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of comparisons between these media, and comparisons of them—with agar 
alone, with soil, wheat and leaf extract media, with ammonium nitrate and 
starch media, both alone and in combination—showed that organisms once 
grown on media will generally grow when transferred to other media. 
The rate of development seemed more important than the fact that the 
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 be 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. 
