284 



CHAPTER VIII. 



ARTIFICIAL SOURCES OF THE FOOD OF PLANTS — ACTION 

 OF MANURES. 



746. When we remember, that although plants 

 derive a large portion of their nourishment from the 

 air, yet at the same time they absorb considerable 

 quantities of saline matters from the soil, it is evident 

 that all plants must more or less impoverish the 

 soil, by taking away that which causes its fertility. 



747. The natural vegetation of any country en- 

 riches rather than deteriorates the soil, because 

 nothing is carried away from its surface ; the plants 

 which grow on it return to the soil, during their 

 decay, all the earthy and saline substances which 

 they had absorbed from it during their growth, whilst 

 they add to it a considerable quantity of the carbon 

 they had collected from the air. 



748. Very different, however, is the condition of 

 cultivated lands; on them large crops are raised year 

 after year, which are removed and carried away to a 

 distance, to form the food of men and animals. 

 Such land is gradually impoverished ; because with 

 the crops a large quantity of inorganic matter, ne- 



