272 MICROBIOLOGY OF SOIL. 



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

 are widely distributed in cultivated soils of Europe and America, and prob- 

 ably 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 con- 

 taining potassium phosphate, magnesium sulphate, calcium chloride 

 and ferric sulphate, and some organic nutrient, such as dextrose, sac- 

 charose, xylose, mannit, acetate, propionate, 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, Azoto- 

 bacter 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. 



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 necessarily limited 

 by the supply of such potential energy. This limitation was already 

 recognized by Winogradski in his experiments with B. (Clostridium] 

 pasteuriamis. 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 "Clostridhtm 

 americanum" the amounts fixed were, at times, considerably larger. On 

 the whole, however, it has been proved by a number of investigators that 

 A zotobacter can fix much larger quantities of nitrogen than the anaerobic 

 bacilli. The extended investigations of Lipman showed that A. vine- 

 landii has the ability to fix more nitrogen per unit of organic nutrient 

 consumed than either A. chroococcum or A. beyerincki. Under favorable 

 conditions A. mnelandii may at times fix 15 or even 20 mg. of nitrogen 

 per gram of mannit used up. Krzemieniewski found in experiments 

 with A. chroococcum that additions of humates to the culture solutions 



