Relation to Factor Limiting Bacterial Activity in Soil. 453 



evident from this that the bacterial content of the T. + Bacteria has main- 

 tained a consistently high level, the numbers never going down to those of 

 the toluened soil, though there is a decided drop between 32 days and 

 337 days. It might, at first sight, be supposed that high bacterial content 

 was due to the absence of protozoa from the T. + Bacteria soil, the bacteria 

 of the soil left after treatment, together with those added, having no 

 preying organisms around them to check their growth. The T. + Bacteria 

 soil does, however, contain protozoa, no doubt the offspring of those which 

 withstood the partial sterilisation treatment, to the extent of about 

 3000 flagellates and 100 amoebae per gramme. Thus there are almost as 

 many flagellates and many more amoebae per gramme of this soil than in 

 the toluened soil. Yet in spite of these numbers of protozoa the bacteria 

 have maintained a much higher level than those in the toluened soil. 



The high bacterial counts obtained during the first 160 days in all the 

 four soils, viz. : T. + Bacteria, T. + Ciliates, T. + Amoebae, and T. + Flagel- 

 lates, together with the decided drop in all cases, are very interesting. At 

 first sight it might be assumed that in the case of the three soils inoculated 

 with protozoa the drop was due to the limiting action of the latter becoming 

 well established. This would appear to be sound reasoning if it were not for 

 the fact that a similar drop occurs in the T. + Bacteria soil, where no 

 protozoa were added. Moreover, the protozoa found at the end of the 

 experiment in the T. + Bacteria and in the toluened soils are quite com- 

 parable, and if we were to assume that the drop in the bacterial content 

 in the T. + Bacteria soil was due to the activity of the protozoa surviving 

 partial sterilisation, we should be confronted with the difficulty that in 

 one soil the surviving protozoa were exerting a limiting action, whilst in 

 the other they were not doing so, though conditions for trophic existence 

 were equally good in each case. The high counts during the first 160 days 

 may probably be explained by the fact that hay-infusion and very large 

 numbers of bacteria were added to the soils in inoculating them and in 

 this way the conditions brought about were very favourable to extreme 

 bacterial activity as compared with the toluened soil, to which only sterile 

 distilled water was added, and which consequently exhibits no exceptionally 

 high bacterial figures. In the same way the fall in the bacterial counts after 

 about 160 days in these soils may, perhaps, be accounted for by assuming 

 that the food supplies added with the hay-infusion became exhausted, and as 

 a result of this the bacteria dropped to somewhere near the level of the 

 numbers present in the toluened soil. 



Fig. 8 represents the curve for the T. + 5 per cent, untreated soil along 

 with those for the toluened and untreated soils. It is evident that after 

 vol. lxxxviii. — b. 2 o 



