550 PROCEEDINGS OF SECTION G2. 



that there are no toxins in normal soil from the considerations that 

 nitrification is as active as ammonification in normal soils, and 

 that nitrification does not occur in toxic superheated soils. If, 

 therefore, toxins were present the nitrifying bacteria would be 

 unable to exist. 



Some work upon soil bacteriotoxins has been done in the 

 laboratory of the Linnean Society of New South Wales, and the 

 full paper will appear in Part IV of the 1910 volume. 



Bacteriotoxins are undoubtedly present in soils, and I have 

 extracted a greater quantity from a poor soil than from a good 

 one. If several soils of varying fertility are treated for an hour 

 with water and filtered through porcelain, extracts are obtained 

 which either destroy bacteria or restrict their growth. Experi- 

 ments with one of these extracts, preferably the most lethal, will 

 show after the extract has been boiled for an hour that it behaves 

 as a nutritive solution. 



The toxic and reverse actions can best be shown by using a 

 pure culture of a putrefactive organism such as Bac.prodigiosus, 

 but the same occur, though not in so pronounced a manner, with 

 the mixed bacteria of the same soil from which the extract was 

 obtained. With the mixture of soil bacteria, the toxic action is 

 shown as a retardation of the growth of the bacteria as compared 

 with the growth in the same extract after boiling. So far as my 

 experiments have gone, the soil bacteria are not so sensitive to the 

 toxins of their own soil as Bac. prodigiosus ; in other words, they 

 exhibit a certain degree of immunity towards their own toxins. 



The toxic effect may be considerable. For example, the 

 aqueous extract from a poor sandy soil made by extracting 200 

 grms. with 200 cc. of saline for an hour was heated at 94° for an 

 hour. Two days after it was boiled for an hour. The extracts 

 were infected and incubated overnight at 30° ; in the morning the 

 numbers of bacteria in the experimental flasks were determined. 



1,000 bacteria became 



1 saline extract of poor soil , . . . . . 



2 „ „ „ „ heated at 94° .. 2,000 



3 „ control 117,000 



4 ,, extract (1 above) two days after . . 73 



5 „ „ „ ,, boiled .. 3,250,000 



6 „ control 70,000 



The experiment shows that toxin is contained in soil and is 

 soluble in dilute saline ; it is partially destroyed at 94° and rapidly 

 decays in aqueous solution ; boiling either converts it into a nutrient 

 or by destroying the toxin enables the nutrients dissolved in the 

 saline to act. This is one of many experiments, all showing that 

 the action of heat upon soil extracts and inferentially upon soils is 

 to destroy the bacteriotoxin and possibly to convert it into nutritive 

 or fertilising matter. There may be other effects, but this is 

 enough to account for the enhanced fertility of heated soils. 



