SOIL, HOW IT IS FORMED

More elementary than my other SOIL slide show and

pitched more for 3rd or 4th grade students.

By Moira

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.

Thankfully, soil covers most of the Earth’s solid crust (bedrock).

Plant growth, that allows us to live, occurs on the top layer of soil.

bedrock D

layers of soil

http://www.nrcs.usda.gov

Below that layer are several other layers, some that provide minerals and ores for our use.

Why do we care?

What is soil made of?

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Even though soil looks like one thing, it is really made up of many different kinds of things.

Two of the main things are: 1. tiny, tiny pieces of rock and 2. the rotten remains of dead plants and animals.

Soil:

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

First, looking at that part of the soil that is tiny, tiny pieces of rock.Through the process called weathering big rocks are broken into smaller and smaller pieces. The smallest pieces are called soil.

People can break rock into smaller pieces with a sledge hammer.

But people are not the main reason that rocks are continually being broken up into smaller and smaller pieces until they become soil.

Weathering is caused in nature by such things as wind, water, and temperature changes that break the rock into ever smaller pieces.

The weathered rock in soil at some point in time started out as a bigger rock, maybe a large boulder.

In the process of being weathered, the size of the particles of rock become smaller and smaller—boulders to large rocks, to smaller pieces of rock to pebbles to soil.

Big rocks become smaller

and smallerand smaller until they are soil.

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

SoilSoil

Some rocks break down into smaller pieces more easily than other rocks.

Those rocks that are more easily worn away are called soft.

And those that are less easily worn away are called hard.

Activity to show differences in hardness of rock.

•Give the students a small hard rock, a small piece of soft sandstone rock, a sugar cube and a piece of chalk. •Ask them to weather the rocks by rubbing them against one another. •Tell the students that the chalk and sugar cube are models of rock.

Ask the students if some of the “rocks” were more easily weathered than others. Explain those rocks are called soft and the ones less easily worn are called hard.

Now let us discuss, some of the things in nature that cause rocks to break into smaller and smaller pieces.

In other words, what are some of the things that cause rocks to weather.

Running water in rivers and streams carry rocks downstream causing them to hit one another and break apart.

One of these things is moving water.

Water running over large rocks causing them to break into smaller rocks.

As the sand and gravel carried by the water rub against the rocks in the river, they become smooth and rounded and are called river rocks.

Imagine rocks tumbling along the bottom.

Notice how smooth and rounded these river rocks are.

To see an animation of this process, open http://whs.moodledo.co.uk/mod/resource/view.php?inpopup=true&id=26639and choose “click direct link to file”. When finished, close the window to the internet and you will be back on the slide show.

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When a river flows rapidly over a plateau, it breaks the rocks into smaller and smaller pieces of rock. The fast moving water carries these pieces of rock down the river creating a canyon.

The following activity may be used to demonstrate weathering by moving water.•Place a piece of soft sandstone in a plastic jar or vial along with a harder rock and some gravel. •Add some water. •Have the students shake the container back and forth. •After several minutes pour the contents out on a plastic plate and observe what has happened.Discuss how this activity models weathering of rock due to moving water and rocks banging together.

Another thing that weathers rock is blowing sand. As it rubs against rocks can break them into smaller and smaller pieces.

Blowing sand rubs away the softer rock changing it into soil.

The harder rock is left behind.

Activity to demonstrate weathering by blowing sand.•Place a piece of colored chalk and salt in a plastic jar or vial. •The chalk is a model for rock and the salt a model for sand. •Have the students shake the container and then pour its contents on to a plastic paper plate and observe.

Discuss the results of this activity and how it models weathering by blowing sand.

Water gets into cracks in the rock. If this water freezes and melts and freezes and melts time and time again it can break the rock apart.

http://www.phschool.com/webcodes10/index.cfm?fuseaction=home.gotoWebCode&wcprefix=cul&wcsuffix=2052

and play the small video. When finished, close the window to the internet and you will be back on the slide show.

To see an animation of frost wedging, open

Rocks break as water freezes in cracks

Activity to demonstrate weathering by water that expands when freezes expands and can break open a rock.

•Place a can of liquid coke in the freezer overnight and observe what happens.

Discuss how this activity models weathering of rock due to the fact that water when it freezes expands and can cause rocks to break.

The Grand Canyon was formed by these three things in nature: moving water in a river, wind carrying sand, and the repeated melting and freezing of water in the cracks in the rocks.

Colorado River

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, etc. are called weathered rock.These pieces of rock may be a huge boulder or the the size of a grain of soil.

Some of these pieces of rock may be small that we can only see them under a microscope.

The smallest pieces of weathered rock is called soil.

Humus is formed when dead plants and animals rot or decay.

Now for the other part of soil humus.

How is it formed?

Humus is the organic part of soil—that which was once living.

Bacteria are the smallest living organisms, and the most numerous of the decomposer (living things that cause decay).

A teaspoon of fertile soil generally contains between 100 million and 1 billion bacteria.

As they eat the dead plants and animals, they carry out most of the decomposing that occurs in the soil.

http://soils.usda.gov

Magnified bacteria found in the soil.

Fungi are not plants; they can't make their own food.

Fungi is the name for simple organisms (living things) including mushrooms, molds and yeasts.

Next to bacteria, fungi are the most efficient decomposers.

They absorb their nutrients from the organisms they are decomposing. In the process they release enzymes that cause dead plants and animals to rot or decompose.

http://www.flickr.com Benimoto

http://www.flickr.com Futurilla

http://www.flickr.com scoobygirl

Mushrooms growing on logs

http://www.flickr.com zimpdenfish

Pill bugs

Important invertebrate decomposers are:

Invertebrates (animals without backbones) are also decomposers.

Decomposing mites

Photo credit: Joseph O'Brien, USDA Forest Service,

Millipedes

•Place samples of compost on the students’ tables and ask them to observe. •Be sure to give the students samples of various degrees of decomposition. •Explain the bacteria that cause most decomposition are too small for us to see.

Activity to show how leaves and other organic material can decompose and form rich black soil.

Discuss how dead plants and animals can decompose and change into blackish colored soil and how most soil is a mixture of weathered rock and rotten plants and animals.

So in review: What are two parts of soil?

Soil:

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

What is the process in nature called that causes rocks to break into smaller and smaller pieces?

What are some forces in nature that causes the weathering of rocks?

Yes, weathering.

And finally, you should remember how the other part of soil, humus, is formed?

Soil—it is the dirt we in science choose to love.