Apeil 4, 1913] 



SCIENCE 



519 



DISCUSSION AND COBEESPONDENCE 



THE COMPLEXITY OF THE MICROORGANIC POPULA- 

 TION OF THE SOIL 



During the last few years a series of ex- 

 periments have been carried out in this lab- 

 oratory by Dr. Hutchinson and myself which 

 we can only interpret as showing that bac- 

 teria are not the only active inhabitants of the 

 soil. The results in our view point conclu- 

 sively to the presence of another group of or- 

 ganisms, detrimental to bacteria and diifering 

 from them by their larger size, slower rate of 

 multiplication under soil conditions, and 

 lower power of resistance to heat and anti- 

 septics. They are, therefore, more readily 

 killed than bacteria, and we regard their sup- 

 pression as an important factor in bringing 

 about the increased bacterial activity known 

 to set in after soil has been partially steril- 

 ized or treated in any other way detrimental 

 to active life. Such properties as we could 

 ascertain agree with those of protozoa ; we 

 were thus led to look for these organisms in 

 the soil and found numbers of them. We ad- 

 duced reasons for provisionally identifying 

 the detrimental organisms with the soil 

 protozoa. 



Recently several papers have been published 

 in the United States controverting these con- 

 clusions. We are not satisfied, however, that 

 the criticisms affect the validity of our argu- 

 ments, and therefore desire to set out briefly 

 the experimental facts and the conclusions we 

 draw from them. The actual data are to be 

 found in our papers in the Journal of Agri- 

 cultural Science; many of the figures were 

 also presented to the Graduate School of 

 Agriculture at East Lansing last July. 



1. We begin with the fact that partial ster- 

 ilization of soil, i. e., heating it to a tempera- 

 ture of 60° C. or more, or treatment for a 

 short time with vapors of antiseptics such as 

 toluene, causes first a fall and then a great 

 rise in bacterial numbers. The rise sets in 

 soon after the antiseptic has been removed 

 and the soil conditions once more made f avor- 



^ Journal of Agricultural Science, 1909, 3: 111- 

 144; 1912, 5: 27-47, 86-111; 1913, 5: 152-221. 



able for bacterial development; it goes on till 

 the numbers far exceed those present in the 

 original soil. 



2. Simultaneously there is a considerable 

 increase in the accumulation of ammonia. 

 This sets in as soon as the bacterial numbers 

 begin to rise, and the connection between the 

 two quantities is normally so close as to indi- 

 cate a causal relationship ; the increased am- 

 monia production is, therefore, attributed to 

 the increased numbers of bacteria. There is 

 no disappearance of nitrate; the ammonia is 

 formed from organic nitrogen compounds. 



3. The increase in bacterial numbers is the 

 result of improvement in the soil as a medium 

 for bacterial growth and not an improvement 

 in the bacterial flora. Indeed the new flora 

 per se is less able to attain high numbers than 

 the old. This is shown by the fact that the 

 old flora when reintroduced into partially 

 sterilized soil attains higher numbers and ef- 

 fects more decomposition than the new flora. 

 Partially sterilized soil plus 0.5 per cent, of 

 untreated soil soon contains higher bacterial 

 numbers per gram and accumulates ammonia 

 at a faster rate than partially sterilized soil 

 alone. 



4. The improvement in the soil brought 

 about by partial sterilization is permanent; 

 the high bacterial numbers being kept up even 

 for 200 days or more. The improvement, 

 therefore, did not consist in the removal of 

 the products of bacterial activity, because 

 there is much more activity in partially ster- 

 ilized soil than in untreated soil. Further 

 evidence is afforded by the fact that a second 

 treatment of the soil some months after the 

 first produces little or no effect. 



It is evident from (3) and (4) that the fac- 

 tor limiting bacterial numbers in ordinary 

 soils is not bacterial, nor is it any product of 

 bacterial activity, nor does it arise spontane- 

 ously in soils. 



5. But if some of the untreated soil is intro- 

 duced into partially sterilized soil the bac- 

 terial numbers, after the initial rise (see (3)), 

 begin to fall. The effect is rather variable, 

 but is usually most marked in moist soils that 

 have been well supplied with organic ma- 



