Where does most of the extra mass of a growing plant come from?

Carbon dioxide and water, used in photosynthesis

How plants get water and minerals

Plants make their own food by photosynthesis, but they still need water and minerals from the soil.

Key Stage 3
About 15 minutes

Plants do not eat soil

Plants make carbohydrates (glucose, then starch and cellulose) by photosynthesis, using carbon dioxide from the air and water.
In the 1600s, Jan van Helmont grew a willow tree in a pot for five years. The tree gained a huge mass, but the soil lost only a tiny amount. So the mass of a plant does not come mainly from the soil.
Plants do take in water and small amounts of mineral nutrients (minerals) from the soil.

Roots and water

Root hair cells have a large surface area to take in water and dissolved minerals.
Water travels up the stem to the leaves in tiny tubes called xylem.
Water evaporates from the leaves through the stomata, which pulls more water up the plant. This flow is called transpiration.

Minerals and fertilisers

Each mineral has a job.

Farmers add fertilisers to replace minerals that crops take from the soil.
Mineral	Needed for	If it is missing
Nitrates	Making proteins for growth	Poor growth; older leaves turn yellow
Phosphates	Healthy roots	Poor root growth
Potassium	Healthy leaves and flowers	Yellow leaves with dead patches
Magnesium	Making chlorophyll	Yellow leaves

Worked example

A gardener spreads fertiliser at 30 g per m² on a bed of 12 m². How much fertiliser does she need?

Mass per m² × area
30 × 12

Answer: 360 g

Key idea

Plants make carbohydrates by photosynthesis, not from soil. Root hair cells absorb water and minerals; xylem carries water up. Nitrates for proteins and growth, magnesium for chlorophyll. Fertilisers replace minerals.

Key words

Water: Pure water melts at 0 °C and boils at 100 °C. Its density is 1 g/cm³. Ice is less dense than water, so it floats.
Photosynthesis: How plants make glucose using light: carbon dioxide + water → glucose + oxygen.
Mass: The amount of matter in an object, measured in kilograms (kg). It is the same everywhere.
Stomata: Tiny holes, mostly on the underside of a leaf, that let gases in and out. Guard cells open and close them.

All KS3 Science key terms and definitions →

Common mistakes

Not “Soil eaten through the roots”. Plants do not eat soil.
Not “Sunlight turning into matter”. Light provides energy, not mass.
Not “Stomata”. Stomata are holes in the leaf where water evaporates.

Quick check

Try two quick questions

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Extra practice

2 more questions

Try each one, then tap to check.

A tree grows from a small seed into a huge tree. Where does most of its mass come from?
1. Soil
2. Sunlight turning into matter
3. Carbon dioxide from the air and water
4. Minerals only
Show the answer
Answer: Carbon dioxide from the air and water
Photosynthesis builds glucose from carbon dioxide and water.
How are root hair cells adapted to absorb water?
1. They are very short
2. They have a large surface area
3. They have many chloroplasts
4. They have thick walls
Show the answer
Answer: They have a large surface area
More surface area means faster absorption.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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