174 OF MANURE. 



leaves of which are food for animals ; for they pre- 

 pare the soil for plants of every other kind, but par- 

 ticularly for those to which, as to rape and flax, the 

 presence of humus is the most essential condition 

 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 deve- 

 lopment. 



Now, although the quantity of humus in a soil 

 may be increased to a certain degree by an artifi- 

 cial 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 un- 

 impaired, unless we replace in it all those substances 

 of which it has been thus deprived. 



Now this is effected by 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- 



