BACTERIA OF OLD SOILS 



113 



provide insufficient nitrates for the proper development of plants 

 is not unlikely, and in such cases suitable inoculation with viru- 

 lent strains might invigorate the depleted flora. The effects of 

 drying on the soil organism in the soil are of a scientific and 

 agricultural interest in estabhshing the relative resistance and 

 distinguishing features between groups of organisms involved 

 in the economic processes of anmionification, nitrification, and 

 nitrogen fixation. 



It is interesting to compare the resistant powers of seeds of 

 various plants with those of the soil organisms. McCarthy^' 

 from his own observations and those of Nobbe, concludes that 

 the life of seeds of most commercially grown plants under mer- 

 cantile conditions does not exceed six years, with a limit of ten 

 years for the most resistant. In contrast to the observations of 

 McCarthy, DuveP" has found that seeds of economic plants 

 buried in dry clay soil will not maintain their vitality for periods 

 longer than eleven months. Seeds exist for a comparatively 

 short period in the soil, bacteria remain ahve, as far as known 

 indefinitely. Quite the contrary is observed with respect to 

 direct exposure to the air where seeds remain viable for at least 

 six or seven years, while the life of bacteria is comparatively 

 short, depending upon the material on which they are exposed. 

 This striking contrast between the resistant power of seeds and 

 bacteria brought about by reversing the conditions of exposure 

 is agricultiu-ally important, especially in connection with inocula- 

 tion experiments with Bacillus radicicola. 



It is quite evident from the literature cited that organisms 

 are able to Uve longer in the soil than on other media. In short, 

 the soil exhibits a protective function, perhaps involving one or 

 more factors. As indicated by Hilgard,^i hygroscopic moisture 

 has a protective function for plants in cooUng the soil by evapo- 

 ration, and it is more than likely that the cooUng influence pro- 

 duces a more favorable condition for the bacterial flora of soils 

 exposed to direct rays of the sunomer sun in the arid region. 



" North Carolina Exp. Sta. Bui. 108, pp. 378-379. 

 " Bui. 83. Bureau of PI. Ind. U. S. D. A.. 

 " Soils, p. 199. Macmillan, 1911. 



