BACTERIA IN RELATION TO SOIL FERTILITY During the middle ages and the early part of the present era the fertility of soil was the subject of much speculation; varied and fantastic, and very wide of the truth were the views regarding the food requirements of plants. These views however were with out their influence upon the methods of farming practice. These methods have throughout the ages been slowly evolved by experi ence, and even the science of the past century, apart from the in troduction of artificial manures, has done little to change them. The composition of soil and its condition of cultivation have remained constant so long that gradually certain combinations or associations of micro-organisms specially adapted to the soil habitat have established themselves. Thus soil has come to have a special microscopic flora and fauna which vary only slightly in different localities and at different times of the year. These asso ciations consist of certain well-defined groups of organisms each with its characteristic function to perform and such variations as exist show themselves in the relative ascendancy of one group over others as indicated by the prevalence, permanent or tern porary, of the specific function which attaches to that group.
The combined efforts of all these groups result in the complete breakdown of organic remains of plants and animals and of inor ganic rock particles and in the elaboration from them of simple substances which may act as plant food ; in other words, they re sult in the fertility of the soil.
The soil population is so complex, and the functions of one group are so closely associated with the activities of others, that the bacteriologist can often do no better than study the work ing of the soil as a whole under artificially imposed changes in the soil conditions. Variations in the access of air, in the amount of moisture, in the temperature and in the H. ion concentration of the soil have led to considerable knowledge of the biological activities of loose well-aerated and dry soils (sandy soils and sandy loams) as compared with closely packed and relatively wet soils (heavy loams, clays and peaty loams) . On the other hand, individual species isolated from soil have been studied in pure culture, and the chemical phenomena that they exhibit under these conditions have led to much enlightenment of the changes brought about in the soil by the group of organisms to which they belong.
The organic matter in the soil, the dark brown substance which distinguishes the fertile surface layers from the light coloured infertile subsoil, is termed "humus" and consists of plant and animal remains in all stages of decomposition.
The most easily decomposed part of plants are rapidly fermented by yeasts, moulds and bacteria, and being themselves free from nitrogen their removal leaves the remaining humus relatively richer in nitrogen, and since these fer mentations are oxidation processes they occur with greatest rapid ity in well aerated soils and in the upper layers of the soil. The humus is therefore found to be progressively richer in nitrogen as one passes from depth to depth. Moreover, the rapidity of ac tion of these oxidizing organisms is so great in sandy soils and in hot climates that the maintenance of sufficient organic matter in such soils is a matter of considerable difficulty. The fermentation of cellulose is but little slower than that of starch ; many anaerobic bacteria are known to be capable of destroying cellulose and until recently anaerobic organisms alone had been shown to possess this power. It is, however, clear that the cellulose of buried plant material is rapidly attacked in open soils and that such parts lose their identity even more rapidly in well aerated than they do in badly aerated soils. Some oxidative processes of decay are obvi ously at work and recently it has been shown that English soils contain two organisms at least, Spirochaete cytophaga and Micro spira agar lique f aciens, which under aerobic conditions in labora tory cultures have been found to destroy the cellulose of filter paper in so short a time as three weeks. Some part of the cellulose is converted into a mucilaginous material which is incapable of further oxidation, and which has its counterpart in the mucilag inous nature of well rotted farmyard manure.
Under this term is included a number of complicated and little understood chemical phenomena which re suit in the breakdown of the nitrogenous matter of the humus and its conversion into ammonia. The nitrogen of plant and animal remains is for the most part locked up in very large molecules, the proteins, and as such is not available as food for plants. Many bacteria are known to be capable of attacking the protein mole cule with ease; by virtue of an enzyme which is exuded from them, the proteins of gelatine that form the basis of the medium on which the bacteria are cultivated are digested and liquefaction of the medium follows. E. Marchal (1893) showed that of 31 species of organisms isolated from soil no less than 17 liquefied gelatine and a great many others have since been added. All these have the power of converting humus proteins into simple soluble nitrogenous compounds, amino acids, many go farther than this and produce ammonia. Besides these there are in soil many or ganisms which, though not so vigorous as the above, have the power of peptonizing the protein molecule much in the same way as digestion of protein takes place in the human stomach. All these organisms, since they are associated with the ultimate conversion of protein into ammonia, are included in the group of ammon ifiers. Another group which is also included here, and which de serves special notice on account of the importance of its par ticular function, is a small group of urea bacteria, organisms which bring about the conversion to ammonium carbonate of the large quantity of nitrogen eliminated by animals in the form of urea. So rapidly do they effect this conversion and so common are they in the air that urine always smells of ammonia within a short time of its leaving the animal's body.
Now, while it is possible for plants to assim ilate ammonia, the chief form in which nitrogen is taken up by plants is that of nitrate and it has been recognized for a long time that ammonium carbonate is readily .converted into nitrate, but it was not till near the end of the 19th century that this was definite ly shown to be a biological problem. In 1877 two Dutchmen, J. H. Schloessing and C. A. Muntz, showed that the process came to an immediate stop when the soil was heated or was treated with chloroform. Although this pointed to a living agent, and experi ments which followed indicated this to be bacterial, for some time all attempts to isolate these bacteria failed. Among the foremost workers were R. Warington in England and S. Winogradski in Russia. These two independently and almost simultaneously found the cause of their frequent disappointments lay in the fact that the nitrifying bacteria would not grow on ordinary nutrient media. It was Winogradski who first published his results; he cultivated the organisms in a solution of ammonium sulphate con taining potassium phosphate and a small quantity of basic mag nesium carbonate. In this medium nitrification occurred vigor ously and from it he inoculated plates of gelatinous silica im pregnated with these salts and isolated two kinds of bacteria. He found that neither of these organisms produced nitrate from am monia. But having no trace of any other organisms, Winogradski thought to try a double inoculation of the ammonia solution and found that both organisms were required, and that the process took place in two stages.
The first stage in the oxidation of ammonia is brought about by a class included by Winogradski in the genus Nitrosomonas. They produce nitrite only, pure cultures can carry nitrification no far ther. In the stages of culture an incubation period of five or six days at 25° C. occurs during which the organisms are found in a zoogloeal state attached to the particles of magnesium carbonate, and during this period little or no oxidation takes place. After this the zoogloea loosens and free-swimming organisms are found, and the first stage in the oxidation at once sets in. The other group which oxidizes nitrites to nitrates but which is powerless to oxidize ammonium salts is included in the genus Nitrobacter. These are smaller than the nitrite formers and are non-motile. With the conversion of ammonia to nitrite thus explained the complete return of the nitrogen contained in dead plants and animals to a form in which it is readily available for the building up of young plants again is completely accounted for. When one considers that in the process of cropping the greater part of the nitrogen taken up by the plant is lost to the land, only the small amount present in the roots and the stubble remaining, it is obvi ous that this wastage of nitrogen must in some way be made good or the soil would soon become exhausted and would no longer support the growth of plants. The farmer makes up a large part of this wastage by the application of dressings of farmyard manure and of sulphate of ammonia, etc. Forest land, however, never receives dressings of fertilizers, yet one knows that for ests have continued to produce trees with unabated vigour for many centuries. Moreover it is well recognized that one way of enriching land is to allow it for a time to remain fallow, that is, bare of vegetation, and it can easily be shown by analysis that such fallow land gains in nitrogen content.
Fixation.—"Vegetable earth contains not only dead organic matter but living organisms. The mycoderms have only an ephemeral existence and they leave their detritus in the soil which in turn may give rise to ammonia and nitric acid." Thus wrote J. B. J. D. Boussingault in 1858 and though he was not aware of the fact the statement contains the explanation of the gain of nitrogen in fallow land. There are in the soil certain organisms, the most notable of which are Azotobacter chroococcum and Clostridium pastorianum, which, in the absence of other forms of nitrogen, can utilize the nitrogen gas dissolved in the soil solu tion. These organisms were not discovered till 35 years after
Boussingault's writing but it is now known that the enrichment
of the soil is due to their
tion of nitrogen from the air and
the storage of it in their own
bodies till such time as they are
absorbed by the Protozoa. On the
death of the protozoa the
bined nitrogen becomes food for
the plant through the agency of
the ammonifiers and nitrifiers.
Till recently it was believed that
the nitrogen-fixers required to
have nitrogen presented to them
in the gaseous form. It is now
recognized that they do not differ
from other forms of life in
ing the path of least resistance.
They fix nitrogen not because
they must but because they are
able to do so. If it is possible for
them to acquire their necessary
nitrogen with smaller expenditure
of energy no nitrogen will be
fixed. In the presence of much
ammonia and relatively little
bohydrate material; as when too
heavy dressings of rich
enous fertilizers like dried blood or cow dung are made, these organisms will multiply very rapidly, and will actually go into competition with the plant for the avail able supply of ammonia, and the plants temporarily will actually suffer nitrogen starvation.
A further way in which the soil is enriched is through the growth of plants of the family Leguminosae (including peas, beans, clover, etc.) .
By analysis it was shown by Schulz-Lupitz in 1887 that the way in which these plants enrich the soil is by increasing the nitrogen content. The only possible source for this increase was the at mospheric nitrogen. It had been, however, an axiom with botanists that the green plants were unable to use the nitrogen of the air. The apparent contradiction was explained by the experiments of H. Hellriegel and H. Wilfarth in 1888. They showed that, when grown on sterilized sand with the addition of mineral salts, the Leguminosae were no more able to use the atmospheric nitrogen than other plants such as oats and barley. Both kinds of plants required the addition of nitrates to the soil. But if a little water in which arable soil has been shaken up was added to the sand, then the leguminous plants flourished in the absence of nitrates and showed an increase in nitrogenous material. They had clearly made use of the nitrogen in the air. When these plants were ex amined they had small swellings or nodules on their roots, while those grown in sterile sand without soil-extract had no nodules. Now these peculiar nodules are characteristic of the roots of leguminous plants grown in ordinary soil. The experiments above mentioned made clear the nature and activity of these nodules. They are the result of infection (if the soil extract was boiled before addition to the sand no nodules were produced), and their presence enabled the plant to absorb the nitrogen of the air.
The work of recent investigators has made clear the whole process. In ordinary arable soil there exists motile rod-like bac teria, Bacterium radicicola. These enter the root-hairs of legumi nous plants, and passing down the hair in the form of a long, slimy (zoogloea) thread, penetrate the tissues of the root. As a result the tissues become hypertrophied, producing the well-known nod ule. In the cells of the nodule the bacteria multiply and develop, drawing material from their host. Many of the bacteria exhibit curious involution forms ("bacteroids"), which are finally broken down and their products absorbed by the plant. The nitrogen of the air is absorbed by the nodules, being built up into the bac terial cell and later handed on to the host-plant. It appears from the observations of Maze that the bacterium can even absorb free nitrogen when grown in cultures outside the plant. We have a very interesting case of symbiosis, the green plant always keeps the upper hand, restricting the bacteria to the nodules and later ab sorbing them for its own use. Different genera require different races of the bacterium for the production of nodules.
Another case of symbiotic nitrogen fixation is met in certain members of the plant families Rubiaceae and Myrsinaceae. Here the bacteria invade the leaf tissue through a large stomatal open ing which seems to have evolved specially for this purpose, they become established in spherical glands which are conspicuous as blisters on the surface of the leaves. These glands were previously regarded as protein glands, but through the researches of H. Miehe, A. Zimmerman and F. C. von Faber it is now known that what were formerly regarded as protein crystals are in truth bac teroids of a species of bacteria, Pseudomonas rubiacearum, which, like Pseudomonas radicicola, has the power of fixing atmospheric nitrogen.
There are in soil certain organisms which bring about a sulphur cycle similar to that of nitrogen. F. Cohn long ago showed that certain glistening particles in the cells of Beggiatoa consist of sulphur and S. Winogradski and M. W. Bei gerinck have shown that a whole series of sulphur bacteria exist. In the process of decay the sulphur contained in protein is for a large part converted into hydrogen sulphide and in its presence sulphur bacteria thrive ; they oxidize the molecule into sulphuric acid, at the same time storing up some of the sulphur in their own bodies, so that, if the hydrogen sulphide in the soil runs short, they have this reserve of sulphur, and so long as it lasts the organ isms remain alive but death by starvation occurs when the last traces of sulphur are gone. The sulphuric acid formed finds its use in the soil in rendering soluble the insoluble calcium phosphate (apatite, bones, fish meal, etc.) Beigerinck has shown that Spirillum desulphuricans, a definite anaerobic form, attacks and reduces sulphates, thus undoing the work of the sulphur bacteria. This phenomenon has its parallel in the action of certain de-nitrifying bacteria in reversing the opera tions of the nitrifying bacteria.
This group of organisms is present in all soils
and especially abundant in regions bordering on iron-stone
its; ponds in such regions may show a bright rust colour due to
the presence of these bacteria for they are characterized by the
possession of a thick sheath-like envelope in which ferric
ide, a substance akin to iron rust, is deposited. They are all
thread-like forms, some species being flattened filaments having
the appearance under the microscope of twisted ribbons. It is
doubtful whether they are in any way useful but, on the other
hand, they may prove extremely troublesome in the maintenance
of the water supply for large towns for, given suitable conditions,
they grow with extreme rapidity and the cells, when they die,
leave behind minute tubes of
iron oxide, and these have been
known to choke completely the
service pipes. Berlin had trouble
of this kind in 1877 and the whole
of the water mains had to be
renewed. But perhaps the most
striking calamity on record befell
Rotterdam in, 1887. The
voirs first became contaminated
by growths of mussels, polyzoa
and sponges with which the walls
of the reservoirs became thickly
encrusted. These, however, did
little harm, they produced a
tain amount of increase of
ganic matter through their
creta, but by their habit of
ing they reduced the bacterial
content of the water and they
were consequently allowed to
main. The trouble came when the
organic matter increased to such
an extent that iron bacteria had
their chance and all this mass of
molluscs became coated with a
to 1-inch covering of filamentous iron bacteria. These found their
way into the service mains and made the water quite undrinkable.
