Grade by Grade
E. Earth and Space Systems · Soils in the Environment

What Is Soil Made Of?

E2.1 A1.2 A1.4 A1.5About 40 minutes
Learning goals
  • I can name the non-living parts of soil: bits of rock, humus, water and air.
  • I can explain where the bits of rock and the humus in soil come from.
  • I can describe the layers of soil: topsoil, subsoil and bedrock.
  • I can observe a soil sample safely and record what I find.

Words to know

soil
The loose top layer of the ground where plants grow. It is a mix of bits of rock, humus, water, air and living things.
minerals
The tiny bits of broken rock that make up most of soil.
humus
The dark, spongy part of soil made from dead plants and animals that have rotted. Say it "HYOO-muss".
decompose
To rot and break down into tiny pieces. Dead leaves decompose and become part of the soil.
weathering
When rock is slowly broken into smaller and smaller pieces by water, ice, wind and plant roots.
topsoil
The top layer of soil. It is usually the darkest layer and has the most humus and living things.
subsoil
The layer under the topsoil. It has less humus and more bits of rock and clay.
bedrock
The solid rock deep under the soil.

Dig in!

Pick up a handful of soil from a garden. Squish it. Smell it. Look closely. Is it just "dirt"?

Scientists say no! Soil is a special mixture of many things. Plants grow in it, animals live in it, and almost all of our food starts in it. In this unit you will become a soil scientist. Today you will find out what soil is made of.

A garden trowel holding a scoop of dark soil with small stones, bits of roots and a dead leaf in it.
Look closely and you can see that soil is a mixture: bits of rock, dark humus, pieces of dead leaves and thin roots. AI-generated image

Remember from Grade 2

In Grade 2 you learned that earthworms help make soil rich, that plant roots help hold soil in place, and that rain soaks into the ground in the water cycle. This year you will find out much more about the soil itself.

Soil is a mixture

Soil has non-living parts and living parts. Today we look at the non-living parts. There are four:

  1. Bits of rock (also called minerals). This is the biggest part of most soils. Some bits are big enough to see, like sand and tiny pebbles. Some are much too small to see.
  2. Humus. This is the dark, spongy part. It comes from dead leaves, plants and animals that have decomposed (rotted). Humus is full of nutrients, which are like food for plants. It also soaks up water like a sponge.
  3. Water. Water fills some of the tiny spaces between the bits of soil.
  4. Air. Air fills the other tiny spaces. Roots and soil animals need this air to breathe.

The living parts, like worms, roots and tiny living things too small to see, are next lesson.

🪨🪨🪨🪨🪨🪨🪨🪨🪨🍂💧💧💧💧💧💨💨💨💨💨
A recipe for good garden soil, in 20 parts: about 9 parts bits of rock, 1 part humus (from dead leaves and plants), 5 parts water and 5 parts air.

Look at the picture above. In good garden soil, about half is solid stuff (bits of rock and humus) and about half is spaces filled with air and water. Surprised? Soil has lots of tiny holes, like a sponge!

Where do the bits of rock come from?

A long, long time ago, the bits of rock in soil were part of big rocks. Big rocks slowly break into smaller pieces. This is called weathering.

  • Water and ice: Water gets into cracks in a rock. In our cold Canadian winters, the water freezes. Ice takes up more space than water, so it pushes the crack wider. Freeze, thaw, freeze, thaw... after many years, pieces break off.
  • Wind and moving water rub and bump rocks, wearing them down.
  • Plant roots grow into cracks and push them apart.

This is very, very slow. It can take hundreds of years for nature to make a layer of soil just a few centimetres thick. That is why soil is so precious.

1. Water fillsa crack2. Water freezes andpushes the crack wider3. After many winters,a piece breaks office
Freeze and thaw weathering. Ice takes up more space than water, so it pushes a crack wider every winter until a piece breaks off.

Where does the humus come from?

In the fall, maple and oak leaves drop to the ground. Over time, worms, bugs and tiny living things called decomposers eat the dead leaves and break them into smaller and smaller pieces. The leaves decompose. What is left is dark, crumbly humus. The same thing happens to dead plants and dead animals.

Humus is what gives rich soil its dark colour.

plantstopsoilsubsoilbedrock
Soil has layers. Plants grow at the top. Topsoil is darkest and has the most humus, roots and living things. Subsoil has more bits of rock and less humus. Bedrock is solid rock deep down.

Soil has layers

If you dug a very deep hole (or looked at the side of a hill where the ground has been cut away), you would see layers:

  • Topsoil is on top. It is usually the darkest layer because it has the most humus. Most roots and soil animals live here. It is the best layer for growing plants.
  • Subsoil is under the topsoil. It is usually lighter in colour. It has less humus and more bits of rock and clay. Some long roots reach down here.
  • Bedrock is the solid rock at the bottom.

In northern Ontario, on the Canadian Shield, the soil is often thin. In many places you can see the bedrock poking right out of the ground!

Worked example

Let's observe a soil sample

Maya collects a cup of soil from her garden with her dad. How can she find out what is in it, like a scientist?

  1. She puts on gloves and spreads the soil on a white paper plate, so things are easy to see.
  2. She looks with her eyes and a magnifying glass. She sees tiny pebbles, grains of sand, bits of dead leaf and one tiny root.
  3. She feels a pinch between her fingers. It feels a bit gritty and a bit damp.
  4. She smells it. It smells earthy, like a forest after rain.
  5. She records what she found in a table (below) and sorts each thing into a part of soil.

Maya's soil has bits of rock (pebbles and sand), humus and bits of dead plants, water (it feels damp) and a root (part of a living plant). Recording helps her share her findings.

What Maya observedLiving or non-living?Which part of soil?
tiny pebbles and sand grainsnon-livingbits of rock (minerals)
small brown bits of dead leafnon-living now (once living)becoming humus
soil feels damp and coolnon-livingwater
crumbs with little gaps between themnon-livingspaces filled with air
a thin white rootlivingpart of a plant
Maya's soil observation chart
Worked example

Let's prove that soil has air and water

Air and water in soil are hard to see. How can we show they are there?

  1. Air test: Put a lump of dry soil into a clear cup of water. Watch closely. Little bubbles rise up! The water is pushing the air out of the tiny spaces in the soil.
  2. Water test: Put a scoop of damp soil in a clear jar and close the lid. Leave it in a sunny window for an hour.
  3. Tiny drops of water appear on the inside of the jar. Water evaporated from the soil and then condensed on the jar, just like in the water cycle you learned about in Grade 2.

The bubbles show there is air in soil. The drops show there is water in soil.

Stay safe, soil scientist!

  • Wear gloves, or wash your hands with soap after touching soil.
  • Keep your hands away from your eyes and mouth. Never eat soil.
  • Dig only where a grown-up says it is OK: not beside busy roads, not where pets go to the bathroom, and not in places that might have garbage or chemicals.
  • Watch for sharp things like glass or nails. If you see one, don't touch it. Tell a grown-up.
Just for fun

Why do soil scientists never get bored with their work?

Because they really dig it!

Hands-on activity

Soil detective

You need: gloves, a small shovel or big spoon, a cup, a white paper plate, a magnifying glass (if you have one), a clear cup of water, a pencil and paper.

  1. With a grown-up, collect one cup of soil from a garden or a park flower bed.
  2. Spread it on the plate. Look, touch (with gloves) and smell. Use the magnifying glass.
  3. Make a chart like Maya's. Write or draw everything you find. Sort each thing: bits of rock, humus or dead plant bits, water, air, or living thing.
  4. If you find a living thing, like a worm or a bug, gently put it back outside where you found it.
  5. Do the air test: drop a lump of dry soil into the cup of water. Count the bubbles you see in 10 seconds.
  6. Share your findings with your family. Use the words humus, minerals and topsoil.
  7. Wash your hands with soap when you are done.

Your turn

1

Name the four non-living parts of soil.

2

What is humus made from?

3

Which soil layer is usually the darkest? Why?

4

What is the solid rock under the soil called?

5

You drop dry soil into water and see bubbles. What do the bubbles tell you?

6

How does ice help make soil?

7

Why should you wash your hands after touching soil?

More practice: warm-up

1

True or false: Soil is just one thing.

2

Which part of soil is the biggest part in most soils?

3

Name one thing that can turn into humus.

4

Which layer is on top: topsoil or subsoil?

5

Which sense do you use when you say soil smells earthy?

More practice: on your own

1

A dead maple leaf lies on the forest floor. Describe what happens to it over time.

2

Why do roots and soil animals need the spaces in soil?

3

Sam digs a deep hole. The soil near the top is dark. Deeper down it is lighter and has more stones. Name the two layers he found.

4

How can you show that soil has water in it?

5

Why is soil precious (very valuable)?

Challenge

1

In good garden soil, about how much of the soil is spaces filled with air and water? Explain why the spaces matter.

2

On the Canadian Shield, you can often see bare rock on the ground. Use the word bedrock to explain why.

3

Jen says, "Weathering happens in one day." Is she right? Explain.

Check your understanding

Quiz

1.

Which of these is not a natural part of soil?

2.

Humus comes from...

3.

Which layer usually has the most living things and roots?

4.

Bubbles rise when you put dry soil in water. This shows that soil has...

5.

Freezing water helps break rocks apart because...

6.

What should you do after handling soil?

For parents and teachers

Big idea (E2.1, non-living part): Soil is a mixture of mineral particles (weathered rock), humus (decomposed organic matter), water and air, plus living things (next lesson). Good garden soil is roughly half solids and half pore space; the 9:1:5:5 picture is a typical textbook ideal (about 45% minerals, 5% organic matter, 25% water, 25% air), not an exact rule for every soil.

How to help: Do the soil detective activity outside or on newspaper. Ask your child to predict before each test and to say what they observed afterward, using the new words (A1.2, A1.5). A cheap magnifying glass makes this much more exciting.

Common difficulties:

  • Calling everything "dirt". Gently model the word soil and the names of its parts.
  • Thinking humus is alive. Humus is non-living, but it was once living. Dead leaves are "non-living now".
  • Thinking soil is quick to form. Stress that it takes hundreds of years.

Safety (A1.4): Gloves or hand-washing, no hand-to-mouth contact, and choose soil away from roads, pet areas and anywhere chemicals might have been used. If anyone in the family has a cut on their hands, wear gloves.