Vli.] CONDITIONS FAVOURING NITRIFICATION I9J 



with the nitrous or nitric acids produced, for nitrification 

 ceases as soon as the medium becomes at all acid. 

 While calcium carbonate is the substance which, 

 as a rule, is effective to this end, many organic salts will 

 also supply the necessary base. Ammonium salts of 

 the strong acids will not nitrify directly in the absence 

 of a base, and the function of the calcium or magnesium 

 carbonate is usually added to form, by double decomposi- 

 tion, ammonium carbonate, which the nitrifying organ- 

 isms can attack. The complex salts formed by the 

 interaction of the zeolites of clay with ammonium 

 salts can be nitrified directly, but not, however, the 

 ammonium humate formed by the corresponding 

 interaction of ammonium salts and humus. Humus 

 itself does not inhibit nitrification, and, indeed, the 

 organisms can be brought to tolerate considerable 

 quantities of other organic matter, by transferring 

 them into successively stronger solutions. The organ- 

 isms are able to obtain the carbon necessary to their 

 growth from carbonates in the culture medium or 

 carbonic acid in the air ; the energy necessary to 

 decompose the carbon dioxide and fix the carbon is 

 derived from the oxidation of the ammonia, about 

 35 parts of nitrogen being oxidised for each part of 

 carbon that is fixed. The nitrifying organisms are 

 chiefly confined to the cultivated surface layer of 

 the soil. Warington found that, in the close-textured 

 Rothamsted soil they were by no means uniformly 

 distributed below the top 9 inches, and that they were 

 never present, except accidentally, in the subsoil below 

 a depth of 2 feet. It has also been shown that they 

 are entirely absent from many heath and moor soils, 

 even in the surface layer. They are abundantly found 

 in the water of shallow wells and rivers. 



Summing up the above facts, it is seen that for the 



N 



