THE MICRO-ORGANIC POPULATION OF THE SOIL 285 



and not an improvement in the bacterial flora. Indeed, the 

 new flora per se is less able to attain high numbers than the 

 old. This is shown by the fact that the old flora when re- 

 introduced into partially sterilised soil attains higher numbers 

 and effects more decomposition than the new flora. Partially 

 sterilised soil plus 0-5 per cent, of untreated soil soon contains 

 higher bacterial numbers per grm. and accumulates ammonia 

 at a faster rate than partially sterilised soil alone. 



(4) The improvement in the soil brought about by partial 

 sterilisation is permanent, the high bacterial numbers being 

 kept up even for 200 days or more. 1 The improvement, there- 

 fore, did not consist in the removal of the products of bacterial 

 activity, because there is much more activity in partially steri- 

 lised soil than in untreated soil. Further evidence is afforded 

 by the fact that a second treatment of the soil some months 

 after the first produces little or no effect. 



It is evident from (3) and (4) that the factor limiting 

 bacterial numbers in ordinary soils is not bacterial, nor is it 

 any product of bacterial activity, nor does it arise spontaneously 

 in soils. 



(5) But if some of the untreated soil is introduced into 

 partially sterilised soil, the bacterial numbers, after the initial rise 

 (see (3)), begin to fall. The effect is rather variable, but is 

 usually most marked in moist soils that have been well sup- 

 plied with organic manures ; e.g. in dunged soils, greenhouse 

 soils, sewage farm soils, etc. Thus the limiting factor can be 

 reintroduced from untreated soils. 



This is controverted by J. M. Sherman at Wisconsin (261), 

 who failed to find any evidence that the introduction of un- 

 treated soil brought down the bacterial numbers in partially 

 sterilised soil. 



(6) Evidence of the action of the limiting factor in untreated 

 soils is obtained by studying the effect of temperature on 



1 Subsequent work indicates that a rapid and considerable rise followed by a 

 fall takes place in the first few days after treatment with certain antiseptics. 

 This is attributed to the utilisation by certain organisms of the trace of anti- 

 septic left in the soil ; it is regarded by the writer as distinct from the sustained 

 but lower rise referred to here. 



