil moisture stress
Notice the concentration
of roots in the more
nutrient-rich, aerated,
looser organic layers
near the surface
No crop residues or
fertilizers
Fertilizers and crop
residues received
Mineral constituents of soils
*
*
The smallest
clays (<0.001 mm)
display colloidal
properties, as does
very fine organic
matter
Soil Texture
Particle Size
Distribution
Particle Size Differences
• Different properties based on the size of the
particles, even if same mineral.
• Function of surface area.
LARGE
SAND
CLAST
mm2*6
LARGE
CLAY
CLAST
mm2
4
= 24
Surface area: 24 x 106 um2
4 um2*6 = 24 um2
Potential surface area within
sand grain volume: 24 x 109 um2
Note: Most clasts are not square and would not fit together, leaving pore space.
Soil texture is
of great
significance to
plant growth
Eg. Clays hold
water more tightly
than do sands
Later, we’ll learn
why loamy soils
with a high organic
fraction provide the
most ‘available’
water
Hand Texturing (see Box 4.2)
• Used to determine the relative contributions
of the fine fraction.
• Very useful in the field to determine soil
texture.
• Based on physical properties and “feel”.
• Sand feels gritty as you can feel the
individual particles. Silts are smooth, and
clays are sticky.
Start by Making a Ball
1. Falls apart? SAND (or not enough water)
Does not fall apart? Continue by making a ribbon.
2. Will not form ribbon? LOAMY SAND
3. Ribbon breaks <2.5cm
SANDY LOAM, SILTY LOAM or LOAM
4. Ribbon moderately sticky, firm, 2.5 – 5.0 cm
SANDY CLAY LOAM, SILTY CLAY LOAM or CLAY LOAM
5. Ribbon sticky and firm, >5.0 cm
SANDY CLAY, SILTY CLAY or CLAY
Ribbon Test
SILT LOAM
SANDY LOAM
CLAY
Why Hand Texturing Works:
SAND
Lowest surface area (weak particle attraction).
Won’t hold together unless saturated
Loses water easily
SILT
Particles are small enough to hold water well (0.05
– 0.002 mm
Too large to feel sticky, just smooth
CLAY
Clay particles are the smallest (<0.002 mm)
Cohesive particles are so small, that they feel
sticky.
Soil Texture
• Different relative amounts of sand, silt, and
clay (see soil texture triangle).
• Coarse fraction not considered in texture
assessment.
– Not important for soil texture.
– Important for soil structure.
• Fine fraction describes the soils ability to
hold moisture and store nutrients.
Soil Structure
•Particles sometimes remain independent
•May also form aggregates
- roundish granules
- cube-like blocks
- flat plates
•Both texture and structure affect water
and air movement within soils
•Important for plant growth
Soil Organic Matter
What is organic matter?
•remains of plants, animals and microorganisms
•soil biomass (living organisms)
•Organic compounds produced by floral and
faunal metabolism
Relevance to carbon balance
•atmospheric CO2 sequestered by plants and
stored in soils
•CO2 is also lost to atmosphere via microbial
decomposition
Organic matter as a ‘glue’
•plant roots and soil organisms produce gluelike substances
•mineral particles are bound by this ‘glue,’
resulting in a granular soil structure
•causes productive, loose, easily managed soil
Organic matter as a ‘sponge’
•Increases volume of water that can be held
•Increases proportion of water a plant can use
(difference between wilting point and field
capacity)
Organic matter as a ‘fertilizer’
•primary source of N, P and S
•nutrients released as soluble ions as organic
matter decays
•food and energy source for soil organisms
What is humus?
•stable, colloidal fraction of organic matter
•acts as contact bridge between larger
particles
•surface charges hold soluble nutrients
•water held tightly when pores small
•Stimulates plant growth more effectively
than colloidal fraction of clays
What is humus?
•stable, colloidal fraction
of organic matter
•acts as contact bridge
between larger particles
•surface charges hold soluble
nutrients
SUCTION
•water held tightly when pores small, especially
when soil is dry (see figure)
•stimulates plant growth more effectively than
colloidal fraction of clays
Figure 1.21
The Soil Solution
•Contains soluble, inorganic compounds that
supply elements for plant growth
•Organic and inorganic colloidal particles release
these elements to the soil solution
Acidity vs. Alkalinity
•H+ and OH- ions in soil solution
•Affects solubility and availability of soil nutrients
•pH is the negative logarithm of H+ ion activity
(pH=6 has 100 times more H+ ions than pH=8)
Nutrients
taken up
through
hydrophilic
channels
(binding sites on
protein carrier
molecules)
(soil water flows)
(roots grow)
Soil Air
•Pores filled either with air or water
•High [CO2]; Low [O2]
•Effects exacerbated if pore size is small
or if soil moisture is high
Soil Formation
FACTORS AFFECTING SOIL FORMATION
1.
2.
3.
4.
5.
Parent Materials (resistance, composition)
Climate (precipitation, temperature)
Biota (vegetation, microbes, soil fauna)
Topography (slope, aspect, hillslope position)
Time (period since parent material exposed)
1. PARENT MATERIAL
Review of Minerals
• Basic building blocks of rocks.
• All started as igneous rocks (even metamorphic
and sedimentary rocks/clasts), but most have
been altered and redistributed at surface.
• Chemical composition is a reflection of
environmental conditions & parent material.
• Different levels of stability.
– Quartz (SiO2) more stable than Olivine (Mg2SiO4).
Time for a quick review of Geography 1010/2030 – the rock cycle…
Mineral
A natural, inorganic compound with a
specific chemical formula and a
crystalline structure
Examples
silicates (quartz, feldspar, clay minerals),
oxides (eg., hematite)
carbonates (eg., calcite)
A rock is an assemblage of minerals
bound together
• Igneous (solidify and crystallize from
molten magma)
• Sedimentary (settling)
• Metamorphic (altered under pressure)
Existing rock is digested by
weathering, picked up by erosion,
moved by transportation, and
deposited at river, beach and ocean
sites.
Lithification follows (cementation,
compaction and hardening)
Laid down in horizontally-layered beds
Conglomerate
Sandstone
Siltstone
Shale
Limestone
Coal
largest clasts
sand cemented together
derived from silt
mud/clay compacted into
rock
calcium carbonate, bones
and shells cemented or
precipitated in ocean waters
ancient plant remains
compacted into rock
Any type of rock is transformed, under
pressure and increased temperature
• Often harder and more resistant to
weathering
• Compressional forces: (i) collision of
plates, (ii) rock thrust under crust,
(iii) weight of sediment above
Shale
Slate
Granite
Gneiss
Basalt
Schist
Limestone, dolomite
Marble
Sandstone
Quartzite
Mineral composition affects resistance to weathering
Most Common Elements
Oxygen
Silicon
Aluminium
Iron
Calcium
Magnesium
Sodium
Potassium
Percentage by Weight
Relative
susceptibility
to weathering
Ca
Mg
K, Al
Si
Al
Fe
K, Al
Sample minerals and their products
Mineral
Residual Products
Material in Solution
Quartz
quartz grains
silica
Feldspar
clay minerals
silica, K +, Na+, Ca2+
Amphibole (hornblende)
clay minerals, limonite,
hematite
silica, Mg2+, Ca2+
Olivine
limonite, hematite
silica, Mg2+
(SiO2)
PHYSICAL WEATHERING
Rocks broken down into smaller rocks,
sand, silt and clay
(i) Temperature (cracking, exfoliation, freeze-thaw)
Expansion and contraction
Differential stresses since mineral composition varies
Cracking or exfoliation may occur
Freeze-thaw weathering in temperate and arctic regions
(ii) Abrasion (water, ice and wind)
Sediment carried by water, ice and wind abrades
(iii)Plants and animals
Roots enter cracks and pry apart rock
Burrowing animals
Frost Wedging
•Adequate
moisture
•Cracks in rocks
•Freeze/thaw
cycles
Glacier National Park,
USA – formed due to
freeze-thaw
weathering)
Abrasion by
sediments carried
by water
Freeze-thaw
weathering
SLATE
RESISTANT SILICATE
CLAY MINERALS
MARBLE
LESS RESISTANT CALCITE
Biological Wedging
• Biological wedging –
plant roots penetrate into
cracks causing cracks to
widen.
• Must have:
– Climate hospitable for
plants.
– Adequate moisture and
temperature.
Unloading
Removal of pressure of deep burial.
Exfoliation Dome
Abrasion and
Plucking
Glacial ice is not
clean…loaded with
sediment that
abrades the surface.
Transport by Ice
Wind Erosion
Particles of sand and
dust wear away
relatively soft rock.
More resistant
Less resistant
BIOGEOCHEMICAL WEATHERING
(i) Hydration
H2O molecules bind to a mineral through HYDRATION
Oxides of Fe and Al are common
(ii) Hydrolysis
Water molecules split into hydrogen and hydroxyl components
H often replaces a cation in the mineral
Releases nutrients (eg. K+) and forms secondary minerals
(iii) Dissolution
Cations and anions hydrated until they dissociate
(iv) Carbonation
Acids such as carbonic, nutric and sulphuric acid accelerate
dissolution
(v) Oxidation-reduction
Fe, Mn and S can be oxidized (loses and electron) in the presence
of air and water during soil formation
Causes destabilizing adjustments in crystal structure
May be visible as a change in colour
Iron-rich rock weathered by oxidation: Trout River, NL
Photo source: http://www.stmarys.ca/conted/webcourses/GEO/GEO99/pubweather/chemcombined.html
Crustal warping
(eg. due to
compressional
forces) followed by
weathering and
erosion near surface
Leads to abrupt
changes in
parent material
(complexity),
soil quality and
even vegetation
composition
Parent material sediment can be classified by its
method of deposition
Alluvial/fluvial sediments deposited in a floodplain
Alluvial Fans
Glacial Deposits
1
2
3
4
–
–
–
–
till
glaciolacustrine deposits
loessial blanket (aeolian)
unglaciated (loess)
*nearly all of Canada
was glaciated!
Glaciated, U-shaped Valley
Deposition from Outwash Plain
Aeolian Deposits
Organic Deposits
Stages in peatland
formation
N.B. Many wetland
ecologists now believe
that forested peat is not
necessarily the final stage!
Mer Bleue Bog,
Ontario
Climate
• Most influential of the five soil forming
factors over large areas.
• Determines the nature and intensity of
weathering.
• Greater precipitation = greater degrees of
weathering.
• Water percolates through the profile
transporting soluble ions and suspended
materials (clays).
Climate
• Water deficiencies can cause problems.
– Soluble salts are not carried away.
– Over time, these salts can cause salinity
problems.
• What are the dominant climatic
characteristics of Lethbridge?
• How do these conditions affect soil
development?
From de Blij & Miller, 1996, Physical Geography of the
Global Environment. Adaptation by M.J.
Pidwirny,Okanagan University College
Credit: Government of Alberta, 2002
Different Dimensions
Soil Zones of Western Canada
Black
Dark Brown
Brown
Grey
Dark Grey
Alberta
Saskatchewan
Manitoba
Temperature & Moisture
• For every 10° rise in temperature,
biochemical reactions more than double.
• Temperature and moisture influence the
amount of organic matter.
• If you have moisture and temperature
present at the same time, weathering and
leaching are maximized.
• Is this the case in our environment?
Biota
• Biological activity is the primary
contributor to the organic constituent of the
soil.
• Organisms play a strong role in profile
mixing and nutrient cycling. Which ones?
• Grassland soils have large accumulations of
organic matter.
– Beneficial for moisture retention, nutrient
storage, and defense against fire.
Biota
• Forested soils.
– Generally lower in soil organic matter.
– Not really necessary as the environment has
plenty of moisture.
– Leaves on forest floor are the principal source
of OM.
• Very acidic, inhibits the action of soil organisms
used to decompose.
• Most trees can withstand low pH.
Same parent
material.
Different
environment.
Crotovinas
Topography
• Three essential factors
– Elevation, slope, landscape position.
• Can change in response to climate factors.
– More gentle slopes in warm, moist climates.
• Causes change in local microclimate.
– Different slope aspects.
– Lateral changes in soil moisture conditions.
Soil Catena
Poorly developed B
Development of B
Deeply weathered B
• Depressions also have greater depths of
weathering.
• Can get the development of very different
soils along a slope from top to base.
• Same parent material…just different
topographic position / characteristics.
• Milne (1935) recognized this property and
called it a catena (chain).
• Steeper slopes have larger amounts of soil loss
due to erosion.
• Less complete vegetation cover.
• Shallower soil development.
• Depressions tend to accumulate runoff of
moisture and sediment.
– Not generally connected to external drainage
networks.
Time
• Takes time to form soils.
• Difficult property to gauge.
• Over what sort of time scales are soil
forming processes significant enough to
develop a soil.
• Complex system.
• Easier to solve if we can control the time
factor…known disturbance.
Soil Formation in Loess Over Time.
Time:
Buried
Horizons
Soil Forming Processes
• So we have the five factors…what
are the processes that create a soil.
• Also known as pedogenic
processes.
• All processes are in action, but the
relative importance is variable.
• Transformations, translocations,
additions, losses.
SYNERGISTIC
INTERACTIONS OF
MULTIPLE VARIABLES
OVER TIME
Transformations
• Soil constituents are chemically or
physically modified.
• Primary minerals are converted into
secondary products.
• Decomposition of organic material into
organic matter.
• Change of particle sizes.
Translocations
• Movement of inorganic and organic
materials laterally within a horizon or
vertically from one horizon to another.
– Percolation down (vertically and laterally due
to gravity and slope).
– Capillary action drawing materials to the
surface.
• Incorporation of surface organic material
into A and B horizons.
Losses
• Loss of material due to groundwater flow,
and erosion of surface materials.
– Erosion affects clays and silts more than sands
Net effect: Leaves a more sandy profile
– Agricultural activities can lead to the removal
of large amounts of OM.
The Master Horizons