Bacteria at Different Depths 143 



numbers of bacteria are present, not immediately at the 

 surface, but at a slight distance below it. It was also 

 shown that the number diminishes rapidly as the dis- 

 tance from the surface is increased. 



The smaller number of bacteria at the surface, as com- 

 pared with that four or five inches below, is due, largely, 

 to less favorable conditions in regard to moisture and 

 organic matter. Beyond the zone of root-development, 

 the decrease of humus naturally involves a rapid de- 

 cline in the number of bacteria. Moreover, the air is 

 not renewed as readily at greater depths from the sur- 

 face, and the aerobic organisms find conditions there 

 unfavorable for survival. There is a tendency, of course, 

 for the bacteria to be carried to the deeper soil layers by 

 the rain-water percolating downward. Yet this tendency 

 is checked by the filtering action of the soil, the organ- 

 isms being held back in the fine pores of the latter. 

 It is evident, likewise, that in the more open sandy 

 soils the bacteria are not filtered out as rapidly, and are, 

 therefore, scattered through a greater depth. 



