ORGANIC SOIL CONSTITUENTS 259 



proteinaceous material of their bodies. It is certain that the benefit 

 derived by Azotobacter from the sulphates of iron and calcium is due 

 in a large measure to the sulphur which these compounds supply. No 

 evidence has as yet been produced which would lead us to believe 

 that the organisms can use sulphur as a source of energy. 



Calcium carbonate and calcium oxid, in addition to furnishing a 

 base which neutralizes the acid formed in the metabolic processes 

 of the Azotobacter, also furnish calcium to the organism. Christensen 

 brought out the fact that Azotobacter can derive their calcium from 

 dibasic calcium phosphate and some calcium salts of organic acids. 

 They could not, however, utilize the calcium of tribasic phosphate, 

 of calcium chlorid or sulphate. 



Iron is essential and either the ferric or ferrous sulphate is especially 

 beneficial. Rosing found the amount of nitrogen fixed increased 

 from 2.23 mgms. to 10.3 mgms. per gram of mannite when iron 

 sulphate was added to the cultural media. This is due, in a great 

 degree, to the iron which serves as food for the organisms, yet its 

 colloidal nature may play a part, for both organic and inorganic 

 colloidal substances have an especially favorable action on Azoto- 

 bacter, although the action of the inorganic colloids is fully manifest 

 only in the presence of organic colloids. If used alone, large quan- 

 tities of the ferric hydroxid are essential for the maximum effect, 

 but in the presence of organic colloids very small quantities of iron 

 are effective. This has been attributed to the action of the colloidal 

 iron which adsorbs the nitrogen and oxygen of the air and brings 

 them into more intimate contact with the Azotobacter. This would 

 not only accelerate the normal processes of the aerobic Azotobacter 

 by furnishing theni with nitrogen and oxygen, but it w^ould tend to 

 suppress the anaerobic processes which are extremely wasteful of 

 the food. According to Kaserer, these organisms also require 

 aluminum. Although this may accelerate, it has not been proved 

 to be essential to their growth. 



While not essential to the organisms, manganese is an extremely 

 active catalyzer in increasing proportions up to 6 mgm. per 100 c.c. 

 of media. Above this concentration the reaction falls off rapidly, 

 and at 20 mgm. it is less than in the absence of manganese. It is 

 oxidized by Azotobacter, and in the proportion of 1 part to 200,000 

 parts of soil it is an active stimulant. Olaru considers it likely that 

 the increased yield obtained after the application of manganese 

 compounds to a soil is due to its accelerating the action of the 

 nitrogen-fixing organisms of ^he soil. 



Organic Soil Constituents. Reed found urea, glycocol, formamid, 

 and allantoin active in depressing nitrogen-fixation* This he 

 attributes to the compounds furnishing the Azotobacter an available 

 source of combined nitrogen and not to a direct toxicity* But 

 Walton found that the addition of urea, peptone, acetamid, aspar- 



