14 MICROBES AND TOXINS 



ammonia frequently very considerable. Schloesing has shown 

 that in the soil ammonia does not prevent the nitric ferment 

 from acting. These apparent contradictions between theory 

 and practice are capable of explanation. Ammonia prevents 

 the development of the nitric ferment but scarcely acts at all on 

 the same ferment in the adult state. The soil in nature being 

 populated by the adult ferment, the inhibiting action of 

 ammonia is much more limited than in an experimental 

 flask (Boullanger and Massol). 



The nitrous ferment will stand neither an excess of ammonia 

 nor an excess of its own product, the nitrite of magnesium. 

 The nitric ferment ceases to act when too much nitrite has 



Fig. 4. — Nitrous ferment Fig. 5. — Nitric ferment : the nitro-bacte- 



from Gennevilliers. rium from Quito (after Kayser). 



been produced around it and when it has itself developed a 

 certain quantity of nitrates. Thus each demands suitable 

 proportions both of the primordial material and of the products. 

 It is, no doubt, to maintain these favourable conditions that 

 we have the denitrifying microbes which partially undo the 

 labour of the nitrifying organisms. They return the nitrates 

 and nitrites to the condition of ammonia, liberating protoxide 

 of nitrogen, dioxide of nitrogen, or simply nitrogen. Wheat 

 straw, the straw of maize and of lucerne and oilcakes contain 

 denitrifying ibacteria. Animal excrement also contains them, 

 for soil to which cow dung is added loses part of its nitrates. 

 When soil is treated with nitrate of soda too soon after 

 receiving farmyard manure it loses nitrogen. The denitrifying 

 organisms act best in presence of the excess of organic matter 



