278 BIOCHEMICAL PROCESSES OF THE SOIL 



antiseptics and by heating the soil to 60 C., and once out of 

 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 gram 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. 



(d) It can be reintroduced into soils from which it has been 

 permanently extinguished by the addition of a little untreated 

 soil. 



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

 time may show little or no effect; then it causes a marked 

 reduction 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 

 glasshouse " 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 considerable 

 numbers of protozoa were found. The ciliates and amcebse 

 are killed by partial sterilisation. Whenever they are killed 

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



* This is dealt with fully in J. Agric. Sci., 5 (1912), 27-47, 86-111. 



