THE BIOLOGICAL CONDITIONS IN THE SOIL 117 



mainly the work of micro-organisms. The curves belong to the type 

 associated with bacterial, rather than purely chemical, change. Soil 

 which has been heated to 125 C. (at which temperature all organisms 

 are killed) shows no increase in ammonia-content after the first small 

 gain. There is no rapid period of gain if enough toluene is left in to 

 inhibit bacterial action, nor if the water supply is insufficient. Further, 

 the numbers of bacteria increase paripassu with the amount of ammonia ; 

 and as addition of ammonium salts to untreated soil led to no such 

 increase as is observed here, it was concluded that the increased pro- 

 duction of ammonia was due to the increased numbers of bacteria. 



The new flora arising after partial sterilisation was found to be more 

 active than the original flora in effecting the decomposition of nitro- 

 genous organic matter, such as peptone, and in hydrolysing urea. But 

 there is no evidence that the individual species surviving the treatment 

 have become more active on the contrary organisms isolated from the 

 partially sterilised soils proved less active than others of the same kind 

 from the untreated soil. Nor can the difference in the rate of ammonia 

 production be attributed to a change in the type of bacterial flora. 

 Examination of the gelatin plates showed that the flora establishing 

 itself in the heated soil is altogether different from that originally pre- 

 sent ; while the flora of the soil treated with toluene did not appear to 

 have altered very much. The curves showing the amount of ammonia 

 produced in the soil treated with toluene and in the heated soil are very 

 much alike, but the bacterial flora of the soils is very different ; the curves 

 for the untreated soil and the soil treated with toluene are fundamen- 

 tally different, whilst the bacterial flora is not Lastly, reintroduction 

 of bacteria from the untreated soil to the partially sterilised soil led to 

 still further increases both in bacterial numbers and in the rate of de- 

 composition, whilst addition of bacteria from partially sterilised to un- 

 treated soils had little or no effect. 



The experiments, therefore, indicate that the increased production 

 of ammonia in the partially sterilised soil is due to the increased num- 

 bers of the bacteria rather than to any other cause ; the problem reduced 

 itself to finding out why the bacteria can increase so much more 

 rapidly in the partially sterilised than in the untreated soils. No 

 evidence could be obtained that partial sterilisation produced any 

 sufficient increase in bacterial food to account for the results, it appears 

 rather that the untreated soil contained some factor detrimental to bac- 

 teria and put out of action by heat or antiseptics. All attempts to 

 find a soluble toxin in the untreated soil failed. The factor appeared 

 to be biological and could be reintroduced into the partially sterilised soil 



