282 BIOCHEMICAL PROCESSES OF THE SOIL 



has already been observed by practical men in various countries 

 that certain soil conditions harmful to the growth of organisms 

 were ultimately beneficial to productiveness, such as long 

 continued and severe frosts, long drought (especially if 

 associated with hot weather), sufficient heat, treatment with 

 appropriate dressings of lime, gas lime, carbon disulphide, etc. 



Further, it has been observed that conditions which are 

 undoubtedly favourable to life, such as the combination of 

 warmth, moisture, and organic manures found in glass-houses, 

 lead to reduced productiveness after a time. This phenomena 

 is spoken of as " sickness " by the practical man. 



It is difficult to account for these results 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 new view 

 that detrimental organisms are also present readily explains 

 the observed facts. 



The " sickness " that affects the soil of glass-houses run at 

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

 lower pitch (such as tomato houses), has been investigated in 

 some detail owing to its great technical importance. It was 

 traced to two causes : an accumulation of various pests, and 

 an abnormal 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. On the other 

 hand, some remarkably interesting protozoa and allied organisms 

 have been picked out from these sick soils and described by 

 Martin, Lewin, and 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 M'Lennan, and of 

 various antiseptics by Buddin. 



While these laboratory investigations are going on, a detailed 



