When you’re playing in your back yard or walking down the sidewalk, what do you see when you look down at the ground? Grass, cement, leaves, trash? Most of the time, you’ll also see something many people call “dirt.”

You might think, “What’s the big deal about dirt? It’s everywhere.” That dirt is actually soil, which is very important to life on Earth. It’s one of the three most important natural resources on our planet, along with air and water. Without soil, there wouldn’t be life on Earth.

WORDS to KNOW

soil: the top layer of the earth, in which plants grow.

natural resource: something from nature that people can use in some way, such as water, stone, and wood.

In the same way that you have skin covering your whole body our planet has soil covering its entire surface. Soil covers the ground in natural places, such as meadows, gardens, forests, and your back yard. Some places have only a thin layer of soil. In other spots, it can be several feet thick.

Soil does a lot of work for us. It is a habitat for tiny organisms. Most of our plants need it to grow. And soil recycles nutrients and filters our water, too. There’s a lot going on under your feet!

WHAT’S IN SOIL?

Soil is made up of lots of different materials. The organic matter in soil includes little bits of dead plants and animals. There are tiny bits of inorganic matter, too, such as broken-down rocks and pebbles. There are also even smaller particles in soil, such as sand, silt, and clay.

WORDS to KNOW

habitat: a plant or animal’s home.

organism: a living thing.

nutrients: substances in food, soil, and air that living things need to live and grow.

substance: the material that something is made of.

organic matter: rotting plants and animals that give soil its nutrients.

organic: something that is or was living.

inorganic: not living.

particle: a tiny piece of matter.

silt: particles of fine soil, rich in nutrients.

Sand is the largest particle you’ll find in soil. Have you ever scooped up a handful of soil at the beach and looked at it closely? You’ll be able to see each little grain of sand.

Silt particles are smaller than sand. If you drag your hand lightly along the bottom of a running stream, you can snag a little bit of soil that’s being carried along with the current. If you look at it very closely you might be able to see individual particles—but they’re really small!

Clay is made of the smallest particles. You won’t be able to see the individual particles just by looking at a lump of clay. You need a microscope to see them because they’re so small. When clay particles get wet, they turn really sticky and clump together. Have you ever played with clay?

Squeezed between all these soil particles are air and water. That’s how plants grow in soil—they push their roots between the particles to get the water and nutrients they need.

THE FIRST STEP

Rocks are very hard. If you drop one on your foot, it hurts! But they can be broken into smaller pieces under the right conditions. That’s how soil starts.

Rock is called the parent material of soil because it’s where most soil comes from. Soil comes from the parent material of rock just as you come from your parent! Water, ice, and wind break down bedrock through time.

Water can move forcefully enough to push rocks, tumble them around, and knock them into each other. As the water continues to roll them, little pieces break off, and then they start tumbling around, too. In time, the pieces get smaller and smaller.

SLIP-SLIDING AWAY

There are some locations where soil can’t form. Have you ever seen craggy mountain peaks? They are often bare, just peaks of exposed rocks. That’s because gravity pulls any soil-making materials, such as sand particles and organic matter, down the sides before any layer of soil has a chance to form. Water and wind wash materials down from mountaintops, too.

WORDS to KNOW

parent material: material that eventually becomes soil, such as rock.

bedrock: the layer of solid rock under soil.

gravity: the force that pulls things down toward the surface of the earth.

Ice can break down rocks, too. Rainwater seeps into cracks in the bedrock. When the water freezes, it expands. This forces the cracks to get wider and wider, sometimes splitting the entire rock into smaller pieces.

Wind wears away rocks by blowing small particles such as grains of sand against them. This grinds down the rock, just as when you rub sandpaper against a piece of wood. All these little particles of rock become part of the soil.

A LITTLE BIT OF THIS AND THAT

Rock particles aren’t the only things in soil. When plants and animals die, they decay, or rot. Bacteria and fungi help this organic matter break down.

Bacteria are microscopic living things that are too small to see with your eyes—but they have a very big job. You have some kinds of bacteria in your mouth and digestive system to help you digest your food.

WORDS to KNOW

expand: to spread out and take up more space.

decay: to break down and rot.

bacteria: tiny organisms found in animals, plants, soil, and water.

fungi: organisms that grow on and feed on rotting things.

microscopic: something so small that it can be seen only with a microscope.

Maybe you’ve seen mold when an old piece of bread has started to grow something green and fuzzy on it. That’s fungi! Fungi include mold and mushrooms.

Bacteria and fungi work hard to break down dead plants and animals into tiny pieces called humus. Humus is very dark brown or black, moist, and extremely rich in nutrients.

WORDS to KNOW

humus: soil formed from decaying leaves and organisms.

The particles in soil might be small, but there’s still room for air to fit around them. This air helps the creatures that live in the soil, such as earthworms, move around. Earthworms leave more air space in the holes they leave behind. These spaces allow plants to push their roots through the soil.

Soil is also made of water. Just as the air fills in the empty spaces, water does too. It gets absorbed by some of the material in soil and moves around to fill any empty pockets, carrying nutrients as it goes.

Soil isn’t the same all over the planet, though. Think about the desert. Because there isn’t much water, there aren’t as many plants or animals living in the desert, so the soil there has less humus from decaying organic matter. A prairie has a lot of plants and animals, so it’s far richer in humus.

WHAT DO YOU GET WHEN YOU CROSS DIRT WITH A DOG?

A Land Rover!

There are more than 50,000 different types of soil in the United States! That’s because there are different kinds of rocks, organisms, land features, and weather conditions all across the country. What lives in the soil in your yard? Let’s find out!

GOOD SCIENCE PRACTICES

Every good scientist keeps a science journal! Choose a notebook to use as your science journal. As you read through this book and do the activities, keep track of your observations and record each step in a scientific method worksheet, like the one shown here. Scientists use the scientific method to keep their experiments organized.

Each chapter of this book begins with an essential question to help guide your exploration of soil.

ESSENTIAL QUESTION

Keep the question in your mind as you read the chapter. At the end of each chapter, use your science journal to record your thoughts and answers.

 

PAINT WITH SOIL

Soil from different locations can be different colors, depending on what it’s made of. Since ancient times, soil has been used for painting. You, too, can create works of art with soil!

SUPPLIES

several dry soil samples from different places, about ¼ cup each

several pieces of paper

goggles

hammer or mallet

plastic Ziploc bags

rolling pin or mortar and pestle

paper cups

small section of old pantyhose

paper plates

marker or pen

tape

clear acrylic artist paint (you can use white paint or even just water)

small artist paint brush or old bits of sponge or rag

1 Put one soil sample at a time on a piece of paper and use the hammer or mallet to crush it into very small pieces. Wear goggles for safety!

2 Put the pieces into a plastic Ziploc bag and use the rolling pin to crush it into a very fine powder. If you have a mortar and pestle, you can grind it into a powder that way. Repeat with all of your samples until they are all very fine powders. Keep them separate!

3 Place one soil powder sample into a paper cup. Stretch the pantyhose over the top of the cup and shake the cup over a new piece of paper or a paper plate. You’re going to end up with a super-fine powder on your paper to use as a base for your paint. Repeat with the other colors on separate pieces of paper. When you’re finished, put each colored powder into a new paper cup.

These famous cave paintings in Lascaux, France, were made with soil.

They have survived for more than 17,000 years! How are they different from paintings you see on the walls of museums?

 

 

KEYWORD PROMPTS

Lascaux cave paiting

4 On another piece of paper, sketch out a drawing with your marker or pen. You can skip this step if you want to just start painting! Use the tape to secure your paper flat to the surface you’re working on. It will need to stay very still until it’s done drying.

WORDS to KNOW

topsoil: the top layer of soil.

5 Add a small bit of clear paint to each color of powdered soil and mix until they’re at the consistency of paint. If you’re using water, get your sponge or paintbrush wet and dip it in the powder.

6 Begin painting! When you’re finished, let the painting stay flat and undisturbed until it’s dry.

TRY THIS! Mix your powder and clear paint into different consistencies. Does a thinner mixture look different on paper? How? How do the different colors and consistencies look when they’ve dried?

 

ANALYZE SOIL

You might live in an area that has a lot of sand mixed in with the soil. Or maybe most of the soil around you looks like thick, red clay. With this project, you can find out what’s in your soil. Start a scientific method worksheet to organize your ideas and observations.

SUPPLIES

science journal and pencil

small shovel or scoop for dirt

small container (anything that will hold soil, such as a bag, cup, bucket)

clear jar with lid

water

1 In your yard or a nearby field, scoop up about a cup of soil and put it in your container.

2 Fill your jar about halfway with water. Add the soil to the jar, cover, and shake well. Then put the jar in a place where it won’t be disturbed and watch what happens as the soil settles. Which particles do you think will reach the bottom of the jar first, the large ones or the small ones?

3 Check your jar after 1 minute, 30 minutes, and an entire day. Does it look different every time? Do you see layers forming in the soil that settles on the bottom of the jar? Record your observations in your science journal. Draw a picture of the layers you see in your jar. Look at the picture here to discover what each layer is made of.

4 Different areas in your own neighborhood might have different soil. Collect small samples from other areas, such as near a pond or in a garden. Be sure to get permission before scooping up someone’s property, though!