A potted willow tree gains hundreds of kilograms of wood over five years while the soil barely changes in weight. Most of this new dry mass comes directly from

Core Concept

Most of the dry mass added as a plant grows ultimately comes from carbon dioxide fixed during photosynthesis and incorporated into carbohydrates and other organic molecules. Soil supplies essential mineral nutrients but contributes far less to total dry mass. Van Helmont's willow experiment showed that the soil lost little mass, but historically he concluded that water accounted for the plant's gain; the experiment itself did not identify atmospheric carbon as the source.