THE MICRO-ORGANIC POPULATION OF THE SOIL 295 



It is difficult to account for this result on the old view that 

 the useful plant-food making bacteria are the only active 

 micro-organisms in the soil. On the other hand, the view 

 that detrimental organisms "are also present readily explains 

 the observed facts. 



The " sickness" that affects the soils of glass-houses run 

 at a high pitch (such as cucumber-houses) and less slowly 

 those run at a lower pitch (such as tomato-houses) has been 

 investigated in some detail owing to its great technical im- 

 portance (Russell and Petherbridge, 2412). It was traced to 

 two causes : an accumulation of various pests, and an ab- 

 normal development, especially in cucumber-houses, of the 

 factor detrimental to bacteria. The properties of this factor 

 show that it is identical in character with that present in 

 normal soil, and strongly indicate its biological nature. No 

 evidence of a soluble toxin could be obtained but the pos- 

 sibility of insoluble toxins was not excluded. On the other 

 hand, some remarkably interesting protozoa and allied organ- 

 isms have been picked out from these sick soils and described 

 by Martin and Lewin (191) and T. Goodey. Finally it has 

 been shown that the whole trouble can be cured by partial 

 sterilisation, and methods suitable for large scale work have 

 been investigated and are now in use in practice. Steam 

 heat at present proves most convenient, but the suitability 

 and detailed effects of lime have been studied by Hutchinson 

 and McLennan (140*:) and of various antiseptics by Buddin 

 (61), Russell, and others. 1 



Again, it should be stated that the views set out here are 

 not generally accepted by American workers (see J. M. Sher- 

 man, 261). Bolley attributes the whole trouble in North 

 Dakota to plant diseases, 2 which, of course, may be the pre- 

 dominating factor there. It is possible that there are im- 

 portant differences in soil population in British and American 



1 See Journ. Roy. Hort. Soc., 1920, 45, and Reports of the Nursery and 

 Market Garden Expt. Station, Cheshunt, Herts, 1917, onwards. 



2 H. L. Bolley, North Dakota Bull., 107, 1913 ; Science, 1910, 32, 529-541, 

 and 1913, 38, 249-259. 



