The tub of soil that stayed full

By the end you can say where the wood in a tree actually came from, and show that the soil did not go anywhere.

A very common way to think about this

“A plant is made out of the soil it grows in. The roots take soil up into the stem, which is why a plant in a pot needs the pot filled with it.”

You put a seed in soil and a heavy plant comes out. Nothing else visibly goes in except water, and water is not wood. Soil is solid, brown and there in exactly the quantity a growing thing seems to need.

1. Predict first

A willow is planted in a tub holding 90 kg of dried soil. Five years later the tree is tens of kilograms heavier. What will the tub of soil weigh when you dry it and put it back on the balance?

2. Try it

Years the willow has been in the tub (0–12 yr) — The tree gains tens of kilograms while the balance under the soil moves by grams — two readings that cannot both be the same stuff.
The tub's air supply — With the air sealed off the tree stops gaining, even though the soil is untouched and the watering never stops.

Both balances are read dry. The tree is lifted out, roots and all, before the soil is weighed.

Tree, dry2.30 kg

Soil, dry90.70 kg

Soil has lost0.0 g

Tree has gained0.0 g

Of that, from air0.0 g

Years in the tub0 yr

The tub's air supply

Run the tub out to eight years or more with the hood open, and keep one eye on the soil balance.

3. The check

Keep experimenting above. This question opens once you have seen the thing it asks about.

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