48 PRESIDENT’S ADDRESS. 
upon, and this accounts for the advantage of allowing fields 
to lie fallow or in grass. 
Bacteria do not appear to require carbonaceous food 
beyond carbonates, and preferably bi-carbonates. For their 
nitrogenous food, nitrogen, ammonia, nitrites and nitrates, 
seem to be sufficient. 
Bacteria can bring about the requisite chemical changes 
in the dark, but green plants can only build up chemical 
compounds from carbon dioxide and water in the presence 
of hght. 
The nitrogen oxides formed by electric discharges and 
other means in the atmosphere, and carried down by rain, 
are also of great importance and value. <A large amount 
of the nitrous and nitric acids, nitrites and nitrates, present 
in the soil are carried off by drainage, since they are all 
very soluble in water. 
The nitric acid in living plants is not carried off by water, 
but is held’in some way at present unknown. Froma dead 
plant it is readily extracted by water. 
There are also other ferments present in soils which 
reduce or destroy nitric acid, whereby it is lost; these are 
known as denitrifymg ferments. Fortunately these imimi- 
cal ones are usually present in smaller numbers than the 
friendly ones ; the conditions favourable to these are also 
well worthy of the closest investigation. 
Nitrifying organisms are widely distributed; they are 
found even on the bare mountain tops and in the deep- 
seated chalk obtained from the deepest artesian wells. 
The culture of the useful nitrifying organisms has become 
a matter of business, and, under the name of nitragin, they 
are put up in bottles for the market. Different cultivations 
or varieties can be purchased to suit the particular soil or 
kind of crop which it is desired to inoculate, but they do not 
always appear to be successful. 
Closely connected with the foregoing is the oceurrence of 
nitrogen and other gases in rocks, @.e., as distinguished from 
