Soil Protozoa and Soil Bacteria. 



79 



8. This factor, as already shown, is put out of action by 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 gramme 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. 



(cl) 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 amoebae are killed by partial sterilisation. Whenever they 

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

 bacterial numbers rise and maintain a high level. Whenever the detri- 

 mental factor is not put out of action the protozoa are not killed. To these 

 rules we have found no exception. Further, intermediate effects are 

 obtained when a series of organic liquids of varying degrees of toxicity is 

 used in quantities gradually increasing from small ineffective up to completely 

 effective doses. The detrimental factor is not completely suppressed but 

 sets up again after a time, so that the rise in bacterial numbers is not 

 sustained. But the parallelism with ciliates and amoebae is still preserved : 

 they are completely killed when the detrimental factor is completely put 

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



