Rocket pats the soil in his cup. "My bean is getting bigger. So the soil must be shrinking!"
Raven tilts the cup. "What do you notice? Does the soil look smaller?"
Nova blinks thoughtfully. "A scientist tested that idea long ago," she says. "Let's look at the numbers."
Rocket grabs a pencil. "Let's check the math!"
In the 1600s, a scientist named van Helmont asked a question.
Where does a growing plant's weight come from?
He dried 200 pounds of soil in an oven and put it in a pot.
He planted a young willow shoot that weighed 5 pounds.
For five years, he added only water.
Then he weighed the tree and dried and weighed the soil again.
| What he weighed | At the start | After five years |
|---|---|---|
| the willow tree | 5 pounds | about 169 pounds |
| the soil | 200 pounds | 200 pounds, less about 2 ounces |
Van Helmont thought all the new weight came from water.
He was partly right. He did not know plants take in air too.
Today we know plant matter comes mostly from air and water.
The carbon dioxide from the air is a big part of it.
Soil gives the plant small amounts of minerals.
| Statement | True or false? |
|---|---|
| The soil lost most of its weight as the tree grew. | ? |
| Plant matter comes mostly from air and water. | ? |
| Van Helmont added only water for five years. | ? |
| Soil gives a plant small amounts of minerals. | ? |
Super evidence work! Tomorrow we set up our own plant test with light and shade.