BACTERIA IN NATURAL PROCESSES.--AGRICULTURE.
Thus far, in considering the relations of bacteria to mankind, we
have taken into account only the arts and manufactures, and have
found bacteria playing no unimportant part in many of the
industries of our modern civilized life. So important are they
that there is no one who is not directly affected by them. There
is hardly a moment in our life when we are not using some of the
direct or indirect products of bacterial action. We turn now,
however, to the consideration of a matter of even more fundamental
importance; for when we come to study bacteria in Nature, we find
that there are certain natural processes connected with the life
of animals and plants that are fundamentally based upon their
powers. Living Nature appears limitless, for life processes have
been going on in the world through countless centuries with
seemingly unimpaired vigour. At the very bottom we find this
never-ending exhibition of vital power dependent upon certain
activities of micro-organisms. So thoroughly is this true that, as
we shall find after a short consideration, the continuance of life
upon the surface of the world would be impossible if bacterial
action were checked for any considerable length of time. The life
of the globe is, in short, dependent upon these micro-organisms.
BACTERIA AS SCAVENGERS.
In the first place, we may notice the value of these organisms
simply as scavengers, keeping the surface of the earth in the
proper condition for the growth of animals and plants. A large
tree in the forest dies and falls to the ground. For a while the
tree trunk lies there a massive structure, but in the course of
months a slow change takes place in it. The bark becomes softened
and falls from the wood. The wood also becomes more or less
softened; it is preyed upon then by insect life; its density
decreases more and more, until finally it crumbles into a soft,
brownish, powdery mass, and eventually the whole sinks into the
soil, is overgrown by mosses and other vegetation, and the tree
trunk has disappeared from view. In the same way the body of the
dead animal undergoes the process of the softening of its tissues
by decay. The softer parts of the body rapidly dissipate, and even
the bones themselves eventually are covered with the soil and
disintegrated, until in time they, too, disappear from any visible
existence. This whole process is one of decay, and the result is
that the solid mass of the body of the tree or of the animal has
been decomposed. What has become of it? The answer holds the
secret of Nature's eternal freshness. Part of it has dissipated
into the air in the form of gases and water vapour; part of it has
changed its composition and has become incorporated into the soil,
the final result being that the body of the plant or animal
disappears as such, and its substance is converted into gaseous
form, which is dissipated in the air or into simple compounds
which sink into the earth.
This whole process of decay of organic life is one in which
bacteria play the most important part. In the case of the
decomposition of the woody matter of the tree trunk, the process
is begun by the agency of moulds, for this group of organisms
alone appears to be capable of attacking such hard woody
structure. The later part of the decay, however, is largely carried
on by bacterial life. In the decomposition of the animal tissues,
bacteria alone are the agents. Thus the process by which organic
matter is dissipated into the air or incorporated into the soil is
one which is primarily presided over by bacterial life.
Viewing this matter in a purely mechanical light, the importance
of bacteria in thus acting as scavengers can hardly be
overestimated. If we think for a moment of the condition of the
world were there no such decomposing agents to rid the earth's
surface of the dead bodies of animals and plants, we shall see
that long since the earth would have been uninhabitable. If the
dead bodies of plants and animals of past ages simply accumulated
on the surface of the ground without any forces to reduce them
into simple compounds for dissipation, by their very bulk they
would have long since completely covered the surface of the earth
so as to afford no possible room for further growth of plants and
animals. In a purely mechanical way, then, bacteria as
decomposition agents are necessary to keep the surface of the
earth fresh and unencumbered so that life can continue.
BACTERIA AS AGENTS IN NATURE'S FOOD CYCLE.
But the matter by no means ends here. When we come to think of it,
it is a matter of considerable surprise that the surface of the
earth has been able to continue producing animals and plants for
the many millions of years during which life has been in
existence. Plants and animals both require food, animals depending
wholly upon plants therefor. Plants, however, equally with
animals, require food, and although they obtain a considerable
portion of their food from the air, yet no inconsiderable part of
it is obtained from the soil. The question is forced upon us,
therefore, as to why the soil has not long since become exhausted
of food. How could the soil continue to support plants year after
year for millions of years, and yet remain as fertile as ever?
The explanation of this phenomenon is in the simple fact that the
processes of Nature are such that the same food is used over and
over again, first by the plant, then by the animal, and then again
by the plant, and there is no necessity for any end of the process
so long as the sun furnishes energy to keep the circulation
continuous. One phase of this transference of food from animal to
plant and from plant to animal is familiar to nearly every one. It
is a well-known fact that animals in their respiration consume
oxygen, but exhale it again in combination with carbon as carbonic
dioxide. On the other hand, plants in their life consume the
carbonic dioxide and exhale the oxygen again as free oxygen. Thus
each of these kingdoms makes use of the excreted product of the
other, and this process can go on indefinitely, the animals
furnishing our atmosphere with plenty of carbonic acid for plant
life, and the plants excreting into the atmosphere at the same
time an abundant sufficiency of oxygen for animal life. The oxygen
thus passes in an endless round from animal to plant and from
plant to animal.
A similar cycle is true of all the other foods of animal and plant
life, though in regard to the others the operation is more complex
and more members are required to complete the chain. The
transference of matter through a series of changes by which it is
brought from a condition in which it is proper food for plants
back again into a condition when it is once more a proper food for
plants, is one of the interesting discoveries of modern science,
and one in which, as we shall see, bacteria play a most important
part. This food cycle is illustrated roughly by the accompanying
diagram; but in order to understand it, an explanation of the
various steps in this cycle is necessary.
It will be noticed that at the bottom of the circle represented in
Fig. 25, at A, are given various ingredients which are found in
the soil and which form plant foods. Plant foods, as may be seen
there, are obtained partly from the air as carbonic dioxide and
water; but another portion comes from the soil. Among the soil
ingredients the most prominent are nitrates, which are the forms
of nitrogen compounds most easily made use of by plants as a
source of this important element. It should be stated also that
there are other compounds in the soil which furnish plants with
part of their food--compounds containing potassium, phosphorus,
and some other elements. For simplicity's sake, however, these
will be left out of consideration. Beginning at the bottom of the
cycle (Fig. 25 A), plant life seizes the gases from the air and
these foods from the soil, and by means of the energy furnished it
by the sun's rays builds these simple chemical compounds into more
complex ones. This gives us the second step, as shown in Fig. 25
B, the products of plant life. These products of plant life
consist of such materials as sugar, starches, fats, and proteids,
all of which have been manufactured by the plant from the
ingredients furnished it from the soil and air, and through the
agency of the sun's rays. These products of plant life now form
foods for the animal kingdom. Starches, fats, and proteids are
animal foods, and upon such complex bodies alone can the animal
kingdom be fed. Animal life, standing high up in the circle, is
not capable of extracting its nutriment from the soil, but must
take the more complex foods which have been manufactured by plant
life. These complex foods enter now into the animal and take their
place in the animal body. By the animal activities, some of the
foods are at once decomposed into carbonic acid and water, which,
being dissipated into the air, are brought back at once into the
condition in which they can serve again as plant food. This part
of the food is thus brought back again to the bottom of the circle
(Fig. 25, dotted lines). But while it is true that animals do thus
reduce some of their foods to the simple condition of carbonic
acid and water, this is not true of most of the foods which
contain nitrogen. The nitrogenous foods are as necessary for the
life as the carbon foods, and animals do not reduce their
nitrogenous foods to the condition in which plants can prey upon
them. While plants furnish them with nitrogenous food, they can
not give it back to the plants. Part of the nitrogenous foods
animals build into new albumins (Fig. 25 C); but a part of them
they reduce at once into a somewhat simpler condition known as
urea. Urea is the form in which the nitrogen is commonly excreted
from the animal body. But urea is not a plant food; for ordinary
plants are entirely unable to make use of it. Part of the nitrogen
eaten by the animal is stored up in its body, and thus the body of
the animal, after it has died, contains these nitrogen compounds
of high complexity. But plants are not able to use these
compounds. A plant can not be fed upon muscle tissue, nor upon
fats, nor bones, for these are compounds so complex that the
simple plant is unable to use them at all. So far, then, in the
food cycle the compounds taken from the soil have been built up
into compounds of greater and greater complexity; they have
reached the top of this circle, and no part of them, except part
of the carbon and oxygen, has become reduced again to plant food.
In order that this material should again become capable of
entering into the life of plants so as to go over the circle
again, it is necessary for it to be once more reduced from its
highly complex condition into a simpler one.
Now come into play these decomposition agencies which we have been
studying under the head of scavengers. It will be noticed that the
next step in the food cycle is taken by the decomposition
bacteria. These organisms, existing, as we have already seen, in
the air, in the soil, in the water, and always ready to seize hold
of any organic substance that may furnish them with food, feed
upon the products of animal life, whether they are such products
as muscle tissue, or fat, or sugar, or whether they are the
excreted products of animal life, such as urea, and produce
therein the chemical decomposition changes already noticed. As a
result of this chemical decomposition, the complex bodies are
broken into simpler and simpler compounds, and the final result is
a very thorough destruction of the animal body or the material
excreted by animal life, and its reduction into forms simple
enough for plants to use again as foods. Thus the bacteria come in
as a necessary link to connect the animal body, or the excretion
from the animal body, with the soil again, and therefore with that
part of the circle in which the material can once more serve as
plant food.
But in the decomposition that thus occurs through the agency of
the putrefactive bacteria it very commonly happens that some of
the food material is broken down into compounds too simple for use
as plant food. As will be seen by a glance at the diagram (Fig. 25
D), a portion of the cleavage products resulting from the
destruction of these animal foods takes the form of carbonic-acid
gas and water. These ingredients are at once in condition for
plant life, as shown by the dotted lines. They pass off into the
air, and the green leaves of vegetation everywhere again seize
them, assimilate them, and use them as food. Thus it is that the
carbon and the oxygen have completed the cycle, and have come back
again to the position in the circle where they started. In regard
to the nitrogen portion of the food, however, it very commonly
happens that the products which arise as the result of the
decomposition processes are not yet in proper condition for plant
food. They are reduced into a condition actually too simple for
the use of plants. As a result of these putrefactive changes, the
nitrogen products of animal life are broken frequently into
compounds as simple as ammonia (NH3), or into compounds which the
chemists speak of as nitrites (Fig. 25 at D). Now these compounds
are not ordinarily within the reach of plant life. The luxuriant
vegetation of the globe extracts its nitrogen from the soil in a
form more complex than either of the compounds here mentioned;
for, as we have seen, it is nitrates chiefly that furnish plants
with their nitrogen food factor. But nitrates contain considerable
oxygen. Ammonia, which is one of the products of putrefactive de-
composition, contains no oxygen, and nitrites, another factor,
contains less oxygen than nitrates. These bodies are thus too
simple for plants to make use of as a source of nitrogen. The
chemical destruction of the food material which results from the
action of the putrefactive bacteria is too thorough, and the
nitrogen foods are not yet in condition to be used by plants.
Now comes in the agency of still another class of micro-organisms,
the existence of which has been demonstrated to us during the last
few years. In the soil everywhere, especially in fertile soil, is
a class of bacteria which has received the name of nitrifying
bacteria (Fig. 26). These organisms grow in the soil and feed upon
the soil ingredients. In the course of their life they have
somewhat the same action upon the simple nitrogen cleavage
products just mentioned as we have already noticed the vinegar-
producing species have upon alcohol, viz., the bringing about a
union with oxygen. There are apparently several different kinds of
nitrifying bacteria with different powers. Some of them cause an
oxidation of the nitrogen products by means of which the ammonia
is united with oxygen and built up into a series of products
finally resulting in nitrates (Fig. 26). By the action of other
species still higher nitrogen compounds, including the nitrites,
are further oxidized and built up into the form of nitrates. Thus
these nitrifying organisms form the last link in the chain that
binds the animal kingdom to the vegetable kingdom (Fig. 25 at 4).
For after the nitrifying organisms have oxidized nitrogen cleavage
products, the results of the oxidation in the form of nitrates or
nitric acid are left in the soil, and may now be seized upon by
the roots of plants, and begin once more their journey around the
food cycle. In this way it will be seen that while plants, by
building up compounds, form the connecting link between the soil
and animal life, bacteria in the other half of the cycle, by
reducing them again, give us the connecting link between animal
life and the soil. The food cycle would be as incomplete without
the agency of bacterial life as it would be without the agency of
plant life.
But even yet the food cycle is not complete. Some of the processes
of decomposition appear to cause a portion of the nitrogen to fly
out of the circle at a tangent. In the process of decomposition
which is going on through the agency of micro-organisms, a
considerable part of the nitrogen is dissipated into the air in
the form of free nitrogen. When a bit of meat decays, part of the
meat is, indeed, converted into ammonia or other nitrogen
compounds, but if the putrefaction is allowed to go on, in the end
a considerable portion of it will be broken into still simpler
forms, and the nitrogen will finally be dissipated into the air in
the form of free nitrogen. This dissipation of free nitrogen into
the air is going on in the world wherever putrefaction takes
place. Wherever decomposition of nitrogen products occurs some
free nitrogen is eliminated. Now, this part of the nitrogen has
passed beyond the reach of plants, for plants can not extract free
nitrogen from the air. In the diagram this is represented as a
portion of the material which, through the agency of the
decomposition bacteria, has been thrown out of the cycle at a
tangent (Fig. 25 E). It will, of course, be plain from this that
the store of nitrogen food must be constantly diminishing. The
soil may have been originally supplied with a given quantity of
nitrogen compound, but if the decomposition products are causing
considerable quantities of this nitrogen to be dissipated in the
air, it plainly follows that the total amount of nitrogen food
upon which the animal and vegetable kingdoms can depend is
becoming constantly reduced by such dissipation.
There are still other methods by which nitrogen is being lost from
the food cycle. First, we may notice that the ordinary processes
of vegetation result in a gradual draining of the soil and a
throwing of its nitrogen into the ocean. The body of any animal or
any plant that chances to fall into a brook or river is eventually
carried to the sea, and the products of its decomposition pass
into the ocean and are, of course, lost to the soil. Now, while
this gradual extraction of nitrogen from the soil by drainage is a
slow one, it is nevertheless a sure one. It is far more rapid in
these years of civilized life than in former times, since the
products of the soil are given to the city, and then are thrown
into its sewage Our cities, then, with our present system of
disposing of sewage, are draining from the soil the nitrogen
compounds and throwing them away.
In yet another direction must it be noticed that our nitrogen
compounds are being lost to plant life--viz., by the use of various
nitrogen compounds to form explosives. Gunpowder, nitro-glycerine,
dynamite, in fact, nearly all the explosives that are used the
world over for all sorts of purposes, are nitrogen compounds. When
they are exploded the nitrogen of the compound is dissipated into
the air in the form of gas, much of it in the form of free
nitrogen. The basis from which explosive compounds are made
contains nitrogen in the form in which it can be used by plants.
Saltpetre, for example, is equally good as a fertilizer and as a
basis for gunpowder. The products of the explosion are gases no
longer capable of use by plants, and thus every explosion of
nitrogen compounds aids in this gradual dissipation of nitrogen
products, taking them from the store of plant foods and throwing
them away.
All of these agencies contribute to reduce the amount of material
circulating in the food cycle of Nature, and thus seem to tend
inevitably in the end toward a termination of the processes of
life; for as soon as the soil becomes exhausted of its nitrogen
compounds, so soon will plant life cease from lack of nutrition,
and the disappearance of animal life will follow rapidly. It is
this loss of nitrogen in large measure that is forcing our
agriculturists to purchase fertilizers. The last fifteen years
have shown us, however, that here again we may look upon our
friends, the bacteria, as agents for counteracting this
dissipating tendency in the general processes of Nature. Bacterial
life in at least two different ways appears to have the function
of reclaiming from the atmosphere more or less of this dissipated
free nitrogen.
In the first place, it has been found in the last few years that
soil entirely free from all common plants, but containing certain
kinds of bacteria, if allowed to stand in contact with the air,
will slowly but surely gain in the amount of nitrogen compounds
that it contains. These nitrogen compounds are plainly
manufactured by the bacteria in the soil; for unless the bacteria
are present they do not accumulate, and they do accumulate
inevitably if the bacteria are present in the proper quantity and
the proper species. It appears that, as a rule, this fixation of
nitrogen is not performed by any one species of microorganisms,
but by two or three of them acting together. Certain combinations
of bacteria have been found which, when inoculated in the soil,
will bring about this fixation of nitrogen, but no one of the
species is capable of producing this result alone. We do not know
to what extent these organisms are distributed in the soil, nor
how widely this nitrogen fixation through bacterial life is going
on. It is only within a short time that it has been demonstrated
to exist, but we must look upon bacteria in the soil as one of the
factors in reclaiming from the atmosphere the dissipated free
nitrogen.
The second method by which bacteria aid in the reclaiming of this
lost nitrogen is by a combined action of certain species of
bacteria and some of the higher plants. Ordinary green plants, as
already noted, are unable to make use of the free nitrogen of the
atmosphere It was found, however, some fifteen years ago that some
species of plants, chiefly the great family of legumes, which
contains the pea plant, the bean, the clover, etc, are able, when
growing in soil that is poor in nitrogen, to obtain nitrogen from
some source other than the soil in which they grow. A pea plant in
soil that contains no nitrogen products and watered with water
that contains no nitrogen, will, after sprouting and growing for a
length of time, be found to have accumulated a considerable
quantity of fixed nitrogen in its tissues The only source of this
nitrogen has been evidently from the air which bathes the leaves
of the plant or permeates the soil and bathes its roots This fact
was at first disputed, but subsequently demonstrated to be true,
and was found later to be associated with the combined action of
these legumes and certain soil bacteria. When a legume thus gains
nitrogen from the air, it develops upon its roots little bunches
known as root nodules or root tubercles. The nodules are sometimes
the size of the head of a pm, and sometimes much larger than this,
occasionally reaching the size of a large pea, or even larger.
Upon microscopic examination they are found to be little nests of
bacteria In some way the soil organisms (Fig 27) make their way
into the roots of the sprouting plant, and finding there congenial
environment, develop in considerable quantities and produce root
tubercles in the root. Now, by some entirely unknown process, the
legume and the bacteria growing together succeed in extracting
the nitrogen from the atmosphere which permeates the soil, and
fixing this nitrogen in the tubercles and the roots in the form of
nitrogen compounds. The result is that, after a proper period of
growth, the amount of fixed nitrogen in the plant is found to have
very decidedly increased (Fig 25 E).
This, of course, furnishes a starting point for the reclaiming of
the lost atmospheric nitrogen. The legume continues to live its
usual life, perhaps increasing the store of nitrogen in its roots
and stems and leaves during the whole of its normal growth.
Subsequently, after having finished its ordinary life, the plant
will die, and then the roots and stems and leaves, falling upon
the ground and becoming buried, will be seized upon by the
decomposition bacteria already mentioned. The nitrogen which has
thus become fixed in their tissues will undergo the destructive
changes already described. This will result eventually in the
production of nitrates. Thus some of the lost nitrogen is restored
again to the soil in the form of nitrates, and may now start on
its route once more around the cycle of food.
It will be seen, then, that the food cycle is a complete one.
Beginning with the mineral ingredients in the soil, the food
matter may start on its circulation from the soil to the plant,
from the plant to the animal, from the animal to the bacterium and
from the bacterium through a series of other bacteria back again
to the soil in the condition in which it started. If, perchance,
in this progress around the circle some of the nitrogen is thrown
off at a tangent, this, too, is brought back again to the circle
through the agency of bacterial life. And so the food material of
animals and plants continues in this never-ceasing circulation. It
is the sunlight that furnishes the energy for the motion. It is
the sunlight that forces the food around the circle and keeps up
the endless change; and so long as, the sun continues to shine
upon the earth there seems to be no reason why the process should
ever cease. It is this repeated circulation that has made the
continuation of life possible for the millions and millions of
years of the earth's history. It is this continued circulation
that makes life possible still, and it is only this fact that the
food is thus capable of ever circulating from animal to plant and
from plant to animal that makes it possible for the living world
to continue its existence. But, ah we have seen, one half of this
great circle of food change is dependent upon bacterial life.
Without the bacterial life the animal body and the animal
excretion could never be brought back again within the reach of
the plant; and thus, were it not for the action of these micro-
organisms the food cycle would be incomplete and life could not
continue indefinitely upon the surface of the earth. At the very
foundation, the continuation of the present condition of Nature
and the existence of life during the past history of the world has
been fundamentally based upon the ubiquitous presence of bacteria
and upon their continual action in connection with both
destructive and constructive processes.
RELATION OF BACTERIA TO AGRICULTURE.
We have already noticed that bacteria play an important part in
some of the agricultural industries, particularly in the dairy.
From the consideration of the matters just discussed, it is
manifest that these organisms must have an even more intimate
relation to the farmer's occupation. At the foundation, farming
consists in the cultivation of plants and animals, and we have
already seen how essential are the bacteria in the continuance of
animal and plant life. But aside from these theoretical
considerations, a little study shows that in a very practical
manner the farmer is ever making use of bacteria, as a rule, quite
unconsciously, but none the less positively.
SPROUTING OF SEEDS.
Even in the sprouting of seeds after they are sown in the soil
bacterial life has its influence. When seeds are placed m moist
soil they germinate under the influence of heat. The rich
albuminous material in the seeds furnishes excellent food, and
inasmuch as bacteria abound in the soil, it is inevitable that
they should grow in and feed upon the seed. If the moisture is
excessive and the heat considerable, they very frequently grow so
rapidly in the seed as to destroy its life as a seedling. The seed
rots in the ground as a result. This does not commonly occur,
however, in ordinary soil. But even here bacteria do grow in the
seed, though not so abundantly as to produce any injury. Indeed,
it has been claimed that their presence in the seed in small
quantities is a necessity for the proper sprouting of the seed. It
has been claimed that their growth tends to soften the food
material in the seed, so that the young seedling can more readily
absorb it for its own food, and that without such a softening the
seed remains too hard for the plant to use. This may well be
doubted, however, for seeds can apparently sprout well enough
without the aid of bacteria. But, nevertheless, bacteria do grow
in the seed during its germination, and thus do aid the plant in
the softening of the food material. We can not regard them as
essential to seed germination. It may well be claimed that they
ordinarily play at least an incidental part in this fundamental
life process, although it is uncertain whether the growth of
seedlings is to any considerable extent aided thereby.
THE SILO.
In the management of a silo the farmer has undoubtedly another
great bacteriological problem. In the attempt to preserve his
summer-grown food for the winter use of his animals, he is
hindered by the activity of common bacteria. If the food is kept
moist, it is sure to undergo decomposition and be ruined in a
short time as animal food. The farmer finds it necessary,
therefore, to dry some kinds of foods, like hay. While he can thus
preserve some foods, others can not be so treated. Much of the
rank growth of the farm, like cornstalks, is good food while it is
fresh, but is of little value when dried. The farmer has from
experience and observation discovered a method of managing
bacterial growth which enables him to avoid their ordinary evil
effects. This is by the use of the silo. The silo is a large,
heavily built box, which is open only at the top. In the silo the
green food is packed tightly, and when full all access of air is
excluded, except at its surface. Under these conditions the food
remains moist, but nevertheless does not undergo its ordinary
fermentations and putrefactions, and may be preserved for months
without being ruined. The food in such a silo may be taken out
months after it is packed, and will still be found to be in good
condition for food. It is true that it has changed its character
somewhat, but it is not decayed, and is eagerly eaten by cattle.
We are yet very ignorant of the nature of the changes which occur
m the food while in the silo. The food is not preserved from
fermentation. When the siloxis packed slowly, a very decided
fermentation occurs by which the mass is raised to a high
temperature (140 degrees F. to 160 degrees F.). This heating is
produced by certain species of bacteria which grow readily even at
this high temperature. The fermentation uses up the air in the
silo to a certain extent and produces a settling of the material
which still further excludes air. The first fermentation soon
ceases, and afterward only slow changes occur. Certain acid-
producing bacteria after a little begin to grow slowly, and in
time the silage is rendered somewhat sour by the production of
acetic acid. But the exclusion of air, the close packing, and the
small amount of moisture appear to prevent the growth of the
common putrefactive bacteria, and the silage remains good for a
long time. In other methods of filling the silo, the food is very
quickly packed and densely crowded together so as to exclude as
much air as possible from the beginning. Under these conditions
the lack of moisture and air prevents fermentative action very
largely. Only certain acid-producing organisms grow, and these
very slowly. The essential result in either case is that the
common putrefactive bacteria are prevented from growing, probably
by lack of sufficient oxygen and moisture, and thus the decay is
prevented. The closely packed food offers just the same
unfavourable condition for the growth of common putrefactive
bacteria that we have already seen offered by the hard-pressed
cheese, and the bacteria growth is in the same way held in check.
Our knowledge of the matter is as yet very slight, but we do know
enough to understand that the successful management of a silo is
dependent upon the manipulation of bacteria.
THE FERTILITY OF THE SOIL.
The farmer's sole duty is to extract food from the soil. This he
does either directly by raising crops, or indirectly by raising
animals which feed upon the products of the soil. In either case
the fertility of the soil is the fundamental factor in his
success. This fertility is a gift to him from the bacteria.
Even in the first formation of soil he is in a measure dependent
upon bacteria. Soil, as is well known, is produced in large part
by the crumbling of the rocks into powder. This crumbling we
generally call weathering, and regard it as due to the effect of
moisture and cold upon the rocks, together with the oxidizing
action of the air. Doubtless this is true, and the weathering
action is largely a physical and chemical one. Nevertheless, in
this fundamental process of rock disintegration bacterial action
plays a part, though perhaps a small one. Some species of
bacteria, as we have seen, can live upon very simple foods,
finding in free nitrogen and carbonates sufficiently highly
complex material for their life. These organisms appear to grow on
the bare surface of rocks, assimilating nitrogen from the air, and
carbon from some widely diffused carbonates or from the CO2 in the
air. Their secreted products of an acid nature help to soften the
rocks, and thus aid in performing the first step in weathering.
The soil is not, however, all made up of disintegrated rocks. It
contains, besides, various ingredients which combine to make it
fertile. Among these are various sulphates which form important
parts of plant foods. These sulphates appear to be formed, in
part, at least, by bacterial agency. The decomposition of proteids
gives rise, among other things, to hydrogen sulphide (H2S). This
gas, which is of common occurrence in the atmosphere, is oxidized
by bacterial growth into sulphuric acid, and this is the basis of
part of the soil sulphates. The deposition of iron phosphates and
iron silicates is probably also in a measure aided by bacterial
action. All of these processes are factors in the formation of
soil. Beyond much question the rock disintegration which occurs
everywhere in Nature is chiefly the result of physical and
chemical changes, but there is reason for believing that the
physical and chemical processes are, to a slight extent at least,
assisted by bacterial life.
A more important factor of soil fertility is its nitrogen content,
without which it is completely barren. The origin of these
nitrogen ingredients has been more or less of a puzzle. Fertile
soil everywhere contains nitrates and other nitrogen compounds,
and in certain parts of the world there are large accumulations of
these compounds, like the nitrate beds of Chili. That they have
come ultimately from the free atmospheric nitrogen seems certain,
and various attempts have been made to explain a method of this
nitrogen fixation. It has been suggested that electrical
discharges in the air may form nitric acid, which would readily
then unite with soil ingredients to form nitrates. There is little
reason, however, for believing this to be a very important factor
But in the soil bacteria we find undoubtedly an efficient agency m
this nitrogen fixation. As already seen, the bacteria are able to
seize the free atmospheric nitrogen, converting it into nitrite
and nitrates. We have also learned that they can act in connection
with legumes and some other plants, enabling them to fix
atmospheric nitrogen and store it m their roots. By these two
means the nitrogen ingredient in the soil is prevented from
becoming exhausted by the processes of dissipation constantly
going on. Further, by some such agency must we imagine the
original nitrogen soil ingredient to have been derived. Such an
organic agency is the only one yet discerned which appears to have
been efficient in furnishing virgin soil with its nitrates, and we
must therefore look upon bacteria as essential to the original
fertility of the soil. But in another direction still does the
farmer depend directly upon bacteria The most important factor in
the fertility of the soil is the part of it called humus. This
humus is very complex, and never alike in different soils It
contains nitrogen compounds in abundance, together with sulphates,
phosphates, sugar, and many other substances. It is this which
makes the garden soil different from sand, or the rich soil
different from the sterile soil. If the soil is cultivated year
after year, its food ingredients are slowly but surely exhausted.
Something is taken from the humus each year, and unless this be
replaced the soil ceases to be able to support life. To keep up a
constant yield from the soil the farmer understands that he must
apply fertilizers more or less constantly.
This application of fertilizers is simply feeding the crops. Some
of these fertilizers the farmer purchases, and knows little or
nothing as to their origin. The most common method of feeding the
crops is, however, by the use of ordinary barnyard manure. The
reason why this material contains plant food we can understand,
since it is made of the undigested part of food, together with all
the urea and other excretions of animals, and contains, therefore,
besides various minerals, all of the nitrogenous waste of animal
life. These secretions are not at first fit for plant food. The
farmer has learned by experience that such excretions, before they
are of any use on his fields, must undergo a process of slow
change, which is sometimes called ripening. Fresh manure is
sometimes used on the fields, but it is only made use of by the
plants after the ripening process has occurred. Fresh animal
excretions are of little or no value as a fertilizer. The farmer,
therefore, commonly allows it to remain in heaps for some time,
and it undergoes a slow change, which gradually converts it into a
condition in which it can be used by plants. This ripening is
readily explained by the facts already considered The fresh animal
secretions consist of various highly complex compounds of
nitrogen, and the ripening is a process of their decomposition.
The proteids are broken to pieces, and their nitrogen elements
reduced to the form of nitrates, leucin, etc, or even to ammonia
or free nitrogen. Further, a second process occurs, the process of
oxidation of these nitrogen compounds already noticed, and the
ammonia and nitrites resulting from the decomposition are built
into nitrates. In short, in this ripening manure the processes
noticed in the first part of this chapter are taking place, by
which the complex nitrogenous bodies are first reduced and then
oxidized to form plant food. The ripening of manure is both an
analytical and a synthetical process. By the analysis, proteids
and other bodies are broken into very simple compounds, some of
them, indeed, being dissipated into the air, but other portions
are retained and then oxidized, and these latter become the real
fertilizing materials. Through the agency of bacteria the compost
heap thus becomes the great source of plant food to the farmer.
Into this compost heap he throws garbage, straw, vegetable and
animal substances in general, or any organic refuse which may be
at hand. The various bacteria seize it all, and cause the
decomposition which converts it into plant food again. The rotting
of the compost heap is thus a gigantic cultivation of bacteria.
This knowledge of the ripening process is further teaching the
farmer how to prevent waste. In the ordinary decomposition of the
compost heap not an inconsiderable portion of the nitrogen is lost
in the air by dissipation as ammonia or free nitrogen. Even his
nitrates may be thus lost by bacterial action. This portion is
lost to the farmer completely, and he can only hope to replace it
either by purchasing nitrates in the form of commercial
fertilizers, or by reclaiming it from the air by the use of the
bacterial agencies already noticed. With the knowledge now at his
command he is learning to prevent this waste. In the decomposition
one large factor of loss is the ammonia, which, being a gas, is
readily dissipated into the air. Knowing this common result of
bacterial action, the scientist has told the farmer that, by
adding certain common chemicals to his decomposing manure heap,
chemicals which will readily unite with ammonia, he may retain
most of the nitrogen in this heap in the form of ammonia salts,
which, once formed, no longer show a tendency to dissipate into
the air. Ordinary gypsum, or superphosphates, or plaster will
readily unite with ammonia, and these added to the manure heap
largely counteract the tendency of the nitrogen to waste, thus
enabling the farmer to put back into his soil most of the nitrogen
which was extracted from it by his crops and then used by his
stock. His vegetable crops raise the nitrates into proteids. His
animals feed upon the proteids, and perform his work or furnish
him with milk. Then his bacteria stock take the excreted or refuse
nitrogen, and in his manure heap turn it back again into nitrates
ready to begin the circle once more. This might go on almost
indefinitely were it not for two facts, the farmer sends
nitrogenous material off his farm in the milk or grains or other
nitrogenous products, which he sells, and the decomposition
processes, as we have seen, dissipate some of the nitrogen into
the air as free nitrogen.
To meet this emergency and loss the farmer has another method of
enriching the soil, again depending upon bacteria. This is the so-
called green manuring. Here certain plants which seize nitrogen
from the air are cultivated upon the field to be fertilized, and,
instead of harvesting a crop, it is ploughed into the soil. Or
perhaps the tops may be harvested, the rest being ploughed into
the soil. The vegetable material thus ploughed in lies over a
season and enriches the soil. Here the bacteria of the soil come
into play in several directions. First, if the crop sowed be a
legume, the soil bacteria assist it to seize the nitrogen from the
air. The only plants which are of use in this green manuring are
those which can, through the agency of bacteria, obtain nitrogen
from the air and store it in their roots. Second, after the crop
is ploughed into the soil various decomposing bacteria seize upon
it, pulling the compounds to pieces. The carbon is largely
dissipated into the air as carbonic dioxide, where the next
generation of plants can get hold of it. The minerals and the
nitrogen remain in the soil. The nitrogenous portions go through
the same series of decomposition and synthetical changes already
described, and thus eventually the nitrogen seized from the air by
the combined action of the legumes and the bacteria is converted
into nitrates, and will serve for food for the next set of plants
grown on the same soil. Here is thus a practical method of using
the nitrogen assimilation powers of bacteria, and reclaiming
nitrogen from the air to replace that which has been lost. Thus it
is that the farmer's nitrogen problem of the fertile soil appears
to resolve itself into a proper handling of bacteria. These
organisms have stocked his soil in the first place. They convert
all of his compost heap wastes into simple bodies, some of which
are changed into plant foods, while others are at the same time
lost. Lastly, they may be made to reclaim this lost nitrogen, and
the fanner, so soon as he has requisite knowledge of these facts,
will be able to keep within his control the supply of this
important element. The continued fertility of the soil is thus a
gift from the bacteria.
BACTERIA AS SOURCES OF TROUBLE TO THE FARMER.
While the topics already considered comprise the most important
factors in agricultural bacteriology, the farmer's relations to
bacteria do not end here. These organisms come incidentally into
his life in many ways. They are not always his aids as they are in
most of the instances thus far cited. They produce disease in his
cattle, as will be noticed in the next chapter. Bacteria are
agents of decomposition, and they are just as likely to decompose
material which the farmer wishes to preserve as they are to
decompose material which the farmer desires to undergo the process
of decay. They are as ready to attack his fruits and vegetables as
to ripen his cream. The skin of fruits and vegetables is a
moderately good protection of the interior from the attack of
bacteria; but if the skin be broken in any place, bacteria get in
and cause decay, and to prevent it the farmer uses a cold cellar.
The bacteria prevent the farmer from preserving meats for any
length of time unless he checks their growth in some way. They get
into the eggs of his fowls and ruin them. Their troublesome nature
in the dairy in preventing the keeping of milk has already been
noticed. If he plants his seeds in very moist, damp weather, the
soil bacteria cause too rapid a decomposition of the seeds and
they rot in the ground instead of sprouting. They produce
disagreeable odours, and are the cause of most of the peculiar
smells, good and bad, around the barn. They attack the organic
matter which gets into his well or brook or pond, decomposing it,
filling the water with disagreeable and perhaps poisonous products
which render it unfit to drink. They not only aid in the decay of
the fallen tree in his forests; but in the same way attack the
timber which he wishes to preserve, especially if it is kept in a
moist condition. Thus they contribute largely to the gradual
destruction of wooden structures. It is therefore the presence of
these organisms which forces him to dry his hay, to smoke his
hams, to corn his beef, to keep his fruits and vegetables cool and
prevent skin bruises, to ice his dairy, to protect his timber from
rain, to use stone instead of wooden foundations for buildings,
etc. In general, when the farmer desires to get rid of any organic
refuse, he depends upon bacteria, for they are his sole agents
(aside from fire) for the final destruction of organic matter.
When he wishes to convert waste organic refuse into fertilizing
material, he uses the bacteria of his compost heap. On the other
hand, whenever he desires to preserve organic material, the
bacteria are the enemies against which he must carefully guard.
Thus the farmer's life from year's end to year's end is in most
intimate association with bacteria. Upon them he depends to insure
the continued fertility of his soil and the constant continued
production of good crops. Upon them he depends to turn into plant
food all the organic refuse from his house or from his barn. Upon
them he depends to replenish his stock of nitrogen. It is these
organisms which furnish his dairy with its butter flavours and
with the taste of its cheese. But, on the other hand, against them
he must be constantly alert. All his food products must be
protected from their ravages. A successful farmer's life, then,
largely resolves itself into a skilful management of bacterial
activity. To aid them in destroying or decomposing everything
which he does not desire to preserve, and to prevent their
destroying the organic material which he wishes to keep for future
use, is the object of a considerable portion of farm labour; and
the most successful farmer to-day, and we believe the most
successful farmer of the future, is the one who most intelligently
and skilfully manipulates these gigantic forces furnished him by
the growth of his microscopical allies.
RELATION OF BACTERIA TO COAL. Another one of Nature's processes in
which bacteria have played an important part is in the formation
of coal. It is unnecessary to emphasize the importance of coal in
modern civilization. Aside from its use as fuel, upon which
civilization is dependent, coal is a source of an endless variety
of valuable products. It is the source of our illuminating gas,
and ammonia is one of the products of the gas manufacture. From
the coal also comes coal tar, the material from which such a long
series of valuable materials, as aniline colours, carbolic acid,
etc, is derived. The list of products which we owe to coal is very
long, and the value of this material is hardly to be overrated. In
the preparation of these ingredients from coal bacteria do not
play any part. Most of them are derived by means of distillation.
But when asked for the agents which have given us the coal of the
coal beds, we shall find that here, too, we owe a great debt to
bacteria.
Coal, as is well known, has come from the accumulation of the
luxuriant vegetable growth of the past geological ages. It has
therefore been directly furnished us by the vegetation of the
green plants of the past, and, in general, it represents so much
carbonic dioxide which these plants have extracted from the
atmosphere. But while the green plants have been the active agents
in producing this assimilation, bacteria have played an important
part in coal manufacture in two different directions. The first
appears to be in furnishing these plants with nitrogen. Without a
store of fixed nitrogen in the soil these carboniferous plants
could not have grown. This matter has already been considered. We
have no very absolute knowledge as to the agency of bacteria in
furnishing nitrogen for this vegetation in past ages, but there is
every reason to believe that in the past, as in the present, the
chief source of organic nitrogen has been from the atmosphere and
derived from the atmosphere through the agency of bacteria. In the
absence of any other known factor we may be pretty safe in the
assumption that bacteria played an important part in this nitrogen
fixation, and that bacteria must therefore be regarded as the
agents which have furnished us the nitrogen stored in the coal.
But in a later stage of coal formation bacteria have contributed
more directly to the formation of coal. Coal is not simply
accumulated vegetation. The coal of our coal beds is very
different in its chemical composition from the wood of the trees.
It contains a much higher percentage of carbon and a lower
percentage of hydrogen and oxygen than ordinary vegetable
substances. The conversion of the vegetation of the carboniferous
ages into coal was accompanied by a gradual loss of hydrogen and a
consequent increase in the percentage of carbon. It is this change
that has added to the density of the substance and makes the
greater value of coal as fuel. There is little doubt now as to the
method by which this woody material of the past has been converted
into coal. The same process appears to be going on in a similar
manner to-day in the peat beds of various northern countries. The
fallen vegetation, trees, trunks, branches, and leaves, accumulate
in masses, and, when the conditions of moisture and temperature
are right, begin to undergo a fermentation. Ordinarily this action
of bacteria, as already noticed, produces an almost complete
though slow oxidation of the carbon, and results in the total
decay of the vegetable matter. But if the vegetable mass be
covered by water and mud under proper conditions of moisture and
temperature, a different kind of fermentation arises which does
not produce such complete decay. The covering of water prevents
the access of oxygen to the fermenting mass, an oxidation of the
carbon is largely prevented, and the vegetable matter slowly
changes its character. Under the influence of this slow
fermentation, aided, probably by pressure, the mass becomes more
and more solid and condensed, its woody character becomes less and
less distinct, and there is a gradual loss of the hydrogen and the
oxygen. Doubtless there is a loss of carbon also, for there is an
evolution of marsh gas which contains carbon. But, in this slow
fermentation taking place under the water in peat bogs and marshes
the carbon loss is relatively small; the woody material does not
become completely oxidized, as it does in free operations of
decay. The loss of hydrogen and oxygen from the mass is greater
than that of carbon, and the percentage of carbon therefore
increases. This is not the ordinary kind of fermentation that goes
on in vegetable accumulations. It requires special conditions and
possibly special kinds of fermenting organisms. Peat is not formed
in all climates. In warm regions, or where the woody matter is
freely exposed to the air, the fermentation of vegetable matter is
more complete, and it is entirely destroyed by oxidation. It is
only in colder regions and when covered with water that the
destruction of the organic matter stops short of decay. But such
incomplete fermentation is still going on in many parts of the
world, and by its means vegetable accumulations are being
converted into peat.
This formation of peat appears to be a first step in the formation
of denser coal. By a continuation of the same processes the mass
becomes still more dense and solid. As we pass from the top to the
bottom of such an accumulation of peat, we find it becoming denser
and denser, and at the bottom it is commonly of a hard consistence,
brownish in colour, and with only slight traces of the
original woody structure. Such material is called lignite. It
contains a higher percentage of carbon than peat, but a lower
percentage than coal, and is plainly a step in coal formation. But
the process goes on, the hydrogen and oxygen loss continuing until
there is finally produced true coal.
If this is the correct understanding of the formation of coal, we
see that we have plainly a process in which bacterial life has had
a large and important share. We are, of course, densely ignorant
of the exact processes going on. We know nothing positively as to
the kind of microorganisms which produce this slow, peculiar
fermentation. As yet, the fermentation going on in the formation
of the peat has not been studied by the bacteriologists, and we do
not know from direct experiment that it is a matter of bacterial
action. It has been commonly regarded as simply a slow chemical
change, but its general similarity to other fermentative processes
is so great that we can have little hesitation in attributing it
to micro-organisms, and doubtless to some forms of plants allied
to bacteria. There is no reason for doubting that bacteria existed
in the geological ages with essentially the same powers as they
now possess, and to some forms of bacteria which grow in the
absence of oxygen can we probably attribute the slow change which
has produced coal. Here, then, is another great source of wealth
in Nature for which we are dependent upon bacteria. While, of
course, water and pressure were very essential factors in the
deposition of coal, it was a peculiar kind of fermentation
occurring in the vegetation that brought about the chemical
changes in it which resulted in its transformation into coal. The
vegetation of the carboniferous age was dependent upon the
nitrogen fixed by the bacteria, and to these organisms also do we
owe the fact that this vegetation was stored for us in the rocks.