Normal Soil and Its Requirements 13 



A. The Transformation of Carbon 



Cellulose, which is but a form of carbon, consti- 

 tutes a large per cent, of the woody tissue of plants. 

 Soils contain large amounts of cellulose and this un- 

 doubtedly helps to maintain their proper physical con- 

 dition. Straw manure, or green vegetable matter all 

 contain large amotints of cellulose. When it is in- 

 corporated in the soil, living plants cannot make use 

 of it, because of its complex form. It therefore must 

 first undergo a certain decomposition. This is ac- 

 complished by a group of soil bacteria known as 

 Afnylobacter. These feed on the dead vegetable 

 cellulose, breaking it up and reducing it back to car- 

 bon diojdde, hydrogen, and fatty adds. The carbon 

 dioxide either returns to the air to replenish the at- 

 tnospheric supply, or unites with water to form car- 

 bonic acid and soil carbonates. The carbon dioxide 

 is taken in by the plants either directly from the air 

 through the leaves, or from the soil in some carbon- 

 ate form. Thus we see that it is not the cellulose nor 

 the product of its decomposition that furnishes plant 

 food, but certain inorganic elements which are set free 

 in its decomposition. 



B. Elaboration of Available Nitrogen 



From the viewpoint of plant nutrition, nitrogen 

 is imquestionably the most important of all elements. 

 The nitrogen of the air, although totalling about 79 

 per cent, of it, is not in an available form. In the 

 transformation of protdds into available nitrogen 



