482 University of California Publications in Agricultural Sciences [Vol. 3 



Bacteriological Relations 



1. The ammonifying power shows rather larger variations from 

 type to type than between the samples of a type. 



2. The nitrogen fixation data do not show characteristic differences 

 for the several types. 



3. Regarding nitrification as a whole there may be a greater 

 divergence between the samples of a type than between types. The 

 relative nitrification of the soil's own nitrogen varies with the type, 

 as does the relative nitrification of the several nitrogenous materials 

 added. 



Pot Cultures in the Greenhouse 



In addition to the effect of the probable error, the impossibility 

 under the conditions herein described of growing the same crops on 

 all the soils, during the same season of the year in the greenhouse, 

 prevents close comparisons between the types, or between the first and 

 second crops on a given soil. The comparison of several samples of a 

 given soil type and the comparisons of various soil types, according 

 to the previously outlined greenhouse methods show that : 



1. Different representatives of a given type are not the same in 

 their ability to produce crops. 



2. The arrangement of the samples of a given type according to 

 their fertility may or may not vary with the special crops used as the 

 indicators. 



3. The types are distinct with respect to their fertility, considering 

 their average production. 



Therefore it is concluded that with regard to the 24 soils of 4 

 types examined, all soils mapped under a given name by the Bureau 

 of Soils method may or may not be closely similar, depending upon 

 the criteria used. The greater number of the criteria show the soils 

 of a type to be not closely similar, and the types to be but litle differ- 

 entiated from each other. 



In connection with the results of the author's study of the soils, 

 there is given an historical sketch of the development of soil classifica- 

 tion and mapping, also a discussion of certain of the methods em- 

 ployed by the Bureau of Soils of the United States Department of 

 Agriculture. It is pointed out that despite its defects, the work of 

 the Bureau of Soils is of value, and is practically the only type of soil 

 classification and mapping possible under the conditions imposed. 



