Soil does much more than simply blanket our planet and provide a habitat for plants and animals. Soil also has a role to play in the health of our planet.
SOIL LAYER CAKE
Soil is made of different layers. What if you drilled a tube straight down into the earth and lifted out a long sample core of soil? You’d see different colors in the sample, like the layers in a cake.
ESSENTIAL QUESTION
Is all soil the same?
Each layer of soil is called a horizon. Each horizon is made of a different type of material, and is named with a letter.
ORGANIC LAYER: The organic layer, called the 0 layer, is what you usually see on the surface. It’s made of humus and bits of grasses and other plants that haven’t broken down yet. This layer can be really thick. Think of the cushy surface you walk on in a forest. The 0 layer can also be very thin or not even there at all, as on rocky mountaintops.
A LAYER: The A layer comes next. This is the first layer of real soil. It has organic material mixed in with mineral matter, such as bits of rocks. This is the best layer for plants and organisms to live and grow in.
E LAYER: The E layer is beneath the A layer. E stands for eluviated. This describes how materials, such as clay, minerals, and organic matter, are carried by water from the layer above it. When material moves down from the A level, it’s called leaching. Together, the A and E layers make the topsoil.
WORDS to KNOW
horizon: a layer of soil.
minerals: nutrients found in rocks and soil.
eluviate: when materials move down through the soil to the layer below.
leaching: when minerals and other matter move out of the topsoil.
B LAYER: Next is the B layer. This is called subsoil, and it’s usually very rich in minerals that have leached down from the layers above it.
C LAYER: The C level is made of the parent material. This is the layer of loose rock that soil develops from.
R LAYER: Finally there’s the R level. This is the bedrock below all the soil. It includes granite, limestone, or sandstone. These are the rocks that form the parent material for some soils.
SOIL AT WORK
What happens when you add water to soil? Do you just get plain old mud? Water moving through soil is actually an important step in the water cycle.
Soil works hard to filter the planet’s water. Have you ever watched someone cook spaghetti? When it’s done you pour the contents of the pot into a strainer so the water passes through and you’re left with just yummy noodles. Soil is like the strainer letting the water go through.
WORDS to KNOW
subsoil: the layer of soil below the topsoil.
water cycle: the continuous movement of water from the earth to the clouds and back to the earth again.
filter: to pass a liquid through something to remove unwanted material.
When it rains, the rainwater seeps into the soil. What if that rainwater had fallen on a field that had been sprayed with chemicals to kill insects? As the water moves through the soil, the soil traps many of those toxic substances that can harm the environment.
Remember the bacteria and fungi that help break down organic matter in the soil? They also help the soil get rid of chemicals and other harmful things.
Sometimes, the soil and the organisms can break down the unhealthy stuff altogether. Other times, the soil holds onto it. Through many years, if more and more chemicals are added to the soil, the chemicals can build up so much that the soil can’t be used to grow anything anymore. That’s when some of that toxic material gets leached back into the air or water.
Soil’s ability to filter is important, because a lot of the water we drink comes from the ground. Rainwater seeps downward until it is stopped by solid rock. That place is called the water table. You can find the water table at different depths depending on where you are.
WORDS to KNOW
toxic: something that is poisonous.
environment: everything in nature, living and nonliving, including plants, animals, soil, rocks, and water.
water table: the underground water supply for the planet.
The water stays underground until someone taps it by drilling a well to bring the water to the surface. It can also come up naturally through a spring. A spring is a low point in the ground that touches the water table and lets the water out of the ground.
WETLANDS
Wetlands are large areas that hold a lot of water naturally. These are really important spaces on our planet because they act as giant filters. When wetlands are destroyed, the earth loses an important resource for keeping our planet healthy.
Chances are, there’s a wetland near you! Visit the U.S. Fish and Wildlife Service’s interactive National Wetlands Inventory map.
KEYWORD PROMPTS
National Wetlands Inventory 
Did you know soil can breathe? It doesn’t have lungs, so it doesn’t breathe like you do, but soil respiration can be thought of as breathing. Soil respiration happens when the soil releases carbon into the air. Carbon is an element that is stored in the soil. Carbon is released in the form of carbon dioxide during soil respiration.
WORDS to KNOW
soil respiration: when carbon dioxide is released into the atmosphere from the soil.
carbon: an element found in all living things.
carbon dioxide: a gas formed by the rotting of plants and animals and when animals breathe out.
Every time you take a breath, you breathe out carbon dioxide gas. So does every animal, even the ones that live in the soil. Where does all that carbon dioxide go? The soil releases some of this carbon dioxide into the air.
Carbon dioxide also comes from dead organisms. All living things are made of carbon, so when plants and animals decay into the ground, that carbon is released into the soil. In the soil, the carbon mixes with oxygen to form carbon dioxide, which is released back into the air.
If the temperature is higher than usual, there’s more soil respiration. That’s not always a good thing. Too much carbon dioxide released into the air contributes to the greenhouse effect. That’s when the atmosphere above the earth gets clogged with too many thick gases and acts like a greenhouse. This warms the earth’s surface more and more and can contribute to climate change.
WORDS to KNOW
atmosphere: the blanket of air surrounding the earth.
climate change: changes to the average weather patterns in an area during a long period of time.
You can see that soil is a very important resource to the earth! For thousands of years, humans have learned how to use the soil in many ways, as you’ll discover in the next chapter.
CARBON GOES ROUND AND ROUND
Just like the water cycle, we also have a carbon cycle. This is when carbon is released from the soil into the air, where plants breathe it in. When the plants die, they rot back into the soil, and the carbon gets released into the air again. Carbon also gets absorbed into the oceans, where ocean animals use it to make shells!
ESSENTIAL QUESTION
Now it’s time to consider and discuss the Essential Question: Is all soil the same?
SOIL LAYERS
The soil is made of horizons stacked up like the layers in your birthday cake! You can make your own soil horizons with this easy, edible project.
SUPPLIES
large, clear jar or glass bowl
pudding, any flavor
shelled nuts or chocolate chips
rolling pin
vanilla cookies, wafers, or sandwich cookies
chocolate cookies
shredded coconut and green food coloring
1 Spoon the pudding into the bowl or jar, keeping it piled up in the center. You don’t want the pudding to hit the sides, because you don’t want the pudding visible from the outside once you’re done. Think of it as a core of pudding.
2 Place a layer of the nuts or chips at the bottom of the container, around the core of pudding. Be sure the nuts push up against the glass, because you do want to see them from the outside.
3 Using the rolling pin, crush the vanilla cookies until they’re very fine. Set aside a small amount, then carefully pour a layer of crumbs on top of the nuts. Make sure the crumbs go up against the glass, because you want to be able to see them from the outside.
4 Crush the chocolate cookies into very fine crumbs, too. Take equal amounts of chocolate crumbs and leftover vanilla crumbs and mix them together. Then, pour a layer of this mixture on top of the vanilla crumbs in your bowl or jar. Again, make sure you can see them from the outside of the jar.
5 Pour the remaining chocolate crumbs in a layer on top, touching the sides of the bowl or jar and covering the pudding core.
6 Use the food coloring to tint the coconut green. Then sprinkle your coconut “grass” over the top of your chocolate crumbs.
THINK ABOUT IT: You should be able to see all the layers from the side of the bowl or jar. Can you identify the different horizons in your “soil”? Which food is the A layer? Which food is the B layer? What is the bedrock made out of?
GEORGE NELSON COFFEY
George Nelson Coffey was the first American to suggest that soil comes from organic materials. He started studying soil in 1900. How do you think the study of soil is different now than it was 100 years ago? What tools do scientists have that they didn’t have back when George Nelson Coffey was studying soil? What do you think we know now about soil that we didn’t know then?
You can view photos of George Nelson Coffey and his gravesite.
KEYWORD PROMPTS
“George Nelson Coffey”
SOIL FILTER
When water filters down through the soil, it gets cleaned of many toxins as it moves through the different horizons. With this project, you’ll see how soil can filter water.
SUPPLIES
clean play sand
screen or another mesh
toothpick
5-ounce paper or plastic cups
3-ounce paper or plastic cups
dirty water from a puddle or mix outdoor water with some soil
fine soil
water
grape powdered drink mix
1 Use the screen to filter the play sand. You want the sand to be as fine and clean as possible.
2 Using the toothpick, poke about five holes in the bottom of one of the large cups. Put the large cup inside one of the smaller cups. It will rest partway into the smaller cup, leaving room for liquid underneath.
3 Fill the top cup about half full with sand. Pour some of the dirty water into the top cup. What happens?
4 After a few minutes, remove the top cup and look at the water underneath. Does it look any different than before? Is it the same color?
5 Dump out the water and place the big cup back into the smaller cup.
6 Next, mix together the grape drink according to the package directions. Pour some of the prepared grape drink into the top cup.
7 When it’s finished dripping, remove the top cup and look at the liquid below. Is it the same color it was when you poured it in?
8 Prepare a second set of cups the same way you did the first. Poke holes in the bottom of a large cup and put it into a smaller cup.
9 Put a thin layer, about one-third of an inch, of play sand in the bottom of the large cup. Put topsoil on top of the sand until the cup is three-quarters full.
10 Pour grape drink into the top cup. Wait until it’s finished dripping, then remove the top cup and look at the liquid in the bottom cup. What color is it? What does it look like? How is this second cup of filtered grape drink different from the one you poured through just sand? How do you think the soil filtered that drink? What conclusions can you make about how soil filters things such as chemicals out of water?
SOIL MOISTURE
Even though you can’t see the water in soil, it’s there. Soil holds on tightly to both water and air. This project will show you this is true.
SUPPLIES
three containers about the same size
black construction paper
tape
regular soil from a garden area or other moist place
dry sandbox sand
science journal and pencil
1 Line the containers up in a sunny window. Leave one empty and tape a piece of black construction paper over the top of it.
2 Fill the next container half full with the moist soil from the garden. Then tape a piece of black construction paper over the top of that one.
3 Fill the last container half full with sand from the sandbox. Again, tape a piece of black construction paper over the top of it.
4 Make sure all the containers are in the sunlight for a couple of hours. Start a scientific method worksheet in your science journal. After the time is up, check the paper over each container. What do you find? Which of the papers is completely dry? Which is damp? Why is one paper more wet than another? Record your observations in your science journal.
THINK ABOUT IT: When water warms up from sitting in a puddle or swimming pool or from getting absorbed into soil, it begins to evaporate. That means it changes from a liquid to a gas. If that gas hits paper on its way into the atmosphere, it will dampen the paper. Can you explain why your construction paper was damp or dry?
