CONSTITUTIOX OK THE SOIL. 805 



of little practical importance, since ammonia is so sparse- 

 ly supplied by nature, and the ammonia of fertilizers is 

 almost invariably subjected to the conditions of speedy 

 nitrification. 



CHAPTER VI. 



THE SOIL AS A SOURCE OF FOOD TO CROPS.— INGRE. 



DIENTS WHOSE ELEMENTS ARE DERIVED FROM 



ROCKS. 



§ 1. 



GENERAL VIEW OF THE CONSTITUTION OF THE SOIL AS 

 RELATED TO VEGETABLE NUTRITION. 



Inert, Active, aaid Reserve Matters. — In all cases the 



soil consists in gre.at ])art of matters thnt are of no direct 

 or present use iu feeding the plant. The cliemical nature 

 of this inert portion may vary gre.atly Avithout coi-respond- 

 ingly influencing the fertility of tho soil. Sand, either 

 quartzose, calcareous, micaceous, fi'ldspatliic, hoi-nblendic, 

 or augitic ; clay in its many varieties ; chalk, ocher (oxide 

 of iron), humus ; iu short, any porous or granular material 

 that is insoluble and little alterable by ^veather, may con- 

 stitute the mass of the soil. The physical and mechanical 

 characters of the soil are chielly influenced by those ingre- 

 dients which preponderate in quantity. Hence Ville has 

 quite appropriately desi^'nated them the "mechanical 

 agents of the soil." They aifect fei'tility principally as 

 they relate the plant to moisture and to temperature. 

 They also have an influence on croj)S by gradually assum- 

 ing moie active forms, and yielding nourishment as the 

 result of chemical changes. In generalj it is probable 



