Download Unit 3 Lesson 9 Soils and Soil Profiles

Grade 9 Geography - Unit 3 Lesson 9
Soil and Soil Profiling
Climate helps to create wet- and dry-climate soils. We know there are eight
climate zones in Canada including the very dry Prairie and the very wet,
temperate rainforest Pacific Maritime. If climate partially creates soil, Canada
should have, at least, eight soils region. In fact, there are four major soil regions
in Canada:
Tundra in Canada far north
Dry-climate soils of the Prairie region
Complex soils of the mountain region
Wet-climate soils covering all other parts of Canada including Ontario, the
Maritime provinces, most of Quebec, the northern parts of the Prairie
provinces and most of the Northwest Territories.
Soil is very complex. It is not just a collection of fine mineral particles. Soil
consists of four major components: (1) minerals, (2) dead organic matter, (3) air
and (4) moisture.
Minerals – come from the parent material (…usually rock). Minerals
become part of soil when the rock breaks apart due to weathering into
smaller particles of sand, silt and clay. Many minerals are the nutrients
needed for plant growth such as phosphorus.
Bacteria and Organic Matter – When plant and animals dies, bacteria
decomposition occurs. Decomposition releases nutrients into the soil.
The decaying organic material forms humus. Humus gives soil its dark
colour.
Air – Spaces between decaying materials serves as air pockets. Air
spaces are also formed by burrowing worms, insects and small, tunneling
animals. Plants need air.
Moisture – Water dissolves nutrients in the soil. Water is also need for
weathering of rock and decay of organic material,
The formation of a soil
is influenced by
organisms, climate,
topography, parent
material and time.
The values presented
in Figure 1 are
estimates for an
average soil profile
Figure 1. Percent composition of materials in soil for an average soil profile.
Soil Profile – Background
If you look in a soil pit or on a roadside cut, you will see various layers in the soil.
These layers are called Soil Horizons.
The arrangement of these horizons in a soil is known as a Soil Profile. Soil
scientists or pedologists describe soil profiles and soil horizons to classify and
interpret the soil for various uses.
Soil horizons differ in a number of easily seen soil properties such as color,
texture, structure, and thickness. Other properties are less visible. Properties,
such as chemical and mineral content, consistence and reaction require special
laboratory tests. All these properties are used to define types of soil horizons.
Capital letters O, A, B, C, and E are
used to identify the primary
horizons, and lowercase letters for
distinctions of these horizons (see
Figure 2).
Most soils have three major
horizons:
Surface Horizon (A)
Subsoil (B)
Substratum (C).
Some soils also have an Organic
Horizon (O) on the surface, but this
horizon can also be buried.
The Master Horizon (E) is used for
subsurface horizons that have a
significant loss of minerals.
Hard bedrock, which is not soil, is
identified with the letter R.
Figure 2. Typical Soil Profile
Soil has distinct layers or horizons. Together, these layers form the soil profile.
Ground level - Plants grow and animals live here. A thick cover of plants
can keep the soil cool and keep it from drying out. Decomposers recycle
dead plants and animals into humus.
Topsoil - Plants grow and animals live on top of the soil. This is
sometimes called the organic layer. A thick cover of plants can keep the
soil cool and keep it from drying out. Decomposers recycle dead plants
and animals into humus.
Subsoil - This is a mix of mineral particles and some humus near the top.
Subsoil is very low in organic matter compared to the topsoil. This is the
layer where most of the soil's nutrients are found. Deep plant roots come
here looking for water. Clays and minerals released up above often stick
here as water drains down.
Weathered Parent Material - This horizon can be very deep. There's no
organic matter here at all. We're out of reach of all living and dead
organisms down here. It's all rock particles, full of minerals. The entire soil
profile used to look like this all the way to the surface. Physical weathering
broke the parent material up into small pieces. Don't be fooled! This layer
may contain rock particles that are different from the bedrock below. A
river or a glacier might have brought it from somewhere else.
Bedrock: We finally found solid rock! The bedrock formed before the soil
above it. It will wait here until erosion or an earthquake exposes it to the
surface. Then some of it will be weathered to become the next batch of
parent material. The soil-making process will start all over again.
Interpreting Soil Colour
Colour can be used to identify the mineral content of a soil. Iron minerals provide
the greatest variety of pigments in earth and soil (see Table 1).
Table 1. Some Properties of Minerals
Mineral
Formula
Humus
Colour
Black
Iron sulfide
FeS
Black
Ferrihydrite
Fe(OH3)
Dark Red
Calcite
CaCO3
White
Hematite
Fe2O3
Red
Quartz
SiO2
Light Gray
Gypsum
CaSO4X 2H2O
Very Pale Brown
Colour, or lack of colour, can also tell us something about the environment.
Anaerobic environments occur when a soil has a high water table or water settles
above an impermeable layer. In many soils, the water table rises in the rainy
season. When standing water covers soil, any oxygen in the water is used
rapidly, and then, the aerobic bacteria go dormant. Anaerobic bacteria use ferric
iron (Fe3+) in goethite and hematite. In this process, the iron is reduced to
colorless, water-soluble ferrous iron (Fe2+) which is returned to the soil. Other
anaerobic bacteria use Mn4+ which is reduced to colorless, soluble Mn2+. The
loss of pigment leaves gray colors of the underlying mineral. If water stays high
for long periods, the entire zone turns gray.
When the water table edges down in the dry season, oxygen re-enters the soil.
Soluble iron oxidizes into characteristic orange colored mottles of lepidocrocite
(same formula as goethite but different crystal structure) on cracks in the soil.
If the soil aerates rapidly, bright red mottles of ferrihydrite form in pores and on
cracks. Usually ferrihydrite is not stable and, in time, alters to lepidocrocite.
Soil Texture
Soil texture refers to the size distribution of the mineral particles found in a
representative sample of soil. Particles are normally grouped into three main
classes: (1) sand, (2) silt and (3) clay.
Table 2. Particle size range for sand, silt and clay
Type of Material Particle Particle Size Range
(mm)
Sand
0.06 – 2.0
Silt
0.002 – 0.06
Clay
<0.002
Soil pH
Soils support many inorganic and organic chemical reactions. Most reactions
depend on soil chemical properties. One of the most important soil chemical
properties is pH. Soil pH is mainly controlled by the concentration of free
hydrogen ions in the soil matrix. Soils with a large concentration of hydrogen
ions are acidic; whereas, alkaline soils have a relatively low concentration of
hydrogen ions.
The pH scale goes from 1 to 14. A soil with pH of 7 is neutral. Values greater
than 7 are alkaline (basic) while pH less than 7 is acidic (see Figure 3).
Figure 3. pH scale.
Canada’s Soil Profiles
As we have learned, Canada has four soil regions:
Tundra
Wet-climate
Dry-climate
Complex soils of the mountain areas
Each region has its own unique soil profile. Two processes related to climate
contribute to soil formation: (1) leaching and (2) calcification.
Leaching is a natural process by which chemicals and minerals are transported
downwards through a soil profile. This soil is typical in a wet climate region.
Water is continually moving downward taking dissolved materials with it. Horizon
A is usually thin and poor in quality. Horizon B also contains little nutrients and
other soluble materials. Typically, minerals such as iron and aluminum occur in
Horizon B.
Calcification occurs in warm, semi-arid environments, usually under grassland
vegetation. Horizon A in this soil tends to be rich in organic matter and high in
soluble bases. The B Horizon of the soil is enriched with calcium carbonate
precipitated from water moving upward from below…called Capillary Action.
Calcium is the main material deposited at the surface.
Task
Using the materials provided and the
information about soils in your
textbook (Page 147), make soil
profiles representing (1) wet-climate
soil and (2) dry-climate soil.
Make a soil profile for an area with
the following characteristics:
(1) low-lying
(2) heavy rainfall
(3) waterlogged
NOTE: In a waterlogged setting, bacterial activity (i.e., decomposition of plant
material) is slowed.
NOTE: Decaying plant material often releases acids that will react with the
iron.