176 CONTRIBUTIONS TO OUR KNOWLEDGE OF SOIL-FERTILITY, xvi. 

 EXPERIMEJ^T xxi. 



The figures are instructive, inasmuch as they show a consider- 

 able reduction in the number of bacteria originally added. There 

 was something in the solution which was strongly toxic towards 

 the test-organism added to the extract. It is unfortunate that 

 the extract was so alkaline, for this undoubtedly clouds the issue, 

 but, even when an allowance is made for it, according to the 

 information previously obtained with extract of vegetable mould 

 (p. J 72), a strongly rising dilution-curve is still apparent. 



The alkalinity was determined by boiling 25 c.c. of the extract 

 with 35 c.c. of distilled water and 1 c.c. of centinormal sulphuric 

 acid for ten minutes, rapidly cooling the solution, adding 1 c.c. 

 of centinormal soda, and titrating back until the tint became 

 that of the control. Phenolphthalein was used as the indicator. 

 It gave a true indication of the reaction in extracts of leaf-mould, 

 but, as will be shown subsequently, it is not so good for solutions 

 of hay-infusion. 



The experiments so far showed that some means must be 

 adopted to eliminate the excessive alkalinity of the fermented 

 cultures. On a previous occasion, humic acid had been used, 

 but, as it had been soluble, it had not served the desired purpose. 

 On standing, however, a solution of the acid had precipitated, 

 and this precipitate of insoluble humic acid was washed and 

 used. The employment of soil for maintaining a neutral reaction 

 was suggested by the fact that, when it is put into hay-infusion 

 and incubated, there is developed a mixed flora and fauna, the 

 appearance of which is so healthy, that a pronounced acidity or 

 alkalinity of the infusion is unlikely. 



