B. Life Systems · Growth and Changes in Plants

What Plants Need to Survive

B2.1 A1.2 A1.4About 35 minutes

Learning goals

I can name the basic needs of plants: air, water, light, heat, nutrients and space.
I can explain how a plant gets each thing it needs.
I can name conditions in the environment that can threaten a plant's survival.
I can plan a fair test to find out what a plant needs.

Words to know

basic needs
The things a living thing must have to stay alive and grow.

nutrients
Materials in soil and water that help a plant grow strong and healthy, a bit like vitamins.

survive
To stay alive.

threaten
To put something in danger.

drought
A long time with little or no rain.

fair test
An experiment where you change only one thing and keep everything else the same.

Remember from Grade 1

In Grade 1 you learned that all living things have basic needs, like air, water, food and space. In Grade 3 we look closely at plants. What exactly does a plant need? How does it get those things when it cannot walk to the kitchen or the lake?

A sad spider plant

Omar's family went to the cottage for two weeks in July. They forgot to ask anyone to water the spider plant on the windowsill. When they came home, its leaves were brown and droopy.

The plant had light, air and warmth. It was missing only one thing. Can you guess what? (Water!) A plant needs all of its basic needs to survive.

The six basic needs of plants

A plant needs:

Air. Plants take in a gas from the air called carbon dioxide. They use it to make their food. Roots also need air in the soil.
Water. Roots soak up water from the soil. Water moves up the stem to every leaf.
Light. Most plants use sunlight to make their own food in their leaves.
Heat. Plants need the right amount of warmth. Most plants in Ontario grow in spring and summer, when it is warm.
Nutrients. Roots take in nutrients from the soil along with the water. Nutrients help the plant grow strong.
Space. Roots need room to spread underground. Leaves need room to reach the light.

A healthy green-and-white spider plant in a clay pot on a sunny windowsill, with small baby plants hanging down. — This spider plant has light from the window, air, warmth, water and room for its roots. With all its needs met, it even grows baby plantlets. AI-generated image

Basic need	Where the plant gets it	Which part takes it in
air (carbon dioxide)	the air around it	leaves (through tiny openings)
water	rain, snow melt, a watering can	roots
light	the Sun (or a grow light indoors)	leaves
heat	the Sun and warm air	the whole plant
nutrients	the soil (for example from compost and rotting leaves)	roots
space	room in the soil and in the air	roots and leaves

How a plant meets each basic need.

A plant's six basic needs: light, water, air, heat, nutrients and space. If even one is missing, the plant is in trouble.

Plants do not eat food like we do

A plant does not eat soil. It makes its own food using light, air and water. (You will learn how in the lesson about photosynthesis.) Nutrients from the soil are more like vitamins that keep the plant healthy.

When plants are in danger

Sometimes the environment changes and a plant cannot get what it needs. These conditions can threaten a plant's survival:

Drought: no rain for a long time, so the soil dries out.
Flooding: too much water, so roots cannot get air.
Frost: a cold night in late spring can freeze young plants and new buds.
Too much heat: very hot, dry days can wilt and burn leaves.
Not enough light: a plant in deep shade or a dark room grows weak and pale.
Crowding: too many plants close together fight for water, light and nutrients.
Poor or polluted soil: soil with no nutrients, or with oil, garbage or road salt in it.
Being stepped on or eaten: people trampling plants, or too many insects munching the leaves.

Worked example

Let's figure out what went wrong

A tomato plant in a garden was healthy in June. After three weeks of hot, sunny weather with no rain, its leaves curled and drooped. What need was not met? What condition threatened it?

List the six needs: air, water, light, heat, nutrients, space.
Light? It was sunny, so it had light.
Heat? It was hot, so it had heat (maybe too much).
Water? There was no rain for three weeks. The plant did not have enough water.
A long time with no rain is called a drought.

The plant did not get enough water. The condition that threatened it was a drought (and very hot days dried the soil even faster).

Testing what a plant needs: a fair test

Scientists find out what a plant needs by doing an experiment. To be fair, they change only one thing and keep everything else the same.

The thing we change is called the variable. If you want to test whether a plant needs water, the only thing you change is water.

A fair test: two of the same plant, same pot, same soil, same window. Only water is different. After a week the plant in Pot A stands up. The plant in Pot B droops.

Worked example

Let's plan a fair test

Ella wants to find out if bean plants need light to stay healthy. Plan a fair test.

Question: Do bean plants need light to stay healthy?
Prediction: "I think the plant in the dark will get pale and weak."
Change only one thing (the variable): light. One plant goes on a sunny windowsill. One plant goes in a dark cupboard.
Keep the same: kind of plant, size of plant, pot, soil, amount of water, how often you water, room temperature.
Observe and record: check every 2 days for 2 weeks. Measure the height and draw the colour of the leaves.
Conclusion: compare the two plants and say what you found out.

Only light changes. Everything else stays the same, so any difference must be because of the light.

Science safety

Ask a grown-up before using soil, potting mix or garden tools.
Wash your hands with soap after touching soil or plants.
Some house plants are poisonous if eaten. Never put plant parts in your mouth unless a grown-up says the plant is food.
Wipe up spilled water right away so no one slips.

Just for fun

What do you call a plant that never gets watered? Thirsty!

And one that gets watered ten times a day? Soggy! Too much water is a problem too, because roots need air.

Hands-on activity

The thirsty plant test

You need: 2 small plants of the same kind and size (or 2 cups of sprouted beans), the same kind of pot and soil for each, a measuring cup, a ruler, masking tape and a marker.

Label the pots A and B.
Put both pots on the same sunny windowsill.
Every 2 days, give Pot A 50 mL of water. Give Pot B no water.
Every 2 days, measure each plant's height in centimetres and write down how its leaves look (standing up, drooping, colour).
After 10 days, compare. Then water Pot B and see if it recovers.

Think like a scientist: "What did I change? What did I keep the same? Was my test fair?"

Grown-up note: Wash hands after handling soil. Choose a plant that is safe if touched, like beans, peas or grass seed.

Your turn

Name the six basic needs of plants.

Which part of a plant soaks up water and nutrients?

What gas does a plant take in from the air to make its food?

Why do plants need space?

What is a drought? How does it threaten plants?

Can a plant get too much water? Explain.

In a fair test, how many things do you change?

More practice: warm-up

True or false: Plants need light.

Where do most plants get their light?

Where do plants get nutrients?

What season do most Ontario plants grow in: summer or winter?

What should you do after touching soil?

More practice: on your own

A cold night in May turns the leaves of new bean plants black and limp. What condition threatened them?

Mo planted 20 carrot seeds in a tiny pot. They all sprouted but stayed thin and small. Why?

Why might plants beside a busy road in winter have trouble in spring?

Jay tests if plants need water. He waters Pot A and puts it in the sun. He does not water Pot B and puts it in a dark closet. Is his test fair? Why?

Is soil food for a plant? Explain.

Challenge

A plant was moved from a sunny window to a dark hallway. After two weeks it was tall, thin and pale. Which need was missing? What does this tell you?

Plan a fair test to find out if plants grow better with compost (nutrients) mixed into the soil. Say what you change and three things you keep the same.

A forest fire burns a forest in summer. Name two plant needs that are suddenly a problem for the plants that are left, and one need that might become easier to get.

Check your understanding

Quiz

Which list shows the basic needs of plants?

Which part of a plant takes in water from the soil?

A long time with no rain is called...

Why can too much water harm a plant?

Sami wants to test if plants need light. Which is a fair test?

How does a plant get its food?

For parents and teachers

Big idea (B2.1): Plants need air, water, light, heat, nutrients and space, and changes in the environment (drought, flooding, frost, heat waves, shade, crowding, polluted or salty soil, trampling) can threaten their survival. The fair-test activity practises the scientific experimentation process (A1.2) and safe handling of materials (A1.4).

Common difficulties:

"Plants eat soil." This is a very common idea. Soil supplies water, nutrients and support; the plant makes its own food (sugar) in its leaves. The photosynthesis lesson builds on this.
Changing two things at once in a test. Ask, "If the plants end up different, how would you know which change caused it?"
Thinking more water is always better. Overwatered house plants often die because the roots cannot get air.

How to help: Let your child take charge of watering one house plant for a week and decide, by feeling the soil, when it needs water. Bean seeds from the grocery store sprout quickly in a cup of soil and are handy for this whole unit; start a few now. Supervise with soil and potting mix (wash hands afterward), and check that any house plant is not toxic if you have young children or pets.