Student's Pages
Activity 9: Dr. Van Helmont - Is Soil Food for Plants?
Activity 9 - Teachers Pages
How many people in your class think that soil might be food for
plants? _________
How many people in your class think that water might be food for
plants? ________
What are their reasons?
Traveling Back in Time
We have started doing experiments to find out about how plants
get their food. Scientists have been doing experiments for many, many years to find out about food for
plants. Aristotle lived years ago, and he thought about plants and how they got their food to grow. In
fact, people have probably been wondering about how plants make their food for thousands of years. Even
before we called people "scientists", people were wondering about plants and trying to come up with
explanations about how they live and grow. And scientists today are still doing experiments to find
out about how plants get their food -- we still do not have all the answers!
Let's travel back in time 350 years. It is now the year 1642. We
are in Europe. It is a time of excitement and exploration and travel in this part of the world. Some people
have found what they call a New World across the ocean (it is actually a very old world to the Native
Americans living in this "New" World)! And more people are getting interested in finding out about the why's of
the world around us -- more people are interested in and finding benefactors who will pay them to do
science experiments.
We are going to meet one of these early scientists. He is a
physician but he also does experiments with plants. His name is Jan Van Helmont. He is from the country
of Belgium and the year 1642. He wants to visit with us today. He is going to help us think about our
hypothesis that soil is food for plants. He was very interested in that hypothesis. Almost everyone back in 1642
was sure was the soil was the food for plants. Jan Van Helmont decided to prove them right (or wrong?).
Is soil food for plants?
Suppose a child was given 200 pounds of food to eat. Predict what
would happen to the weight of that child as he or she gobbled up the food. Does the child's weight go up,
go down, or stay the same? Write your answer under the box marked "Weight of Child":
Weight of Child
Weight of Food
What would happen to the weight of the food on the table as the
child ate it? Does the weight of the food go up, go down, or stay the same? Write your prediction under the
box marked "Weight of Food".
Now think about a young tree planted in a bucket of soil. As the
tree grows it gains weight. Does it gain weight from the soil the way a child gains weight from food? Is
the soil a kind of food for the plant? What do you think? Write down below whether you think the weight of the
soil will go up, go down, or stay the same as the tree grows:
Weight of the Tree
Weight of the Soil
In the 1600's everyone thought that the soil and minerals in the
soil were the food for the plants. A scientist named Jan Van Helmont did an experiment to see if this was true.
He planted a five pound young tree in a bucket containing 200 pounds of soil. He watered the tree
regularly but he did not add any more soil. Look at the picture below and see what happened to the weight of the
tree and the weight of the soil after five years.
Weight gain by tree
Weight loss by soil
The tree gained a whole lot of weight, but the soil did not lose
hardly any weight! What do you think Van Helmont concluded? Is soil a food for plants? Why or why not?
________________________________________________________
________________________________________________________
________________________________________________________
________________________________________________________
Van Helmont decided that soil is NOT a food for plants. The tree
did not use any of the soil to grow bigger.
In order to grow bigger, the tree (like all living things) needs
___________________________ that is in food.
Think about our scientific definition of food.
Does Van Helmont's experiment give us evidence to say that soil
is or is not food for
plants?____________________________________________________
Explain your thinking.________________________________________
___________________________________________________________
___________________________________________________________
Are minerals in the soil food for plants?
Everyone says that plants take in minerals from the soil.
Minerals do not have very much weight, but they do weigh something. So do you think Dr. Van Helmont’s tree
took in minerals from the soil? _______
About how much weight did the tree get from the minerals?_______
Do you think this amount of minerals could explain how the tree
gained 164 pounds?
_________ Explain your
thinking.____________________________________
_____________________________________________________________
Think about Van Helmont's experiment. Does that experiment give
us any evidence to say whether or not minerals in the soil are food for plants? ___________
Explain your thinking.
______________________________________________________________
______________________________________________________________
______________________________________________________________
Is water food for plants?
Van Helmont thought that his experiment was evidence that water
must be food for plants. He thought that if soil and minerals in the soil were not giving the tree its
food, then the tree must be gaining weight by getting food from the water. After all, he had been watering the
tree everyday for five years.
But remember our scientific definition of food. Water helps the
tree to grow, but does it give the tree energy? Could the tree live and grow if all it took in was water?
Let's think about the evidence from our experiment with the grass plants. It might help us decide whether
or not water is energy-giving food for plants.