240 



MODIFICATION OF THE SOIL POPULATION 



TABLE 56 



Determination of Potash Requirements of Soils Using Growth of 

 THE Fungus Aspergillus niger as the Index (from Niklas) 



Summary. — The invisible soil inhabitants are the victims of the 

 circumstances in the regions where they are located. They have 

 no means of migrating long distances except by occasional for- 

 tuitous transportation by wind, water, or similar agencies. They 

 must be able to sustain themselves from the food materials in their 

 vicinity or else die. They must be able to tolerate sudden changes 

 in the environmental conditions, since they have little or no means 

 of regulating the conditions under which they Hve. They must be 

 able to tolerate high and low temperatures, periods of deficient and 

 excess moisture, change in reaction, and a variety of other factors 

 related to their capacity to absorb and utilize the nutrients con- 

 tained in the soil. 



Organisms cannot all tolerate the same conditions, and conse- 

 quently there is a selective change following any modification in 

 their habitat. If the conditions are not too severe, numerous 

 organisms may persist even though they are relatively inactive. 

 With a change to more favorable conditions they often quickly 

 respond with renewed growth and activity. These relationships 

 are responsible for the occurrence of a greater variety of organisms 



