CONSTITUTION OF THE SOIL. 305 
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. 
SI, 
GENERAL VIEW OF THE CONSTITUTION OF THE SOIL AS 
RELATED TO VEGETABLE NUTRITION. 
Inert, Active, and Reserve Matters,—In all cases the 
soil consists in great part of matters that are of no direct 
or present use in feeding the plant. The chemical nature 
of this inert portion may vary greatly without correspond- 
ingly influencing the fertility of the soil. Sand, either 
quartzose, calcareous, micaceous, feldspathic, hornblendic, 
or augitic; clay in its many varieties; chalk, ocher (oxide 
of iron), humus; in short, any porous or granular material 
that is insoluble and little alterable by weather, may con- 
stitute the mass of the soil. The physical and mechanical 
characters of the soil are chiefly influenced by those ingre- 
dients which preponderate in quantity. Hence Ville has 
quite appropriately designated them the “mechanical 
agents of the soil.” They affect fertility principally as 
they relate the plant to moisture and to temperature. 
They also have an influence on crops by gradually assum- 
ing more active forms, and yielding nourishment as the 
resuat of chemical changes. In general, it is probable 
