200 Mr. Hall, Dr. Miller, and Mr. Gimingham. _[Dec. 19, 
some of the grasses characteristic of the plots under discussion in a medium 
containing unnitrified ammonium salts have, so far, failed. | 
The next step in the enquiry was, if possible, to determine the nature and 
amount of the acid present in the soil, in order to ascertain if it presented 
any special features which would explain the continuance of nitrification in 
its presence. 
No satisfactory method exists of determining the acidity of a soil, owing to 
the sparing solubility of the humic acids and the difficulty of removing acid 
from so extensive a surface as that presented by soil particles. Extracting 
the soil with lime water, or dilute solutions of alkaline salts, gives results 
that are too high, because of the errors introduced by the presence of 
phosphates, and the interactions which may be set up with the zeolitic 
double silicates, ferric hydrate, and other soil constituents. Since we were 
chiefly concerned with the acidity of the soil water, we have used water only, 
accepting the fact that the results will be too low. When large quantities of 
the undried soil from the most acid plot, 11-1, were rapidly washed with small. 
quantities of hot water, an extract was obtained which showed an acidity 
equivalent to 1°71 grammes of hydrogen per million of dry soil. After this 
extraction the soil still remained acid to litmus paper, even though the washing 
was repeated with large quantities of water, so that the acidity observed can 
only represent the easily soluble acids, leaving the greater part of the “ humic ” 
acids still in the soil residue. The chlorides and sulphates present in the 
water extract were equivalent to 1°91 and 3:15 grammes respectively of 
hydrogen per million of soil, the two together being equivalent to about 
three times the acidity. The manures supplied every year to this plot 
would add sulphates equivalent to 5°2 grammes and chlorides equivalent 
to 2:3 grammes of hydrogen per million of soil; consequently there was 
rather less than one year’s stock of chlorides and sulphates in the top layer 
of soil at the time of sampling. 
Under ordinary field conditions, the amount of water present in the soil 
will vary between 10 and 25 per cent., so that the easily soluble acids 
extracted by washing would give rise to an acidity of the soil water varying 
between one-sixtieth and one-hundred-and-fortieth of normal. 
It is hardly necessary to enquire into the nature of the free acid present ; 
the acid water extract contains humates, chlorides, and sulphates, hence it 
follows that such external factors as relative mass, temperature, and concen- 
tration, will determine which of them will be in the condition of a free 
acid. When the extract is concentrated, a certain amount of “ humic” acid 
is thrown out of solution; when it is brought to complete dryness, hydrogen 
chloride is given off as gas. The real question is, whether the acid 
