Soil is formed by the weathering of rock and the decomposition of organic material by soil organisms. It is composed of minerals from broken down rock, organic matter from dead plants and animals, air, and water. The solid components are classified by particle size as sand, silt, or clay. Humus, or organic matter, is formed through the decay of dead plant and animal matter by decomposers like bacteria, fungi, earthworms, and other invertebrates. Together, these components support plant growth and life in the soil.

1. SOIL, HOW IT IS FORMED

2. Hey, listen to this. Have I got
the dirt on....
Soil
By Moira Whitehouse PhD

3. 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.

4. 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.
There are different
layers of soil—similar in
characteristics such as
composition, texture,
and color.
Bedrock
(crust of the
Earth)
USDA

5. 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
D provide minerals and
bedrock
ores for our use.
http://www.nrcs.usda.gov

6. Speaking mainly of top soil and the upper soil
horizons
Soil is made of.....?
Four things:

7. • 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

8. The weathered rock pieces
(minerals) make up about
half of the total mass of
most soil.
Less than 10% is organic
matter-- dead plants and
animals.
The other half is made up of many, many,
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.

9. 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

10. 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.

11. 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)

12. First we will explore the weathered rock part
of the soil.
Weathered rock is formed through the process
of weathering which breaks rock into smaller
and smaller pieces.
Weathering is caused by agents in nature
(wind, water, temperature variations) that
break rock down into smaller pieces.

13. In review, ...so what is weathered rock?
Pieces of rock that have been broken down
into smaller pieces by the forces of nature—
water, wind, ice, acid water, plant roots, etc.
are called weathered rock.
These pieces of rock may be the size of a
boulder or a grain of sand.
The smallest pieces of weathered rock is
called soil.
Some of these pieces of rock may be small that
we can only see them under a microscope.

14. 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.

15. Over hundreds, maybe thousands of years,
it could have happened something like this.
Soil
Soil

16. Looking again at the solid part of soil, first we
will consider the three types of soil that are
the result of weathering:
Soil: Later we will discuss
humus.
Weathered rock Humus
(remains of dead
plants & animals)
Sand Silt Clay
largest smallest particles

17. 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
2. Silt—medium sized particles
3. Clay—smallest particles

18. The following guidelines are use to
talk about the different sized particles
of sand, silt and clay.

19. 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

20. Wikipedia Commons

21. 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

22. What is humus?
The organic part of soil—
that which was once living.
How is it formed?
Humus is formed when dead plants
and animals decay.

23. 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. recycle dead plants and animals
Decomposers
into nutrients plants need.

24. Bacteria are the smallest living organisms,
and the most numerous of the
decomposers.
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.

25. http://soils.usda.gov
Magnified bacteria found in the soil.

26. 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.

27. http://www.flickr.comBenimoto
http://www.flickr.comFuturilla
Mushrooms growing on logs
http://www.flickr.comscoobygirl

28. Mushrooms growing on a forest
floor
http://www.flickr.com mill56
Mushrooms growing in dead grass
http://www.flickr.com photogirl7

29. http://soils.usda.gov/
Fungus beginning to decompose leaf veins
in grass clippings.

30. Other important decomposers found in
the soil are numerous invertebrates—
animals without backbones.
The initials “FBI” can be use to help us
remember the three main decomposers
types:—fungi---bacteria---invertebrates.
Invertebrate decomposers include worms
called nematodes, mites, pillbugs and
millipedes.

31. 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 http://soils.usda.gov/
nematodes. Nematodes magnified in soil.

32. Other important
invertebrate
decomposers
Pill bug
http://www.flickr.comzimpdenfis

33. Decomposing
mites
Millipedes

34. 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.
The lowly earthworm is also an important
decomposer.

35. 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 and phosphorous 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.

36. Decomposition creates soil that contains
the nutrients plants need in a form that they
can use to carry out their life processes.
USDA

37. The 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

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

39. 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.

40. In review, we learned that soil is made up
of four main things. Can you remember
them? (two solids, one liquid and one gas)

41. 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

42. Next, what do we call tiny particles of
weathered rock?
Yes, we call them soil.
What are the three main types of soil that
result from weathering of rocks?
Sand, silt and clay
What is the main feature that distinguishes
sand, silt and clay?
The size of the particles, sand being the
largest and clay the smallest

43. Weathered rock makes up one part of solid
soil. What makes up the other part?
Yes, it is humus.
What is humus?
The organic part of soil
which was once living.
How is it formed?
Humus is formed when dead plants
and animals decay.

44. This stuff is an important natural resource for man.
When we love and
honor it we call it soil.

45. But when grownups don’t like it, they call it dirt or mud.