.S14 CONTRIBUTIONS TO OUR KNOWLEDGE OF SOIL-FERTILITY 



The Toxic Action of Filtered Soil-Extracts. 

 Experiment iii. 



Tap-water (control) 



Extract of untreated soil 



Extract of chloroformed soil 



10 bacteria became 



one day 



three days 



75 

 



7,950 



The same(Expt. iv.) untreated soil-extract was, two days after- 

 wards, heated in boiling water. The temperature was tested 

 several times, and found to be 94°. Portions were abstracted at 

 intervals, infected and incubated. 



Experiment iv.a. 



Extract not heated 



Extract heated 15 minutes.. 

 Extract heated 30 minutes.. 

 Extract heated GO minutes.. 

 Extract heated 120 minutes 

 Extract heated 180 minutes 



1,000 bacteria became 



24,800 

 272,000 

 1,200,000 

 3,000,000 

 3,850,000 

 3,710,000 



It is evident from (iii.) and (iv.) that the aqueous extract of 

 soil, as made by treating the soil for an hour, contains a toxic 

 substance. In two days, at laboratory temperature, the toxin 

 has apparently disappeared; but that it has only been partly 

 destroyed, is shown from the effect of heat in (iv.a). Two hours' 



