Yvon Chouinard (2016). “Let My People Go Surfing: The Education of a Reluctant Businessman--Including 10 More Years of Business Unusual”, p.270, Penguin
Justus von Liebig (1855). “Principles of agricultural chemistry with special reference to the late researches made in England: By Justus von Liebig. (Ed. by William Gregory)”, p.31