Ciencias

Where the Mass of a Tree Comes From

Almost all of it came out of the air as carbon dioxide. Van Helmont's willow gained 164 lb while his 200 lb of soil lost about two ounces in five years.

Respuesta breve

Where does the mass of a tree come from?

Mostly out of the air. The carbon that makes up the bulk of wood arrived as carbon dioxide through pores in the leaves, not as nutrients drawn up from the soil.

La respuesta corta
Almost all of it comes from the air, not the soil. Carbon dioxide enters through pores in the leaves and photosynthesis fixes the carbon into sugar, which becomes the cellulose and lignin of wood. Soil supplies water and small quantities of mineral nutrients, which is why a potted tree never empties its pot.

Qué cambia la respuesta

Whether you mean fresh mass or dry mass, because a living tree is largely water and that water did come up from the soil.
Which element you are tracking: the carbon came from the air, the hydrogen and much of the oxygen from water, and nitrogen, magnesium, phosphorus and potassium from the soil in small quantities.
The species and the conditions — a tree on poor soil grows slowly because a trace mineral is limiting it, even though that mineral is a tiny fraction of the final mass.

The experiment that ruled the soil out, four centuries ago

The intuition that a tree eats the soil is old and was taken seriously by educated people for a long time. It was tested, in one of the first quantitative experiments in biology, by the Flemish physician Jan Baptista van Helmont, and published after his death in 1648.

He dried 200 pounds of earth in an oven, put it in a pot, and planted a willow shoot weighing five pounds. For five years he added nothing but water and kept the pot covered against dust. At the end of it he weighed the tree: 169 pounds and about three ounces. Then he dried the soil again and weighed that: 200 pounds, less about two ounces.

The tree had gained roughly 164 pounds and the soil had given up two ounces. Whatever the willow was made of, it had not come out of the pot. Van Helmont concluded that it had all come from the water, and there he was wrong — but he was wrong in an interesting way. Carbon dioxide had not yet been identified as a substance, so the gas going into the leaves was not available to him as an explanation. He had correctly eliminated the answer everyone believed, using a balance, which is the part that matters.

Follow the carbon

Carbon dioxide enters a leaf through stomata, pores on the underside that open and close. Inside the chloroplasts, energy from sunlight drives a reaction between carbon dioxide and water that produces glucose and releases oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Every carbon atom in that glucose arrived through a pore, from the air.

Glucose is then joined into long chains to make cellulose, the fibrous material of plant cell walls, and combined with lignin, which stiffens those walls and is what makes wood woody. This is where the mass ends up. The repeating unit of cellulose is C₆H₁₀O₅, and of its 162 mass units, 72 are carbon — about 44 per cent by mass, an arithmetic check anyone can do from the formula.

So the sequence is short and entirely traceable: gas in through a pore, carbon fixed into sugar by sunlight, sugar polymerised into structure, structure laid down as a ring of wood. A mature tree is several tonnes of atmosphere that has been rearranged.

What the soil does supply, and why it still matters

None of this means soil is optional. It supplies water, taken up by root hair cells and used both as a raw material in photosynthesis and as the medium everything in the plant is transported in. That water accounts for a large share of the mass of a living tree, which is why freshly cut timber weighs so much more than seasoned timber.

It also supplies mineral ions in small quantities, and their scarcity is often what limits growth. Nitrate ions are needed to make amino acids and therefore proteins; a plant short of nitrate is stunted and its older leaves go yellow. Magnesium ions are needed to make chlorophyll; a plant short of magnesium develops chlorosis, yellowing between the veins while the veins themselves stay green. Phosphates and potassium are needed for respiration and for enzyme function.

The point to hold on to is the mismatch between importance and quantity. These ions decide whether the tree grows well and contribute almost nothing to how much it weighs. That is precisely why the question is worth asking — being essential and being the raw material are not the same thing.

Two checks anyone can run

A tree in a large pot on a balcony can be watched for a decade. It gains height, girth and branches. The level of the compost does not visibly drop, and topping it up occasionally is about texture and nutrients rather than replacing lost bulk. The tree grew and the pot did not empty.

Now run it backwards. Burn a seasoned log and what remains is a small heap of ash — a fraction of the original mass. The carbon left as carbon dioxide, the hydrogen left as water vapour, and both went back into the air they came from. What sits in the grate is the mineral fraction: the potassium, calcium, magnesium and phosphorus the tree took from the soil. That handful of ash is, near enough, the tree's soil contribution made visible, and it is the reason wood ash has been spread on gardens for centuries.

Weight of van Helmont's willow after five years, grown from a shoot of 5 lb: 169 lbWeight of van Helmont's willow after five years, grown from a shoot of 5 lb[1]
All that his 200 lb of oven-dried soil had lost over the same five years: 2 ozAll that his 200 lb of oven-dried soil had lost over the same five years[1]
Share of cellulose's mass that is carbon, worked out from the formula C₆H₁₀O₅: 44%Share of cellulose's mass that is carbon, worked out from the formula C₆H₁₀O₅

Preguntas frecuentes

So does fertiliser not make plants grow?

It does, but not by supplying bulk. Fertiliser provides nitrogen, phosphorus and potassium, which plants need in small quantities to build proteins, transfer energy and run enzymes. Supplying a limiting nutrient lets a plant use the carbon dioxide and light it already has more effectively. The mass still comes from the air.

How much of a living tree is water?

A large share of the fresh mass, which is why the question is usually asked about dry mass. Newly felled timber is noticeably heavier than the same timber after seasoning, and the entire difference is water that has evaporated. The water came from the soil; the material left behind did not.

Where does the oxygen in wood come from?

From both sources. Cellulose contains oxygen atoms that trace back to carbon dioxide and to water, so the oxygen has a mixed origin. The oxygen released into the air during photosynthesis, by contrast, comes from the water molecules that are split — not from the carbon dioxide.

Was van Helmont's conclusion wrong then?

His conclusion was wrong and his experiment was sound. He showed convincingly that the soil was not the source, which was the belief he set out to test. He credited water alone because the role of atmospheric gases in plant growth was unknown in his lifetime. It took another century and a half of work on gases to complete the picture.

Fuentes

Analyzing van Helmont's Classic Experiment — College of Saint Benedict and Saint John's University
Jan Baptista van Helmont — Encyclopaedia Britannica

Última actualización 21 de agosto de 2026