1912] * Lipman : Bacteria in Soils of Arid Eegions 17 



alkali, and a lack of humus and moisture decrease the ammoni- 

 fying powers of our soils or are not favorable to the develop- 

 ment of vigorous ammonifying flora, but their effects are just as 

 pronounced in the upper laj^ers of these abnormal soils as in 

 the lower layers which, therefore, cannot be fairly compared 

 with our deep average soils as to bacterial content. To what a 

 serious extent alkali salts may affect ammonification has been 

 shown by me in a recent paper.^ That the humus content alone 

 may profoundly affect the number and vigor of bacteria is well 

 exemplified in .both soils No. 6 and 10, where all other condi- 

 tions but the humus content are favorable and where both the 

 number and physiological efficiency of the organisms is small. 



It would therefore seem, in brief, that ammonification is 

 vigorously active in the lower soil layers in soils of the arid region 

 where humus is present and hardpan and alkali are absent. 

 Since these conditions are complied with in the average of our 

 cropped soils, the opinion is justified that the deep penetration 

 of bacteria is a distinctive characteristic of soils in arid regions 

 which results from much better aeration, as a starting point, 

 than can be attained in soils of the humid region. The experi- 

 mental data above given amply confirm this opinion and help 

 to explain why deep plowing is not only harmless in our soils 

 but directly beneficial, and why three or four feet of upper soil 

 may be removed in grading, and alfalfa and fruit trees may be 

 grown on the newly uncovered subsoil without difficulty, a feat 

 which cannot be accomplished on soils of the humid regions. 



As for nitrification my data present again features of striking 

 interest. They go to prove that nitrate formation, like ammoni- 

 fication, goes on at much greater depths in soils of arid than in 

 soils of the humid region, and thus render distinctly sectional the 

 observations of Dyer* on this subject, and makes them applicable 

 only to soils of the humid region. While the nitrifying organ- 

 isms are doubtless more susceptible to a lack of oxygen than the 

 ammonifying bacteria, the differences obtained above between 

 the two groups of organisms, so far as soil fertility is concerned, 

 are rather those of degree than of kind. The same relationships 



* Bulletin 106, p. 55, O. E. S., U. S. D. A. 

 3 Centrallblatt fiir Bakt., 2 Abt., vol. 32, p. 58. 



