136 EFFECT OF HEAT ON SOIL BACTERIA 



(a) If a filtered soil extract containing bacteria from an untreated 

 soil is added to a toluened soil, there is an increase in the rate of 

 ammonia production, and also in the number of bacteria. (6) 

 However, if untreated soil is added to toluened soil, there is no 

 increase, but on the contrary a reduction, (c) As pointed out above, 

 an extract of the toluened soil is more active than an extract of 

 untreated soil, (d) But when the extract of toluened soil is added 

 to the untreated soil there is no increase in ammonia production. 



"The conclusion drawn is that 'the untreated soil contains a 

 factor, not bacterial, limiting the development of bacteria, this 

 factor being put out of action by toluening or heating.' 



" Having determined the presence of a limiting factor in untreated 

 soils an examination of its nature reveals that : (a) it is not a toxin, 

 for if it were it would be sure to affect the nitrifying bacteria most; 

 (6) barley seedlings grown in aqueous extracts of untreated and 

 toluened soils showed no difference in growth over a period of four 

 weeks; (c) the limiting factor is probably biological, for when 

 untreated soil is added to toluened soil the reduction in the rate of 

 ammonia is not at once operative. It is also a large organism, since 

 it is only in the soil and not the filtered extract of the untreated soil 

 that is effective in reducing the rate of ammonia production in 

 toluened soil. An examination of treated and untreated soil was 

 made, and the latter revealed the presence of large organisms, 

 protozoa, etc., which constitute the factor, or one of the factors, 

 limiting the bacterial activity, and therefore the fertility of untreated 

 soil. Direct evidence is furnished by inoculating toluened soil or 

 soil extract with cultures of large organisms and studying the 

 effect produced which is a consequent depression in the rate of 

 ammonia formation." 



Although accepted by many workers, there are many of what 

 appear to be fatal objections that have been brought against this 

 theory: (1) It has been demonstrated that the soil contains many 

 species of fungi which are capable of producing considerable quan- 

 tities of ammonia and these would withstand the actions of the anti- 

 septic or partial sterilization by heat and may develop later and 

 produce large quantities of ammonia. (2) There may be a great 

 difference in the physiological efficiencies of the surviving ammoni- 

 fiers. (3) The work of Russell and Hutchinson does not consider 

 the probability of the protozoa being in soil as cysts. (4) The 

 direct laboratory work of Fred and Gainey cannot be interpreted in 

 the light of this theory. Kopeloff and Coleman analyze the work 

 of Fred in the light of the protozoan theory as follows: 



"In order to test the validity of Russell and Hutchinson's con- 

 clusion that the absence of protozoa (by treatment with toluene) 

 is responsible for increased production of ammonia, Fred, using 

 ether instead of toluene, subjected one series of flasks containing 



