FIXATION OF ATMOSPHERIC NITROGEN 405 



of nitrogen than the anaerobic bacilli. The extended investigations 

 of Lipman showed that A . mnelandii 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 g. of mannit used up. Krze- 

 mieniewski found in experiments with A. chroococcum that additions 

 of humates to the culture solutions increased the nitrogen fixed from a 

 maximum of 2.4 mg. to a maximum of 14.9 mg. 



The practical bearing of the foregoing data lies in the fact that the 

 fixation of nitrogen in cultivated soils is limited, among other things, by 

 the energy available, that is, by the quantity of readily decomposable 

 organic residues. An indication as to the extent of these is given by the 

 amount of humus present; nevertheless, this must remain an indication 

 merely, for most of the humus is too inert to serve as a source of energy 

 to Azotobacter. From the data at present available different investi- 

 gators have estimated the quantity of nitrogen fixed by Azotobacter 

 at 6.8 kg. to 18 kg. (15 to 40 pounds) per acre, per annum. Assuming 

 favorable conditions for fixation, so that 500 g. (i pound) of nitrogen 

 could be fixed for every 50 kg.(ioo pounds) of carbohydrate consumed, 

 it would still take an equivalent of 680 kg. to 1,814 kg. (1,500 to 4,000 

 pounds) of sugar to produce this quantity of combined nitrogen. It may 

 be noted in this connection that Azotobacter have been demonstrated 

 to live in symbiosis with algae, obtaining thereby the necessary energy 

 for their activities. This may explain, perhaps, the remarkable facts 

 observed by Headden in Colorado, relating to the accumulation of such 

 enormous quantities of nitrate in the soil as to destroy all vegetation. 

 In some instances the nitrates were found to be present to the extent of 

 90,718 kg. (100 tons), or more (per acre), to a depth of a few inches. If 

 the accumulation of combined nitrogen was due to Azotobacter, as is 

 claimed by Headden, and the bacterial residues oxidized by nitrifying 

 bacteria to nitrates, it is difficult to account for the source of the 1,000 

 or 2,000 tons of carbohydrates necessarily used up in the process of 

 fixation, unless it could be proved that the energy was furnished by 

 algae. 



SYMBIOTIC FIXATION 



HISTORICAL. Empirical observations extending well back into 

 ancient agriculture have led to the recognition of the soil-enriching 



