NITRIFICATION 37 



organisms. For example, NO^ is immediately nitrified by 

 soil which has been previously percolated with NHt, but 

 there is a lag in the nitrification of NHt by soil previously 

 treated with NO^. Hence the 'saturated soil' technique is 

 of value in testing substances believed to be intermediates 

 in the nitrification process. Thus hydroxylamine (NHgOH) 

 has long been postulated as a possible intermediate in the 

 conversion of NH3 to HNO2 , but there is little direct 



i20-| f^ r-l-9 



FIG. 3.2. — Course of nitrification in soil percolated with o-oi M.- 

 NH4CI: A, first percolation of fresh soil; B and C, linear rela- 

 tionship between time and logio /xg. N07-N/ml. formed 

 respectively in the first (B) and second (C) percolations [22] 



evidence [15]. Free NHgOH is toxic to bacteria and soon 

 stops nitrification. However, when combined, as in pyruvic 

 oxime, it is nitrified by soil enriched with nitrifiers, but 

 only after a lag. This is due to the development of hetero- 

 trophs, species of Archromohacter and Corynebacteria, which 

 convert the NHgOH into NOT without the intermediate 

 formation of NH3 [23]. The percolation technique is also 

 valuable for determining the effects of various substances 

 on nitrification under conditions simulating those found in 



