DECOMPOSITION OF ORGANIC MATTER IN THE SOIL. 259 



and many others. A large number of bacteria, yeasts and molds were 

 tested with negative results. Warington, who gathered a great mass 

 of valuable information about nitrification, almost succeeded in securing 

 pure cultures of nitrifying bacteria. Finally, Winogradski showed in 

 1890 not only that nitrification is caused by specific bacteria, but explained 

 also why the others failed in securing pure cultures. He proved that 

 these organisms do not develop colonies on the ordinary gelatin and 

 other organic media, a fact whose recognition was largely responsible 

 for his successful solution of the problem. The medium subsequently 

 employed by him consisted of silicate jelly properly supplied with inorganic 

 nutrient salts. After him other investigators proved that agar, deprived 

 of its soluble organic matter, gypsum and sandstone disks, filter-paper 

 pads, etc., could be used effectively as solid media. 



Nitrous and Nitric Bacteria. Winogradski's investigations led to the 

 conclusion, foreshadowed by the earlier work of the Franklands and 

 Warington, that the oxidation of ammonia proceeds in two stages, viz., 



(i) 2 NH 3 +3O 2 =2HNO 2 +2H 2 O 

 ( 2 )2HNO 2 +O 2 =2HN0 3 



The organisms oxidizing ammonia to nitrites, and designated as 

 nitrous or nitrite bacteria, were called by Winogradski Nitrosomonas 

 and Nitrosococcus. The former include species or varieties isolated 

 from soils in Europe, Asia and Africa, and the latter those isolated from 

 soils in America and Australia. The organisms oxidizing nitrites to 

 nitrates and known as nitric or nitrate bacteria, were included by Wino- 

 gradski in the genus Nitrobacter. 



Apart from these bacteria there is an organism, according to Kaserer, 

 that can oxidize ammonia directly to nitrate. He named it B. nitrator. 

 The reaction is illustrated by the following equation: 



NH 3 +H 2 CO 3 +O 2 =HNO 3 +H 2 O+CH 2 O-4i Cal. 

 CH 2 O+O 2 =H 2 CO 3 -f-i32 Cal. 



Enough energy for the completion of the reaction is obtained by the 

 oxidation of the formaldehyde (CH 2 O). Beyond the preliminary 

 announcement of Kaserer's there are no experimental data to prove the 

 existence of this organism, even though other evidence of an indirect 

 nature may be construed to lend support to his theory. But whether it be 

 proved or not that ammonia may be oxidized to nitrate by a single species, 



