THE BIOLOGICAL CONDITIONS IN THE SOIL 119 



is heated to 98 C. for a short time, or dried at lower temperatures 

 (e.g. 40) for a longer time, the phagocytes are killed, but not the bac- 

 terial spores. On removing the toluene and moistening the soil, the 

 spores germinate, and the resulting organisms multiply with great 

 rapidity, since they now have a clear field. The individual species may 

 be less virulent than the old races, but they more than make up for 

 any deficiency in this direction by their enormously increased numbers. 

 The rate of decomposition is considerably hastened and a larger amount 

 of ammonia is produced, so that the fertility of the soil is increased. 

 Some, if not all, of the parasites are also killed, and the plant has a 

 better chance of development. 



The Action of the Plant on the Soil. 



The plant reacts in several ways on the soil. It takes up nitrates 

 and other substances, thus preventing too great an accumulation of 

 soluble material in the soil. It also effects a certain amount of de- 

 composition. Water cultures in which plants are growing are known 

 to become alkaline ; the explanation offered is that the plant takes up 

 the acid radicle of the sodium nitrate and leaves behind the base, 

 which immediately appears as the carbonate. Hall and Miller (n 8) 

 have obtained evidence of a similar action in the soil, the calcium 

 nitrate formed during nitrification being converted into calcium car- 

 bonate while the nitrate radicle is taken by the plant. These effects 

 are favourable to micro-organisms ; others are unfavourable, such as 

 the removal of moisture by the plant and the evolution of carbon 

 dioxide from the roots. 



