XIX. 



THE IIUMUS THEORY. 



THE first attempt, says Dr. R. C. Kedzie, to 

 explain the nutrition of plants was founded on 

 the analogy of animal life; that, as an animal may 

 live in vigorous health while feeding upon the 

 remains of another animal, so vegetable remains, 

 or the humus of the soil, is the principle food of 

 plants. It was found, however, that the mere 

 presence of humus or vegetable matter in the soil 

 was not the sole condition of its fertility, and that 

 some soils, such as our peaty swamps, might be 

 composed almost entirely of humus, and yet for 

 most plants be unproductive. Liebig, of Germany, 

 in studying the question of plant nutrition con- 

 clusively refuted the humus theory, and proved 

 that plants obtain their chief supply of carbon or 

 woody material from the atmosphere. 



Liebig's own theory of plant nutrition is stated 

 by the same writer substantially as follows: 



(117) 



