FIXATION OF ATMOSPHERIC NITROGEN. 273 



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 investigators 

 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 one pound of nitrogen could be fixed for 

 every one hundred pounds of carbohydrate consumed, it would still take 

 an equivalent of 680 kg. to 1,814 kg. (1500 to 4000 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 sym- 

 biosis with algae, obtaining thereby the necessary energy for their activi- 

 ties. 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 90718 kg. 

 (100 tons), or more (per acre), to a depth of a few inches. If the accumu- 

 lation 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 qual- 

 ities of certain crops of the legume family. Columella mentions the fact 

 that many Roman farmers regarded beans as possessing these qualities, 

 but does not accept this belief for himself. On the other hand, he points 

 out that luzerne (alfalfa), lupins and vetches improve the land and act as 

 manure. He points out, also, that it was the practice of Roman farmers 

 to plow under lupines in order to enrich the soil. In the centuries follow- 

 18 



