SOIL ORGANISMS 235 



that they make more available many substances already in the soil but 

 difficult of assimilation by higher plants. 



1. NITROGEN FIXATION 



For the synthesis of proteins the plant requires nitrogenous com- 

 pounds. To a small extent these are supplied in inorganic form from 

 the atmosphere. During thunderstorms considerable quantities of 

 oxygen combine with the nitrogen of the air. Thus by the aid of 

 atmospheric moisture nitrous and nitric acids are formed, which reach 

 the soil in rain. Nitrogen in the form of nitric acid, HNO3, is easily 

 assimilated by plants, while free nitrogen is absolutely useless to most 

 plants. In addition to this inorganic source of nitrogen, vegetation has 

 another, not less important, organic one in the subterranean associa- 

 tions of nitrogen-fixing bacteria, fungi, and algae. 



The discovery of the anaerobic schizomycete, Clostridium pastoria- 

 7ium, by Winogradsky, has furnished us the key to the scientific 

 explanation of the long-recognized fixation of atmospheric nitrogen by 

 plants. With adequate aeration this schizomycete fixes 0.0025 to 

 0.003 g. of atmospheric nitrogen for each gram of glucose consumed. A 

 great increase in the nitrogen content of the soil is followed by the 

 cessation or retardation of the nitrogen-fixing activity of this 

 bacterium. 



The investigations by Truffaut and Bezssonoff (1925) make it seem 

 probable that Clostridium pastorianum, which fixes atmospheric 

 nitrogen abundantly in soil free from nitrogen and carbon, is able to 

 draw its food from the root secretions of the higher plants (these experi- 

 ments refer to Zea mais). 



Two species of Clostridium (C. americanum and C. aerohicum) have 

 been isolated by Pringsheim as nitrogen fixers. C. aerohicum, as the 

 name indicates, functions in the presence of air. Equally aerobic are 

 some of the later-discovered bacteria, especially of the genus Azoto- 

 hader (A. choococcum, A. beijerinckii, A. vinelandii, A. agilis, and A. 

 vitreum). A. choococcum, a notable nitrogen collector, is distributed 

 in most soils. Out of 562 soil samples from Bavaria it was found in 

 341 (61 per cent). It was absent in soils of high H ion concentration 

 (below pH 5.6) (Niklas, 1925). It thrives best in lime soils at an 

 optimum temperature of about 28°C., while sodium chloride and 

 sulphates are destructive. The former acts toxically even in doses of 

 0.5 per cent; it can endure sodium sulphate up to 1.25 per cent 

 (Andr6, 1921). 



Anaerobic and aerobic nitrogen fixers often occur associated 

 together in the same soil, and with them certain fungi {Aspergillus, 



