The unknown ecosystem
 What is soil? What is it made of?
Where do we get if from? What is
your “definition” of soil


The number of organisms in one shovelful of soil
The number of humans on earth

More in one
shovelful than all
the humans that
have ever lived on
earth

Many don’t even
have names yet
A Recipe for Soil
 Start with
minerals
 Add organic
materials
(humus)
 Water
 Air
 It takes 100 to 100,000 years to
form one meter of soil
 Soil forms by weathering and
decomposition.
 Faster in warmer climates
 Why?
 Mineral (weathered rock) = 45%
 Air = 25%
 Water = 25%
 Organic matter (humus) = 5%




Dark organic
matter from
decomposed plants
and animals
Provides nutrients
for crops
Soil organisms feed
on it
Absorbs and holds
water


1000 fit on the head
of a pin.
Divide to reproduce
every 20 minutes.
 Arachnids
 No eyes (it’s
dark in the soil)
 Eat only fungus
Also called round
worms
 Only 1 mm long
 Some eat
bacteria, some
eat detritus,
some eat fungus

 Live 3 to 6
years
 Eat detritus,
also swallow
soil
 Add bacteria
back to soil
 Aerate soil
Tardigrade


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
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The texture is caused by the size of the soil particles
Sand – Between 0.06-2.0
mm in size
 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.

Silt – Between
.002-.05mm in
size. Need
microscope to see
 Silt allows for air
to circulate but
still retains some
water.
 Silty soils do not
usually hold
enough water for
most plants.



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
Clay - < .002 mm in size. 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.
Clay is easily compacted, crushing air and water pore space. Plants
root cannot penetrate through dense clay soils.
Clay is a poor material to build on because it swells with water.

Loam is soil with a mixture of sand, silt and clay. It is the best
type of soil for most crops