174 OF MANURE. 



V 



the presence of humus is the most essential condi- 

 tion of growth. 



The reasons why this interchange of crops is so 

 advantageous, — the principles which regulate this 

 part of agriculture, are, therefore, the artificial pro- 

 duction of humus, and the cultivation of different 

 kinds of plants upon the same field, in such an order 

 of succession, that each shall extract only certain 

 components of the soil, whilst it leaves behind or 

 restores those which a second or third species of 

 plant may require for its growth and perfect devel- 

 opment. 



Now, although the quantity of humus in a soil may 

 be increased to a certain degree by an artificial 

 cultivation, still, in spite of this, there cannot be the 

 smallest doubt that a soil must gradually lose those 

 of its constituents which are removed in the seeds, 

 roots, and leaves of the plants raised upon it. The 

 fertility of a soil cannot remain unimpaired, unless 

 we replace in it all those substances of which it has 

 been thus deprived. 



Now this is effected by manure. 



CHAPTER IX. 



OF MANURE. 



When it is considered that every constituent of 

 the body of man and animals is derived from plants, 

 and that not a single element is generated by the 

 vital principle, it is evident that all the inorganic 

 constituents of the animal organism must be re- 

 garded, in some respect or other, as manure. During 

 their life, the inorganic components of plants which 

 are not required by the animal system, are disen- 

 gaged from the organism, in the form of excrements. 

 After their death, their nitrogen and carbon pass 

 into the atmosphere as ammonia and carbonic acid, 



