H. B. Hutchinson and A. C. Thavsen 5;> 



of the three other soils there was apparently no such recovery within 

 72 hours. The latter results are thus in accordance with those given by 

 Greig Smith, and might without further examination be assumed to be 

 due to the presence in the extracts of substances inimical to bacterial 

 growth. 



Assuming for the moment that this decrease in numbers is of similar 

 character to those observed by Greig Smith and is due to a like cause 

 to that operating in his extracts, it might be expected that subjecting 

 the extracts to heat would to some extent render them suitable for 

 growth. This, however, is precisely the reverse of what actually takes 

 place with the six soils mentioned above. The value of only one extract 

 is maintained but not increased by heat, while that of the remaining 

 five is so affected that the bacterial numbers after 72 hours are but 

 one-fifteenth of those of the untreated extracts. Hence we are compelled 

 to conclude either (1) that the toxins if present in these six soils must 

 be heat-stable while those in Australian soils are labile; or, (2) that the 

 lack of growth in the untreated e.xtracts of the soils is not determined 

 by the presence of toxins, but by some other factor. Further reference 

 to the latter point will be made later on. 



The Extracts of Soils treated ivith Mild Antiseptics. 



The increased fertility of soils which have undergone partial sterilisa- 

 tion by volatile antiseptics has been attributed by Greig Smith to the 

 localisation, on the soil particles, of a soil wax or " agricere." The extrac- 

 tion of nutrients is thus facilitated and the equilibrium which is held to 

 exist between these substances and soil toxins presumably becomes 

 temporarily disturbed. It is difficult to reconcile this view with observed 

 facts ; an increased extraction of the soil should also lead to corresponding 

 increases of the soil toxins as well as of nutrient .substances, and the 

 equilibrium ought consequent!}^ to be maintained. 



The results, however, can be explained solely on the basis of a 

 difference of nutritive values without assuming the presence of toxins. 

 Treatment leads to the liberation of soluble organic nitrogen compounds 

 from the soil, and this action is reflected in the high bacterial numbers in 

 the toluened soil extracts. The extracts of three of the soils have been 

 examined and the following results obtained: 



Extractable Organic Nitrogen (mgrm. per kilo of Soil) 



