In the early 1600s, Jean Baptista van Helmont investigated how plants acquire new mass as they grow. He weighed a willow shoot and planted it into soil that he had also weighed. After 5 years of adding nothing but water to the plant, he found that the soil had lost only a little weight, while the plant had grown from 2 kg to about 76 kg. He therefore concluded, incorrectly, that water was the sole source of the added plant material. What other source did he fail to consider in his experiment?