120 BACTERIOLOGY OF THE SOIL 







year, but careful observations have shown that this is 

 more due to alterations in the degree of moisture of the 

 soil than to temperature changes. The experiments of 

 Engberding and others indicate that the optimum 

 development of the majority of organisms occurs when 

 the soils contain 70 to 80 per cent, of their maximum 

 water-holding capacity. 



Dry soils contain few bacteria, and dryness has been 

 shown by Lohnis to have an injurious effect on nitrifica- 

 tion and nitrogen assimilation. 



Acid peaty soils and marsh land are poor in bacteria. 

 After drainage the number increases only slightly, but by 

 cultivation and the application of lime, sand, clay, and 

 dung the bacterial content soon approximates to that of 

 ordinary cultivated soils. 



. The following figures of the change in the number of 

 bacteria after draining and cultivation of " ling, cotton 

 grass, and sphagnum moor " in Sweden are given by 

 Fabricius and von Feilitzen : 



Farmyard dung and other organic manures contain 

 enormous numbers of bacteria and large amounts of 

 compounds upon which bacteria thrive. After these 

 are applied to the soil the numbers, especially of putre- 

 factive and denitrifying organisms, rapidly rises for a 

 time. Heavily manured soils, or those to which sewage 

 has access, may contain from 50 to 120 millions per 

 gram. Later, however, the number falls to the original 



