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Chapter 4

Chapter 4: Soil & Agriculture


Department
Geography
Course Code
GGR107H1
Professor
Sarah Wakefield
Chapter
4

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CHAPTER 4: SOIL AND AGRICULTURE
Mer Bleue Conservation Area, a provincially protected wetland area located in Ottawa, Ontario classified as a wetland site
of international importance
Hosts a number of unusual plant species that are adapted to boggy and acidic conditions
Those plant species are sphagnum moss, bog rosemary, blueberry, cotton grass, cattails, and tamarack
The wetlands provide a specific type of soil (peat) which has occurred due to the last ice age
Canada has the most extensive peat lands in the world, covering 14% of land area
Bogs which are 6m deep took 8000 years to form
Peat lands are natural carbon reservoirs and hold 1/3 of the carbon stored in soils
Through decomposition peat releases soil gasses which function as GHGs in the atmosphere and warming can lead the peat
to release these GHGs at a faster rate, and result in positive feedback
Storage of carbon in peat depends on the balance between production and decomposition
Plants store/sequester carbon due to photosynthesis
Stored carbon accumulates in peat through plant litter
Temperature and light influence the amount of carbon stored in the soil
The study of soil, enables scientists to understand how soils behave in a global context (especially climate change)
4.1 SOIL AS A SYSTEM
Soil in everyday language means “dirt”
Soil is not merely composed of rocks, it is composed of
Disintegrated rocks
Organic matter
Water
Gasses
Nutrients
Microorganisms
The composition of soil is half mineral matter and half organic matter with spaces holding air, water, and soil gasses
The organic matter is dead/living microorganisms and decaying matter from plants and animals
One teaspoon of soil can hold
100 million bacteria
500, 000 fungi
100, 000 algae
50, 000 protists
Soil provides habitat to earthworms, insects, mites, millipedes, centipedes, other invertebrates and burrowing animals.
Soil is part of the ecosystem

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4.1.1. SOIL FORMATION
Soil formation occurs in the lithosphere (surface of earth) where the parent material is exposed to the effects of the
atmosphere, hydrosphere and biosphere
Parent material: The base geological material in a particular location
Lava or volcanic ash
Rock or sediment deposits by glaciers
Wind-blown dunes
Sediment deposits (rivers, lakes, oceans)
Bedrock (mass that makes up earth’s crust)
Weathering, erosion, and deposition:
Weathering: The physical, biological, and chemical processes that break down rocks and minerals turning large
particles into smaller ones.
Physical weathering:
Breaks down rocks without chemical change to parent material
Rain and wind are the primary forces of weathering
Daily and seasonal temperatures affect expansion and contraction of parent material
Water freezes and expands rocks, causing cracks
Chemical weathering
When water and other chemicals interact with the parent material
Warm and wet conditions accelerate chemical weathering
Happens in acidic conditions (bogs)
Biological weathering
When living things breaks down parent material physically or chemically
Tree roots rubbing against rocks
Decomposition of leaves and branches creates chemicals
Chemicals from roots release chemicals
Erosion
Movement of soil from one area to another
Erosion promotes physical weathering
Can happen from wind and water
Usually in dry places with no vegetation
Organic matter creates humus The soil at Mer Bleue is called peat as it is compressed organic matter
Soil formation table (pg. 76 fig. 4.1)

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4.1.2 SOIL PROFILE (HORIZONS)
Once weathering has occurred, wind, water, and organisms begin to move and sort them
Distinct layers called horizons are developed or known as soil profile
The five major horizons are O, A, B, C, and R horizons
O Horizon
organic or litter layer (leaves, branches, and animal waste)
A Horizon
Inorganic mineral components with organic matter and humus mixed in
Often referred to as topsoil
This part of the soil is the most nutritive for plants
Surface minerals are carried downwards due to leaching
B Horizon
Often called subsoil
Minerals and organic matter from topsoil are leeched into B Horizon and accumulate there
C Horizon
If present below B Horizon consists of parent material that is slightly altered or unaltered
Rock particles are larger
Sits above the R Horizon (Parent Matrial)
R Horizon
R stands for rock
W Horizon
Presence of distinct layer of water between layers
Soils contain segregated layer of frozen ice (permafrost)
4.1.3. SOIL CHARACTERISTICS
Soil can be characterised by
Colour
Red soils: high iron content
Black or dark brown: high content of organic matter
Pale grey/white: low organic matter and leaching
Texture
Three categories: Clay, silt, and sand
Loam is when all three particle sizes are present
Farmers determine usability of soil by looking at texture as it offers understanding of the porosity and
permeability of the soil
Structure
The measure of clumpiness of the soil
Structure enables soil productivity and biological activity
Clumps that are too large can discourage plant roots from establishing if they are too tightly compacted
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