72 THE SOIL. 



bonic acid and ammonia in the soil ; its decay also 

 causes heat. 



Organic matters containing nitrogen, such as ani- 

 mal substances, etc., furnish ammonia, and other ni- 

 trogenous substances to the roots of plants. 



CHAPTER III. 



USES OF EAKTHY MATTER. 



The offices performed by the earthy constituents of 

 the soil are many and important. 



These, as well as the different conditions in which 

 the bodies exist, are necessary to be carefully consid- 

 ered. 



Those parts which constitute the larger proportion 

 of the soil, namely the clay, sand, and limy portions, 

 are useful for purposes which have been named in the 

 lirst part of this section, while the day has an addi- 

 tional effect in the absorption of ammonia. 



For this purpose, it is quite as effectual as charcoal ; 

 the gases escaping from manures, as well as those ex- 

 isting in the atmosphere, and in rain-water, being 

 arrested by clay as well as by charcoal. 



The more minute ingredients of the soil — those 

 which enter into the construction of plants — exist in 

 conditions which are more or less favorable or in- 

 jurious to vegetable growth. The principal con^^i- 



