1907. | | Nitrification in Acid Souls. 201 
originates with the ammonium salts, or results from the attack of humic 
acid upon them. But free humic acid is not a normal product of decay 
in the soil; the unmanured plot, for example, is not acid; the “humus” of 
soil usually consists of calcium salts.* Further, the acidity of the grass 
plots under discussion increases with the amount of ammonium salts annually 
applied, so that it is to the ammonium salts we may look for the origin of 
the acid. Moreover, as the amount of freely soluble acid in the soil is of the 
same order of magnitude as the sulphuric and hydrochloric acids contained 
in one year’s application of ammonium salts, we can suppose that it has 
been recently split off from them, but it does not accumulate from year to 
year, because it slowly acts upon the calcium humate present, forming the 
sparingly soluble free humic acid, and calcium sulphate and chloride which 
are removed in the drainage water. In confirmation of this opinion, it was 
found that a solution of ammonia would dissolve considerable quantities of 
humic acid from the soil of Plot 11-1, though in the case of normal soils it 
is necessary to break up the humates with hydrochloric acid before they will 
yield any humic acid to ammonia solution. | 
The next step in the enquiry was to find if any agency existed in the soil 
capable of splitting up ammonium salts so as to set free the acids therein. 
In a previous paper,f two of the authors have shown that no free acid 
results from purely chemical or physical interactions of ammonium salts 
and the soil constituents. Neither the double silicates nor the calcium 
humate in the soil exercise a selective absorption of the base to set free 
the acid, nor is there hydrolysis of the salt followed by adsorption or 
evaporation of the ammonia; the action between ammonium salts and the 
soil constituents is always in the nature of a double decomposition attended 
by no change in the neutrality of the medium. Being thus constrained to 
look for a biological explanation of the acidity in the grass soils, nutrient 
solutions were made up, containing, besides dextrose and the usual nutrient 
_ salts, ammonium sulphate or chloride as the only nitrogenous compound. 
These were rendered faintly acid and seeded either with the fresh soil or 
with a cold water extract from one of the acid grass plots. A vigorous 
crop of moulds and other fungi soon appeared, at the same time the liquid 
became markedly acid. The dominant organisms isolated in this way were 
forms of Penicilliwm glaucum and a mould allied to Mucor, and when 
pure cultivations of any of these species were seeded into solutions con- 
taining a carbohydrate and ammonium salts, the organisms were found to 
grow and to remove ammonia from the solution, which, at the same time, 
* Hall and Gimingham, ‘Chem. Soc. Trans.,’ 1907, vol. 91, p. 677. 
+ Hall and Gimingham, loc. cit. 
