VII.] CONDITIONS FAVOURING NITRIFICATION 221 



ammonia immediately it is presented to them, and the 

 real limiting factor in nitrate production is the activity 

 of the ammonia - producing organisms. The amount 

 of free ammonia in neutral soils is always very small, 

 one to two per million, whereas the nitrate may 

 amount to 50 to 100 parts per million. This is 

 because the ammonia is transformed as fast as it is 

 produced, and nitrification organisms are always able 

 to keep pace with the output of ammonia if by any 

 means that can be stimulated. In acid soils where 

 nitrification is slowed down or at a standstill, the 

 higher plants doubtless take in the nitrogen they 

 require as ammonia ; indeed certain modifications in 

 the colour of the crop and its habit of growth appear 

 to follow when it is deriving its nitrogen from ammonia 

 and not from nitrates. In considering the fertility of 

 the soil the ammonia-making and nitrifying organisms 

 must be regarded as one group, which do the final 

 preparation of the organic nitrogen residues for the 

 food of the higher plants and cipon the activity of 

 which, particularly of the former, the rate of pro- 

 duction of available plant food depends. 



Summing up the above facts, it is seen that for the 

 active production of nitrates from the organic com- 

 pounds of nitrogen present in the soil and this is 

 necessary if the crop is to be kept supplied with the 

 nitrogen required for its growth the following condi- 

 tions are requisite : The presence of the ammonia- 

 making and nitrifying organisms in sufficient quantities, 

 a certain degree of temperature, sufficient moisture 

 for the development of the bacteria, free aeration 

 of the soil to supply the oxygen necessary, and a 

 base to neutralise the acids as they are produced. 



The scanty number of ammonia-making and nitrify- 

 ing bacteria in any subsoil below the cultivated layer 



