294 SOIL CONDITIONS AND PLANT GROWTH 



be briefly summed up. Most workers now recognise protozoa 

 as a normal component of the soil population. Many admit 

 the existence of a trophic protozoan fauna. Some, e.g. 

 Waksman (292<5), recognise that certain of the trophic fauna 

 act detrimentally on soil bacteria, but few are prepared 

 to admit that they act detrimentally on the decomposition 

 processes, such as ammonification, nitrification, etc. In the 

 writer's opinion the evidence is in favour of the existence of 

 the trophic protozoan population acting detrimentally on soil 

 bacteria, but as yet there is no direct evidence to show whether, 

 or to what extent, the decomposition processes in the soil are 

 affected. 



The Effect of the Complexity of the Soil Population on 

 Productiveness. 



Apart from the effect of partial sterilisation there is other 

 evidence that simplification of the soil population may lead to 

 increased productiveness, and conversely that a more complex 

 population may be relatively less effective as producers of 

 plant nutrients. It has been observed by practical men in 

 various countries that certain soil conditions harmful to the 

 growth of organisms were ultimately beneficial to productive- 

 ness ; 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 

 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 phenomenon 

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



1 For references see E. J. Russell, Journ. Roy. Ag. Soc, 1910, 71, 9; also 

 J. A. Prescott (228). 



^ " Sickness " is a very vague term and no doubt includes many dissimilar 

 cases. Other instances of "sickness," which may refer to something quite dif- 

 ferent, are recorded by Hudig (Landw. Jahrb., 1911, 40, 613-644) and Loew 

 Porto Rico Report, 1910). Some soil in Orkney was said to be " oat-sick " but 

 experiment revealed nothing (Aberdeen Leaflet 15, 1911). 



