3IO 



NATURE 



[November 19, 1914 



which they are found in the film is the state in 

 which they actually occur in the soil. It was with 

 no little interest, therefore, that the work of the 

 protozoologists was watched, and when they dis- 

 covered that the amoebae were in the trophic state, 

 and not all present as cysts, it was felt that a 

 distinct advance had been made. Mr, Lewin has 

 since devised a method of getting- the protozoa 

 alive out of the soil. Thus, satisfactory proof is 

 furnished that an active protozoan fauna exists 

 in normal field soils. Some of the organisms are 

 of special zoological interest, and have therefore 

 been studied in detail. 



Even yet, however, the problem is not solved, 

 because there is nothing to show how numerous 

 the active protozoa are. Neither Martin's nor 

 I.ewin's method is susceptible of quantitative use, 

 and at present the only guide is the ease with 

 which the organisms can be picked out. 



I however, the main interest of the " sick " 



I soils, i.e., the soils known to be heavily 



stocked with the detrimental factor, is that they 



contain numerous active amoebae, and never fail 



to give good crops of organisms by either of the 



j methods in use. 



I On the other hand, the detrimental factor is 

 i reduced considerably by drying the soil, especiallv 

 I at high summer temperature. So also the harvest 

 j of protozoa becomes \'ery thin in field soils during 

 I dry summer weather. 



[ This, therefore, is the present position so far 



I as the identification of the detrimental factor is 



i concerned. There seems no doubt that it is liv- 



1 ing, and there is strong indirect evidence to con- 



; nect it with the soil protozoa. But the direct 



evidence is lacking, partly because the difficulties 



of introducing mass cultures into partially 



sterilised soils have not yet been overcome, and 



CHLOROFORtl. 



M/50roM 



40 



PAKTS 



PER 



MILL. 



U 



\(i 



AMMONIA AND NITRATE 

 PKODUCEB. 



30 



DAYS 



Fig. 3. — Effect of a typical volatile antiseptic on bacterial numbers and production of ammonia in soils. (Buddin.) 



It was found from the soil investigations that 

 the detrimental factor increased in intensity when 

 the soil was kept moist, warm, and well supplied 

 with organic matter. In these circumstances the 

 bacteria might reasonably have been expected to 

 increase enormously ; as a matter of fact, they did 

 not. Moreover, the fertility of these soils did not 

 increase as much as was anticipated ; it became 

 less, and after a time no longer justified the cost 

 of intense cultivation : this fact is well known 

 among nurserymen, who speak of such soils as 

 "sick." The treatment in the past has been to 

 throw the soils out in despair, and to sacrifice the 

 great manurial residues they contain. But it was 

 obvious that the proper course was to adopt 

 partial sterilisation methods ; and laboratory 

 and pot culture experiments alike proved the 

 success of the method. For the moment, 

 NO. 2351, VOL. 94I 



partly because no satisfactory method of enu- 

 merating active protozoa in the soil has yet been 

 devised. The zoological work is being continued : 

 it has already more than justified itself by reveal- 

 ing the presence in natural soil of a trophic pro- 

 tozoan fauna the members of which are of con- 

 siderable interest : the most important further 

 development will be to ascertain whether the pre- 

 sent indications are correct that the fauna is most 

 numerous when the detrimental factor is most 

 fully developed, and least numerous when the 

 factor is not much in evidence. 



The " sick " soils mentioned above are so im- 

 portant from the technical point of view that they 

 have been further investigated. Sickness is 

 proved to be an excessive development of the 

 detrimental factor and also of disease organisms. 

 The demonstration that it could be cured by 



