INTRODUCTION 5 



absorption of the gaseous nitrogen of the air by vege- 

 table earth, he says : " I am not acquainted with a 

 single irreproachable observation that establishes it ; 

 not only does the earth not absorb gaseous nitrogen, 

 but it gives it off." 2 



The investigations of DeSaussure and Boussingault, 

 and the writings of Davy, Thaer, Sprengel, and Schub- 

 ler prepared the way for the work and writings of 

 Liebig. In 1840 he published "Organic Chemistry 

 in its Applications to Agriculture and Physiology." 

 Liebig's agricultural investigations were preceded by 

 many valuable discoveries in organic chemistry, which 

 he applied directly in his interpretations of agricul- 

 tural problems. His writings were of a forcible char- 

 acter and were extremely argumentative. They pro- 

 voked, as he intended, vigorous discussions upon 

 agricultural problems. He assailed the humus theory 

 of Thaer, and held that humus was not an adequate 

 source of the plant's carbon. In the first edition of 

 his work he showed that farms from which certain 

 products were sold naturally became less productive, 

 because of the loss of nitrogen. In a second edition 

 he considered that the combined nitrogen of the air 

 was sufficient for crop production. He overestimated 

 the amount of ammonia in the air, and underestimated 

 the value of the nitrogen in soils and manures. A 

 study of the composition of plant-ash led him to 

 propose the mineral theory of plant nutrition. De- 

 Saussure had shown that plants contained certain 

 mineral elements, but he did not emphasize their im- 

 portance as plant food. Liebig's writings on the com- 



