Excerpts from Books and Wikipedia
"Until it was known that plants synthesize their proteins from inorganic nitrogen compounds in the soil, the need to replace the all-important nitrogen could not be appreciated. Indeed, nitrogen itself was not discovered until c. 1770." [Blanning: Pursuit of Glory, p. 144] "Nitrogen is the chemical element with the symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772." [Wikipedia]