132 AGRICULTURAL BACTERIOLOGY. 



bound to increase as the soil needs them more and more, and 

 as the supply diminishes. Clearly enough the supplying of 

 the lost nitrogen will become more and more expensive as the 

 great nitrogen stores are used up. The seriousness of this 

 problem of a constant draining of nitrogen from the soil has 

 been quite prominent in the minds of chemists and agriculturists 

 as they have, in the last few years, learned the significance of 

 nitrogen for agriculture. 



The continuation of agriculture depends upon the existence 

 of some means of reclaiming the nitrogen out of the atmosphere 

 for the use of plants. If there is no such means it is evident 

 that the nitrogen store of the soil will be used up and vegeta- 

 tion will eventually, and, in highly cultivated lands, speedily 

 die of nitrogen starvation. If, on the other hand, there is some 

 means of reclaiming such lost nitrogen there is no need of 

 nitrogen starvation, since there is an absolutely unlimited store 

 of this element in the form of the free nitrogen of the air. It 

 is quite evident that there is some means within the reach of 

 organic nature for making use of this atmospheric nitrogen. 

 Vegetation has continued on the earth for an unknown number 

 of centuries without any apparent diminution of the nitrogen 

 supply. This would not have been possible unless there were 

 some means by which the soil could obtain from the air a stock 

 of nitrogen to replace that which has been lost by the proc- 

 esses already indicated. 



It is well known that plants obtain their carbon directly from 

 the carbon dioxide of the air, and that their water is also ob- 

 tained from the rain. These facts would naturally lead us to 

 believe that they could also obtain nitrogen from the atmos- 

 pheric store. But experiments have shown that this, under 

 ordinary circumstances, does not occur. Most careful and 

 thorough experiments have shown that, when plants are grow- 

 ing under ordinary conditions, unassisted by secondary aids 

 (which will be noted presently), and guarded from any possible 



