Soil Note Guide #2 – How is Soil Made?
I.
II.
Soil is made from weathered rock fragments
a. Parent rock – the source of mineral fragments which make soil (what gets
broken down to make soil)
i. Chemical weathering: During this process, the mineral composition of
the rock is changed due to the influence of water, oxygen, and carbon
dioxide.
1. Example: Feldspar changes to clay
2. Example: Acid rain dissolves calcite in limestone
a. Leading causes are emissions from factories, coal fire power
plants and automobiles
ii. Mechanical weathering: The breakdown of rock into smaller pieces by
physical means.
1. Example: River breaks down rock
2. Example: Tree roots widen cracks in rock
b. Bed rock – layer of rock beneath the soil
i. Residual soils - stay on top of the parent rock (in this case, the parent
rock IS the bedrock)
ii. Transported soils – Blown or moved away from parent rock (in this case,
the parent rock and the bed rock are DIFFERENT)
1. Can be moved by
a. Wind
b. Water
c. Glaciers
All Soils are Not the Same – Soil Properties
a. Soils created by different parent rock have different minerals
b. Texture – the AMOUNT of sand, silt, or clay particles
i. Impacts infiltration
1. Definition: The process by which water on the ground surface
enters the soil
2. Clay has low infiltration
3. Sand has high infiltration
c. Structure – the ARRANGEMENT of sand, silt, or clay particles
i. Impacts infiltration
1. Blocks or plates of soil make it more difficult for water to
penetrate the soil’s surface!
2. Granular soil high higher infiltration
d. Fertility
i. Definition: Soil with the ability to hold nutrients and pass them on to a
plant.
ii. Soil nutrients come from
1. Parent rock
2. Humus – Dark, organic material formed in soil from the decayed
remains of plants and animals
a. Humus is broken down by decomposers
i. Fungi
ii. Bacteria
e. Horizons
i. Definition: A layer in a soil profile
1. MATCH THE LAYERS TO THE PICTURE (*hint* they are NOT in
alphabetical order!)
O – leaf litter and animal scat,
_A – Topsoil contains nutrients for plants
E – Lots of leaching (removal of nutrients
due to passing water)
B – Collects the dissolved substances
from upper horizons
C – Partially weathered bedrock
R – Bedrock with little to
weathering
E – Lots of leaching (removal
of nutrients due to passing
of water)
R – Made of bed rock that
has little or no weathering
B – Collects the dissolved
substances from upper
horizons
O- Present in forests; made
of leaf litter and dead plants
C- Partially weathered bed
rock
A – Topsoil (contains the
nutrients for plant growth)