1052 Su2)plement to the “ Tropical Agriculturist.” [March 1, 1898. 
experiments with reference to which Dr. Somer- 
ville first made a communication to the R.A.S.E. 
Journal of September last. We have already had 
Dr. Bernard Dyer’s deliverance (summarised in 
the Observer of the 3rd instant,) and now follows 
an exhaustive paper, covering 30 pages, written 
by no less an authority than Prof. Warrington, on 
the same subject. The Professor, after clearly 
stating the conclusions arrived at by the German 
experimenters, begins by tracing the history of the 
denitrifying question, referring to the labours 
of Dr. Angus Smitn (1867), Mensel (1875), 
Deherain and Marquenne ^1882), Dr. Prankland, 
Gayon and Dupetit (1886), Burri and Stutzer 
(1895.) It has been proved to satisfaction that 
the reduction of nitrates is due to certain bacteria 
present in abundance in the soil, in the atmo- 
sphere and all forms of organic matters ; in facts 
these organisms may be said to be universally 
distributed. Their action, however, is controlled 
and regulated by conditions and circumstances. 
There is reason to believe that animal excre- 
ments contain a greater abundance of the denitri- 
fying organism. For one thing it may be taken 
for granted that denitrifying bacteria, like other 
bacteria present in food, pass uninjured through 
the intestines of animals, and will therefore be 
present in larger relative proportion in the solid 
excrement than in the original food. Again, the 
proportion of organisms will depend largely on 
the extent and character of the exposed surface, 
and on the length of its exposure to the atmosphere. 
The denitrifying bacteria belong to a class of 
organisms requiring oxygen, free or combined, to 
accomplish their work, titrates contain a rela- 
tively large proportion of oxygen. It has been 
found that nitrates in the soil steadily diminish 
as the proportion of oxygen present decreases, and 
that denitrification is at a maximum when no free 
oxygen is preseno, as in soils saturated with water. 
Where then does tlie denitrifying organism get its 
essential oxygen ? From the nitrates which con- 
tain it in a combined form, and which, in giving 
it up, become reduced to lower compounds of 
nitrogen, and even to nitrogen gas itself. 
What, perhaps, most controls the process of 
denitrification is the supply of organic matter. 
The abundance of the organisms is of little im- 
portance, since they are always present in some 
proportion, and will increase and multiply if the 
conditions are favourable. If the organic matter 
is small in quantity the denitrifying action will 
be proportionately limited, however large the 
supply of bacteria. Here, says Dr. Warrington, 
i.s where the German experimenters erred, — they 
sought to explain denitrification as due to the 
supply of additional organisms in the manures, 
when the results were really due to the supply 
of an excess of organic matter. In the absence 
of oxidisable organic matter it is the nitrifying 
organisms that command the situation, while 
with abundance of a combustible carbonaceous 
substance the denitrifying bacteria spring into 
activity. It is pointed out that in the German 
experiments with pot culture, while the quantities 
of nitrogen (estimated as such) was the same in 
different experiments, the quantity of organic 
matter was very different in each case. With 
intrate of soda, for iustante, no organic matter 
was supplied 5 with urine and dried blood the 
supply was very small ; with cattle manure it 
became very large, and indeed the German ex- 
periments themselves bear out the fact that the 
denitrifying action increased with the increase «n 
the supply of organic matter in the manure. 
Some may demur to the statement that organic 
matter induces denitrification, audit would, there- 
fore, be necessary to explain that the influence of 
decomposable organic matter affects the process 
both of nitrification and denitrification. The de- 
composition of organic matter and its partial 
oxidation must precede its nitrification. If much 
of this preliminary work has to be done, the com- 
mencement of nitrification will be delayed ; the 
products of the decomposition of carbonaceous 
matters are, indeed, inimical to nitrification. 
When the proportion of organic manure ex- 
ceeas a certain proportion therefore, it will bo 
understood that a nitrifying medium may be 
converted for a time into a denitrifying medium, 
the oxygen demanded by the decomposing 
organic matter being now obtained by the des- 
truction of the nitrates in the soil. An organic 
manure which is effective when applied in 
moderate doses, may thus become injurious when 
in excess. Dr. Warrington then goes on to show 
that this fact, viz., that large doses of ferment- 
able organic matter retard the nitrification of 
other easily nitrified organic manures, must ex- 
plain the evil effects resulting in the German 
experiments, since in these pot experiments the 
additions of dung were equivalent to 40, 70, 100, 
and in some cases 300 tons per acre ! These are 
quantities which, of course, are never heard of 
in practice, so that the great delay of the process 
of nitrification and the production of active deni- 
trifying conditions, was only what was to be 
naturally expected under the circumstances of the 
experiments, and is no proof that the same 
action will occur to the same extent in ordinary 
cultivation. 
But again, all kinds of organic matter have not 
an equal effect, and quantity alone does not 
determine results. We must grasp the fact, 
which Prof. Warrington tries hard to impress 
on us, that denitrification is determined by the 
presence of fermentable organic matters. Hence 
it was that even in the German experiments 
black humified horse manure, and dung applied 
two months before the addition of nitrate of 
soda, did not cause any great loss by denitrification, 
while the use of fresh manure resulted in 
considerable loss ; and again sheep manure, 
which contains less organic matter per unit 
of nitrogen, and that too in an easily uitrifiable 
form, gave much djelter results than horse and 
cattle manure. Prof. Warrington adds that 
superphosphate and kainit decreased the fer- 
mentation of manure when added to it, and 
in this way intensified its denitrifying power. 
Reference is made to a very significant German 
experiment made with the object of arresting 
the denitrifying action, by destroying the deni- 
trifying germs in dung by treating it previously 
with bisulphide of carbon, with the result that 
denitrification was as active as ever ! We could 
not, says Prof. Warrington, have a more con- 
clusive proof of the fact that the presence or 
absence of the organisms in the manure is a 
matter of indifference so long as the neces- 
