NITRATES AND NITRITES 159 



nitrogen compounds of the soil may be increased at the 

 expense of the nitrate in it. Such syntheses of insoluble 

 nitrogenous material temporarily reduce the amount of 

 nitrogen available for the growth of ordinary field crops : 

 after deatn of the bacteria, however, nitrification sets 

 in, and the nitrogen becomes again of service for the 

 nutrition of green plants. Experience of farmers and 

 experiments at Rothamsted and elsewhere have shown 

 that under ordinary conditions of farming a good return 

 is generally obtained from the nitrogen of nitrate of 

 soda, even when applied with dung, so long as the latter 

 is used in moderate amounts. It is only in gardens 

 where 50 to 100 tons of fresh dung per acre are 

 occasionally employed that denitrifying effects such as 

 those observed in Wagner and Maercker's pot experi- 

 ments are likely to be encountered. 



A large number of species of bacteria are known to 

 possess the power of reducing nitrates to nitrites ; a 

 much smaller number, however, are found capable of 

 destroying nitrates with the evolution of free nitrogen. 



From the recent investigations of Beijerinck and 

 Minkman, it would appear that the denitrification of 

 a nitrate in the soil usually results in the formation of 

 considerable amounts of carbon dioxide and nitrous 

 oxide gases, from the latter of which free nitrogen is 

 ultimately evolved. The decompositions occurring in 

 the process may be expressed thus : 



2KNO 3 + C... 2KNO 2 + CO 2 

 2KNO 2 + C... = 2K 2 CO 3 + N 2 O 

 2N 2 O + C... =CO 2 + 2N 2 

 where C... represents an organic carbon compound. 



They found that in some instances the mixed gases 

 given off from bouillon with 5 to 1 2 per cent, of nitrate 



