This article smells like a fine bullshit. It's based on one diagram and a meta-analysis of papers.
However, the diagram has these nice annotation: "Asterisks indicate numbers could not be independently verified". And there are asterisks on all the historic data except for 1948.
The only reliable reference is from "35. Firman, B. Ash and Mineral Cation Content of Vegetables. Soil Sci. Soc. Am. Proc. 1948,13, 380–384."
However, the diagram has these nice annotation: "Asterisks indicate numbers could not be independently verified". And there are asterisks on all the historic data except for 1948.
The only reliable reference is from "35. Firman, B. Ash and Mineral Cation Content of Vegetables. Soil Sci. Soc. Am. Proc. 1948,13, 380–384."