Where Does Plant Matter Come From?

This is exactly what a scientist named Jan Baptist van Helmont observed nearly 400 years ago. He grew a willow tree in a carefully weighed pot of soil for five years. The tree gained about 74 kilograms, but the soil lost only about 57 grams. Where did all that new tree material come from, if it didn't come from the soil?

• If the tree gained over 74 kg but the soil barely changed, where did the new material come from?
• What was the tree taking in during those five years besides soil?
• How could something invisible — like air — become something solid like wood?

For a long time, people assumed that plants "eat" soil to grow. But hundreds of years of scientific investigation have revealed a very different — and more amazing — truth. Let's look at what scientists have discovered about where plant matter actually comes from.

Scientists use evidence from investigations to support the claim that plant matter comes mostly from air and water. Here's an investigation that helps us see this in action.

Tracking Where the Mass Goes

The question: If a plant grows in a sealed container with only air, water, and light, can it still gain mass?

Setup: A student places a small plant in a clear, sealed container. The plant has access to water (in a small reservoir), air (sealed inside the container), and light (the container is transparent). No new soil is added, and no new air enters the container. The student measures the plant's mass at the start and every week for 6 weeks.

What they would observe: The plant gains mass even though no soil is added. The water level in the reservoir decreases. When scientists measure the CO₂ in the air inside the container, they find it decreases over time. This is evidence that the plant is converting air (CO₂) and water into new plant material.

Notice: The plant gained 4.8 grams while using 255 mL of water and decreasing the CO₂ level in the sealed container. No soil was consumed.

How Photosynthesis Builds Plant Matter

The investigation data gives us strong evidence for a claim that might seem hard to believe at first: most of a plant's mass comes from carbon dioxide gas in the air and water from the ground. Let's trace exactly where the atoms go.

Carbon dioxide (CO₂) is a gas made of one carbon atom and two oxygen atoms. When a plant pulls CO₂ into its leaves during photosynthesis, it strips out the carbon atom and uses it as a building block. These carbon atoms are linked together with hydrogen atoms (from water) to form sugar molecules. The plant then rearranges those sugars into all the complex molecules it needs — cellulose for strong cell walls, starches for stored energy, and many other substances.

If you could weigh all the carbon in a large tree and trace where it came from, you would find that almost all of it entered the tree as CO₂ through the stomata on the leaves. And if you could trace all the hydrogen atoms, you would find they came from the water the roots absorbed. Together, carbon from the air and hydrogen and oxygen from water account for about 95% or more of a plant's total dry mass.

What about the soil? The soil does provide minerals — substances like nitrogen, phosphorus, potassium, and iron — that plants need in small amounts. These minerals are important, but they make up only a tiny fraction of the plant's total weight. That is why, in van Helmont's experiment, the soil lost almost no mass even though the tree gained dozens of kilograms.

Where Does a Plant's Mass Come From?

The crosscutting concept in this lesson is Energy and Matter: Flows, Cycles, and Conservation. This idea shows up all across science, not just in plants. The key insight is that matter is made of atoms that cycle between different forms — atoms are not created or destroyed, they just move from one substance to another.

In photosynthesis, carbon atoms don't appear out of nowhere. They move from CO₂ molecules in the air into sugar molecules inside the plant. Hydrogen atoms move from water molecules into those same sugars. The atoms are rearranged, not created. This is a pattern scientists see everywhere in nature.

Notice the pattern: in every example, matter is not created or destroyed — it just changes form. When a plant grows, it isn't making atoms from nothing. It is pulling atoms from the air and water and rearranging them into new substances. When wood burns, those same carbon atoms go back into the air as CO₂. It's a cycle!

Understanding that plants build themselves from air and water has big implications for the real world — from farming to fighting climate change.

• Photosynthesis — The process by which plants use sunlight energy to convert carbon dioxide and water into sugar and oxygen. This is how plants make their own food and build new material.
• Carbon dioxide (CO₂) — A colorless, odorless gas found in the air. It is made of one carbon atom and two oxygen atoms. Plants absorb CO₂ through their leaves and use the carbon to build plant matter.
• Stomata — Tiny pore-like openings on the surface of leaves that allow gases (CO₂ and O₂) to move in and out of the plant.
• Matter — Anything that has mass and takes up space. Plants, air, water, and soil are all made of matter. Matter is made of tiny particles called atoms.
• Mass — The amount of matter in an object, usually measured in grams or kilograms. When a plant grows, its mass increases because it has gained new matter.
• Minerals — Substances like nitrogen, phosphorus, and potassium that plants absorb from the soil in small amounts. Minerals are necessary for plant health but make up only a small fraction of a plant's total mass.
• Sugar (glucose) — The molecule that plants produce during photosynthesis. Plants use sugar as a source of energy and as a building block for making other molecules like cellulose and starch.
• Cellulose — A strong molecule made of linked sugar units that forms the walls of plant cells. Cellulose gives plants their rigid structure and makes up a large portion of wood.