Soil science for teachers (massive-the real dirt on soil)

The Complete
Soil

By Moira Whitehouse PhD

The continents have a
layer of solid rock, the
crust, covering the hot
stuff in the mantle.

The continental crust, then, is mostly covered
with thick layers of soil.

Here we see a slab of Earth taken out of
the crust with the soil on top showing
where we live.
Soil, from the bedrock
to the top, is our
subject.

Below the top, there
are coherent layers of
soil—similar in
characteristics such as
composition, texture, a
nd color.

Bedrock
(crust of the
Earth)
USDA

Thankfully, soil covers most of the Earth’s solid
crust (bedrock); however, in some places it is
thin or nonexistent. Why do we care?
Plant growth, that
allows us to live,
occurs on the top
layer of soil.
layers of Below that layer are
soil several other layers,
some that provide
D minerals and ores for
bedrock
our use.
http://www.nrcs.usda.gov

We should understand that soil is a valuable natural
resource that is essential to life on Earth.
Since top soil can be nurtured and protected, or
squandered, our well being as a specie may well
depend on which we choose.
Listing the good things provided by soil might be
useful for our students: soil is where we grow our
food; it helps clean our water; it provides the
material for constructing many of our buildings
and roads; it absorbs rainwater and helps prevent
floods.

Speaking mainly of top soil and the upper soil
horizons

Soil is made of.....?

Four things:

• weathered pieces of rock made up of
minerals (All rock is a mixture of
minerals.)
• organic material (remains of dead plants
and animals)
• air (containing oxygen)

• water

The weathered rock pieces
makes up about half of the
total mass of most soil.
Less than 10% of the
mass would be organic
matter and living things.
The other half is made up of vast numbers of
interconnecting holes between the rock pieces
and organic matter.
Depending on location, the time of year or
rainfall, these holes, called pores, are filled with
either air or water.

Soil

Tiny pieces of Remains of Air with oxygen Water
weathered dead plants
rock and animals In pores, spaces between
the pieces of weathered
rock

air
water

Those ingredients that make up soil are necessary
for the plants and micro-organisms that live there.
• The weathered pieces of rock and the organic
material provide many of the nutrients such as
iron, nitrogen, potassium that plants need to grow
and to carry out their life processes.
• Plant roots and soil microorganisms get the
oxygen and water they need to live from the
spaces (pores) in the soil.
• The weathered pieces of rock anchor the plants
roots so a plant doesn’t blow or wash away.

Though we usually think of soil as the solid
combination of weathered rock (minerals)
and organic material we can hold in our
hands, the liquid and the gas components
are crucial to the life processes that are
supported by soil.
Liquid—the water and the minerals
dissolved in the water that plants need.
Gas—oxygen is needed for respiration by
the roots, the bacteria and other
microorganisms.

In this presentation we will focus mainly on the
solid parts of soil—weathered rock and the
humus—the organic material (dead plants and
animals). Soil:

Weathered rock Humus—organic matter
(tiny pieces of rock) (remains of
dead plants and animals)

First we will explore the weathered rock part
of the soil.
What is weathered rock?

To answer, we will review “weathering”.

Weathering can happen as a
result of physical processes

Knowing that soil is made up of tiny pieces of
weathered rock, it is obvious that the rocks in
these pictures have a long way to go before
becoming soil.

Weathering will take many years to change
this sized rocks into soil sized pieces.

We will find, however, that some rock
weathers (breaks down) into soil size
fragments faster than other rock.

Some of the reasons for rock to weather at
different rates include:
Particle size--The smaller the particles making
up the rock, the faster it will weather.

Exposed surface area--The more surface area
that is exposed, the more it will weather.

Porosity--the more holes in the rock the faster
it weathers.

The minerals that make up the rock-- Softer
minerals weather faster than harder ones.

Knowing that weathering of rock takes
time, and that some rocks weather at
different rates than others, we will now
consider what causes weathering.

The two major types of weathering are
physical and chemical.
1. Physical weathering causes the parent
rock to break into smaller fragments
without changing the chemical makeup of
the rock.

Physical weathering

•Breaking bigger rocks into smaller
fragments without changing the chemical
makeup of the rock can be done by
nonliving things and by living things.

We will look first at non living things that
break up rock.

Non living things
that cause
weathering

Nonliving things that break rocks into pieces.
1. Water running over the rock
2. Water freezing in cracks in the rocks
3. The temperature of rocks changing
from hot to cold
4. The abrasion of rock by the blowing
wind carrying sand
5. Salt wedging

Rapidly moving water particularly high up in the
mountains or a canyon can wear away (breaking
off fragments of rock).
http://www.ngu.no/en-
gb/hm/

Seen here, a small,
fast running
stream of water is
dramatically
cutting through
this huge granite
monolith.

Rocks carried by fast moving water hit other rocks
breaking them into pieces. Moving sand and silt
acts like sandpaper on the larger rocks in the river
bed rubbing off pieces of rock. Theses smaller
pieces
become part
of the
suspended
rock and
sediment
carried by
the fast
moving river.
http://www.flickr.com kia4067

Gradually larger rocks are broken into smaller
and smaller rocks eventually becoming sand and
silt.

http://www.flickr.com Wildcat Dunny

Here you can see fragments of rock created
by water that was moving quickly.

http://www.flickr.com Randy OHC

Rocks that have
been tumbled for
a long time in
rivers and streams
become smooth
and rounded.

http://www.flickr.com
Dawn

2. Water freezing in cracks in the rocks
This process occurs most often in cold wet climates
usually in high mountain ranges. It does need to
periodically warm up enough for liquid water to run
into the cracks.

http://www.flickr.com/
reurinkjan

Here we can see
cracks in large
mountain rock.

Copyright 2004 by Andrew Alden,
geology.about.com, reproduced under
educational fair use http://www.ct.gov/

Rain fills these cracks with water.
When the water freezes, it expands within
the crack pushing the rock apart.

Image courtesy of the geology department umd

As the ice melts, the water penetrates
more deeply into the fracture.
Then the water freezes again, further
cracking the rock.

This repeated cycle
of freezing and
thawing forces the
crack open more
and
more, eventually
shattering the large
rock into smaller
pieces.

3. Changing temperature also causes
physical weathering.
As rocks heat up, they expand. As they
cool, they contract.
This process is repeated over and over again
in nature.
Eventually this process causes them to
break apart.

The expansion and contraction of rock due to
changes in temperature occurs most often in
desert environments where the temperature
fluctuates dramatically from day to night.
The sky above a desert is usually clear and dry.
As a result, during the day, the ground heats up
and at night cools down rapidly.
The surrounding air is likewise heated and
cooled quickly by the ground.
This repeated expansion and contraction of
weakens the rock causing it to break.

Here we see
large rock in a
desert
environment
that has
probably
been exposed
to the freeze-
melt cycle.

Hoggheff aka Hank Ashby aka Mr. Freshtags'

4. Abrasion by windblown sand also
weathers rocks particularly in deserts. It
is similar to rubbing sandpaper over a
piece of wood or sandblasting concrete.

http://www.flickr.com beige alert

Frequent sandstorms in the desert
weather exposed rocks.

http://www.flickr.com nukeit1

The scouring of the rock by the wind
carrying sand wears off fragments of rock.

http://www.flickr.comlumierefl

So much for those non living things that
cause physical weathering, now we will
consider the living things that also cause

Living things
that cause
weathering

Living things also break rocks into
smaller pieces:

1. Plant roots—particularly tree roots

2. Lichen growing on rocks

3. Burrowing animals

The roots of plants, particularly tree
roots, are amazingly strong. When they start
growing as tiny root hairs they can fit into
the smallest of cracks.

As these tree roots continue to grow in
diameter, they put stress on the rock and
cause the cracks to expand breaking the
rock apart.

Here the
roots of the
tree are
growing in
the cracks in
the rocks
making the
fractures
larger.

Chazz Layne

http://media.photobucket.com/ city bumpkins

2. Lichen growing on rocks.
Lichens are unusual creatures, they
are not single organisms the way
most other living things are.
Rather, lichens are a combination of two
organisms—a fungus and an algae-- which
live together in a symbiotic relationship.
A little memory prodder for this, “Freddy
fungus met Alice algae and took a lichen to
her.”

Lichens appear in the form
of small patchy crusty
outgrowths in colors of
green, brown, orange and
other colors. They often
grow on rocks.

http://www.flickr.combrian http://www.flickr.comSeaDavid

Lichens expand and contract as they are
moistened and then dry out. In this
process they loosen microscopic rock
fragments creating tiny cracks in the rock.
Their rock-breaking power is greatly
enhanced when seeds fall into cracks
and then germinate. This is particularly
true in the case of woody shrubs and
conifers with powerful expanding root
systems.

3. Burrowing animals
When animals burrow in rocks or between
the rocks, they introduce seeds which
germinate, or water that freezes, both of
which encourage rock breakup.

So far, we have looked at the agents of
physical weathering—the process of
breaking up parent rock into smaller
pieces without changing the chemical
makeup of the rock.

Now we will investigate
chemical weathering.

Chemical weathering creates holes or
soft spots in rock, so the rock breaks
apart more easily.
Causes of chemical weathering:
1. Acidic water
2. Oxidation (rusting of iron in rock)
3. Hydrolysis

1. Acidic water:
Carbonic acid is very common in nature. It is
produced when carbon dioxide is released
from decaying material or when carbon
dioxide in the air combines with water.
When this weak carbonic acid trickles
into cracks in limestone, it can dissolve
large amounts of calcium carbonate
(calcite) in the form of calcium
bicarbonate.

The mildly acidic rain
water seeps into
cracks in the ground.

The carbonic acid in
the water dissolves
the calcium
carbonate in
limestone rock
leaving behind holes
in the rock. http://www.esi.utexas.edu

Carbonic acid also works on limestone found
on the surface. We don’t see the classic
holes here, but there is evidence that softer
parts of the rock have been dissolved away
allowing the rock to break apart more easily.

http://www.geolab.unc.edu/

2. Oxidation, another type of chemical
weathering, is the process in which oxygen
chemically combines with another
substance.
Many rocks contain iron. When water and
air are present, this iron often oxides
(rusts), similar to an iron bar rusting.

The rust formed by oxidation makes
rocks softer. As a result, the rock
deteriorates.

When you see red
rocks you know
oxidation (rusting)
is happening and
that those rocks
are weathering.

This process is
common is arid
regions.
pics4learning

3. Hydrolysis occurs when minerals in rocks
react with water to form other weaker
compounds.
These weaker compounds are then more
easily worn away by physical weathering.

Feldspar changes
to clay.

To review, both physical and chemical weathering
are important in creating soil out of rock.
Physical Weathering Chemical Weathering
Wind with sand
Freezing and
thawing of Running water
water Acids
Heat and cold sand
Water
Ice silt
clay

soil

Weathering breaks the rock into
parent rock smaller and smaller pieces

In the study of soil, the rock that is weathered to
produce soil is called the “parent rock”. The parent
rock may be igneous, sedimentary or metamorphic.
Soil:

Weathered rock
Humus
(remains of dead
plants & animals)
igneou
s metamorphic
sedimentary

Igneous rock:
is rock that is formed from
solidified magma.

It may be cooled above the
surface, usually on the
ocean floor (as in basalt).
http://rst.gsfc.nasa.gov/

Or the magma may be
extruded underground
where it gradually cooled
and solidified (as in granite)

http://rst.gsfc.nasa.gov/

Sedimentary rock:
is rock that is formed
from sediment deposited
by water or wind or
minerals deposited when
water evaporated.

Seventy-five to eighty
percent of the rock
exposed on the Earth’s
surface is sedimentary rock; therefore most of the
parent rock is sedimentary.

Metamorphic rock: changed chemically by
is rock that has been
heat and pressure deep under the surface of
the Earth.

Gniess

Igneous and metamorphic rock tend to be
buried underground and therefore, generally not
subject to weathering.

In science, it is often found that things occur in
cycles. The rock cycle is one of the more
interesting ones, but it takes a very long time
to complete. This picture of the
rock cycle shows
that each type of
rock can be
weathered and
changed into
sediments
including soil
(small pieces of
weathered rock).

Looking again at the solid part of soil we
will consider the three types of soil:
Soil:

Weathered rock Humus
(remains of dead
plants & animals)

Sand Silt Clay
largest smallest particles

These three types of soil, sand, silt and clay
are identified largely based of the size of
their weathered rock pieces.
1. Sand—largest particles (created by

2. Silt—medium sized particles (created by

3. Clay—smallest particles (created by
chemical weathering.

The weathered rock in soil probably
started out as a huge boulder.
In the process of being broken down, the
size of the particles of rock become
smaller and smaller—boulders to large
rocks, to smaller pieces of rock to
pebbles to sand, silt and clay.
The following particle size guidelines are
used to differentiate the various rocks as
well as the three types of soil.

Size of Particles of Rock (Diameter)
•2 m boulders
• coarse fragments such as pebbles > 2 mm
• sand < 2 mm to 0.05 mm
• silt< 0.05 mm to 0.002 mm
• clay < 0.002 m

Still looking at the solid part of soil, we
now examine the humus:
Soil:

Weathered rock Humus
(remains of dead
plants & animals)

Sand Silt Clay
largest smallest particles

What is humus?

The organic part of soil.

How is it formed?

Humus is formed when dead plants
and animals decay.

What causes these dead things to change
into soil?
Special organisms in the soil, called
decomposers, cause dead plants and animals
to decay or rot changing their bodies into the
humus part of soil.
When plants and animals die, they become food
for these decomposers--bacteria, fungi,
arthropods, nematodes and earthworms.
Decomposers recycle dead plants and animals
into chemical nutrients like carbon and nitrogen
that are released back into the soil, air and water.

Bacteria are the smallest living organisms,
and the most numerous of the
decomposers; they make up 90% of the
billions of microorganisms typically found
in a gram of soil.
A teaspoon of fertile soil generally
contains between 100 million and 1 billion
bacteria.
They carry out the majority of decomposing
that occurs in the soil.

Bacteria are important in the carbon cycle.
During respiration, bacteria release carbon
dioxide into the atmosphere.

USDA

Bacteria are an essential parts of the nitrogen cycle.

Wikipedia commons

http://soils.usda.gov

Magnified bacteria found in the soil.

http://soils.usda.gov

Bacteria dot the surface of strands of fungal hyphae.

http://soils.usda.gov/

Actinomycetes are a large group of bacteria that grow
as hyphae like fungi. They are especially important in
degrading hard-to-decompose compounds, such as
chitin and cellulose.

Fungi is the name for simple organisms
including mushrooms, molds and yeasts.
Next to bacteria, fungi are the most efficient
decomposers.
Fungi are not plants; they can't make their own food.
They absorb their nutrients from the organisms
they are decomposing. In the process they release
enzymes that decompose dead plants and animals
Many fungi can break down tough organic materials
such as the cellulose in leaves and the cellulose and
lignin in wood.
Many plants depend on fungi to help extract
nutrients from the soil.

http://www.flickr.comBenimoto

http://www.flickr.comFuturilla

Mushrooms growing on logs

http://www.flickr.comscoobygirl

Mushrooms growing on a forest
floor

http://www.flickr.com mill56

Mushrooms growing in dead grass

http://www.flickr.com photogirl7


Fungus beginning to decompose leaf veins
in grass clippings.

Other important decomposers found in
the soil are numerous invertebrates—
animals without backbones.

Thus, the saying “F B I
ungi ” as the
acteria nvertebrates

three main decomposers.

Included under the invertebrate
grouping are worms called
nematodes, mites, pillbugs and
millipedes.

Nematodes, a group of invertebrate decomposers
living in the soil are tiny non-segmented worms
typically 1/500 of an inch in diameter and 1/20 of
an inch in length.
One square yard of woodland or agricultural soil
can contain up to several million nematodes.

Nematodes are important in releasing nutrients
into the soil in forms plants can use.

Nematodes are play an crucial role in the return
of nitrogen to the soil (nitrogen cycle).


Nematodes magnified in soil.

Other important
invertebrate
decomposers

Pill bug

http://www.flickr.comzimpdenfis

Decomposing
mites

Millipede

Organisms such pill bugs, millipedes and mites
are important to the soil because they stir up
and churn the soil, mixing in air which is
needed by other organisms in the soil habitat.
They shred organic matter into small
pieces, assisting other soil organisms in the
decomposition process.
Finally, along with other decomposers, they
change nutrients into forms plants can use.
The lowly earthworm is also an important
decomposer.

Earthworms eat dead plants and
animals, thereby, absorbing the
nutrients that they need to
survive.
Earthworms excrete wastes in
the form of casts which
are rich in nutrients such as
nitrogen, phosphorous and
potash that plants need.
In addition to breaking down organic materials and
adding nutrients to the soil, earthworms also help
loosen the soil, thereby, creating space for the oxygen
that plant roots and microorganisms need to live.

All these decomposers along with the
dead plants and animals form a soil food
web.

And how are soil food webs different than
food webs on the surface of the Earth?

In a food web above ground, the
decomposers are at the end of the energy
transfer in the food web.

In a food web below ground the decomposers
are at the beginning of the web.

While living plants are the largest source of
energy above ground, the most abundant
energy resource in the soil is dead plant and
animal matter.

The primary consumers in soil are often
microbes such as bacteria, fungi and
nematodes (tiny microscopic worms).

They consume the remains of dead plant
and animal.

In turn, these primary consumers are the
food for many other secondary consumers
that live in the soils such as mites, protozoa
and other nematodes.

The natural cycle of
plants absorbing
minerals from the soil
and and these minerals
being returned to the
soil through
decomposition is
repeated over and
over in nature.

http://www.flickr.com/ angus clyne

Decomposition creates fertile soil containing
the nutrients plants need in a form that they
can use to carry out their life processes.
There are 13 main mineral nutrients that when
dissolved in water and absorbed through a plant's
roots provide what is needed by plants to grow
healthy forests, prairies, or even front lawns.
These are: nitrogen, phosphorous, potassium,
calcium, magnesium, sulfur, boron, copper, iron,
copper, manganese, molybdenum and zinc.

Where there is lots of vegetation to decay and
enrich the soil, such as in deciduous forests and
grasslands, the soil is rich in humus and very
fertile.

Wikipedia Commons

Wikipedia Commons http://www.flickr.com/ Cory Leopold

Desert in Saudi Arabia The Chihuahuan Desert along the Rio Grande

Where there is little or no vegetation to
provide the organic debris, such as at the
seashore or in the desert, the soil has little or
no humus and is not very fertile.

Now that we have explored how
the two solid parts of soil are
formed (weathered rock and
humus), let’s move on to another
topic.

What are the physical properties
of these kinds of soil, sand, silt,
clay and humus?

Remember physical properties are the
things we can observe about a substance
using our five senses.

For soil, the two main physical properties
are:
•color
•texture

Soil Color
The most obvious property when looking
at soil is its color.

Geologist officially recognize over 170
different soil colors.

The most common color of soils are shades
of black, brown, red, gray, and white.

Generally speaking, the darker a soil
is, the more nutrients it contains.

The darker color often indicates an increase in
decomposed organic matter known as humus.

Gray soils often indicate poor drainage, while
red soils may indicate a lack of nutrients.

black fertile soil infertile red soil
Photos courtesy of USDA

• Soil texture is the identifying characteristic
of sand, silt and clay, the three types of soil
formed from weathered rock. Texture is
based on the size of the particles making up
the soil and is the single most important
physical property of the soil. The size of the
particles making up a soil, therefore, greatly
affects:
• 1) how much water will flow through it
• 2) water holding capacity

• 3) ability to sustain plant growth

Sand
• particles are visible
without microscope
• < 2 mm to > 0.05 mm
• rounded or angular in
shape
• if the sand grains look
white it is usually
weathered quartz; if
they look brown it
contains many
minerals Images from Wikipedia Commons

Particles under a microscope

Sand
• feels gritty
• does not stick
together in a mass
unless it is very wet.
• pores between
sand particles promote free
drainage and entry of air
• fewer nutrients for plants than silt or clay
• holds little water and prone to drought

Sandy soil
Sandy soil has large particles with large
air spaces between them.
Therefore, it drains very quickly and
dries out faster than the other soils.
Also, water-soluble nutrients leach out
rapidly before the plants can use them.
Humus added to sandy soil acts like a
sponge, absorbing and holding moisture and
any nutrients dissolved in it.

Silt
• particles not visible
without a microscope
• particles < 0.05 mm
to > 0.002 mm

• erosion by glaciers
often responsible for
formation of silt

• does not feel gritty.
• floury powdery feel –
smooth like silly putty
when wet.
• wet silt does not stick
together and cannot mold
it into different shapes.
• bigger particles than sand -- retains
more water for plants and have slower
drainage than sand.
• easily washed away by flowing water –
highly erosive.
• has more plant nutrients than sand.

Clay
• particles are flat plates or tiny flakes
• < 0.002 mm
• when stirred in water small particles of clay
donot settle but remain suspended.

Clay
particles
under a
microscope

http://photojournal.jpl.nasa.gov

• wet clay is very sticky
and slippery and can
be molded readily into
any shape like a rod or
a long ribbon.
• pores (spaces between
particles), like the particles are very small so
movement of water and air through clay is slow
giving clay a tremendous ability to hold water.
• clay swells when water is added and shrinks
when the water evaporates. This results in
bothersome shrinking and swelling that can
adversely affect buildings, roads and walls.

Clay soil
The tiny particles that make up clay tend to stick
together causing water to fill up the air spaces.

Since moisture does not drain from this soil
well, if clay is too wet plant roots cannot
respire so they rot.
Adding humus to clay soils discourages the
small particles from sticking so tightly
together, resulting in larger spaces that drain
water more easily and hold more air.

Determining Soil Texture - Feel Method
• Wet a sample of the soil and place in your
hand
• Try to roll it into a ribbon.
• If it makes a ribbon, it contains mostly clay.
• The longer the the more clay in the soil.
• If it does not roll into a ribbon, it is sand or
silt
• If it feels grainy or gritty it is mostly sand
• If it feels smooth, silky or flowery, it is
mostly silt

Properties of humus

Humus is the soil formed
when dead plants and
animals decay.
http://www.edupic.net/sci_gr.htm
Free clip are for educational use

It is a dark brown or black (color).

It feels crumbly and loose when dry and
spongy when wet (texture).

When dead plants and
animals decay leaving behind
the humus, it accumulates
just under the top layer of
soil (shown as Horizon “A”)
Humus inherits the minerals that were
contained in the decayed plants and animals.
Humus therefore contains the nutrients
(minerals such as, nitrates, phosphates,
potassium, copper, zinc dissolved in water)
that plant roots can absorb.

Comparing soil... ability to hold water
Clay soils hold more water than sand, silt or
humus.
For growing plants, clay holds too
much water for most kinds to survive.
Humus holds more water than sand or silt.
But water does drain through humus
quickly.
Sand and silt hold the least water, often too
little for many kinds of plants to survive.

Comparing soil...nutrients (minerals) provided
Humus soils have an abundance of nutrients
for plants.
Adding humus to infertile soil increases its
fertility.
Clay has more nutrients (minerals) than
sandy soils mainly because it hangs on to
its mineral abundant water for longer.
Since clay soils hold a lot of water, fewer
nutrients are leached out of the soil.
However, as clay soil ages and plants
remove its nutrients, it becomes infertile.

Sandy soils have fewer minerals
becausewhen water drains through sandy
soils, it often dissolves the minerals in the
rock and carries them along with it.
This condition
is called leaching.
When nutrients leach out of the
soil, they are not available for
plants to use.

Loam is a mixture of clay, silt, sand and humus
and is the best soil for growing plants.

Because loam is a mixture of four kinds of
soil, it holds the proper amount of water and
provides all the nutrients plants need.

Loam is formed in nature when the dead
plants and animals are left to rot and mix
in clay, sand or silt.

We previously mentioned that humus is
found in the “A” soil horizon.

What do we mean by soil horizons?

A soil profile is a vertical
cross-section of soil.
It shows the various
layers or horizons,
beginning at the
layers of
surface with the “O”
soil
horizon and
continuing downward
through the “A”, “B”, bedrock R
“C”, and “R” horizons http://www.nrcs.usda.gov

to the parent material. Soil horizons

The layers in a
cross-section of soil
are easy to spot
here but they are
usually much
harder to find.

Photo coutesy of USDA

The horizons are named O, A, B, C and R.

“O” is the top
horizon. It's
about an inch
thick and is
made up of
dead plants,
animals and
some humus.

The A horizon called topsoil is made up of
humus (decomposed organic matter) mixed
with mineral materials.

The A horizon is alive with plant roots
and tiny microorganism like bacteria,
fungi, and other decomposers such as
nematodes, mites, pill bugs and worms.

It is generally dark-colored because it
contains large amount of humus.

E Horizon -- the Eluviaton Layer
This layer, between the A and B Horizons is
light in color and made up mostly of sand and
silt.

Due to a process known as leaching in
which water with dissolved minerals drips
through the soil above, Horizon E loses
most of its minerals and clay.

Horizon B
The B Horizon is commonly called “subsoil”.
It contains clay and mineral (such as
iron, aluminum oxides, and calcium
carbonate).

Horizon B is formed when clay and minerals
migrate down through Horizon A and E so
sub-soils generally have more clay than top-
soils.

Horizon C
That layer beneath the B Horizon and
above the D Horizon consists of broken-
up bedrock that is only slightly
.
weathered .

Plant roots do not grow down into this
layer and organic material is pretty
much nonexistent.

Layer R
Layer R is the
unweathered rock
(bedrock) layer that
is beneath all the
other layers.

Layer R is the crust,
the outermost layer of
the Earth, that lies
below the soil. NASA image

In discussing soil texture, one of the main
characteristics we discussed was how each
was able to hold or pass through water.

Which brings us to the water table, a
phenomenon dependant upon water
supply but also affected by soil texture.

Surface runoff water seeps through empty
spaces in the soil, sand, or rocks until it
reaches an impermeable layer of rock.

The water then fills the pores, empty
spaces and cracks above that rock layer.
Water table
The upper “surface” of the water filling
those spaces in the soil, sand, or rocks is
called the water table.

Again, the upper “surface” of the
ground water is called the
water table.

usgs

It is in this unsaturated
zone above the water
table that plants absorb
the moisture through
their roots and return it
to the atmosphere
through transpiration.
The dirt and rocks in this
zone contain air and
usually some water. In
dry spells, this layer
sometimes dries out.

A large amount of
surface water can
result in water
seeping through
the upper soil
layers and
accumulating
underground as
ground water.

Ground water of recharged from rain water
A large body is ground water is called an
and snowmelt or from water that leaks from
aquifer.
lakes and rivers.

usgs

Ground water is the water trapped in the
empty spaces under ground.

Thinking long term aboutsoil
• Over long periods (thousands of years) the soil
horizon textures change.
• As soils get older, sand weathers to silt and silt
weathers to clay. Consequently, old soils have
more clay than younger soils.
•Soil, along with oxygen, water, plants and animals, is
considered a renewable natural resource. This is

because soil can be replaced in a relatively short
period of time.
•”Relatively” short because it may take a 1,000 years to
form one inch of soil as compared to the 350 million
years for fossil fuels and around 100 million years for
most rocks on the surface of the Earth.

So there it is, the dirt or soil.

Soil science for teachers (massive-the real dirt on soil)

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