Download Soil Formation and Composition notes

Soil Formation and Composition
I. Soil Formation
A. Soil – the loose, weathered material on Earth’s surface in
which plants
can grow. It is being formed wherever bedrock is
exposed.
B. Bedrock – the solid layer of rock beneath the soil. It weathers
when it is exposed at the surface to form the basic materials of
soil.
II. Soil Composition
A. Soil is a mixture of rock particles, minerals, decayed organic
material, air, and water.
B. All soil is not the same – the type of rock particles and minerals
in the soil depend on the bedrock that was weathered to form
the soil and the type of weathering.
C. Humus – a dark-colored substance that forms as plant and
animal remains decay. It is the decayed organic material in soil.
III. Soil Texture
A. Soil texture depends on the size of individual soil particles
which are classified by size. The texture is very important for
plant growth.
B. From largest smallest –
Gravel (2 mm or larger),
Sand (less than 2 mm),
Silt (less than 1/16 mm),
Clay (less than 1/256 mm) – that is smaller than the period at
the end of this sentence.
C. Soil that is mostly clay is heavy and dense which allows them
to hold a lot of water. Is this good? No, plants may “drown” from
a lack of air.
D. Sandy soil has a coarse texture and water will drain quickly
through it. Is this good? No, plants may die from a lack of
water.
E. Loam is the perfect soil. It is composed of equal parts of clay,
sand, and silt. It has a crumbly texture that holds both air and
water.
IV. Soil Horizons
A. A soil horizon is a layer of soil that differs in color and texture
from the layers above or below it. They form over long periods
of time. Think centuries not decades. It can take hundred of
years for just a few centimeters of soil to form.
B. A horizon – Topsoil – a crumbly, dark brown soil that is a
mixture of humus, clay, and other minerals.
C. B horizon – Subsoil – usually consists of clay and other
particles washed down from the A horizon, but little humus.
D. C horizon – Partly weathered rock
E. Bedrock
V. The Rate of Soil Formation
A. It depends on the climate and type of rock.
B. Occurs most rapidly in warm, wet climates.
C. Limestone weathers more quickly than Granite in warm, wet
climates.
VI. Life in Soil
A. Some soil organisms mix the soil and make spaces in it for air
and water. Other soil organisms make humus which makes the
soil fertile.
B. Fertile soil is rich in nutrients that plants need, such as nitrogen
and phosphorus.
C. As plants shed leaves they form a loose layer of litter on the
ground. Dead plants also add to the litter and over time they
decay to form humus.
D. Humus is formed by decomposition. This is where organisms,
called decomposers, that live in the soil turn the dead organic
material into humus.
E. Fungi, protists, bacteria, and worms are the main
decomposers.
F. Fungi – molds and mushrooms – grow on and digest plant
remains
G. Bacteria – microscopic decomposers – attack dead organisms
and their waste
H. Earthworms – mix humus and other materials in the soil and
aerate, or mix air into, the soil. They also eat the soil and
eliminate it as waste which is enriched in nutrients, such as
nitrogen.
I. Burrowing animals break up hard, compacted soil and add
nitrogen when they excrete waste.