Back in 1648, Johannes Baptista van Helmont took a young willow sapling weighing 5 pounds (2.3 kg*), and planted it into 200 pounds (90.7 kg) of soil. Five years later he uprooted the tree and determined its weight to be 169 pounds and 3 ounces (76.8 kg), while the soil weighed in just shy of the original 200 pounds.
(* Assuming a pound then was of identical mass to today’s pound, which it probably wasn’t due to revised definitions over the centuries.)
The experiment was poorly designed, had wrong conclusions, and was not an original idea. But despite its flaws, it has its place in history as the first known and documented quantitative experiment in biology.
van Helmont concluded that the five years’ growth “arose out of water onely", whereas a better conclusion might be that soil per se contributes very little to a plant’s final weight, and investigations into what actually does could well have advanced botany and soil science much earlier. After all, van Helmont was not only a chemist, but came very close to discovering carbon dioxide (he called it gas sylvestre — John Black is credited with discovering CO2 in 1754) and had even coined the word ‘gas’!
370+ years later we still agree that plants don’t ‘eat’ soil, and soil mass doesn’t turn into plant mass. But we also now know that the vast majority of plants still need to feed from soil in some way, as the vast fertiliser industry will attest to! We also know that plants just naturally grow better in some soils than others, whether fertilised or not.
The physical and chemical properties of soils that makes some poor and some rich are many, and it is these that we’ll be exploring further in this section.
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