136 Partial Sterilisation of Soil 



The toxic substance slowly disappears from the soil and ultimately 

 nitrification once more becomes possible (cf. also § 45). 



§ 32. Denitrification. Organisms decomposing or assimilating 

 nitrates seem to be little influenced by toluene, but they are adversely 

 affected, though not killed, by heat. The nitrate completely dis- 

 appeared in 5 days from 50 c.c. of Giltay's culture solution inoculated 

 with 5 grams of untreated or toluened soil and maintained at a tempera- 

 ture of 30°, but it persisted for 20 or 30 days when inoculated with 

 heated soil. 



§ 33. Organisms suppressed hy partial sterilisation. Even a cursory 

 examination of the soil reveals the fact that the bacterial flora has 

 altered. Neither the heated nor the toluened soils possess the character- 

 istic soil odour : the heated frequently smells somewhat musty and the 

 toluened has a faint but quite distinct odour. The toluened soil often 

 shows white spots like mould, which proved to be white streptothrix. 



§ 34. Gelatine plate cultures were made by Koch's method of 

 untreated and partially sterilised soils immediately after partial steri- 

 lisation, and again on the ninth day after moistening. There had been 

 the usual enormous increase in number in the " toluene evapoi'ated " 

 and, to a less extent, the heated soil : this is recorded in Table 12. 

 The organisms present on the various plates, and the proportions 

 their colonies form to the whole assemblage, are given in Table 12. 



In the untreated soil the white streptothrix and 8 — 11 predominate 

 at first, followed by brown streptothrix and the two organisms 15 and 

 18, then come a number of others: moulds, mycoides, zopfii, fluorescens, 

 13, 17, 18, etc., none of which formed 10 per cent, of the colonies on the 

 plate. After the soils have been kept moist for nine days there is a 

 slight rearrangement: 8 — 11 now predominate, then follow the brown 

 streptothrix, then the white and 13, whilst the other organisms remained 

 as before, so far as could be judged. 



The order in the toluened soil is different. White and brown 

 streptothrix and 8 to 11 suffer less than the others and predominate 

 directly after toluening. Nine days afterwards white streptothrix has 

 gone ahead very considerably and is the principal organism present 

 whilst the brown streptothrix formed less than 20 per cent, of the 

 colonies. After a long period the brown streptothrix was much further 

 diminished and the chief organisms were 8 to 11, 7 and white streptothrix. 

 The difference in appearance of the plates is very striking ; the colonies 

 from the untreated soil look mainly brown, whilst those from the 

 toluened soils are mainly white. It is curious that brown streptothrix 



