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 take their re¬ 
quired nitrogen from this source. Bacteria contain seme 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 Soil Humus. 
Soil humus is the decaying remains 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 formed 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 plant 
becomes food for animals. Both within the plant and the animal, 
