Weathering and Soil Formation
Supplement to ppt.
4.2
Weathering and organic processes form soil.
VOCABULARY
humus- decayed
organic matter (dead leaves) in soil
soil horizon-
a layer of soil with properties that differ from those of the layer above or
below it
soil profile-
horizons in a specific location
Soil
is a mixture of weathered rock particles and other materials.
SOIL
COMPOSITION
Weathered
rock particles (main ingredient)
Water
(20-30%)
Air
(20-30%)
Organic
matter (5%)
Organic means, coming from living
organisms. Organic matter in soil comes from the remains and waste products of
plants, animals, and other living organisms.
*Soils
differ depending on what types of rock the rock particles came from.
Humus
comes from decayed organic matter.
Different
soils are made up of different ingredients and different amounts of each
ingredient.
The
black humus on the left contains much more plant material and water than the
red soil on the right.
Types
of Soil
The kind
of soil that forms in an area depends on:
The
kind of rock in the area
The
areas climate (overall weather pattern)
The
landforms in the area (mountains, valleys)
The
plant cover in the area
The
animals and other organisms in the area
Time
*The
composition of the soil determines what you can grow in it, what you can build
on it and what happens to the rainwater that falls on it.
Soil horizons
Soil
develops in a series of horizontal layers called horizons.
Deeper
soil looks different than that on top.
Further
down you will find larger, less weathered rock particles and less organic
matter.
Main
horizons are labeled A, B, C
The
A Horizon- The upper layer of soil commonly called topsoil. Often includes more organic matter (humus)
and, therefore, is darker in color.
The
B Horizon- Just below the A horizon. It
has little organic matter and is usually brownish or reddish in color. Contains clay and minerals that wash down
from above.
The
C Horizon- The deepest layer of soil. It
contains the largest and least-weathered rock particles. Typically they are light yellowish-brown.
Climate and landforms affect soil
Different
kinds of soils form in different climates.
Soil
that forms in hot, wet climates is different than those that form in cold, dry
climates.
The
shape of the land affects soil development.
Mountain
soils (cold climate) are different than nearby valleys.
Earths
Surface pg. 124
Activities
of organisms affect soil.
There
is a whole world alive below your feet!
Organisms
affecting the soil
Plants-
Trees/ grasses provide much of the organic matter that forms humus.
Microorganisms-
Include decomposers (fungi, bacteria). A
spoonful of soil may contain 1 million microorganisms. They change nitrogen in soil and air to
compounds that plants can absorb.
Bacteria can produce acids that break down rocks.
Cycling
of nutrients pg. 126
Animals-
Earthworms, ants, termites, mice and groundhogs all live in the soil. They add to the air content of the soil by
loosening and tunneling the soil. They
help to improve drainage also. Return
nutrients to the soil when they die.
Observable, measurable properties of
soil.
Texture-
Determined by the size of the weathered rock particles that it contains.
Color-
Comes from iron compounds and humus.
Pore
Space- Spaces between particles. Soils range from 25-60% pore space. Ideal growing soil is about 50% pore space.
Chemistry-
Acidity of water in soil determines how well nutrients dissolve.
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