Lecture 22:
The Geography of Soils: Soil Properties
WHAT IS SOIL AND WHERE IS IT?
“The Soil” = unconsolidated material at
the earth surface (or in a plant pot) that
has been altered over time
“A Soil” = A characteristic soil profile in
the landscape.
The “Pedon” = the smallest identifiable
unit of ‘a soil’.
A combination of these soils make up
“The Soil”.
Geog 1000 Introduction to Physical Geography Fall 2013 : Dr. Hester Jiskoot, University of Lethbridge
SOIL PEDON AND SOIL HORIZONS
THE SOIL PROFILE
Theoretical “complete” soil profile
Soils have a vertically layered
structure that develops over time
from the top down
Soil layers are called horizons and
are differentiated on the basis of
colour, structure, chemistry, texture,
organic content, etc.
illuviation or
accumulation layer
Soil horizons are in principle
organized as A-B-C horizons from
the top down
Transitions between horizons are not
always clearly visible
SOIL PROFILES
SOIL COMPONENTS
vary with soil horizons
A method used to characterize soils based on Horizons, which
are distinguishable layers that are parallel to the surface
Average composition
Organic Material (OM)
Decayed OM
Accumulation / Percolation
Parent Material
A
B
C
50% Water and Air
5% Organic Matter
(dead plants)
45% Mineral/Rock
particles
In VOLUME estimates.
WHAT IS SOIL NOT ?
5 SOIL FORMING FACTORS
™Soil is not “DIRT”
™Sediments or rocks are not soil
• Parent material
WHAT IS SOIL & WHERE ARE ITS BOUNDARIES?
• Living organisms (flora/fauna/people)
™ Soil = Unconsolidated rock particles/minerals from pre-existing rocks
(parent material), and organic material, air and water, that has
together been altered at the earth surface under the influence of
FIVE soil forming factors.
• Climate
• Topography
SOIL BOUNDARIES
• Time
™ Upper limit = air, shallow water, or plants
™ Lower limit = at 2 m, or the lower limit of biological and/or leaching
activity, whichever comes first
THE IMPORTANCE OF SOILS: RESOURCE VALUES
LIVING ORGANISMS INFLUENCING SOILS (BIOTA)
• Soil covers much of the earth surface to a depth of ~10-200 cm
™ Biological activity = contributes to the organic constituent
of the soil
• Life on earth is completely dependent on soil, water & air
• Soil is an ever-changing dynamic entity and the result of many
constructive and deconstructive cycles
™ Living organisms work on profile mixing and nutrient
cycling
6 main roles of soils
™ This soil mixing is called bioturbation
1. Medium for plant growth
2. Regulator of water supplies
3. Recycler of raw materials
4. Modifier of the atmosphere
5. Habitat for soil organisms
6. Engineering medium
Ant hill
2008 Andrew Alden,
licensed to About.com
Orange fly
Writing-on-stone
SOIL PROPERTIES
Root canals
Magrath
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Colour
Texture
Structure
Consistence
Porosity
Moisture
SOIL COLOUR
• Colour
Gives clues to the nature of soil properties & conditions
• Texture
Munsell Colour Charts
• Structure
Hue (colour)
• Consistence
Value (brightness)
• Porosity
Chroma (intensity)
• Moisture
On the Munsell scale each page is a specific Hue
DISTINGUISHING COLOUR
IN SOIL LAYERS
There is often, but not always,
a dramatic change in colour
from one horizon to the next.
Notation: 10YR 5/4 = yellowish brown
FACTORS AFFECTING SOIL COLOUR
1. Organic content
-
Gives darkness to soil
Regulates moisture
Colour
Texture
Structure
Consistence
Porosity
Moisture
SOIL TEXTURE
Soil texture characterizes the particle size distribution
2. Moisture level
- Darker when wet
3. Presence and oxidation state of Fe and Mn oxides
- Oxidized Æ iron oxides = red (rusty) Æ often well-drained soils
- Reduced Æ iron reduced = greys/blues (gley)
4. Presence of certain chemicals
- Calcite gives whitish colour in semi-arid regions.
PARTICLE SIZE ANALYSIS
Measure using a sieve or an electronic particle size analyser
Fine fraction
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Coarse fraction
Based on the relative abundance of
Sand (2 mm - 0.05 µm)
Silt (0.05 - 0.002 µm)
Clay (< 0.002 µm).
µm = micrometer or micron = one-thousands of a millimeter
Æ Different for each horizon
ÆCan also contain some boulders and gravel
SOIL TEXTURE expressed in TERNARY DIAGRAM
HAND TEXTURING: round ribbon test
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Colour
Texture
Structure
Consistence
Porosity
Moisture
SOIL STRUCTURE
= the shape and size of soil aggregates
- affects water movement, heat transfer, aeration and porosity
- is affected by human action (ploughing, grazing, manuring, drainage, etc)
ANALYSIS OF STRUCTURE IN THE FIELD
DESCRIBE:
1. Type of structure
2. Relative size of aggregates (fine, medium, coarse)
3. Distinctness or development (weak, moderate, strong*)
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Colour
Texture
Structure
Consistence
Porosity
Moisture
SOIL CONSISTENCE
Soil consistence = the degree and kind of
cohesion and adhesion between soil particles
*Difficult to assess when the soil is wet*
EXPRESSED AS:
• Plasticity (moldability)
• Loose, firm, soft hard, brittle, rigid
1) Look at structure in soil profile
2) Take a handful of soil and shake
Soil consistence is
related to the water
content of the soil
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Colour
Texture
Structure
Consistence
Porosity
Moisture
SOIL POROSITY
(see also chapter 6)
• How large are the pores?
• How are the pores spaced
(density)?
• How well are the pores
connected?
• Is there vertical or horizontal
structure in the pores?
Æ A high soil porosity increases soil permeability, but can decrease the field capacity
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Colour
Texture
Structure
Consistence
Porosity
Moisture
SOIL MOISTURE
(see also chapter 6)
• Air and soil water make up the
pore space
• Fluctuates with season and
climate (drought = more air)
• Generally more air in the upper
horizons and more water in the
lower horizons (closer to the
groundwater level).
SOIL CHARACTERISATION IN THE FIELD
SOIL AUGER
All soil characterization is based on:
a) digging a soil pit
b) using a soil auger, or
c) finding an exposed soil.
A soil pit is usually several metres deep and ~ 1 m2 at
the top, and gives 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.
ROAD CUTS AND NATURAL EXPOSURES
SOIL PIT
Road cut at Beaver Mines Road
River bank exposure at Cardston
OF THE MANY OTHER SOIL PROPERTIES
http://www.anr.state.vt.us/dec/waterq/bass/pHscale2.gif
SOIL pH RANGE MAY BE THE MOST IMPORTANT
OPTIMUM pH RANGES FOR CROPS
Natural pH range and some common products
Soils usually pH > 5
Alkaline
neutral
Acidic
www.fieldtesting.co.uk
Global variation in soil pH.
Red = acidic soil.
Yellow = neutral soil.
Blue = alkaline soil.
Black = no data.
SOIL CLASSIFICATION
AND
SOIL TAXONOMY
REASONS FOR SOIL CLASSIFICATION
BASIS OF SOIL CLASSIFICATION
1. Organize knowledge ALONG A HIERARCHY 2. Communicate knowledge to other people 3. Remember properties of the objects classified 4. Study relationships among individuals and classes of the population being classified 5. Group objects into classes (similar behaviour, use, management, productivity).
soil‐forming factors
↓
soil‐forming processes
↓
diagnostic horizons, properties, materials
↓
soil taxonomic system
Classification helps us deal with complexity
SOIL FORMING PROCESSES
(example)
SOIL HORIZONS : DETAILS
THE SOIL PROFILE GROUPED INTO CLASSES
Soil profiles differ because of differences in the soils forming factors
Different soil profiles can be grouped
into different classes of soils
Different classes of soils have a specific geographic distribution: both at the global‐scale and the landscape‐scale.
Soils of Canada
Remember the 5 soils forming factors?
climate, living matter, parent material, topography, and time
Æ these are broadly represented in this map of soil types in Canada
Reading for Soils
Chapter 14