8 December 2025 
2 minutes At first glance, it seems obvious that plants need soil in order to grow, and that soil is the source of most of the material they are made of. The idea sounds so reasonable and self-evident that it hardly seems to require testing – but real science doesn’t work that way. Every hypothesis must be tested.
Indeed, about 400 years ago, the scientist Jan Baptist van Helmont conducted a famous experiment in which he planted a willow tree in a large pot and cared for it for five years. To his surprise, although the tree grew significantly, the weight of the soil in the pot barely changed. In other words, he discovered that the soil was not the source of the material that makes up the tree.
Van Helmont concluded from this that plants are formed from the water used to water them. Today we know that this conclusion, too, is incorrect. In fact, the main material from which plants build themselves comes from the air—specifically, from the carbon dioxide it contains.
Plants take carbon dioxide from the air through their leaves and water from their roots, and with the help of sunlight convert them into oxygen and a sugar called glucose. This process, known as photosynthesis, is the basis from which almost all the other substances that make up plants are formed. It is also the source of the materials that make up all living organisms on Earth, since plants and other photosynthetic organisms lie at the base of the food web.
By weight, most of a plant’s dry matter—that is, its organic material excluding water—comes from the air. Water comes next, while soil contributes only a tiny amount of minerals. You may be surprised to learn that plants don’t actually need soil in order to grow: they can grow in mineral-enriched water using a method called hydroponics, which you can see in action at the “Everything Grows” exhibit in the Clore Garden of Science.
