Soil Geography

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LECTURE 1
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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 primarily concerned with the
  specific chemical and biological properties of
  soils, though some spatial analysis is still done.

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Functions of Soils

• Supports growth of higher plants
• medium
• nutrient elements
• Hydrological regulation
• supply
• purification
• Natures recycling system
• role in life cycle
• global climate
• Habitat for living organisms
• mammals, reptiles, insects, bacteria
• Engineering medium
• building material
• foundation

 

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Volume composition of a loam surface soil
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Soil as a Medium for Plant Growth
Physical support anchor root system Ventilation C
O2 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
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• 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

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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)   Organ
ic 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
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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.)
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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)
(b)
Low organic matter content
High organic matter content
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Engineering Medium   Designs for roadbeds or
buildings need to account for soil properties
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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?
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LECTURE 2
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SOIL HORIZONS
Partially decomposed organic material dominates
Good mix of mineral and organic particles
(mainly mineral)
ELUVIATION
E Horizon may be present
Silicate clays, iron oxides, aluminium
oxides, and calcium carbonates accumulate (little
organic matter)
ILLUVIATION
Least weathered part of the soil profile
The exposed wall of a soil pit or road cut is
called the soil profile
Regolith (above bedrock) May be transported (ie.,
can be distinct from parent material)
http//www.physicalgeography.net
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It is not always easy to differentiate between
distinct soil horizons Taking samples from each
level identified can help
(b)
(a)
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• 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

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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
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Mineral constituents of soils

The smallest clays (lt0.001 mm) display
colloidal properties, as does very fine organic
matter
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Soil Texture
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Particle Size Distribution
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Particle Size Differences

• Different properties based on the size of the
  particles, even if same mineral.
• Function of surface area.

LARGE SAND CLAST
LARGE CLAY CLAST
4 mm26 24 mm2
Note Most clasts are not square and would not
fit together, leaving pore space.
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Soil texture is of great significance to plant
growth
Eg. Clays hold water more tightly than do
sands Later, well learn why loamy soils with a
high organic fraction provide the most
available water
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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.

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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 lt2.5cm
• 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, gt5.0 cm
• SANDY CLAY, SILTY CLAY or CLAY

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Ribbon Test
SILT LOAM
SANDY LOAM
CLAY
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Why Hand Texturing Works

• SAND
• Lowest surface area (weak particle attraction).
• Wont 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 (lt0.002 mm)
• Cohesive particles are so small, that they feel
  sticky.

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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.

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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

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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

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• Organic matter as a glue
• plant roots and soil organisms produce glue-
• like 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)

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• 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

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• 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, especially
  when soil is dry (see figure)
• stimulates plant growth more effectively than
  colloidal fraction of clays

SUCTION
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Figure 1.21
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• 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
• (pH6 has 100 times more H ions than pH8)

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Nutrients taken up through hydrophilic channels
(binding sites on protein carrier molecules)
(soil water flows)
(roots grow)
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• 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

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LECTURE 3
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• Soil Formation
• FACTORS AFFECTING SOIL FORMATION
• Parent Materials (resistance, composition)
• Climate (precipitation, temperature)
• Biota (vegetation, microbes, soil fauna)
• Topography (slope, aspect, hillslope position)
• Time (period since parent material exposed)

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Review of Minerals
1. PARENT MATERIAL

• Basic building blocks of rocks.
• All started as igneous rocks (even metamorphic
  and sedimentary rocks), 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
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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)
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Rock
A rock is an assemblage of minerals bound
together

• Igneous (solidify and crystallize from molten
  magma)
• Sedimentary (settling)
• Metamorphic (altered under pressure)

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Sedimentary Rock
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
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Clastic
Conglomerate largest clasts Sandstone sand
cemented together Siltstone derived from
silt Shale mud/clay compacted into
rock Limestone calcium carbonate, bones
and shells cemented or precipitated in
ocean waters Coal ancient plant remains
compacted into rock
Chemical / organic
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Metamorphic 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

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Metamorphic rock
Original rock
Shale Slate Granite Gneiss Basalt
Schist Limestone, dolomite Marble Sandston
e Quartzite
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Mineral composition affects resistance to
weathering
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Most Common Elements
Oxygen Silicon Aluminium Iron Calcium Magnesium So
dium Potassium
Percentage by Weight
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Relative susceptibility to weathering
Ca
Mg
K, Al
K, Al
Si
Al
Fe
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Sample minerals and their products
(SiO2)
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PHYSICAL WEATHERING Rocks broken down into
smaller rocks, sand, silt and clay (i)
Temperature (cracking, exfoliation,
freeze-thaw) Expansion and contraction Different
ial 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
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Frost Wedging

• Adequate moisture
• Cracks in rocks
• Freeze/thaw cycles

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Talus slope
Glacier National Park, USA formed due to
freeze-thaw weathering)
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Abrasion by sediments carried by wind
Freeze-thaw weathering
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SLATE RESISTANT SILICATECLAY MINERALS
MARBLE LESS RESISTANT CALCITE
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Biological Wedging

• Biological wedging plant roots penetrate into
  cracks causing cracks to widen.
• Must have
• Climate hospitable for plants.
• Adequate moisture and temperature.

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Trees (Pinus flexilis and Pinus contorta)
growing on very little soil Roots grow into
cracks, prying them apart
Lakeview Ridge, Waterton Lakes National Park
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Unloading
Removal of pressure of deep burial.
Exfoliation Dome
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Abrasion and Plucking
Glacial ice is not cleanloaded with sediment
that abrades the surface.
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Transport by Ice
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Wind Erosion
Particles of sand and dust wear away relatively
soft rock.
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More resistant
Less resistant
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BIOGEOCHEMICAL WEATHERING

• Hydration
• H2O molecules bind to a mineral through HYDRATION
• Oxides of Fe and Al are common
• 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, nitric and sulphuric acid
  accelerate
• dissolution

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• 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

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Photo source http//www.stmarys.ca/conted/webcour
ses/GEO/GEO99/pubweather/chemcombined.html
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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
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Lecture 4
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Parent material sediment can be classified by
its method of deposition
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Alluvial/fluvial sediments deposited in a
floodplain
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Alluvial Fans
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Glacial Deposits
1 till 2 glaciolacustrine deposits 3
loessial blanket (aeolian) 4 unglaciated (loess)
nearly all of Canada was glaciated!
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Glaciated, U-shaped Valley
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Deposition from Outwash Plain
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Aeolian Deposits
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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
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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).

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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?

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Lecture 5
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From de Blij Miller, 1996, Physical Geography
of the Global Environment. Adaptation by M.J.
Pidwirny,Okanagan University College
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Credit Government of Alberta, 2002
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Different Dimensions
Soil Zones of Western Canada
Black
Dark Brown
Brown
Alberta
Grey
Dark Grey
Saskatchewan
Manitoba
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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?

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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.

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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.

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Same parent material. Different environment.
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Crotovinas
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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.

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Soil Catena
Poorly developed B
Development of B
Deeply weathered B
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• Depressions also have greater depths of
  weathering.
• Can get the development of very different soils
  along a slope from top to base.
• Same parent materialjust different topographic
  position / characteristics.
• Milne (1935) recognized this property and called
  it a catena (chain).

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• 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.

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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
  factorknown disturbance.

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Soil Formation in Loess Over Time.
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Time Buried Horizons
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Soil Forming Processes

• So we have the five factorswhat 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 VARIABLESOVER TIME

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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.

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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.

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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.

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The Master Horizons
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