Dec. 4, 1916 
Nitrification in Semiarid Soils 
43 i 
more probable, however, that the magnesium carbonate in low concen¬ 
trations was toxic to the nitrate formers but not to the nitrite formers, 
whereas in still higher concentrations it was toxic to both groups. 
In the course of some studies on the effects of concentration on the 
nitrification of ammonium carbonate it was observed that notable amounts 
of nitrite began to accumulate as the concentration was increased above 
15 mgm. of nitrogen per 100 gm. of soil; with a concentration of 30 mgm. 
the nitrite content, after four weeks’ incubation, was found to be 268.5 
p. p. m., while at the same time no nitrate was formed. From these 
observations it would seem that the nitrate bacteria are more sensitive 
to high concentrations of ammonium carbonate than the nitrite group, 
as has been definitely shown to be the case by Boullanger and Massol (4). 
The effects on nitrite accumulation brought about by the concentra¬ 
tion of different nitrogenous substances and in different soils, the effects 
produced by the addition of alkali salts, organic matter, etc., have been 
studied at varying intervals of time. The full data will be presented 
in a later paper. Briefly, it may be stated that not more than a few 
tenths p. p. m. of nitrite have been found where low concentrations of 
nitrogenous materials have been used, but that as the conditions become 
increasingly abnormal, either through the use of excessive amounts of 
nitrogenous substances, the addition of alkali salts, or by other means, a 
point is usually reached where nitrite formation proceeds more vigor¬ 
ously than nitrate formation, with a consequent accumulation of con¬ 
siderable amounts of nitrite. In addition, it has been found that under 
certain conditions nitrite formation may proceed vigorously without 
nitrate formation taking place at all, even in a soil where the nitrate 
bacteria are present in abundance. 
The occurrence of notable amounts of nitrite necessitates some depar¬ 
ture from the methods usually employed in the determination of nitrate 
in soils. As already stated, the results obtained by the phenol-disulphonic- 
acid method were found to agree closely with those obtained by the 
aluminum-reduction method except where high concentrations of ni¬ 
trogenous materials had been employed. In such cases the reduction 
method frequently gave much higher results. 1 As shown below, the use 
of the reduction method effects a conversion of nitrite into ammonia, 
just as is the case with nitrates; and consequently the results found 
represent the total of the nitrite and nitrate nitrogen present, although 
it is recorded as nitrate. 
If a solution containing nitrite is evaporated and the residue then 
treated with the phenol-disulphonic-acid reagent, small amounts of nitrite 
may also be converted into nitrate, thus introducing a slight error. 
After considerable experimentation the method adopted for the deter¬ 
mination of nitrate in the presence of nitrite was as follows: The water 
1 As shown by Allen (i), certain soluble organic forms of nitrogen also become reduced to ammonia under 
the conditions employed in this method. 
