The VanHelmont experiment consists of a measure of the amount of soil before and after a plant grows.
He wants to demonstrate the independence of soil.
After five years he measures the amount of soil and only 2oz gone from the initial amount of soil when the plant weight much more.
The conclusion was the plant use water to grow, and not the soil.
Afterward, he discovers CO2 but, not until the discovery of photosynthesis we all knew how everything in plants works.