THE MICRO-ORGANIC POPULATION OF THE SOIL 287 



action it does not reappear. Less drastic methods of treating 

 the soil put it out for a time, but not permanently : e.g. 

 heating to 50°, rapid drying at 35°, treatment with organic 

 vapours less toxic than toluene (e.g. hexane), incomplete 

 treatment with toluene. In all these cases the rise induced 

 in the bacterial numbers per grm. is less in amount than after 

 toluene treatment and is not permanent ; the factor sets up 

 again. As a general rule, if the nitrifying organisms are 

 killed the limiting factor is also extinguished ; if they are only 

 temporarily suppressed the factor also is only put out for a 

 time. 



(9) The properties of the limiting factor are : — 



{a) It is active and not a lack of something (see (7)). 



{b) It is not bacterial (see (3) and (4)). 



{c) It is extinguished by heat or poisons, and does not 

 reappear if the treatment has sufficed to kill sensitive and 

 non-spore-forming organisms ; it may appear, however, if the 

 treatment has not been sufficient to do this. 



id) It can be reintroduced into soils from which it has 

 been permanently extinguished by the addition of a little un- 

 treated soil. 



{e) It develops more slowly than bacteria, and for some 

 time may show little or no effect, then it causes a marked re- 

 duction in the numbers of bacteria, and its final effect is out 

 of all proportion to the amount introduced. 



(/) It is favoured by conditions favourable to trophic life 

 in soil, and finally becomes so active that the bacteria become 

 unduly depressed. This is one of the conditions obtaining in 

 glass-house " sick " soils. ^ 



It is difficult to see what agent other than a living organism 

 can fulfil these conditions. Search was therefore made for 

 larger organisms capable of destroying bacteria, and consider- 

 able numbers of protozoa were found. Active ciliates and 

 amoebae are killed by partial sterilisation. Whenever they are 

 killed the detrimental factor is found to be put out of action, 



^This is dealt with fully in Journ. Agric. Sci., vol. 5, pp. 27-47, 86-111 

 (1912). 



