274 The Ohio Naturalist. [Vol. XIV, No. 5, 



One of these classes of nitrogen-absorbing bacteria is aerobic, 

 requiring the presence of air in the soil. These bacteria are called 

 Azotobacter. They require an abundance of lime, phosphoric acid, 

 an optimum condition of moisture, and a soluable form of organic 

 matter, namely, a carbohydrate. 



The other classes of nonsymbiotic bacteria which absorbs 

 free nitrogen, grows in the absence of oxygen, so is an aerobic. 

 These bacteria are called Clostridium pastorianum. They are 

 not as active nitrogen absorbers as the Azotobacter. 



Azotobacter and Clostridium can both absorb nitrogen from 

 other sources than the free nitrogen of the air. That is, if nitrates 

 are abundant in the soil, then these bacteria will talce their re- 

 quired nitrogen from this source. Bacteria contain some proteid 

 material, as do plants, hence nitrogen is needed by them to build 

 up proteid compounds. 



Nature, when undisturbed in her processes, is able to main- 

 tain a sufficient supply of nitrogen in the soil by means of these 

 absorbing bacteria. However, for man's improved cropping 

 methods, the amount of nitrogen added by nature is not adequate. 



Bacteria and the Decomposition of vSoil Humus. 



Soil humus is the decaying re^nains of plants which in their 

 life process lacking in their body substance, large amounts of 

 carbon, combined chiefly with oxygen, hydrogen, and nitrogen. 

 All of these elements have been obtained from the atmosphere. 

 Deposits of peat and beds of coal have likewise been fonned from 

 the atmospheric air. By the burning of peat and coal, carbon- 

 dioxide is restored to the atmosphere. Other means of the restor- 

 ation of carbon dioxide is the respiration of animals, of plants, and 

 the production of carbondioxide by bacterial action in decomposi- 

 tion processes. 



The organic matter in the soil furnishes food for bacteria and 

 the bacteria in turn furnish food for green plants. Humus may 

 be said to contain practically all of the combined nitrogen in the 

 soil. An exception being the nitrogen contained in the bodies of 

 free nitrogen absorbing bacteria. The term humus would include 

 the nitrogen derived from the decay of leguminous plants. 



While chemical changes take place in the process of decay and 

 putrefaction, the process is biological in character. There would 

 be no decay in the absence of bacteria and other micro-organisms. 



Carbon. 



In the form of carbon dioxide, carbon is taken by plants from 

 the air and built into cellulose, starches, and proteins. Some of 

 the carbon is oxidized directly by cells of the plant and returned 

 to the air. Plants die and are returned to the soil or the ]3lant 

 becomes food for animals. Both within the plant and the animal, 



