110* L. T. SHARP 



Nitrification in all but soils Nos. 6 and 9 is positively and 

 permanently stopped. In these two soils there is a feeble trans- 

 formation toward nitric nitrogen, which is also indicated by the 

 nitrite test. This loss of nitrification power is in accordance 

 with observations previously cited and suggests a possibility that 

 plants in arid regions subject to long dry seasons may suffer 

 from a lack of nitrates as well as from insufficient moisture. 



A comparison of the ammonification and nitrification results 

 gives clear evidence — as pointed out by Winogradsky — that 

 there are distinct physiological differences between the groups 

 of organisms involved in the various chemical transformations 

 in the nitrogen cycle. All of the soils experimented with show 

 strong ammonification floras, indicative of the high resistance of 

 some of the ammonifying organisms, while in all but two of the 

 soils the nitrification bacteria proved unable to withstand the 

 long dry period. The comparative resistance to drying distin- 

 guishes qualitatively between the ammonification and nitrifica- 

 tion floras. As in the case of the ammonifying power, there is 

 no relation between numbers and nitrification. 



NITROGEN FIXATION 



The inaccuracy of the solution method has led many soil 

 bacteriologists to abandon its use for the study of soil fertility 

 problems in relation to the soil flora. The insufficient quantities 

 of the soils under investigation compelled me to use the dis- 

 carded mannite solution method to determine their nitrogen 

 fixing power, rather than the more recent and suitable methods. 

 Accordingly a sufficient number of 50 cc. portions of a 2 % solu- 

 tion of mannite in tap water, containing in addition 0.2 gram 

 K2HPO4, 0.2 gram MgS04, 0.02 gram CaCU, and a drop of a 

 10 % Fe2Cl6 solution were distributed and sterilized in 250 cc. 

 Ehrlenmeyer flasks. These sterile solutions were inoculated with 

 five grams of the soils. The cultures were then incubated for 

 two weeks and the total nitrogen determined by a modified^* 



2* Brief mention of the method employed appears in the Jour, of Biol. Chem. 

 10: no. 3, p. 174. 1911. 



