THE DIRT ON SOIL
The unknown ecosystem
A recipe for soil
• Start with rocks
and minerals
• Weather into
small pieces
• Add organic
materials
(humus)
• Add water and
air
• Soil forms by
weathering
(minerals) and
decomposition
(humus)
Be patient!
• It takes 100 to 100,000 years
to form one meter of soil
• Faster in warmer climates
Soil Parts
•
•
•
•
Mineral (weathered rock) = 45%
Air = 25%
Water = 25%
Organic matter (humus) = 5%
WHICH IS LARGER?
• The number of organisms in one shovelful of soil
• The number of humans on earth
SOIL CREATURES
• More in one shovelful than all
the humans that have ever
lived on earth
• Many don’t even have names
yet
HUMUS
• Dark organic
matter from
decomposed
plants and
animals
• Soil organisms
feed on it
• Absorbs and
holds water
SOIL DECOMPOSERS:
BACTERIA
• 1000 fit on the
head of a pin
• Divide to
reproduce
every 20
minutes
BEETLE MITES
• Arachnids
• No eyes (it’s
dark in the
soil)
• Eat only
fungus
NEMATODES
• Also called
round worms
• Only 1 mm long
• Some eat
bacteria, some
eat detritus,
some eat
fungus
EARTHWORMS
• Live 3 to 6
years
• Eat detritus,
also swallow
soil
• Add bacteria
back to soil
SOIL TEXTURE
• Soil has different
textures
• The texture is
caused by the
size of the
mineral particles
Sand
• Sandy soil drains
quickly so roots can
penetrate. Too much
sand in a soil does
not hold enough
water and nutrients
for most plants.
• Does not shrink or
swell so it makes a
good foundation for
buildings.
• Sand – Between
0.06 – 2.0 mm in
size
Silt
• Silt allows for air to • Silt – Between
circulate but still
0.002 - 0.05 mm in
retains some
size
water. Silty soils do
not usually hold
enough water for
most plants.
Clay
• Water clings to tiny
particles holding larger
soil particles together
forming clumps called
aggregates
• Aggregates like gravel
provide drainage
channels for water. Water
will not drain in soils with
a lot of clay.
• < 0.002 mm in size
• Clay is a poor
material to build on
because it shrinks
and swells with water.
• Clay is easily
compacted crushing
air and water pore
space. Plants root
cannot penetrate
through dense clay
soils.
Loam
• A soil with a
mixture of sand, silt
and clay. It is the
best type of soil for
most crops