2Lecture 31Despite these observations on the value of salts, the belief that humus (organic material) was the “food” of plants was upheld well into the 19thcentury and was supported by such renowned chemists as Theodore de Saussure (1767–1845) and Sir Humphrey Davy (1778–1829). The source and function of the inorganic elements in plant ash was unknown. A prize was offered in Germany to resolve the question of whether the inorganic elements, found in the ashes of plants, are constituents produced by plants or must be absorbed and what was their role. The prize was awarded to A.F. Wiegmann and L. Polstroff based on experiments using synthetic soil vs. sand alone. In the 19thcentury Justus von Liebig (1803–1873), dominant figure in plant nutrition, demonstrated that carbon was supplied by the air and not by humus (although he believed it was absorbed by roots). Liebig assumed most N was absorbed from the air (he was unaware of N fixation by
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