Where Do Trees Get Their Mass?

Where Do Trees Get Their Mass?


Trees are some of the
biggest organisms on the planet, but where do they get that matter to grow? Man: Rich nutrients out the ground. Man: Start with soil or in the air. Man: Goodness out of the soil, I suppose. Derek Muller: Comes out of the soil? Man: Yeah… Derek Muller: Yeah… Derek Muller: Goodness. Man: Goodness. Derek Muller: Why isn’t there a big hole
around the tree where it’s taking out all the soil? Man (right): ‘Cause it does it so
gradually that the soil is time to recover. Derek Muller: Now, I think it’s intuitive to believe that the tree gets most of its mass from the soil because you can see those roots
digging into the soil and they must be taking something out of there and I mean
a tree looks like dirt and it feels solid like dirt but it’s not. In the early 1600 a scientist
named Johann Baptist Van Helmholt tried to figure out where the mass
of a tree was coming from so he got a pot of soil and very carefully
measured the amount of soil in there. Then he planted the tree and
took care of it for five years making sure that no soil left or was added to his pot . At the end of this experiment he was weighted the tree
to find that it was 72 kilograms but the massive soil had
only decreased by about 60 grams. This was pretty strong evidence that the
mass of the tree does not come from the soil. I’ve never thought about that actually
because they don’t really eat anything, trees. They don’t eat anything?
–No. They don’t eat anything? Water, it’s all basically absorbed.
–That’s all they eat? Yeah.
–They don’t eat anything else? No.
–That’s all they eat. Well presumably from the water
and the nutrients from the soil. Is there anything else that you need
besides the soil and the water? I suppose that’s all you need, isn’t it? To make–
–Other than the original seed for that particular tree, I suppose. The seed, the soil, and the water and
that makes this big tree. Derek Muller: Of course Johann Baptist Van Helmholt
did conclude that the tree was made entirely of water. Now while that’s not correct
at least he was on the right track realizing that the matter of a tree
doesn’t come out of the soil. The sun energy, yeah. The sun energy?
–Yeah. Are they converting energy into mass or–? Do you know what I mean?
–Yeah. Like that there wasn’t stuff and then there was. Where did that stuff come from? Magic. [laughing]
I have no idea. My question is where do they
get that mass to grow big? Sun. Get it from the rain and the sun presuably. Light.
–Sunlight. The sunshine.
The sunshine? Does the sunshine add mass to the tree? It would need sunshine to grow. Yes, they wouldn’t grow without it.
I don’t know whether it adds mass, but they wouldn’t go without it. Of course the sun’s energy is needed for
the tree to build the matter into its branches and leaves but the Sun itself
the energy is not matter. Well I suppose you’ve got to put air into this as well. There’s got to be something, you know. What is it about the air?
–A gas in the air that it needs as well. Oxygen.
The trees need the oxygen? And I guess oxygen. The oxygen, of course. The oxygen? Are there any ingredients that we’re missing? Carbon dioxide. Would it surprise you to find out that 95%
of a tree is actually from carbon dioxide. Trees are largely made up of air. Yeah.
–Surprising. I need this reaction like, “Oh holy sh– [laughing] So as it turns out trees are mostly made out of air, out of the carbon dioxide that they take in. What’s interesting is that we breathe
out carbon dioxide and water that’s how we lose mass but it’s the exact same substances
that trees breathe in to gain mass. So if you can imagine a closed system
where it’s just you and a tree. You would breathe out that carbon
dioxide and water the tree would take it in. So you would get smaller
while the tree is getting bigger and in a sense you’re becoming the tree.