PHOSPHATES AND NITROGEN 41 



atmosphere, but as it exists in certain combinations, 

 of which the most notable are the compounds of am- 

 monia. This explains to us the highly beneficial ef- 

 fect of manure on our crops. Manure is composed 

 of the bedding used in stables and the animal excre- 

 ment with which it has become mixed and impreg- 

 nated. Now this excrementitious matter, especially 

 urine, yields ammonia in decomposing, as is proved 

 by the odor arising from latrines in hot weather 

 and so powerfully affecting the eyes and nose. Thus 

 manure may be said to hold ammonia compounds in 

 storage, and from them plants derive their nitrogen, 

 as also many other ingredients. 



"Let us summarize these details. In the nutrition 

 of plants four substances are of prime importance. 

 First, carbonic acid gas, which yields carbon, the 

 most widely diffused of all the elements (but which 

 we need not dwell upon here), since plants take it 

 chiefly from the atmosphere, to which it is supplied 

 unceasingly. After carbonic acid come potash, 

 phosphoric acid, and nitrogen, all of which the roots 

 extract from the soil, where it occurs in some com- 

 pound or other. These are the ingredients that the 

 soil, if it is to remain fertile, must have given back 

 to it as fast as they are exhausted by the crops. 

 Such is the part played by fertilizers, without which 

 the soil becomes exhausted and ceases to produce." 



