A LIMITING FACTOR 135 



with toluene. But the untreated soil contained a variety of them, and 

 the work of Martin and Lewin (193) shows that some at any rate are 

 in the trophic state. The properties of the detrimental factor are 

 identical with those of the soil protozoa. Whenever the ciliates and 

 amoebae are killed it invariably happens that the detrimental factor is 

 extinguished ; whenever the detrimental factor is not extinguished the 

 protozoa also are not killed. No exception has yet been found to 

 these rules, and they afford strong presumptive evidence that the soil 

 protozoa are detrimental to bacteria, although of course it does not 

 follow that protozoa are the only detrimental organisms present. The 

 subject is now being attacked from the zoological side. 



Meanwhile, and pending the results of the zoological survey, it 

 seems safe to divide the soil organisms into two groups in their rela- 

 tions to the processes of food production ; a useful group (most of 

 group I on p. 1 18) and a detrimental group (group II). The latter, 

 are, speaking generally, more readily killed than the former. Con- 

 ditions that are harmful to active life in the soil tend, therefore, to 

 reduce their numbers and lead ultimately to an increased activity of 

 the useful bacteria. On the other hand, conditions favourable to 

 active life tend to keep up the detrimental organisms and therefore to 

 reduce the useful bacterial activity. It is thus possible to account for 

 a number of obscure and paradoxical effects that have hitherto caused 

 considerable perplexity. It 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 are 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 un- 

 doubtedly 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 phenomenon is spoken of as "sick- 

 ness " by the practical man. 



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 new view that detrimental 

 organisms are also present readily explains the observed facts. 



The " sickness " that affects the soils of glasshouses 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 importance (Russell and Petherbridge 



