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CHAPTER VI. 

 NATURAL MANURES. 



FROM what has been said in Chapters III., IV., and V., it 

 is evident that the fertility of a soil depends upon a number of 

 circumstances ; among others, upon the existence of suitable 

 relationships between its properties regarded from 



(1) Mechanical 



(2) Chemical 



(3) Biological 

 points of view. 



(1) The possession of the proper fineness of sub-division, poro- 



sity, water-holding power, and other physical properties 

 is essential to permit of the ready growth of a plant's 

 roots, to afford the necessary mechanical support and 

 access of air to the roots, and to provide them with a 

 due supply of moisture. If these properties are lacking, 

 no matter how rich the soil may be, it cannot possess, 

 in the highest degree, true fertility. 



(2) So, too, from a chemical standpoint, the possession of a 



suitable store of plant food in an available condition is 

 obviously necessary. If the plant food, or any part of 

 it, be present in insufficient amount, or in a completely 

 insoluble or unavailable form, the soil, however suitable 

 in mechanical structure, must be incapable of yielding 

 the best results. 



(3) The conversion of one of the most important constituents 



of plant food, nitrogen, from a state of combination in 

 the insoluble, highly complex, organic substances asso- 

 ciated with humus into the directly assimilable nitrates, 

 is, as already described (Chap. IV.), accomplished by the 

 action of micro-organisms, so that it is evident that the 

 realisation of one of the above conditions as to avail- 

 ability of plant food in the soil can only be effected if 



