286 EXHAUSTION OF SOIL. [BOOK n. 



vegetable tissue ; and, consequently, it is probable that the 

 exhaustion of soil does not depend merely upon the destruction 

 of carbonaceous matter, but also upon the consumption of the 

 azotised matter contained in it. This is a most important 

 fact to consider, in attempting to estimate the action of 

 manures. (Comptes rendus, vi. 106.) 



Those who wish to understand the modern opinions con- 

 cerning the theory of manures should consult De Candolle's 

 Physiologic, p. 1278, and the works of Payen, Boussingault, 

 Thaer, Liebig, Sprengel, Hlubek, Johnson, Dana and Edward 

 Solly. 



