276 BIOCHEMICAL PROCESSES OF THE SOIL 



the two quantities is normally so close as to indicate a causal 

 relationship ; the increased ammonia production is, therefore, 

 attributed to the increased numbers of bacteria. There is no 

 disappearance of nitrate ; the ammonia is formed from organic 

 nitrogen compounds. 



3. The increase in bacterial numbers is the result of 

 improvement in the soil as a medium for bacterial growth 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 reintroduced 

 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, or an unfiltered aqueous 

 extract of untreated soil, soon contains higher bacterial numbers 

 per gram 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. The improvement, therefore, 

 did not consist in the removal of the products of bacterial 

 activity, because there is much more activity in partially 

 sterilised 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 supplied 

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

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

 introduced from untreated soils. (Table XCVI.). 



6. Evidence of the action of the limiting factor in untreated 



