. . . . . . . . suggest that saltpeter (KNO₃), and not water, was the 'principle of vegetation'

J. R. Glauber (1604 - 1668), a German chemist, discovered that nitre or saltpeter is very important in plant nutrition. He has concluded saltpeter came from hydrolysis of urea to ammonium followed by nitrification to NO₃ which is then combined with K⁺.
He showed that nitre is obtained from the cow dung and urine of animals in farmyard and it forms from the fodder formed from plants. The addition of these materials to soil increases the growth of plants. This is called Glauber's vegetation theory.