CARBON AND NITROGEN CYCLES IN THE SOIL 87 



Nitrification. 

 The ammonia formed by the action of soil bacteria, or added in 

 manures, is changed to carbonate which is then rapidly converted by 

 Nitrosomonas into nitrite, and this by Nitrobacter into nitrate, the 

 changes proceeding so rapidly that only traces of ammonia or nitrite 

 are ever found in normal arable soils (241). We may, therefore, 

 infer that the production of nitrates is the quickest of the three re- 

 actions, the production of nitrites is slower, while the formation of 

 ammonia is the slowest of all and sets a limit to the speed at which 

 they can take place. Thus a measure of the speed at which nitrates 

 are formed in soil does not measure the rate of nitrification, as is 

 sometimes assumed, but the rate of ammonia production. 



The essential facts of nitriiication are readily demonstrated by 

 putting a small quantity of soil — -2 to -5 grams — into 50 c.c. of a 

 dilute solution of ammonium sulphate containing nutrient inorganic salts 

 and some calcium or magnesium carbonate, but no other carbon com- 

 pound.^ After three or four weeks at 25° the ammonia has all gone 

 and its place is taken by nitrates. The conversion is almost quantitative, 

 only an insignificant quantity of nitrogen being retained by the organisms. 



The course of the oxidation is unknown, and nothing intermediate 

 between ammonia and nitrous acid has been detected. Omelianski 

 could obtain no evidence of an oxidase in Nitrosomonas (222). The 

 action of both organisms seems to be entirely specific. Nitrosomonas 

 oxidises ammonium carbonate and nothing else ; it will not touch 

 nitrates, urea, or the substituted ammonias. Even ammonium salts are 

 only nitrified in presence of a carbonate that can change them into 

 ammonium carbonate (296). Nitrobacter is equally specific, oxidising 

 nitrites^only and not ammonia. 



Addition to the solution of almost any carbon compound other than 

 calcium or magnesium carbonates retards the rate of nitrification, glu- 

 cose and peptone being particularly harmful (312). Carbon dioxide 

 sufificesjjas the source of carbon for the growth of the organism. God- 

 lewski showed that nitrification proceeds in solutions free from organic 

 matter so long as the air supplied contained carbon dioxide, but stops 

 as soon as the carbon dioxide is removed by passage over caustic 

 potash. But the > synthesis of complex cell substances from carbon 



' Omelianski (221) used 2 grams each (NHJjSOj and NaCl, i gram KHjPO,, 

 •5 MgSO^, •4;;FeS04iin i litre of water, and added -5 gram MgCO, for each 50 c.c. of so- 

 lution used. Nitrite formation goes on in this solution. For nitrate production he used 

 I gram each NaNO^nd NajCOj, -5 each KHjPOj and NaCl, -4 FeSOj and -3 MgSO< in i litre 

 of water. Ashby (5) foundjthat both processes went on simultaneously when he diluted the 

 first of these solutions to one quarter the strength. 



