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. 



