NITKIFICATION IN NATUEE 195 



month later the nitrite had all disappeared, its place being taken by 

 the nitrate. The combination A + B produced in a similar culture 

 fluid first ammonia, and later nitrite, but no trace of nitrate. Finally, 

 the combination B + did not effect a change of any kind, even after 

 10 months. In these experiments, if any further proofs were required, 

 we have a proof both of the validity of the bacteriological explanation 

 of nitrification and also of the serial nature of the process. In nature 

 the absence of ammonium compounds prevents the growth of nitrite- 

 bacteria, and the absence of nitrites that of the nitrate-bacteria. The 

 formation of ammonium compounds, and the consequent diminution 

 of the organic material, due to the activity of the saprophytic bacteria, 

 enables the nitrite-bacteria to multiply. Their multiplication results 

 in the using up of the ammonium compounds and in the accumulation 

 of nitrites, both of which conditions are unfavourable to the continued 

 multiplication of the nitrite-bacteria. But these conditions are 

 exceedingly favourable to the development of the nitrate-bacteria, 

 which consequently predominate, and transform the nitrites into 

 nitrates. 



8. ORGANISMS THAT REDUCE NITROGEN COMPOUNDS : 

 DENITRIFICATION. 



So far in this chapter we have been dealing with organisms that 

 promote the oxidation of the nitrogenous material of the soil, thus 

 rendering it suitable for assimilation by the higher plants. We must 

 now devote a small space to those bacteria that work in an exactly 

 opposite direction, removing the oxygen from nitrogenous compounds, 

 and thus rendering them useless to the higher plants. Some of these 

 organisms take away a part of the oxygen from the nitrates, converting 

 them into nitrites, or they may take away the whole of this element, 

 reducing them to ammonia. Other organisms of this class convert 

 the nitrates and nitrites into nitrous oxide (N 2 0) and nitric oxide 

 (NO), both of which are also useless to the higher plants. Finally, 

 a class of organisms exists which detach the nitrogen altogether from 

 its compounds, when it escapes into the atmosphere as free nitrogen. 

 The last process is termed denitrification. The power of reducing 

 nitrates into nitrites and ammonia is possessed by a very large number 

 of bacteria, though these are not dependent on this process for their 

 existence, as they thrive very well when nitrates are altogether absent 

 from the media in which they are cultivated. Frankland states that 



