MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 369 



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 multiplying 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 fall 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. 



Vass recently pointed out that the apparent increase of bacteria in 

 frozen soils is due to the breaking up of the clumps of cells rather than 

 to growth and multiplication. The bacterial activities are influenced 

 by freezing only in so far as it affects the physical properties of the soil. 



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. 



Conn has shown that non-spore-forming bacteria (mostly immotile 

 rods) are the most abundant of all soil microorganisms. Next to them 

 in abundance are the various types of Actinomyces, referred to elsewhere 

 in this book. Spore-forming bacteria are also quite common, but are 

 apparently of no great importance in normal soil. Among the most 

 prominent soil bacteria are non-spore-forming, slowly liquefying or 

 non-liquefying, short rods; rapidly liquefying, non-spore-forming, short 

 rods with polar flagella represented by Ps. fluorescens; spore formers, 

 which seem to come from spores instead of from active organisms. A 

 few micrococci and members of the B. radicicola group have been 

 demonstrated. 



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