vii.] FORMATION OF PEAT 197 



inaries to the final oxidation process or nitrification, 

 which, as the means by which the higher plants receive 

 their supplies of nitrogen, will be discussed separately. 

 The various oxidation processes in the soil are, like 

 all other bacterial actions, promoted by a certain 

 warmth, the optimum temperature being about 25-30 c , 

 by a sufficiency of moisture, and by the presence of 

 mineral food, like phosphates and potash salts. In 

 any great quantity, however, salts are harmful, 

 particularly sodium chloride ; an acid reaction also 

 diminishes considerably the rate of decay. Speaking 

 generally, bacteria do not thrive as soon as the 

 medium passes the neutral point, and all the decay 

 processes are carried out by the development of fungi 

 when the medium is acid. 



In the soil of certain of the Rothamsted grass plots 

 which have become acid through the long-continued 

 application of ammonium salts, bacterial actions are 

 very much reduced or even brought to a standstill, the 

 bacteria of nitrification, for example, are absent. The 

 decay of the grass roots and stubble is brought about 

 by microscopic fungi which abound, and proceeds to 

 such a slight extent that a layer of peaty material has 

 formed on the surface. The formation of peat must 

 always be associated with this acid reaction of the 

 surface soil. In this way we may account for the fact 

 that tree remains are generally found at the bottom of 

 any peat bog and that evidence of forest growth is 

 apparent on the soil surface of places like the Welsh 

 hills or Dartmoor, now covered with a thin peaty 

 vegetation but where no indigenous trees grow. When 

 the soil first became clothed with vegetation after the 

 glacial epoch a forest developed, but as the soil became 

 acid through the accumulation of organic residues and 

 its initial lack of carbonate of lime the trees died, and 



