6 SOILS AND FERTILIZERS 



agricultural problems. He assailed the humus theory 

 of Thaer, and showed 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-ashes 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- 

 position of plant-ash, and the importance of supplying 

 crops with mineral food led to the commercial prepara- 

 tion of manures, which in later years has developed 

 into the commercial fertilizer industry. The woik of 

 Liebig was not conducted in connection with field 

 experiments. It had, however, a most stimulating 

 influence upon investigations in agricultural chemistry, 

 'and to him we owe, in a great degree, the summari- 

 zing of previous disconnected work and the mapping 

 out of valuable lines for future investigations. 



Liebig's enthusiasm for agricultural investigations 

 may be judged from the following extract : kl I shall 

 be happy if I succeed in attracting the attention of men 



