It was once common to assume that plants were ‘soil eaters’: that they made themselves out of dirt. In the 1640s, a Flemish scientist by the name of Jan Baptist van Helmont set out to understand whether that was truly the case. He performed a five-year-long test known as the Willow Experiment, from which humanity learned two things: first, that van Helmont was very patient; second, that plants do not make themselves out of dirt. Van Helmont planted a 5-pound baby willow tree in a large pot filled with 200 pounds of soil. For the next five years, he watered it and watched it grow. Then, after
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