140 



Partial Sterilisation of Soil 



is poured on to toluened soil, the decomposition of urea is only slightly 

 less than when an extract of toluened soil is added. 



However, when we applied a more sensitive test and mixed the 

 extracts of untreated soil and of toluened soil in equal proportions we 

 found that the limiting factor is also present in the extract. 



160 



140 



:2 120 



« 



100 



80 - 



60 



Toluened soil -i- aqueous 



' extract containing bacteria 



from untreated soil 



Toluened soil alone 



Toluened soil + .5°/^ 

 untreated soil 



Untreated soil 



40 



50 



60 



10 20 30 



Time in days 



Curve 3. Effect of untreated soil, and of aqueous extract containing bacteria from 

 untreated soil, on the bacterial activity in the toluened soil (Table 13). 



§ 39. These facts point to large organisms as the limiting factor. 

 Examination was made for algae and for protozoa by the following 

 methods : 



(1) Algae. A solution containing per litre 2 gms. sodium nitrate, 

 0"5 gm. each monopotassium, phosphate, sodium chloride, magnesium and 

 calcium sulphates was sterilised and inoculated with 5 gms. soil per 

 100 c.c. solution, calcium carbonate was also added. The flasks were kept 

 in a warm place exposed to light, and after a few weeks a vigorous algae 

 growth had developed from the untreated soil, only very little from 

 the toluened, and none from the heated soil. Partial sterilisation has 

 therefore removed algae. 



