34 



THE AGRICULTURAL NEWS. 



February 5, 1910. 



the ammonia content was insignificant in comparison. 

 Other effects of partial sterilization were found to be 

 an increase in the rate of production of unstable nitro- 

 gen compounds, and the destruction of nitrifj'ing 

 organisms. In this case, there is a difference 

 between the action of heat and that of toluene, for in 

 the latter instance, they thrive once more, on being 

 re-introduced, while in the former they do not. It 

 seems that, in the heating of the soil, some substance 

 has been formed which is inimical to the existence of 

 the nitrifying organisms: it may be the same body as 

 was found by Pickering* to interfere with the germina- 

 tion of seeds, under the same circumstances. 



The ne.xt step in the investigations was to dis- 

 cover the cause of the increase in the amount of 

 ammonia. Two suggestions to account for this arise: 

 (1) that some agency is present which causes increased 

 production, (2) that some agent which continually con- 

 sumes the ammonia under ordinary conditions, has been 

 removed by the treatment. In the latter connexion, 

 nitrifying bacteria do not enter into the question, as they 

 have been proved to be completely destroyed by the 

 sterilization. That the ammonia is consumed by any 

 other agency was disproved by adding small quantities of 

 ammonium salts and recovering them unaltered, except 

 for a small proportion that had been converted into nit- 

 rates owing to the action of re-introduced nitrifying 

 organisms. The results obtained led to the adoption of 

 the first conclusion, namely that actual production 

 accounted for the greater amount of ammonia that was 

 present, with the additional inference that this was 

 owing to the action of bacteria in hastening the 

 decomposition of substances in the soil. 



The question then arose as to why the bacteria 

 should produce ammonia at an increased rate, after 

 partial sterilization. Russell and Hutchinson confirm- 

 ed Hiltner and Stormer's discovery that bacteria 

 multiply more quickly and reach much greater numbers 

 after that treatment. The rate of increase was 

 determined, and this was found to be paralleled by the 

 rate of increase of ammonia, thus giving rise to the 

 inference, already stated, that the greater production of 

 ammonia was due to the action of bacteria. 



A detailed examination of this conclusion showed 

 that there was no evidence that the kinds of bacteria 

 ■which continue to exist after the treatment are stimu- 

 lated to greater action by it — an explanation which, as 

 has been stated already, is due to Koch — but that the 

 contrary is actually the case, for certain forms that were 

 isolated from the soil treated with toluene all showed 



*Agric'ulUiral Neirs, Vol.' VIII, p. 281. 



less activity than those of the same kind from the 

 untreated soil. Nor was Hiltner and Stiirmer's susrcres- 

 tion, to which reference has already been made, that 

 the increased activity was due to a change in the type 

 of bacteria that was present in the greatest numbers, 

 upheld. The simple conclusion was reached: 'the 

 increased ammonia production in the partially sterilized 

 soil is due to the increased numbers of the bacteria.' 



As this is the case, there must be some factor 

 present in ordinary, untreated soils which limits the 

 rate at which bacteria are produced, but which is 

 removed by the action of toluene or heat. That this 

 factor is not bacterial was shown by adding a filtered 

 soil extract to a treated soil, when there was an 

 increase of bacterial action, whereas if untreated soil was 

 added to treated soil, there was no increase of such 

 action, but rather a decrease. Thus the limiting factor 

 must be something in the untreated soil which could 

 not pass through the filter that was used for the soil 

 extract. It seems that this factor is not a toxin, as 

 has been suggested by Whitney and others, for this 

 would affect the nitrifying bacteria most, as they are 

 more sensitive than those which produce ammonia, yet 

 nitrates accumulate in untreated soils. 



The conclusion is reached that the limiting factor 

 is a living organism or organisms, for, as was stated in 

 the last paragraph, when untreated soil was added to 

 the treated soil, the bacterial activity, and therefore 

 the rate of production of ammonia, was promntly 

 diminished. The further inference is made that the 

 organism is probably large in comparison with bacteria, 

 for it is only the soil, and not the filtered extract of the 

 ordinary soil, that can produce the effect under consid- 

 eration. Examination of the two kinds of soil showed 

 that no large organisms were present in that which had 

 been heated: in the soil treated with toluene, the larger 

 organisms were also absent, only small ciliated infusoria 

 being found; while all these organisms were present in 

 untreated soil. As the large organisms are about one 

 tliousand times the size of bacteria, it is easy to under- 

 stand that they are serious competitors with the latter, 

 even apart from the fact that they probably effect 

 actual destruction of the bacteria. 



The work of these investigators, then, appears to 

 show that the number of bacteria in the soil is limited 

 by the presence of comparatively large, competing and 

 destructive organisms, and that the increased fertility 

 of soils that have been partially sterilized, is due to 

 the killing of these, and the consequent increase in the 

 rate of production of bacteria, with the concomitant 

 increase in the rate of formation of ammonia. 



