April. 1911. 



KNOWLEDGE. 



125 



results, and the assumption that increased fertiHt\' 

 was due to a bacterial rather than a chemical cause, 

 appeared justified. Such being the case, a closer 

 study of the bacterial flora of the various soils was 

 undertaken and an attempt 

 was made to account for the 

 enormously increased num- 

 bers. Definite quantities of 

 the soils were introduced into 

 solutions containing peptone, 

 urea. ha\-dust. and so on, and 

 the amount of ammonia pro- 

 duced after certain intervals 

 was estimated. In almost ever\- 

 case it was found that the 

 cultures containing toluened 

 soil were more capable of 

 decomposing the substances 

 supplied. This result could 

 be induced ui i bv a greater 

 virulence of the toluened soil 

 bacteria, or (hj the presence 

 of some inhibiting factor in 

 the untreated soil. Experi- 

 ments where toluened soil 

 was inoculated with bacteria 

 from untreated soil gave, 

 however, a more vigorous 

 decomposition, and a higher bacterial content than 

 a toluened soil alone, thus showing the superioritv 

 of normal soil bacteria over those persisting after 



Figure 2. 

 Gelatine Plates showing relative numbers of Bacteria 



in Untreated. Toluened. and Heated Soils. 

 Bottom. Left-Hand: Untreated Soil. Top: Heated 



Soil. Bottom, Right-Hand : Toluened Soil. 



When, for instance, toluened soil was mixed with 

 five per cent, of untreated soil there ensued an initial 

 increase in bacterial growth and arn:"c -lia production, 

 but these changes were retarded ir. :h; course of a 

 few weeks. (Sji Table C.) 



It is apparent from the 

 above, that, while consider- 

 able bacterial growth has pro- 

 ceeded in three out of the 

 five soils, it has remained 

 stationary in the untreated 

 soil and toluened soil with 

 five per cent, untreated soil. 

 This condition, however, onlv 

 begins to be evident some 

 weeks after the beginning 

 of the experiment. In fact, 

 during the period up to the 

 thirtv-eighth day. the toluened 

 soil receiving untreated soil 

 possessed the highest bacterial 

 content. In brief, the limiting 

 factor appeared to be of a 

 biological nature, possessing 

 the following properties : — 

 {a) destroyed by heat and 

 volatile antiseptics, (b) in- 

 capable of passing through 

 filter paper, (c) contained in small quantities of soil. 

 ((/) cajiable of growth, and of reducing the numbers 

 of bacteria. It appeared to exert a limiting effect either 



T.\BLE C. 



treatment with the antiseptic. This point having 

 been cleared, it became necessarv to search for 

 a limiting factor in normal soils. It was not 

 possible to demonstrate the presence of injurious 

 bacteria nor that of any toxic body. On the contrary, 

 an extract from untreated soil, that is, liable to 

 contain any soluble toxin or toxic organisms, 

 produced a vigorous decomposition, but similar 

 experiments where toluened soil was inoculated 

 with small quantities of untreated soil, showed the 

 limiting factor to be transmissible in soil itself. 



by competition for nitrogenous foodstuffs, or by des- 

 tro\ing the bacteria in the soil. .\ search was then 

 made for organisms larger than bacteria, and. by the aid 

 of special media, protozoa were found in the untreated, 

 but not in the treated soils. The influence of soil pro- 

 tozoa upon bacterial change was demonstrated by 

 adding cultures of these organisms to flasks containing 

 extracts of untreated and toluened soils and peptone 

 solution, and estimating the amount of ammonia pro- 

 duced after a given period. In this manner we were 

 able to depress the acti\it\' of the toluened soil bacteria 



