Most of a growing plant's dry mass comes from material absorbed from the soil.

The soil idea belonged to a much older theory of plant nutrition than the familiar schoolroom story about van Helmont’s willow tree. Historical work on plant physiology traces an Aristotelian doctrine in which land plants were understood to derive their substance from the soil. Aristotle treated roots as the route by which prepared nourishment entered the plant, while leaves were not yet understood as the major organs of carbon assimilation.

The transition was not a single experiment. Nicholas of Cusa, Jan Baptist van Helmont, Robert Boyle, Stephen Hales, Joseph Priestley, Jan Ingenhousz, and Nicolas-Théodore de Saussure each changed different pieces of the picture. Van Helmont’s famous willow experiment showed that a plant could gain far more mass than the soil lost, but he concluded that water supplied the increase. Hales later suspected that part of plant substance came from the air. Priestley and Ingenhousz connected plants, air, light, and gas exchange; de Saussure helped put plant nutrition on a quantitative chemical footing.

That is why the representative change date is much later than van Helmont. The old soil-centered model changed in stages. It passed through a water-centered model and then through increasingly quantitative studies of air, light, minerals, and carbon chemistry.

Today the key distinction is fresh mass versus dry mass. Water can dominate a fresh plant’s weight, while most of the carbon in its dry matter ultimately comes from atmospheric carbon dioxide. Soil remains indispensable because it supplies water and mineral nutrients.