LAST LECTURE
Introduction to the course
Importance of soils
THIS LECTURE
Principles of soil formation
Soil profiles
Soil horizons
Soil constituents
Soil texture
THE IMPORTANCE OF SOILS
THE SOIL PROFILE
Theoretical “complete” soil profile
6 main roles of soils
Soils have a vertically layered
structure that develops over time
1. Medium for plant growth
2. Regulator of water supplies
The different soil layers are called
soil horizons and are differentiated on
the basis of colour, structure,
chemistry, texture, organic content,
etc.
3. Recycler of raw materials
4. Modifier of the atmosphere
5. Habitat for soil organisms
(illuviation layer)
6. Engineering medium
Soil horizons are in principle
organized as A-B-C horizons from
the top down
Transitions between horizons are not
always clearly visible
5 SOIL FORMING FACTORS
SURFICIAL GEOLOGY
• Parent material
• Living organisms
• Climate
• Topography
• Time
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BEDROCK GEOLOGY
RAINFALL
ECOZONES
PHYSIOGRAPHIC REGIONS
Physiography
geomorphology
and
topography/relief
PHYSIOGRAPHIC REGIONS: PRAIRIE PROVINCES
SOIL ZONES OF WESTERN CANADA
Alberta
Saskatchewan
Black
Dark Brown
Brown
Grey
Dark Grey
Manitoba
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SOILS OF CANADA
DIFFERENT DIMENSIONS
• Soils are classified
based on their physical
properties.
• Large scale changes
are due to climate or
differences in geology.
• Small scale changes
are due to topography
or history
SOIL CHARACTERISATION
All soil characterization is based on
digging a soil pit or using a soil auger*.
A proper soil pit is usually several metres
deep and a metre or so square at the top,
and exposes a vertical exposure of the
soil profile.
Bring a friend or an assistant!
Soil pit is a more complete exposure: augering doesn’t show you as much detail.
SOIL AUGER
SOIL PIT
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ROAD CUTS AND NATURAL EXPOSURES
SOIL PROFILES
Used to characterize soils based on Horizons.
Not as pure as we would like.
May have indefinite boundaries.
Can be highly variable.
Road cut at Beaver Mines Road
Generally they are parallel to the surface.
River bank exposure at Cardston
SOIL PROFILES
SOIL HORIZONS
Organic Material
Decayed OM
Accumulation / Percolation
Parent Material
SOIL HORIZONS
The MAIN soil horizons are in alphabetic order,
From the top down
The solum is that part of a soil that is significantly
modified.
Everything above bedrock is regolith.
O HORIZON
• The O horizon or the Organic
horizon comes first.
• Decomposing biological
matter at the surface.
• Undergo physical and
biochemical breakdown.
• Especially important for
forested soils. Nutrient cycling
(transported by water).
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A HORIZON
B HORIZON
• The A horizon is nearest the surface.
• Darkened by the accumulation of
organic material from the O horizon.
• Many soils loose clays and other
chemical components from this layer
due to leaching.
• The B horizon is the next
closest to the surface.
• Characterized by the
accumulation of minerals from
the A horizon (clay).
• May also be iron, calcium
carbonate, aluminium oxides or
other erodible / weathered
minerals.
Bt, Bc, Bf
B HORIZON
C HORIZON
• The B horizon is sometimes
difficult to find precisely.
• Transition to A Horizon on the
top and transition to C Horizon
on the bottom.
• The C horizon is the material that
formed the above layers.
• Parent Material.
• Least weathered / altered unit in the
soil profile.
DISTINGUISHING SOIL PROFILES
E HORIZON
O
• The E sometimes occurs if the
bottom part of the A horizon has so
much material leached out that it can
be identified as a separate horizon
• Is by definition leached (and has a
pale colour)
E horizon
• There is often a dramatic
change in colour from one
horizon to the next.
• Sometimes the change is
less dramatic depending on
the nature of the soil
processed at work.
• Humo-Feric Podzol.
A
E
Bf
C
• E stands for eluviation
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TOPSOIL AND SUBSOIL
Topsoil is the common term for the A Horizon.
This is the layer that is often cultivated (the plowed
layer).
TOPSOIL AND SUBSOIL
• Ap horizon is generally deeper
than the A as part of the B horizon
is “worked up”.
• Home for most plant roots.
• The less dense the soil the easier
it is for root penetration.
If a soil is being used for production the A horizon is
modified by plowing = Ap horizon.
DEEP RIP PLOW
TOPSOIL AND SUBSOIL
•
•
•
•
•
Subsoil is that layer below the solum.
Storage of soil water and nutrients.
Can be compacted, waterlogged or very acidic.
Very expensive to modify.
Compaction can be broken up with a deep ripper.
SUBSOIL TREATMENT
SOIL CONSTITUENTS
• Deep ripping is sometimes used in forestry to
break up compacted subsoil.
50% Water and Air.
• Helps drainage.
• Can be combined with lime treatment to help
acidic soils.
• Sort of dramatic.
Only 5% Organic
Matter.
45% Mineral.
All VOLUME
estimates.
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SOIL AIR AND WATER
SOIL ORGANIC MATTER AND MINERALS
• Make up the pore space (zone
between mineral or organic
material).
• Accumulates in the upper horizons.
• Air and water content fluctuates
with season (drought = more
air etc.).
• Far less dense than mineral horizons.
• Generally there is more soil air
in the upper horizons and more
soil water in the lower horizons.
SOIL TEXTURE
Soil texture characterizes the particle size
distribution.
Different for each horizon.
Based on the relative abundance of
Sand (2 - 0.05 µm),
Silt (0.05 - 0.002 µm), and
Clay (< 0.002 µm).
MINERAL CONSTITUENTS
• We don’t classify based on lithology, but
lithology is important because of the mineralogy.
– Defines what is likely to be present.
– Based on the amount of weathering or erosion.
– Certain minerals are more susceptible (quartz is more stable
than feldspar).
• Important for nutrient cycling.
• Minerals are sorted based on lithology
and based on size.
• Particle size distribution of the mineral
fraction is important to determine Soil
Texture.
MINERAL (INORGANIC) CONSTITUENTS
• Main framework for most soils (45%).
• Wide range of particle sizes.
• Varies with depth.
– A-horizon less clay than B.
– B-horizon more clay than C.
– C-horizon original distribution.
PARTICLE SIZE ANALYSIS
• Soils contain a wide variety of particle sizes.
– From boulders (cm – m) to clays (0.002 mm).
– Smaller particles are held in suspension
(Colloidal = particles in suspension, like blood).
– Sometimes particles can become fused
(especially in clays).
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PARTICLE SIZE ANALYSIS
Fine fraction
SOIL TEXTURE
Coarse fraction
Ternary diagram
READING FOR THURSDAY
CHAPTER 2:
FORMATION OF SOILS FROM PARENT MATERIALS
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