BACTERIA OF THE SOIL 263 



Clostridium pasteurianum. While this type of fixation may 

 be shown in the laboratory it is not probable that it is of 

 any great economic significance. The amounts of nitrogen 

 fixed are not large and the utilization of carbohydrate as a 

 source of energy must be much greater under anaerobic 

 than under aerobic conditions. Such organisms are, there- 

 fore, far less efficient nitrogen-fixing machines than are the 

 aerobic species such as Azotobacter. 



Nitrogen Fixation by Nonsymbiotic Molds. Considerable 

 study has been devoted to the question of whether or not 

 molds not living in symbiosis with plant roots are capable 

 in the soil of fixing atmospheric nitrogen. There is some 

 diversity of opinion, but on the whole it may be said that it 

 has not been definitely proved that molds do possess this 

 capacity. However, under certain soil conditions, such as 

 in the leaf mold occurring in forests, molds are undoubtedly 

 very important in breaking down organic compounds and 

 it is not impossible that some species may be found capable 

 of fixing atmospheric nitrogen, adding thereby to the fer- 

 tility of forest soils. Such has not been definitely proved. 



THE CYCLE OF CARBON 



The common forms in which carbon is found in nature 

 are as the element, (that is as carbon), as carbon dioxide, 

 and as relatively complex carbon compounds, the so-called 

 organic compounds. The cycle of carbon, therefore, is not 

 as complex as that of nitrogen, and many species of organ- 

 isms are known which can bring about transformations of 

 carbon from one compound to the next. 



Carbon dioxide, as it exists in the atmosphere and in solu- 

 tion in soil water, is taken up by green plants and these, by 

 utilizing the energy from the sun's rays, are capable of 

 forcing this into combination with hydrogen, producing 

 formaldehyde, and then polymerizing this compound into 

 carbohydrates, first into sugars and then into starch. Some 



