1 88 SOIL CONDITIONS AND PLANT GROWTH 



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 reactions, 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. 1 



The essential facts of nitrification are readily demonstrated 

 by putting a small quantity of soil '2 to '5 grm. 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 compound. 2 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 in- 

 significant 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. On general grounds one would look for hydroxy- 

 lamine. A. Bonazzi 3 has made the significant observation 

 that nitrififation in culture solutions is intensified by thorough 

 aeration or continuous motion of the culture medium. 



Miyake has shown (202^) that the reaction resembles 

 autocatalysis in its course. Omelianski could obtain no 

 evidence of an oxidase in Nitrosomonas (2\gc). 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 



1 See also P. L. Gainey, Soil Sci., 1917, 3, 399-416. 



2 Omelianski (2196) used 2 grms. each (NH 4 ) 2 SO 4 and NaCl, i grm. 

 KH g PO 4 , -5 MgSO 4 , -4 FeSO 4 in i litre of water, and added *5'grm. MgCO 3 for 

 each 50 c.c. of solution used. Nitrite formation goes on in this solution. For 

 nitrate production he used i grm. each NaNO 2 and Na 2 CO 3 , '5 each KH 2 PO 4 and 

 NaCl, -4 FeSO 4 and -3 MgSO 4 in i litre of water. Ashby (ja and b) found that 

 both processes went on simultaneously when he diluted the first of these solutions 

 to one quarter the strength. 



3 A. Bonazzi, y. Bact., 1919, 4, 58. 



