SOIL ORGANISMS 237 



through an ammonifying process wherein both bacteria and fungi 

 participate {Mucor, Aspergillus, Fusarimn, etc.). According to Mar- 

 chal (1893), Bacterium mycoides plays a major role in ammonification, 

 especially in cultivated soils. It is an intensively oxidizing organism, 

 which, given a sufficient air supply, unites oxygen with the complex 

 organic compounds and releases their nitrogen in the form of ammonia, 

 NH3. Numerous other representatives of the soil flora react similarly. 

 The end product of this first transformation in the soil is ammonium 

 carbonate, (NH4)2C03. Ammonium carbonate is also derived from 

 the decomposition of urine and excreta under the influence of urine- 

 fermenting bacteria {B. coli, B. vulgare, B. fluorescens, and species of 

 Micrococcus, Urococcus, etc.). (NH 4)200 3 is the initial substance in 

 the formation of nitrates in the soil, that is, in nitrification. 



Nitrate Formation (Nitrification). — Two specific groups of bacteria 

 are involved in the process of oxidation which converts the amino 

 radical into the nitrate radical. These two groups work hand in hand 

 but cannot replace one another. First, the nitrite formers or nitro- 

 bacteria {Nitrosomonas, Nitrosococcus) begin the process, taking up 

 the ammonia compounds and converting them by partial oxidation 

 into nitrous acid or nitrite, HNOo. The process is favored by the 

 presence of alkaline substances, Ca, Mg, etc. 



Then the nitrate bacteria (Nitrobacter) come into action. The 

 process of oxidation goes on by their activity as far as nitrate, HNO3. 

 This is utihzed readily by higher plants. Nitrohacter, which is morpho- 

 logically different from Nitrosomonas and Nitrosococcus, cannot enter 

 into the cycle earher, since the ammonia compounds hinder its develop- 

 ment. Ammonifying, nitrite, and nitrate bacteria are all three neces- 

 sary for the completion of the cycle. Each group, but especially the 

 nitrite and nitrate formers, has its distinctly limited sphere of action. 



Nitrification is favored by light and by an increase of soil moisture, 

 up to approximately the capillary saturation of the soil. Coarsely 

 dispersed soils demand a smaller water content for favorable conditions 

 of nitrate formation than in the case of finely dispersed soils; good 

 aeration of the soil favors this process. In very acid soils nitrification 

 is much decreased. Nevertheless, C. Olsen (1921) has shown that 

 nitric acid formation does occur even in peat soil at pH 3.6. 



Nitrification starts at a temperature of 5°, reaches its optimum at 

 about 37°, and ceases at 57°C. 



Nitrification Capacity. — The rate at which nitrification goes on 

 seems to be independent of the quantity of organic nitrogen in the soil. 

 But the nature of the humus nitrogen present probably affects the 

 speed of ammonification very decidedly. In forests nitrification is 



