Septembee 30, 1904.] 



SCIENCE. 



433 



to be. Surely tests are abundantly worth 

 trying. 



NITROGEN FIXATION BY FREE BACTERIA. 



It was asserted above that little is known 

 of the method by which leguminous plants 

 in connection with root-tubercle bacteria 

 accumulate nitrogen. It remains to be said 

 that some experiments have shown that 

 these same bacteria in artificial cultures 

 can of themselves absorb and combine 

 aerial nitrogen ; that is, the legume bacteria 

 are nitrogen fixers without any aid from 

 leguminous plants. There is no proof, how- 

 ever, that this is done in nature, that these 

 organisms, while in the soil outside of the 

 root tissues, appropriate free nitrogen and 

 build it into compounds which become suit- 

 able for plant food. It is, however, known 

 that some other bacteria, not tubercle form- 

 ers, do accomplish this task, for in un- 

 sterilized soils free from green vegetation 

 an increase in the nitrogen content is 

 found to take place under circumstances 

 which preclude the accession except from 

 the free nitrogen of the air ; while in steril- 

 ized soils of the same kind under the same 

 conditions no increase occurs. Since there 

 is nothing else but bacteria in the unsteril- 

 ized soil to which the phenomenon can be 

 attributed, search has been carefully made 

 for the active agents. After many tests 

 upon numerous soil-inhabiting species, cer- 

 tain kinds of bacteria have been found 

 Avhich undoubtedly have the power of 

 building the aerial nitrogen into combina- 

 tions which subsequently serve as food for 

 higher plants. In this case we must under- 

 stand that the process is accomplished sole- 

 ly by the bacteria, and it is by different 

 species from those forming root tubercles. 

 In fact, several different kinds of bacteria 

 and some molds are now known which thus 

 contribute to soil fertility, but in most 

 cases the quantity of nitrogen fixed is very 

 small. 



Two species, however, have been discov- 

 ered which are apparently very efficient in 

 the process, viz., Clostridium Pasteurianum 

 Parzm. and Bacillus Ellenhachiensis Stutz- 

 er-Hartleb, and an artificially prepared 

 substance called .alinit has been placed on 

 sale in Germany which appears to be a 

 pure culture of the latter. Practical tests 

 with alinit have not upon the whole shown 

 that its application to soils results in any 

 considerable increase in crops, though in 

 some instances favorable reports have been 

 made. The discovery that the organism 

 has under some conditions the effect of 

 reducing nitrates to nitrites makes it seem 

 less certain that the commercial prepara- 

 tion will prove valuable. 



Here we pause. Our story has been a 

 long one, yet only a meager account has 

 been given of some of those soil bacteria 

 which are efficient agents in preparing for, 

 or furnishing, higher vegetation an essen- 

 tial constituent of their food. We have 

 seen that in the most sterile soils certain 

 nitrifying organisms are constantly at work 

 utilizing ammonia washed from the atmos- 

 phere by rains or convex'ting amLmonia salts 

 into nitrites, which are in turn oxidized by 

 other species of bacteria into nitrates, the 

 form suitable for the sustenance of higher 

 vegetation. Again, the root-tubercle bac- 

 teria make a way for leguminous plants 

 to use the free nitrogen of the air and to 

 build up in their own structures compounds 

 which contribute essentially to the fertility 

 of the land when these structures decay. 

 And then certain other soil bacteria are 

 direct agents in the binding of free nitro- 

 gen into complex combinations within their 

 own bodies, which upon their death are 

 fitted by the agency of other bacteria to 

 combine with the mineral constituents of 

 the soil, to the effect that fertility is in- 

 creased, even where the apparently bare 

 earth seems lying idle, wasting in the sun. 



