664 
THE TROPICAL AGRICULTURIST. [April 1, 1901. 
it is surmised by some that these many possibly 
be a modified stage, so to say, of the former, 
From experiments made iu laboratories it has 
been ascertained that in cultures seeded with soil 
the development of nitrites precedes that of 
nitrates, Thus ammonia is first converted into 
nitrous acid, and this latter into nitric acid. 
This has led to the conclusion that two physio- 
loe;ically distinct organisms are required to accom- 
plish the transformation of ammonia into nitrates. 
Wiaogradsky is of this opinion, and claims to have 
separated the nitrate-forming microbe. 
He has proposed for the group of nitrifying or- 
ganisms the name of nilrohncleria, giving the generic 
term Nitrosomonas to the nitrate-forming types, 
and Nitrohacter to those who transform, nitrites into 
nitrates. 
NiTEOGEN-FixiN0 Bacteria. 
Besides the nitrifying bacteria which are able to 
transform ammonium salts found naturally in soil, 
or have been added thereto, into other nitrogenous 
compounds, a number of related organisms are met 
with in many fields which have the power of utilising 
the free nitrogen of the atmosphere and drawing from 
this vast store of almost inert gas considerable 
quantities for plant-food. These are the nitrogen- 
fixing bacteria. 
On the rootlets of many higher plants, more 
especially on those of the Lef/uminosce, small nodules 
in varying numbers are found produced by and tilled 
with bacteria. It is supposed that by the symbiosit 
(a living together) of these lowest forms of plant-life 
with the higher plants, the latter derive the nitro- 
genous food which it is proved cannot have been 
derived from the soil, and therefore must have been 
obtained from the atmosphere. The process is not 
yet properly understood, but the general opinion 
tends towards the assumption that the bacteria fix 
the free nitrogen within the nodules and that the 
resulting nitrogenous compounds are asiimilated by 
the host-plant. By some also it is thought that 
through the peculiar conditions of '' living together " 
the plant is enabled to fix free nitrogen in its foliage. 
"Whatever may be the correct theory, the effect 
of this remarkable inter-action between the lower 
forms and the higher plants is very striking and 
very variablein extent. Even amongstjthe Leguminosat, 
the plants deriving the greatest advantage from 
this phenomenon, extremes are met with ; some 
deriving apparently but little benefit from it, whilst 
on the other hand many may very largely depend 
upon it. Amongst the Lupines, for instance, the 
yellow-flowering variety is able to entirely dispense 
with nitrogenous substances in the soil, 
Through the exhaustive investigations made first 
by Professors Hellriegel and Willsarth; and later by 
Lawest, Gilbert, and others, on nearly all the 
cultivated leguminous plants, no doubt has been 
left that the nodules found on the roots are formed 
through bacteria, and that these are able to fix free 
nitrogen for the use of the plant they attach them- 
selves to. Until Hellriegel proved that the presence 
of bacteria is necessary to enable plants to utilise 
the nitrogen of tne atmosphere, and that, for this 
reason, Leguminosae may almost entirely dispense 
with nitrogenous manuring of the soil they grow 
upon, and in many instances even enrich the land 
with nitrogen, these observed facts were not properly 
understood. 
It is well-known in practice that clover and lucerne 
would grow vigorously for a period of years without 
beinij manured, and when flagging could often be 
invigorated by a dressing with gypsum. This tends 
to prove that these crops did not sicken for the 
want of nitrogenous food, but on account of other 
elements becoming exhausted or unobtainable for 
some reason from the soil. 
A palpable proof that the help of bacteria 
is almost absolutely necessary to enable plants to 
aaaimilate atmospheric nitrogen is afforded by the 
fact that seedlings, say, of peas, will not thrive 
unless the soil contains at least some traces of 
nitrogenous compounds; but as soon as they have 
made a start and have sent out rootlets upon which 
the bacteria can form colonies, they prosper inde- 
pendently of the presence of this food in the soil. 
During the experiments carried on in several 
German agricultural establishments it was discovered 
that e^'ery species o£ legumes was associated with a 
specially sympathetic bicterium which would not 
perform the office of fixing nitrogen for other 
species. Based upon the acquisition of this know- 
ledge, Professor Nobbe, of Tharand, in Saxony, is 
now preparing a number of pure cultures of these 
specific bacteria for the purpose of s jwing them 
together with their respective culture plants. These 
cultures are placed on the market under the name 
of Kitragen, and for some time have been under- 
going and still undergo practical tests regarding 
the eflioacy of promoting the growth of plants. 
It has not been definitely acertained how long 
these artificial cultures can retain their vitality 
unimpaired ; and, besides, in some instances, ad- 
verse seasons have prevented a definite judgment 
being arrived at as yet, whether the results ob- 
tained in trial plots can be maintained on a larger 
scale in the field. Considerable attention is being 
paid to this question at the agricultural stations iu 
Germany as well as in the United States and else- 
where. 
The De-nitrifying Bactebia. 
In addition to the organisms hitherto referred 
to, all of which are pre-eminently friendly to plant- 
life, and thus indirectly to man, there are a host 
of other bacteria met with in soil and water which 
play an important part in connection with agricul- 
ture. 
Their action is to reduce the compound organic 
substances into less complex combinations, or into 
simple elements, and in this manner make them 
again available for plants to which otherwise they 
would be loit, as these can iitilise nothing but 
elements or simple compounds for their nutrition. 
It is manifest that, were it not for the decompo- 
sition of the many complex substances taken from 
the earth in the shape of plants and animals, after 
these have changed from the active state called 
life to that of inactivity or death, and by this pro- 
cess are redissolved periodically, these substances 
would be entirely lost to succeeding generations of 
plants and animals, and this constant drain from 
the resources now found on the surface of the globe 
would ultimately exhaust their supply and make 
life impossible. 
That putrefaction and other processes of decom- 
positions are produced by bacteria was suspected 
for a considerable time, but it was not definitely 
proved till 1875 by Menz'ii, and subsequently 
verified by others. The action of these orgauisms 
is so variable that in the decomposition of any 
given substance probably a dozen species participate. 
From this indisputable achievement of scientific 
research it will be seen that it is in the first 
instance entirely due to the activity of the minutest 
organisms that what is of earth goes back to it 
again by the dissolution into simpler substances 
of the complex and intricate combinitions. Chemi- 
cal action, no doubt, has also much to do with 
the redissolution ; but it is now accepted that this 
activity in the generality of instances is secondary 
to the bacterial, and takes place after these have 
broken up the compounds. 
Without the one process the othe ■ would not 
take place, and from this it is evidu^nt that micro- 
organisms are more closjly connected with the 
productiveness of the soil than was dreamt of less 
than thirty years ago, and that not only iu medicine 
and industrial pursuits, but.also from an agronomic 
point of view, bacteriology is becoming daily of 
greater importance. 
