202 Mr. Hall, Dr. Miller, and Mr. Gimingham. _[Dec. 19, 
became acid, the acidity being sometimes greater and sometimes less than 
the equivalent of the ammonia withdrawn. The degree of acidity attained in 
this way lay between one-fiftieth and one-two-hundredth of normal; usually,. 
growth stopped at about one-eightieth normal. It is significant that this is. 
about the same degree of acidity as was found to prevail in the soil water of 
the acid grass plots. That the acidity is due to the attack of the organisms. 
on the ammonium salts is seen from the fact that the culture solutions 
remain neutral when, asparagin is substituted for the ammonium salts, or 
become alkaline when peptone is used as the source of nitrogen. Consider- 
ing the abundance of these moulds in the peaty surface soil of the acid 
grass plots, and the rapidity with which they will work (im vitro the: 
maximum acidity is attained in a week or ten days, just as the grass plots 
are most acid a few days after the application of the ammonium salts), the: 
authors conclude that they are the active agents in producing the acidity of 
the soil of the grass plots in question. That the change in the ammonium 
salts takes this direction is due to the initial acid state of the soil; in such 
an acid medium the moulds have free play, while the nitrifying bacteria 
attacking ammonium salts in normal soils are checked or even suppressed.. 
Doubtless the present acidity grew up gradually and locally as the original 
small stock of calcium carbonate in the soil became removed by the solvent. 
action of the ammonium salts. 
Having thus reached the conclusion, both by direct examination and 
consideration of origin, that the acidity observed in the soil is due simply 
to sulphuric and hydrochloric acids derived from the ammonium salts, how it. 
is possible for nitrification still to go forward, since all investigators are agreed 
that the action is stopped as soon as the medium ceases to be neutral or- 
faintly alkaline? To test the point further, water extracts were made from 
the soil of Plot 11-1, and concentrated until they represented the normal 
soil water, a little ammonium sulphate was added, and they were seeded with 
a strongly nitrifying extract from a garden soil and put to incubate.. 
Check solutions were treated in the same way, but were first neutralised 
by the supply of calcium and magnesium carbonates. The nitrates were. 
determined in portions of the soil extract at the outset and after three 
weeks’ incubation, when it was found that no nitrification had taken place. 
except in the neutralised solutions. Dilute cold water extracts from the. 
acid soil also refused to nitrify except when neutralised. 
When the extracts were not neutralised they were always invaded by 
moulds, which were especially abundant in the cold water extracts. 
Thus there is found in the soil a degree of acidity which will inhibit. 
nitrification in vitro; even a water extract of the soil will not nitrify, ye 
