220 THE LIVING ORGANISMS OF THE SOIL [chap. 



the strong acids will not nitrify directly in the absence 

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

 carbonate usually added is 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. 



Further investigations show that though nitrate 

 production is the normal end process in the breakdown 

 of nitrogenous substances into plant food, from which 

 it follows that the rate of production of nitrates is some 

 measure of the fertility of the soil, yet the nitrate-pro- 

 ducing bacteria are, as a rule, able to convert the 



