class 1: introduction to soil formation, texture and structure · pdf...
TRANSCRIPT
![Page 1: Class 1: Introduction to Soil Formation, Texture and Structure · PDF file2/11/2012 · Class 1: Introduction to Soil Formation, Texture and Structure Chris Thoreau February 11, 2012](https://reader031.vdocuments.mx/reader031/viewer/2022021421/5a9fcf6e7f8b9a6c178d39a9/html5/thumbnails/1.jpg)
Class 1: Introduction to Soil Formation, Texture and Structure
Chris Thoreau February 11, 2012
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Soil is a dynamic composition of: • Minerals
• Water and its solutions
• Organic matter (detritus)
• Air and other gas mixtures...
• ...which, through interacting with each other and with plant roots, allows for the growth of photosynthesizing terrestrial plants
• …and acts as a habitat for micro- and macro-organisms …which also interact with plants and plant roots
• Or…
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Physical Aspects: • Minerals (from rocks) Sand
Silt
Clay and Colloids
• Organic Matter Plants and Roots
Detritus (decaying organic matter)
Animal waste (including microbes)
• Pore Space Air/Gases
Water
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Soil is the Mother of All
Terrestrial Life
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Conceptual Aspects:
• Habitat
Micro-organisms
Bacteria, Fungi – both good and bad
Viruses
Macro-organisms
Worms, Arthropods, Detrivores and Predators
Plants
Small Mammals
Birds
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Conceptual Aspects:
• Provider to plant life
Rooting substrate
Water holding and release
Nutrient supply and reserve
Heat sink and release
Soil gases
Symbionts
Bacterial and fungal
Insects
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Habitat • What happens when we disturb this habitat? At micro and macro level?
• What happens when we make additions to, or removals from, this habitat? Carbon:Nitrogen ratio?
How do soil organisms and plants respond?
Nutrient loss or gain?
Providing for plant life
• What are the short-term and long-term results? • Are we providing for the soil as well as the plants? What is the difference?
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As a habitat we need to treat soil like a
living organism, which requires:
• Food
Organic Matter and Minerals
• Water
Irrigation and Natural
• Air
• Shelter
Cover crops, Cash Crops, Mulches
• General tender loving care…
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Questions?
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Where does soil come from?
Soil comes (mostly) from the weathering of rocks
over long periods of time – a process highly
influenced by biological organisms,
topography, aspect and human activity…
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1. Parent Material
2. Climate
3. Biota
4. Topography
5. Time
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1. Parent Material • Transported
Gravity - Colluvial
Water – Alluvial, Marine, and Lacustrine
Wind – Eolian
Ice – Glacial
• Cumulose
Due to plant life and anaerobic conditions
High water table
Peat and muck soils
• Residual
In situ; long periods of weathering
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2. Climate
• Temperature and rainfall are major factors
Affect intensity of weathering
• Increased T and precipitation accelerate
weathering
3. Biota
• Plants influence organic matter
• Arthropods and worms mix soil; add to OM
• Small mammals also mix soil
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4. Topography • Slope influences soil development Water infiltration rate
Surface runoff
Vegetation
• Aspect North and South slopes develop differently
• Elevation Climate changes with altitude
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5. Time
• Often noted as most important soil formation
factor
• Our soils in Lower Mainland are relatively
young
Since last ice age 10-12,000 years ago
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Primary Minerals Sand and Silt • Formed at high T and P (at depth); anaerobic
conditions • Physically and chemically formed Quartz, Feldspars, Micas,
Secondary Minerals Clay • Come from primary minerals • Formed at low T and P (at surface) with Oxygen
present • Mostly chemically formed Silicate Clays
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Weathering of Rocks
Physical
Chemical 1
Chemical 2
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Additions
Losses
Transformations
Translocations
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Organic (O) Horizon • High in organic residue from
plant drop
A Horizon • Mineral component plus OM
• Most fertile part of soil; location of much root activity
B Horizon
• Subsoil
• A horizon leaches here
C Horizon • Little influence by soil-forming
processes
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In a given area, over the period of soil formation,
environmental conditions cause a certain set of soil
processes to occur, which leads to a distinctive set of
soil horizons at the time we observe the soil.
These soil horizons are the basis for classifying the soil
in the Canadian System of Soil Classification.
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Podzol Gleysol
Chernozem Brunisol Luvisol Organic
Solonetzic Vertisol Regosol Cryosol
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Any questions?
Check follow up readings and websites
for more detailed information • UBC Virtual Soil Science
• Soil Orders of Canada Videos
• Canadian System of Soil Classification
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Soil texture refers to the relative
amount of sand, silt, and clay found in
a soil
The mixture of these components
affects the feel of the soil as well as
water, nutrient, and pore space
interactions
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Mineral Components
• Sand
• Silt
• Clay
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Mineral ratios determine soil texture
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Sand
• Largest soil mineral particles (.02 – 2 mm)
• Formed greatly from physical processes
• Spherical/erratic in shape
Sand = little rocks
• Larger pore spaces
Good drainage
• Does not hold a charge
• Difficult to compact
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Silt
• Size between sand and clay (.002 - .02 mm)
• Usually physically formed out of sand
• Hold and releases water well
• Flat or round in shape
• Holds very little charge
• Feels soapy
• Carried in moving water
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Clay • Smallest soil mineral particle (< .002 mm)
• Holds water very well
• Holds strong negative charge for mineral
adsorption
• Susceptible to compaction
• Platy-/flat-shaped particles
• Various lattice structures
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Clay • Understanding structure of clay is important for:
Compaction
Water holding
Cation adsorption
Soil cultivation
Clays are categorized by their layer
structure Relationship of Si-tetrahedral and Al-octahedral sheets
2:1; 1:1; 4:1; 5:2
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2:1 Clay Shrink and swell
1:1 Clay
No change
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Shrink and Swell of Clay
• Interlayer space expands
with increasing water
content in soil
• Space contracts as water
is removed
• Clay can crack when
it shrinks
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Why is Texture Important? • Water Infiltration
• Water Storage
• Fertility
• Aeration
• Trafficability
Soil texture knowledge is the key to developing an overall soil maintenance and improvement plan
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Notes:
• We do not change the texture of soils
We can change the characteristics of certain textured
soils
• We change soil characteristics through:
Additions of organic matter
In soil and on top of soil
Cultivation practices
Raised beds
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Attributes of Different Soil Textures
Property Sand Silt Clay
Water
Holding
Poor Medium to
high
High
Nutrient
Holding
Poor Medium to
High
High
Aeration Good Medium Poor
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We can determine the texture of the
soil by feeling it
• Ribbon test
• Ball test
• Jar test
• Laboratory tests give more accurate results
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Texture Questions?
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Soil Structure: How the soil fits
together
• Primary particles are arranged into
secondary particles called aggregates (or
peds)
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Why is Structure Important?
• Pore space
Air and water movement
Rooting space
• Nutrient storage and release
• Contributes to soil resilience
Cultivation
• Erosion resistance
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How does aggregate formation occur? Flocculation + Cementation = Aggregation
Flocculation:
• Primary pulled close together (into flocs) by attractive forces (electrostatic forces, H bonding)
Cementation • Primary particles held together by cementing
agents Carbonates; clays; OM; Oxides
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Soil Aggregates are classified by their
shape
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• Ideal structure: Spheroidal
Typical in A Horizon
Rounded; loose
Granular (porous) or Crumb (very porous)
Greatly affected by soil management OR
mismanagement
Improved with OM additions and microbial activity
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Soil structure is particularly important
in providing adequate pore space for:
• Root growth
• Water movement
• Gas exchange
• Microbial activity
• Macrobial activity
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Structure can be easily observed in the
soil and structural stability, or aggregate
stability, can be measured in the lab
Structure can be improved, to a point, by
soil cultivation.
Soil cultivation is also a great way to
destroy structure
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Related to texture
Very important when considering soil
cultivation
Dependent on: • Texture/clay content
• Clay type
• Soil water content
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Cultivating soil when too dry
• Breaks aggregates into small pieces
De-aggregates
Can result in dust
• Very damaging to soil structure
• The drier the soil – the more it acts like
powder
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Cultivating soil when too wet
• Where to start?!
Compaction
• Risk and depth of compaction increases in
wet soil
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Cultivating soil when too wet
The wetter the soil - the more it acts
like water
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Soil consistency, determined greatly
by water content and percentage of
clay, plays a major role in when soil
can be cultivated!
Not as crucial when hand digging…
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We promote good structure in soil by: • Minimizing cultivation (especially in sandy soils) and
using appropriate cultivation methods
• Avoiding compaction (especially in clay soils) No tractor in wet soil!
Especially careful with clays
Cultivating at proper soil consistency
• Adding various types of organic matter regularly
• Maintaining a proper pH
• Promoting microbial life – especially fungi
• Always keeping the soil covered Preferably by crops
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Soil texture influences soil’s ability to
aggregate • Clay soils aggregate more readily
• Sandy soils have les stable aggregates
Organic matter, plant growth, and
microbial activity all contribute to
aggregate stability