404 MICROBIOLOGY OF SOIL 



(Fig. 132), A . beyerincki and A . woodstownii, and Lohnis and Westermann, 

 A. vitreum. Of these species A. chroococcum and A. beyerincki are most 

 common and are widely distributed in cultivated soils of Europe and 

 America, and probably also of the other continents. They are absent 

 in acid soils deficient in humus, and most common in limestone regions 

 and in irrigated soils rich in mineral salts. Their food requirements are 

 covered by solutions containing potassium phosphate, magnesium 

 sulphate, calcium chloride and ferric sulphate, and some organic 

 nutrient, such as dextrose, saccharose, xylose, mannit, acetate, pro- 

 pionate, butyrate, malate, ethyl alcohol, etc. An alkaline or neutral 

 reaction and the presence of salts of iron are essential for 'the vigorous 

 development of Azotobacter, while humates have been shown by 

 Krzemieniewski to exert a stimulating influence on the growth of these 

 organisms, even though not acting directly as a source of food and 

 energy. As shown by Lipman and others, Azotobacter may gain an 

 increased power of fixing atmospheric nitrogen in the presence of other 

 organisms. It is resistant to drying, notwithstanding the fact that it 

 produces no spores, and has been successfully isolated from soil samples 

 that had been kept in a dry state for several years. For some reason 

 it may be detected in the soil most readily in the fall and winter 

 months. 



As to the nitrogen-fixation by fungi, it has been shown elsewhere 

 that the evidence is, if anything, of a negative character. Some 

 algae are able to fix atmospheric nitrogen, especially those that live 

 symbiotically with azotobacter. 



ENERGY RELATIONS. In the fixation of nitrogen by bacteria the 

 necessary energy for the process is furnished by the carbohydrates, 

 organic acids, alcohols or other organic nutrients employed in the 

 culture media. Since any given quantity of organic nutrient possesses 

 a definite amount of potential energy the fixation of nitrogen is neces- 

 sarily limited by the supply of such potential energy. This limitation 

 was already recognized by Winogradski in his experiments with B. 

 (Clostridium) pasteurianus. For every gram of dextrose used up there 

 was produced, on the average, 2 to 3 mg. of combined nitrogen. In the 

 experiments of Bredeman with B. amylobacter, and of Pringsheim with 

 "Clostridium americanum r the amounts fixed were, at times, con- 

 siderably larger. On the whole, however, it has been proved by a 

 number of investigators that Azotobacter can fix much larger quantities 



