Download 3. Plants need air around their roots.A high humus level helps

500
CONNECTIONS:
1000
PHYSICAL
O
km
CANADA’S
ECOZONES
Tundra soils
LEJ Wet-climate soils
Dry-climate soils
Complex soils
of mountain areas
]
3. Air
Plants need air around their roots.A high humus level helps produce air
in the soil because the loose, decaying materials allow for many air
pockets. Air spaces are also created by worms, insects, and small animals
that tunnel through the soil.
4. Moisture
Water dissolves nutrients in the soil and is then taken up by plants
through their roots.Water is also necessary in the chemical and physical
processes that weather rock and decay organic materials.
A basic soil proffle is shown in Fig. 12-2. New mineral materials are
added at the bottom of the soil by the weathering of the parent material.
At the same time, organic materials are added at the top. This top layer of
soil containing humus is called topsoil. Topsoil formation is a very slow
process. Over the last 6000 to 10 000 years, only 15 to 25 cm of topsoil have
formed under the forests of Canada. Under the grasslands of the prairies,
between 40 and 100 cm of topsoil have developed.
A well-balanced mixture of sand, silt, and clay, plus humus is called
loam. It is the best soil for growing plants because it encourages root
growth and holds moisture, and allows water to pass through it at a
rate moderate enough to allow plants to take up nutrients.
Fig. 12-1 Soil re,ions of
Canada
‘
You can learn more about
Canada’s soils at:
httn://sis. a ci r. g c ca/ca nsis/
intro. htm
About half the volume of a
high-quality soil is composed of
water and air.
The size of rock particles is an
important part of soil structure.
The larger particles of sand
allow rainwater to drain quickly
through the soil, while the
smaller particles of clay
prevent rapid drainage.
I
I
CHAPTER
12:
SOIL
AND
VEGETATION
--
•Verythick
brown to
“A” horizi
• Dry condi
cause wc
move up
through
and evap
from surf
• Organic i
from dec
plants re
the surta
PARENT
MATERIAL
• Mineral m
from whic
is made
• Usually b
or glacial
SUBSOIL
• Combined
and organi
• Lighter bro
colour
TOPSOIL
• Rich in orga
materials es
ally near su
• Dark brown
black in coli
CONNECTION
Fig. 12-4 A calcjfied soil profile
‘00cm
---fl
1’
NATURAL
‘“B” horizon tends
to be light brown
to greyish in
colour
• Most soluble
compounds are
gone
• Only iron and
aluminum
compounds are
left
• Thin “A” horizon
• Soluble organic
materials are
rapidly washed
downward by
excess rainfall
> Fig. 12-2
A typical soil profile
Two other processes, which are related to
climate, contribute to soil formation. The first is
called leaching. In areas where there is a great
deal of precipitation, there is a continual down
ward movement of water through the soil
(Fig. 12-3).As the water moves down, it dissolves
the chemical nutrients in the soil and carries
them away.This downward movement of water
removes nutrients that plants need. In very wet
climates, leaching can take water—soluble
minerals so deep that plant roots cannot reach
them .You can identify a leached soil by its poor,
often thin, topsoil layer. Examine Fig. 12-1 to see
how much of Canada is covered with soils that
are leached. Leached soils can he developed into
excellent farmland by adding fertilizers.
Fig. 12-3 A leached soil profile