Bacteria in Different Soils 189 



food for the growing crop. On the other hand, the colder 

 months of early spring do not provide the best con- 

 ditions for the rapid transformation of plant-food in- the 

 soil. 



There is a certain relation between the character of 

 the soil and the numbers and kinds of bacteria growing 

 in it. Sandy and sandy loam soils allow the air to pene- 

 trate them rather freely. Thus the development of the 

 so-called aerobic species, that is, the kinds of bacteria 

 that will not grow when the supply of air is excluded or 

 limited, is favored. Heavy clay soils and clay loams do 

 not permit the air to circulate freely in them. For this 

 reason the aerobic bacteria do not find in them the best 

 conditions for their development. However, the anaero- 

 bic species, that is, the kinds that grow by preference 

 when the supply of air is cut off or limited, are favored in 

 their growth. It happens thus that the proportion of 

 the aerobic and anaerobic species is not the same in the 

 two classes of soil. 



The altitude of the field is of some importance in 

 determining the numbers and kinds of its bacteria. 

 The distance above sea-level affects the pressure and, 

 therefore, the circulation of the air in the soil. The 

 bacteria in the soil may also be affected to some extent 

 by the character of the prevailing winds and the expos- 

 ure of the land. Mountain slopes and hill-sides turned 

 to the south offer different conditions for the growth of 

 soil bacteria than those offered by northern exposures. 



There exists unquestionably an important relation 

 between the crop on the soil and the numbers and kinds 

 of bacteria within it. For one thing, the leafy crops that 



