"812 CONTRIBUTIONS TO OUH KNOWLEDGE OF SOIL-FERTILITY 



The extracts were prepared by stirring up 200 gms. of soil in 

 a mortar with 200 c.c. of aqueous solvent for an hour, after which 

 the solution was filtered through paper, and then through a 

 porcelain filter. Generally, 10 c.c. of filtered extract were 

 pipetted into a Freudenreich flask, to which 1 c.c. of suspension of 

 Bac. prodigiosus was added. This suspension was prepared by 

 suspending a portion of a culture in sterile tap-water, and centri- 

 fugalising until approximately half of the cells had been sedi- 

 mented. A portion of the supernatant fluid was removed, and 

 diluted until 1 c.c. contained about 10,000 cells. Having once 

 determined the number, a I'epetition of the same procedure will 

 give approximately the same number. This number was employed 

 in those experiments in which the bacterial change is calculated 

 upon 1,000 bacteria. Where the calculation is based upon 10 

 bacteria, the sowings were much heavier. It was only after 

 much work had been done, that the necessity for having weak 

 sowings for accentuating difl'erences in the method of treatment 

 became apparent. On this account the earlier or preliminary 

 work has not been recorded. 



The experimental error is considerable, and no two experiments 

 were ever done under precisely identical conditions, as can be 

 seen from the control tests. When tested, the error varied up 

 to 10%, but I believe that, in some cases, it may liave ranged to 

 20%; and, on this account, I prefer to consider the results as 

 simply indicating a behaviour or a condition. 



Tlie infected extract was incubated at 30°C. overnight, and, 

 after an interval of from 19 to 24 hours, it was thoroughly 

 shaken, and used for preparing dilutions. A capillary pipette, 

 discharging ^c.c, was employed for blowing the diluted sus- 

 pensions upon plates of nutrient agar. The drops were smeared 

 over the surfaces with curved, elongated loops of iron-wire (such 

 as is used by florists), after which the plates were dried in the 

 incubator at 37" for an hour, covered, inverted and incubated at 

 30°. Plates were also smeared immediately after infecting the 

 extract, in order to obtain the actual number of bacteria present 

 at the start. 



