14() HATCH EXPERIMENT STATION. [Jan. 



A Comparison of the Numbers of Bacteria in Sterilized 



AND UnSTERILIZED SoILS. 

 BY A. VINCENT OSMUN. 



There has been repeated inquiry as to the effect steriliza- 

 tion of soil has upon its bacterial flora. In the absence of 

 any reliable experiments touching upon this question, it has 

 frequently been assumed that by soil sterilization the l)ac- 

 teria, and especially the Ijeneficial ones, are destroyed, hence 

 injuring the soil. In regard to this question, it may be 

 stated that little is known about either the so-called bene- 

 ficial or injurious bacteria of greenhouse soils; and, in con- 

 sequence of a paucity of knowledge upon this phase of the 

 subject, positive statements are out of place. From what is 

 alread}^ known about the effects of soil sterilization upon 

 plant growth and the changes which such soil undergoes, it 

 might be assumed upon a priori grounds that soil bacteria 

 would be found to exist more abundantly in sterilized soil 

 than in unsterilized soil. Moreover, it must be borne in 

 mind that absolute sterilization is never accomplished, but 

 something more closely approaching pasteurization takes 

 place. All bacteria are by no means killed, as has been 

 shown by various tests made at this station. ^ Subjecting 

 soils to steam heat has a marked stimulating eifect on the 

 growth of plants. Observations at this station and experi- 

 ence of growers have shown this to be true. Just why 

 this treatment of soil should stimulate the growth of plants 

 is not known. Sorauer^ suggests that steam heat makes the 

 humus compounds more available to })lants. It is not un- 

 likely that steam flowing through a soil also improves its 

 mechanical condition. 



The stimulating effect of soil sterilization on })lant growth 

 suggested a similar effect on the bacterical content of .soil so 

 treated ; and in order to obtain more evidence regarding 

 this ])oint this experiment was planned : — 



Two boxes about 20 inches -scjuare and !) inches deep and 

 of equal weights were filled to the dc})th of about 5 inches 



' Hatch Exporimnnt Station, Massachusetts Report, 11)02, pp. 77, 78. 

 "^ Sorauer, Tlie Physiology of Plants, pp. 45, 46. 



