1 12 



of comparisons between these media, and comparisons of them -with agar 

 alone, with soil, wheal and leaf extracl media, with ammonium nitrate and 

 starch media, both alone and in combination- sin/wed that organisms once 



grown on media will generally frrow when transferred to other media. 



The rate of development seemed more important than the fact that tin- 

 organism grew. Comparisons of growth at end of different periods of in- 

 cubation were usually the same. Where growth was good it developed 

 slowly enough so that it could not lie termed a flash growth. Where growth 

 was poor, distinguishing characteristics peculiar to the organism were rarely 

 apparent. 



The explanation of the tolerance observed is not that those organisms 

 growing when soil is plated on inferior media are probably the same organisms 

 that yield the best colonies on better media. Picking out organisms plated 

 on the best media and growing them on poorer media supports the above 

 statement. Chromogenesis was augmented by the presence of carbohydrate 

 in the media. 



Comment. 



Many expect that soil biology will explain results for which chemical 

 and physical causes have not been found. Many look to the control of plant 

 growth through the application of principles of microbiology. 



Soils with their large or small amounts of decaying organic matter, of 

 both plant and animal origin, must be a possible medium for the growth of 

 all kinds of bacteria. One reason why the number of bacteria in our prairie 

 soils has not been found to vary with the crop-producing power of the soil 

 may be the tolerance of many kinds of bacteria to all present chemical and 

 physical differences between types of prairie soil. In sandy and poor soils 

 some believe that there is a relationship between the number of bacteria 

 and the crop-producing power of the soil. The factors of temperature, 

 aeration and moisture are more constant in the rich soil, and for this reason 

 the changes in soil moisture, the variation in soil temperature, and the 

 movement of soil gases must exert a more marked influence on the presence 

 of and the activities of certain micro-organisms than the food factor does. 



