38 NATURE'S FOOD-SUPPLY. THE CARBON CYCLE. 
fertility of the soil be assured. While various agencies are concerned 
in this matter of soil fertility, the agency of microorganisms 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. 
Carbonic Dioxid. This gas (CO 2 ) 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. 
Nitrates. These, which are salts of nitric acid (HNO 3 ), 
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 certain that ordinarily they do not obtain much 
from this source. The higher compounds of nitrogen, like proteids, 
ures, or other complex bodies, cannot furnish plants with nitrogen 
directly, nor, on the other hand, can nitrites (salts of nitrous acids, 
HNO 2 ) 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 dissolve 
them. 
Sulphates. These salts are also needed in small amounts 
only. 
Iron Salts. Needed in small quantities only. 
