NITRIFICATION 181 



several key-points, namely the proof of aspartic acid excretion 

 by the nodules and of the growth of Rhizobia in nitrogen-free 

 oxalacetate media, have not so far been confirmed in other 

 laboratories. Also the evidence of the nature of the oximo- 

 succinic acid isolated from soil ic not altogether convincing. 

 Wilson, in America, has carried out much careful and detailed 

 research into the symbiotic nitrogen fixation process, and has 

 not obtained evidence in confirmation of Virtanen's hypothesis. 

 An interesting point discovered by this group of workers is 

 that the fixation process is inhibited by the presence of 

 hydrogen. 



The root-nodules contain a haematin-pigment resembling 

 haemoglobin, but the part played by this substance in the 

 fixation process has not yet been elucidated. 



(6) Interchange of bacterial nitrogen and ammonia 



The interchanges between bacterial protein, amino-acids, 

 and ammonia have been dealt with in detail in these last 

 two chapters. Degradation of amino-acids will take place 

 either by deamination or by decarboxylation, according to 

 the pH holding in the environment at the time. If 

 decarboxylation occurs, the nitrogenous product is an amine, 

 and if the environmental ^H subsequently returns to neutral 

 or alkaline values, then amines themselves are oxidised by 

 certain bacteria, particularly Pseudomonas, with the liberation 

 of ammonia. 



(c) Nitrification 



The oxidation of ammonia to nitrate in the soil is called 

 " nitrification," and the biological nature of the process was 

 first demonstrated in 1877 by Schloesing and Muntz. They 

 poured sewage effluent through a long tube containing sand 

 and chalk, and showed that, after the fluid had been passing 

 through the tube for a few days, ammonia entering the tube 

 was removed during the passage of the effluent through the 

 tube, and replaced by nitrate in the fluid issuing from the tube. 

 They showed that treatment of the contents of the tube with 



