What Do Plants Really Eat?

Think about that for a moment. The tree gained over 160 pounds of solid wood, bark, branches, and leaves. If the soil barely lost any weight, then where did all that new material come from? The soil clearly wasn't the main source. Something else was feeding the tree — but what?

• If the soil barely lost any weight, what other sources of material could the tree have used to grow?
• The tree was only given water and was exposed to air and sunlight. Which of these could have provided the material that made the tree bigger?
• How could you design an investigation to figure out exactly which materials a plant uses from air versus soil?

For a long time, people assumed that plants "eat" soil — that the soil gets lighter as the plant gets heavier. Van Helmont's experiment showed that this isn't what happens. Today, scientists understand that plants get their materials from two very different sources, and those sources play very different roles.

What scientists do: Developing and Using Models

Scientists often use models to represent processes that are too small, too slow, or too complex to observe directly. Photosynthesis happens at the molecular level inside leaf cells, so scientists use diagrams and equations to model what's happening. They also conduct controlled experiments — like van Helmont did — to gather evidence about where materials go.

Investigation question: Can we determine the source of a plant's mass by tracking what goes in and what comes out?

What you would need: A small potted plant, a scale accurate to 0.1 gram, a sealed clear bag, water (measured carefully), and 4 weeks of sunlight.

Procedure: Weigh the plant, pot, and soil together. Over 4 weeks, add only measured amounts of water. At the end, weigh the plant separately from the soil. Compare how much mass the plant gained versus how much mass the soil lost. If the plant gained more mass than the soil lost, the extra mass must have come from somewhere else — the air.

What scientists observe: The plant gains significantly more mass than the soil loses. This confirms that most of a plant's new mass comes from carbon dioxide in the air, not from the soil.

How Photosynthesis Works

When scientists carefully measure what goes into and comes out of a plant, the results are striking. A plant absorbs carbon dioxide gas from the air and water from the soil. Using the energy from sunlight, it rearranges the atoms in CO₂ and H₂O to build sugar molecules (glucose). These sugar molecules contain carbon, hydrogen, and oxygen — all of which came from air and water, not from soil.

The plant then uses these sugars as building materials. Sugars are linked together to form cellulose (the main material in wood and plant cell walls), starch (stored food energy), and other complex molecules. This is how a tiny seed grows into a massive tree — by capturing carbon from the air and hydrogen from water and assembling them into solid plant tissue.

But what about the soil? Scientists have discovered that plants absorb mineral nutrients that are dissolved in soil water. These minerals — including nitrogen, phosphorus, and potassium — are critical for building proteins, DNA, and other molecules the plant needs. However, these minerals make up only about 1–2% of a plant's total dry weight. The other 98% comes from carbon dioxide and water.

Where Does a Plant's Mass Come From?

This data tells a clear story. When van Helmont's soil barely changed weight, it was because the soil was never the main source of plant mass. The tree was building itself primarily from invisible gases in the air and from water. The evidence supports the conclusion that air and water are the primary material sources for plant growth, while soil provides small but essential nutrients.

Crosscutting Concept: Energy and Matter

One of the most powerful ideas in all of science is that matter is conserved — it doesn't appear from nowhere, and it doesn't vanish. When a plant grows, the new mass has to come from somewhere. By tracking where matter enters and leaves a system, scientists can figure out what's really happening. This same pattern — tracking the flow of matter and energy — appears across many areas of science.

In every example, the total amount of matter stays the same — it just changes form or moves from one place to another. When a plant grows, carbon atoms that were part of CO₂ gas in the air become part of the solid wood in a tree trunk. The atoms didn't appear from nothing; they were rearranged. Scientists look for this pattern — tracking matter through a system — to understand processes across all areas of science.

Understanding the difference between what plants get from air/water versus soil has enormous real-world importance, especially in farming, forestry, and environmental science.

🌾 Farming & Fertilizer

Farmers add fertilizer to soil because crops use up the small supply of soil minerals (nitrogen, phosphorus, potassium). Fertilizer replaces those nutrients — but it doesn't replace the main building materials. A farmer can't grow a crop without adequate water and CO₂ in the air, no matter how much fertilizer they add. Understanding this helps farmers use resources wisely and avoid over-fertilizing, which can pollute rivers and lakes.

🌍 Climate & Carbon

Since trees pull carbon dioxide from the air to build their wood, forests act as giant carbon sinks — they store carbon that would otherwise be in the atmosphere. When forests are cut down and burned, that stored carbon is released back into the air as CO₂. Scientists and engineers are developing strategies like reforestation to help remove excess CO₂ from the atmosphere and slow climate change.

🧪 Engineering Connection: Hydroponics

Engineers have designed hydroponic growing systems that grow plants without any soil at all. Instead, the plants' roots sit in water that has the essential minerals dissolved in it. The plants still get CO₂ from the air and energy from artificial lights. This technology proves that soil isn't the main food source for plants — it's just one way plants can access minerals. Hydroponic farms can grow food in places where soil is poor or unavailable, like on rooftops in cities or even on future space stations.

• Photosynthesis — The process by which plants use sunlight energy to convert carbon dioxide and water into sugar (glucose) and oxygen. This is how plants make their own food.
• Carbon dioxide (CO₂) — A gas found in the air. Plants absorb CO₂ through their leaves and use the carbon atoms to build sugars and other molecules that form plant structures.
• Stomata — Tiny openings (pores) on the surface of leaves that allow carbon dioxide to enter and oxygen to exit the plant.
• Chlorophyll — The green pigment in plant leaves that captures sunlight energy. This energy powers the process of photosynthesis.
• Glucose — A type of sugar that plants produce during photosynthesis. It serves as the primary building block and energy source for the plant.
• Minerals/Nutrients — Substances like nitrogen, phosphorus, and potassium found in soil. Plants absorb these through their roots in small amounts. They are essential for plant health but make up only a tiny fraction of a plant's total mass.
• Matter — Anything that has mass and takes up space. In this lesson, matter includes the gases, water, and minerals that plants use to grow.
• Hydroponics — A method of growing plants without soil by providing mineral nutrients dissolved in water.