18 University of California Publications in Agricultural Sciences [Vol. 1 



displayed by the ammouifying bacteria toward hardpan, alkali, 

 and a lack of humus and moisture, hold in a more exaggerated 

 way as regards the nitrifying organisms. jNIore specifically, the 

 writer has also shown^ the distinct effects of each of the alkali 

 salts on nitrifjnng bacteria in work quite recently completed. It 

 would appear in general, however, that in our deep soils, a supply 

 of nitrate as well as of ammonia is at the disposal of plants for 

 a depth of five or six feet. As regards the nitrogen-fixing powers 

 of soils of the arid region, my results show plainly that they do 

 not differ strikingly from those of soils in the humid regions, 

 if the presence and vigor of Azotobacter organisms be taken as 

 a criterion. While it is true that in one or two cases Azotobacter 

 organisms were found in our soil-columns below the depth at 

 which they occur elsewhere, and perhaps at a slightly greater 

 depth in all soils in which they were found, I feel loath to believe 

 that these are expressions of a rule for soils of the arid region. 

 Other observations indeed lead me to believe that Azotobacter 

 development has not gone so far in our soils as it has in soils 

 of other regions. For example, I have studied many soils in Cali- 

 fornia with a favorable physical and chemical constitution which 

 were absolutely devoid of Azotobacter organisms. If therefore 

 the results set forth above, with reference to nitrogen fixation, 

 are to be considered representative, the nitrogen supply in the 

 lower layers of the soil must be replenished in this region as 

 well as in the humid region, not from direct fixation by Azoto- 

 bacter, but from the nitrogen of the upper soil layers. 



With reference to these investigations in general, one or two 

 additional points need more than passing consideration. First, 

 as to the method of collecting the soil-samples for examination, 

 it appears to the writer that every possible precaution was used 

 to prevent contamination and it would be difficult to devise a 

 method which takes into consideration and avoids more of the 

 avenues of contamination by which anj^ results might be vitiated. 

 Moreover, I find strong confirmation of this belief in the facts 

 brought out in the data above given, viz., that any abnormality 

 in the soil was sure to be reflected in the results obtained with 

 cultures prepared from that abnormal soil. Thus hardpan layers 



s Cent, fur Bakt., 2 Abt., vol. 33, p. 305. 



