3IO MICROBIOLOGY OF SOIL 



seems to be due to ah actual multiplication of the organisms rather than 

 to a mere lifting of the bacteria from lower depths by capillary action. 

 The greatest increase was found to occur during the winter in the slow- 

 growing bacteria and not in those that liquefy gelatin rapidly or in the 

 Actinomyces. Conn tries to account for the phenomenon by assuming 

 the existence of two groups of bacteria, winter and summer bacteria. 

 The latter, he thinks, prevents the former from multiplpng rapidly 

 in warm weather. Hence, the increase in frozen soils is not to be 

 ascribed directly to the low temperature, but to the depressing effect 

 of the cold upon the summer bacteria. Brown found that the soil 

 bacteria diminish during the faU season with the lowering of the 

 temperature, but, when the soil is frozen, an increase in numbers 

 occurs. He also found frozen soils to possess a much greater ammoni- 

 fying, denitrifying and nitrogen- fixing power than non-frozen soils. 

 According to him, the lowering of the freezing-point of the capillary 

 water, due in part to the concentration of salts at the time of freezing, 

 may account for the abnormal bacterial activities. 



Morphological and Physiological Groups {Morphological Groups). — 

 Rod-shaped organisms are numerically the most prominent among soil 

 bacteria. They occur at times to the extent of 80 or 90 per cent of 

 the total number. Spherical organisms usually constitute less than 

 25 per cent of the bacterial flora. Spirilla and sarcinae are present in 

 slight numbers. Conditions may occur, however, when the proportion 

 of spherical organisms is markedly increased. This happens, par- 

 ticularly, when large quantities of composted manure (rich in spherical 

 organisms) is added to the soil. 



Among the rod-shaped species B. mycoides, B. subtilis, B. mesen- 

 tericus, B. tumescens and other members of the subtilis group are quite 

 prominent. Members of the amylobacter group are seldom absent. 

 Members of the proteus group and various fluorescens are always 

 present, while Bad. arogenes and allied species are common inhabitants 

 of the soil. 



{Physiological Groups). — In the decomposition of organic matter in 

 the soil certain important changes in both nitrogenous and non-nitro- 

 genous material are accomplished by definite groups of bacteria. The 

 breaking down of protein substances is accomplished by the forma- 

 tionjof ammonia, nitrites and nitrates. These in turn may be trans- 

 formedjback into more complex amino-compounds, peptonesv and pro- 



