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Claim check

Is a tree made from the soil it grows in?

Not supported

No good evidence supports this, or the evidence points the other way.

No. Roughly half the dry mass of wood is carbon that entered as gas through the leaves. The soil supplies about one per cent of a tree’s dry weight, as minerals — and this was demonstrated in the 1640s.

The claim as it circulates

“A tree builds itself out of nutrients it takes up from the soil.”

Where you may have met it: A common intuition, and a frequent answer in classrooms; Gardening advice framing feeding a plant as feeding its bulk

Taking it apart

What was claimed
That the bulk of a tree is assembled from material drawn out of the ground, so that a growing tree consumes its soil.
What was actually observed
Van Helmont grew a willow for five years in a covered pot of pre-dried, weighed soil, adding only water. The tree gained roughly 74 kilograms; the soil lost about 60 grams. Later work identified carbon dioxide as the source of the mass and traced the chemistry in detail.
What the evidence supports
That a tree’s dry mass is roughly 50% carbon and 43% oxygen, effectively all of which arrives as carbon dioxide, with hydrogen from water and about one per cent from soil minerals.
What it does not support
That soil is unimportant. The one per cent includes nitrogen, phosphorus and potassium, without which nothing grows at all — soil supplies what a tree cannot make, rather than what it is made of.

What makes this worth a page is that the experiment settling it is nearly four hundred years old, simple enough to describe in two sentences, and got the right answer to the question asked while reaching the wrong conclusion about the answer. Van Helmont weighed the soil. It had not gone anywhere. He concluded the mass had come from the water, because water was the only thing he had added — and hydrogen in wood does come from water, so he was partly right. The larger part, carbon and oxygen from air, was not something he could weigh or had reason to suspect, and identifying it took another century and a half.

The version worth carrying away is that a tree is a structure assembled out of the atmosphere, standing on ground that supplied the trace elements and the water. Burning wood undoes it: the carbon goes back where it came from.

The mechanism, and the experiment

The claims underneath

Each one carries its own evidence, scope and caveats. Expand any of them to reach the studies.

A tree is built mostly out of air — the carbon in wood came from carbon dioxide

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

The dry mass of a plant is predominantly carbon, hydrogen and oxygen. Carbon enters exclusively as atmospheric carbon dioxide fixed in photosynthesis; hydrogen and oxygen derive from water. Soil supplies mineral nutrients required in comparatively small quantities, not the bulk of the material.

Who this applies to
plants generally, trees most strikingly
Studied in
Plantae

You may have heard

“Trees get their mass from the soil”

Van Helmont tested this in the 1640s by growing a willow in weighed soil for five years. The tree gained 74 kg; the soil lost about 60 g. He concluded the mass came from water, which was half right — most of the dry mass is carbon, and it came out of the air.

Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Established by the carbon-fixation work and by elemental analysis of biomass. Van Helmont’s seventeenth-century willow experiment showed the soil loses almost no mass, though he drew the wrong conclusion from it.

How far it can be extended

Elemental composition of plant biomass is general, and the carbon source is established by isotope tracing.

Caveats

  • Soil nutrients are essential; the claim concerns bulk mass, not necessity.
  • A large fraction of a living tree’s mass is water, which does come through the roots.
  • Mineral nutrient limitation constrains growth even though it contributes little mass.

Still unanswered

  • How will rising atmospheric carbon dioxide alter growth where nutrients limit it?

Last reviewed 2026-08-10

The evidence (2 studies)

A willow gained about 74 kilograms in five years while its pot of soil lost around 60 grams.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Van Helmont’s willow experiment, reported in 1648, recorded a mass gain of roughly 74 kg in a tree grown for five years in a covered pot of pre-dried, weighed soil, against a soil mass loss of about 60 g — establishing that the added mass did not come from the soil, while leaving its actual source unidentified.

Who this applies to
A single willow in a single pot; the conclusion generalises, the measurement does not.
Studied in
Salix

You may have heard

“Van Helmont proved plants are made of water.”

He showed the soil was not consumed, which was the question he asked and the part that held. His own explanation was wrong, and identifying air as the real source took another century and a half.

Why we rate it this way, and what the caveats are
EstablishedHigh confidence

A single historical measurement, but one whose negative conclusion — the soil was not consumed — has never been contradicted and is now explained by the chemistry of photosynthesis.

How far it can be extended

The result has been reproduced in principle by every subsequent measurement of where plant mass comes from, and the underlying mechanism is now known.

Caveats

  • Van Helmont concluded the mass came from water, which is only partly right: hydrogen does, but most of the mass is carbon and oxygen from carbon dioxide.
  • One plant, no replication, and a balance too coarse to resolve the small mineral uptake that did occur.

Last reviewed 2026-08-30

The evidence (2 studies)

  • Supports · primary

    Ortus medicinae

    van Helmont, 1648 · Ludovicus Elzevirius, Amsterdam

    The original measurement of tree mass gained against soil mass lost, over five years in a covered pot.

  • Context · contextual

    Observations on different kinds of air

    Priestley, 1772 · Philosophical Transactions of the Royal Society of London

    Part of the sequence of work that identified air as the missing input Van Helmont could not weigh.

Go deeper

Trees and carbon

Almost every gram of a tree came out of the air. What the soil supplied is roughly one per cent of it.

Related things people ask

Last reviewed 2026-08-30