H. B. Hutchinson and A. C. Thaysbn 61 



Dilution leads to a marked iiiiproveiuent in the suitability of the 

 filtered culture for growth, and is thus distinctly more effective than 

 treatment by heat. Hence it bears little relation to the alleged soil 

 toxinsorto the inhibitory substances formed by B.Jluorescens liqiicfaciens 

 and B. coli which were found by Rahn(e) to be incapable of passing 

 through a porcelain filter and were readily destroyed by heat. 



Summary. 



The experimental results submitted in the preceding pages may be 

 summarised as follows: 



Seven English soils have been examined in order to ascertain the 

 validity of Greig Smith's claim that partial sterilisation effects may be 

 due to the destruction of bacterio-toxins in the soil. The results obtained 

 with six of these soils show that : 



1. The untreated extracts of these soils varied largely in their 

 suitability for the growth of the test organism (B. piodifiiosus). In some 

 instances vigorous growth occurred ; in others the numbers of introduced 

 organisms fell to a minimum. 



2. Treatment of the extracts by heat (which was supposed to result 

 in the destruction of '"toxins") invariably led to still further bacterial 

 decreases. 



3. Extracts of soils treated with antiseptics (which are not supposed 

 to destroy "toxins") were on the whole more favourable for growth 

 than those of untreated soils. Such excracts were found to have appreci- 

 ably more organic nitrogen compounds than extracts of untreated soils. 



4. The addition of minute quantities of peptone to unsuitable ex- 

 tracts sufficed to convert them into favourable media. 



5. Extracts of the two poorest untreated soils were tested with a 

 common soil organism {B. Jluorescens liquefaciens). No evidence of 

 toxicity could be obtained; on the contrary, very abundant growth 

 occurred. Results obtained by the use of an extraneous organism, such 

 as B. prodigiosus, must be accepted with reserve. 



6. The curve of diminished numbers of bacteria in poor untreated 

 soil extracts is practically identical with that obtained when bacteria 

 are introduced into pure salt solutions: the decreases are symjjtomatic 

 of starvation. 



7. The only soil which gave extracts similar in behaviour to those 

 reported by Greig Smith was an acid heath soil. The value of the extract 

 of this soil was distinctly increased after the extract had been subjected 



