Photosynthesis · Grade 7
Where does a plant's mass actually come from?
Overwhelmingly from the air, not the soil. The plant takes in carbon dioxide through its leaves and builds sugar from it, which is why a tree can add hundreds of kilograms while the soil around it barely loses any mass at all. From the soil come water and a small quantity of minerals.
Learning objectives
- Describe van Helmont's willow experiment and state exactly what he weighed
- Explain why soil cannot be the source of most of the mass a plant adds
- Identify carbon dioxide, water and light as the inputs a plant actually uses
- Predict what happens to the mass of a plant kept in the dark for two weeks
The experiment that opened the question
About four hundred years ago Jan Baptist van Helmont planted a willow cutting in a pot, having separately weighed the cutting and the soil. For five years he watered it and nothing else.
Then he weighed both again. The willow had gained tens of kilograms; the soil had lost a few dozen grams — almost nothing. The mass the plant had added to itself could not have come from the soil, for the simple reason that the soil was still there.
Van Helmont concluded the source was the water, which was a reasonable conclusion and turned out to be only part of the answer. He could not weigh air, so he missed the main ingredient.
What actually goes in
The plant uses three things: carbon dioxide from the air, water from the soil, and light as the source of energy. Light is not matter and contributes no mass — it is what drives the process.
Most of a plant's dry mass is carbon, and every atom of that carbon arrived as a gas the leaves took in. In a real sense a tree is air that has been set solid using light.
What happens in the dark
A plant kept in the dark for a fortnight will not grow, and its mass will fall. With no light there is no sugar being made, and the plant carries on consuming the store it built up.
That is a test you can actually run, and it is what makes light necessary rather than merely helpful. Soil and water alone are not enough.
Worked examples
A tree gained 500 kg while the soil in its pot lost 60 g. Where did the mass come from?
- The soil lost less than a tenth of a per cent of what the tree gained
- So it cannot be the source of most of that mass
- That leaves air and water, and the carbon in the tree comes from carbon dioxide
Answer: Almost all from the air, a little from the water
What happens to the mass of a plant kept in a dark cupboard for two weeks, watered normally?
- With no light there is no photosynthesis and no sugar made
- The plant keeps respiring and consuming what it stored
- So the mass balance is negative
Answer: Its mass falls, and it grows pale and stretches towards any light
A student says fertiliser makes plants grow, so the mass comes from fertiliser. What is the reply?
- Fertiliser supplies minerals such as nitrogen and phosphorus, in small quantities
- They are needed to build proteins, not as the source of mass
- The carbon in a plant outweighs them by orders of magnitude
Answer: Fertiliser helps it grow, but is not where most of the mass comes from
Common mistakes
- Thinking the plant eats soil
- Soil supplies water and minerals only. The willow experiment shows it barely diminishes even as the plant adds tens of kilograms to itself.
- Assuming light contributes mass
- Light is energy, not matter. It makes the process possible, but every atom in the plant arrived as a gas or a liquid that went in.
- Concluding from van Helmont that water is the source
- That was precisely his conclusion, and it is partial. He could not weigh the air, so he never saw the carbon dioxide — which is where most of the dry mass comes from.
What to remember
- Most of a plant's mass comes from the air.
- From the soil come only water and minerals.
- Light is energy, not matter.
- In the dark a plant loses mass.