118 Partial Sterilisation of Soil 



III, The nature of the limiting factor. 



§ 8. The limiting factor is not a toxin such as are postulated by 

 Whitney and others. 



(1) If it were, it would be sure to atfect the nitrification bacteria 

 most as they are more sensitive than the ammonia producing groups, 

 as seen : 



{a) in their absence from toluened or heated soils, 

 (b) in the fact that they cannot be reintroduced into a heated 

 soil because the heating has developed some substance toxic to nitrify- 

 ing organisms, but not to ammonia producing organisms. (§ 31.) 



In the untreated soil nitrates but never ammonia accumulate, and the 

 rate of nitrification is at least as great as the rate of ammonia produc- 

 tion. If there is nothing toxic to the nitrifying organisms, a fortioin, it 

 is very unlikely there is anything toxic to the ammonia producers. 



(2) Barley seedlings grown in aqueous extracts of untreated and 

 toluened soils with or without addition of culture solution showed 

 no difference in growth over a period of four weeks. Had any toxin 

 been present it should according to Whitney have produced an effect in 

 much less time. 



§ 9. The limiting factor is probably biological, since when untreated 

 soil is added to toluened soil the reduction in the rate of ammonia pro- 

 duction is not at once operative. (Curve 3 (p. 140), § 36.) It is probably 

 also a large organism, since it is only the soil and not the filtered extract 

 of the untreated soil that is effective in reducing the rate of ammonia 

 production in toluened soil. (Cf. § 7fl, also §§ 38 and 39.) Search was 

 therefore made for large organisms such as infusoria, amoebae, and 

 ciliata. None were found in the heated soil, and only small ciliate 

 infusoria in the toluened soil. All these organisms are found in the 

 untreated soil. Some, e.g. Colpoda cucullus and Amoeba nitrophila, are 

 known to devour bacteria, and all must be severe competitors by reason 

 of their large size (about 1000 times that of the soil bacteria). We 

 conclude then that these large organisms — i^rotozoa, etc. — constitute the 

 factor, or one of the factors (see § 42) limiting the bacterial activity, and 

 tlierefore tJie fertility of our untreated soil. Direct evidence is furnished 

 by inoculating toluened soil or soil extract with cultures of large 

 organisms, and studying the effect produced. Curve 4 shows the conse- 

 quent depression in the rate of ammonia formation. (§ 39, Table 14.) 



