42 NATURE’S FOOD SUPPLY. THE CARBON CYCLE 
apply them properly to cultivated soil. In this way only can the 
continued fertility of the soil be assured. While various agencies 
are concerned in this matter of soil fertility, the agency of micro- 
Organisms is certainly one of the largest. 
PLANT FOODS 
The green plants live chiefly upon the following foods: 
Water.—This material, coming from the rains, is unlimited in 
amount and need not detain us. 
Carbon Dioxid.—This gas (COz) furnishes the carbon which 
is the basis of most plant structures, wood, cellulose, starch, 
sugar, etc. It is present in the air in small percentage only, but 
is kept fairly constant by processes which we shall consider. 
Witrates.—These, which are salts of nitric acid (HNOs), 
constitute the chief form in which plants obtain their nitrogen. 
Nitrogen in considerable amount is an absolute necessity for all 
plant life, and while plants can probably assimilate some nitrogen 
from ammonia, it is probable that ordinarily they do not obtain 
much from this source. The higher compounds of nitrogen, like 
proterds, urea, or other complex bodies, cannot furnish plants with 
nitrogen directly, nor, on the other hand, can mitrites (salts of 
nitrous acid, HNOs) or free nitrogen in the air supply any nitrogen 
directly to plants. Practically all the nitrogen must be obtained 
by the plants in the form of nitrates from the soil, and to keep a 
constant supply of nitrates in the soil must be the first aim of the 
farmer. 
Phosphates.—A small amount of phosphorus is needed by 
plants and is obtained in the form of the soluble phosphates from 
the soil. ‘The mineral soil ingredients contain much phosphorus 
in insoluble compounds, and agencies for rendering these soluble 
are necessary to soil fertility. 
Potash.—Some form of potassium salts is necessary. These 
salts abound in soils, but some agency must be employed to dis- 
solve them. 
