220 



Bacteriological Department. 



[Bull. 161 



5A, stirred four, six and eight inches deep, were 63.86, 53.66 

 and 58.11 million, respectively. In the plot which was stirred 

 ten inches deep the number was slightly less than in the plot 

 which was stirred four inches deep. The plots which were 

 plowed to a depth of two, four, six, eight and ten inches showed 

 an increase in the number of bacteria of 0.71, 16.58, 6.38, 10.83 

 and 2.42 millions per cubic centimeter, respectively. 



Upon examination of table XIV it will be found that in the 

 silt soil the increase in the number of bacteria, due to deeper 

 plowing, is not so rapid as in the sandy soil, but much more 

 uniform. Commencing with plot 2B, the number of bacteria 

 shows a gradual increase from 1.20 million in the two-inch 

 plowing to 9.84 million in the ten-inch plowing. 



The percentage of increase of bacterial content is shown in 

 table XV. 



TABLE XV. 



From the results in table XV it appears that extra deep 

 cultivation tends to greater increase in numbers of bacteria 

 upon the tenacious silt soil than upon the more porous sandy 

 soils. This effect in favor of the silt soil is perhaps due 

 largely to the^increased aeration. At the same time it is well 

 to observe that the silt soil had previously been plowed about 

 six inches deep, while the sandy soil had been plowed on an 

 average of four inches deep. Hence, in stirring the silt soil 

 ten inches deep, four inches of new soil were exposed, while 

 with the sandy soils that were stirred the same depth six 

 inches of new soil were brought into action. This deep plow- 

 ing would tend to place the surface soil, high in humus con- 

 tent, at a depth that is less favorable for highest bacterial de- 

 velopment and at the same time place the previously unstirred 



