The micro-organisms of the soil

I. General and Historical Introduction.

Speaking broadly, the organisms of the soil may be classified into several distinct groups differing conspicuously in their general characters and physiological functions and therefore in their economic significance; among such groups may be mentioned the bacteria, protozoa, algæ and fungi. It is found, however, that though typical members of these groups are conspicuously different from one another, yet there exist a number of unicellular forms which have characters in common with more than one of these big groups, and the lines of demarcation between them become difficult to define. It becomes advisable, therefore, to depart a little from the systematist’s rigid definitions and to adopt a somewhat more logical grouping of the soil organisms based on their mode of life.

To give but a single example: Euglena viridis occurs quite commonly in soil. Through its single flagellum, its lack of a definite cellulose wall, its changeable shape and its ability to multiply by simple fission in the motile state it definitely belongs systematically to the group of protozoa. But its possession of chlorophyll, in enabling it to synthesise complex organic substances from CO₂ and water in a manner entirely typical of plants, connects it physiologically so closely with the lower green algæ that in studying the biology of the soil it seems best to include it and other nearly related forms with the algæ.

On this physiological basis “soil-algæ” may be defined as those micro-organisms of the soil which have the power, under suitable conditions, to produce chlorophyll. Such a definition has the advantage that it is wide enough to include the filamentous protonema of mosses, which, though alga-like in form and in physiological action, is nevertheless separated from the true algæ by a wide gulf. A more accurate name for such a group of organisms would be the “chlorophyll-bearing protophyta” of the soil; they may be classified briefly as follows (Table IX.):—

TABLE IX.

The importance of the lower algæ from a biological standpoint has long been recognised, since their extremely primitive organisation, coupled with their ability to synthesise organic compounds from simple inorganic substances, singles them out as being not very distantly removed from the group of organisms in which life originated upon the earth. But the possibility of their having a very much wider economic significance was completely overlooked until about a quarter of a century ago, when Hensen demonstrated their importance in marine plankton as the producers of the organic substance upon which the whole of the animal life of the ocean is ultimately dependent. In consequence, it has been generally assumed that the growth of algæ, since they contain chlorophyll, is entirely dependent on the action of light. Hence the recent idea of the existence of algæ which actually inhabit the soil has been received with a certain amount of scepticism, though the results of modern physiological research on a number of the lower algæ show that there is very good reason to believe that such a soil flora is entirely possible.

In considering the alga-flora of a soil it is necessary to distinguish between two very different sets of conditions under which the organisms may be growing. In the first place, they may grow on the surface of the soil, being subjected directly to insolation, rain, the deposition of dew, the drying action of wind, relatively quick changes of temperature and other effects of climate. Certain combinations of these conditions present so favourable an environment for the growth of algæ that at times there appears on the surface of the soil a conspicuous green stratum, sometimes so dark in colour as to appear almost black. Strata of this nature are well known, and in systematic works there are constant references to species growing “on damp soil”; for instance, of the 51 well-defined species of Nostoc recognised by Forte, no less than 31 are characterised as terrestrial. Such appearances, however, seem to have been regarded as sporadic and more or less accidental, rather than as the unusually luxuriant development of an endemic population, and have been frequently attributed to an excessively moist condition of the soil due to defective drainage.

In the second place, the algæ may be living within the soil itself, away from the action of sunlight and under somewhat more uniform conditions of moisture and temperature.

Up to the present time the greater number of the investigations carried out in this subject have been of a systematic nature, and extremely little direct evidence has been obtained which can throw any light on the subject of the economic significance of the soil algæ.

The earliest systematic work was carried out by Esmarch, in 1910-11, who investigated by means of cultures the blue-green algæ of a number of soils from the German African Colonies, the samples being taken from the surface and also from the lower layers of the soil. He obtained a considerable number of species and observed that in cultivated soils they were not confined to the surface but occurred regularly to a depth of 10-25 cms. and occasionally as low as 40-50 cms. He attributed their existence in the lower layers to the presence of resting spores carried down in the processes of cultivation, since his samples from uncultivated soils were unproductive.

Later, Esmarch extended his investigations to a far larger number of samples, 395 in all, of soils of different types from Schleswig-Holstein. He found that blue-green algæ were very widely distributed in soils of certain types, though they occurred rarely in uncultivated soils of low water-content, and he described no less than 45 species of which 34 belonged to the Oscillatoriaceæ and Nostocaceæ. Certain of the commoner species were obtained from soils of widely different types, as shown in Table X., while other forms occurred only rarely and with a much more limited distribution.

TABLE X.—FREQUENCY OF OCCURRENCE OF CERTAIN COMMON SPECIES IN ESMARCH’S SOIL SAMPLES.

Taking the number of samples containing blue-green algæ as a rough measure of their relative abundance, Esmarch obtained the following interesting figures (Table XI.):—

TABLE XI.

In noting that the soils fell into two groups, those relatively rich and those poor in blue-green algæ, Esmarch concluded that the two chief factors governing the distribution of the Cyanophyceæ on the surface of soils are, (1) the moisture content of the soil, (2) the availability of mineral salts, cultivated soils being especially favoured in both of these respects. He further distinguished between cultivated land of two kinds, viz. arable land and grass land, and found that on all types of soil grassland was richer in species than was arable land.

Esmarch examined, in addition, 129 samples taken from the lower layers of the soil immediately beneath certain of his surface samples, 107 at 10-25 cms. and the rest at 30-50 cms. depth.

In cultivated soils, whether grassland or arable land, he found that blue-green algæ occurred almost invariably in the lower layers in those places bearing algæ on the surface and that, with rare exceptions, the algæ found in the lower layers corresponded exactly to those on the surface, except that with increasing depth there was a progressive reduction in the number of species.

In uncultivated, moist, sandy soils the agreement was far less complete, for though algæ were rarely absent from the lower layers their vertical distribution was frequently disturbed by the action of wind and rain. Other uncultivated soils not subject to periodic disturbance were found to be uniformly lacking in algæ in the lower layers, but as the limited number of samples examined came completely from places where there were no algæ on the surface this means very little.

By direct microscopic examination of soil Esmarch claims to have found living filaments of blue-green algæ at various depths below the surface. He realised, however, that there was no indication of the length of time that such filaments had been buried, and therefore conducted a series of experiments from which he concluded that the period during which the algæ investigated could continue vegetatively in the soil after burial varied with different species from 5-12 weeks, but that during the later part of the period the algæ gradually assumed a yellowish-green colour.

It is unfortunate that Esmarch’s investigations were directed only towards the blue-green algæ since observations made in this country indicate that such a series of records gives but a very incomplete picture of the soil flora as a whole.

Petersen, in his “Danske Aërofile Alger” (1915) added considerably to our knowledge of soil algæ, especially of diatoms. Unfortunately he confined his investigations of the green algæ to forms growing visibly on the surface of the ground. He observed, however, that acid soils possessed a different flora from that commonly found on alkaline or neutral soils, the former being dominated by Mesotænium violascens, Zygnema ericetorum, and 2 spp. of Coccomyxa, while the latter were characterised by Mesotænium macrococcum var., Hormidium, 2 spp., and Vaucheria, 3 spp.

Of diatoms he obtained no less than 24 species and varieties from arable and garden soils, and five characteristic of marshy soils, while from forest soils and dry heathland they appeared to be often absent. He omitted all reference to blue-green algæ.

Meanwhile Robbins, examining a number of Colorado soils that contained unprecedented quantities of nitrate, obtained from them 18 species of blue-green algæ, 2 species of green algæ, and one diatom. Moore and Karrer have demonstrated the existence of a subterranean alga-flora of which Protoderma viride, the most constantly occurring species, was shown to multiply when buried to a depth of one metre.

In this country attention was first called to the subject by Goodey and Hutchinson of Rothamsted who, in examining certain old stored soils for protozoa, obtained also a number of blue-green forms which were submitted to Professor West for identification. This ability of certain algal spores to retain their vitality for a long resting period was so very striking that an investigation was begun at Birmingham in 1915 to ascertain whether other forms were equally resistant. The investigation was carried out on a large number of freshly collected samples of arable and garden soils which were first aseptically air-dried for at least a month and then grown in culture. No less than 20 species or varieties of diatoms, 24 species of blue-green and 20 species of green algæ were obtained from these cultures (Table XII.). In the majority of the samples there was found a central group of algæ, including Hantzschia amphioxys, Trochiscia aspera, Chlorococcum humicola, Bumilleria exilis and rather less frequently Ulothrix subtilis var. variabilis, while moss protonema was universally present. These species were thought to form the basis of an extensive ecological plant formation in which, by the inclusion of other typically terrestrial but less widely distributed species smaller plant-associations were recognised.

In certain of the soils, associations consisting very largely of diatoms were present, and it is to be noted that the majority of the forms that have been described are of exceedingly small size. It is doubtless this characteristic which enables them to withstand the conditions of drought to which the organisms of the soil are liable to be subjected, small organisms having been shown to be better able to resist desiccation than are larger ones. Since the soil diatoms belong to the pennate type, they are further adapted to their mode of life by their power of locomotion, which enables them in times of drought to retire to the moister layers of the soil.

In the soils examined in this work blue-green algæ were less universally present than were diatoms or green algæ, and the species found appeared to be more local in occurrence. There seemed to be, however, an association between the three species, Phormidium tenue, Ph. autumnale, and Plectonema Battersii, at least two of the three species having been found together in no less than 16 of the samples, while all three occurred in 7 of them.

TABLE XII.—ALGÆ IN DESICCATED ENGLISH SOILS. (BRISTOL.)

It was generally noticeable that those soils found to be rich in blue-green algæ contained only a few species of diatoms, and vice versa. Diatoms appeared most frequently in soils from old gardens, whereas blue-green algæ were more characteristic of arable soils. The green algæ and moss protonema, on the other hand, were distributed universally.

The majority of green algæ typically found in soils are unicellular, but a few filamentous forms occur. With the exception of Vaucheria spp. these are characterised, however, by an ability to break down in certain circumstances into unicellular or few-celled fragments, in which condition identification is often very difficult.

It was also found by cultural examination of a number of old stored soils from Rothamsted that germination of the resting forms of a number of algæ could take place after an exceedingly long period of quiescence. No less than nine species of blue-green algæ, four species of green algæ, and one species of diatom were obtained from soils that had been stored for periods of about forty years, the species with the greatest power to retain their vitality being Nostoc muscorum and Nodularia Harveyana.

II. The Soil as a Suitable Medium for Algal Growth.

Were it not for the recent advances that have been made in our knowledge of the mode of nutrition of many of the lower algæ, it would be very difficult to account for the widespread occurrence of algæ in the soil, for it is undoubtedly true of some of the more highly evolved algæ that their mode of nutrition is entirely typical of that of green plants in general. The application of bacteriological technique to the algæ, however, by Beijerinck, by Artari, and by Chodat and his pupils, and the introduction of pure-culture methods have led to a study of the physiology of some of the lower algæ, in the hope of getting to understand some of the fundamental problems underlying the nutrition of organisms containing chlorophyll. It is impossible here to do more than mention the names of a few of the more important of those who have worked along these lines, such as Chodat, Artari, Grintzesco, Pringsheim, Kufferath, Nakano, Boresch, Magnus and Schindler, and to condense into a few sentences some of their more important conclusions.

It is now established that although in the light the algæ are able to build up their substance from CO₂ and water containing dilute mineral salts, yet in such conditions growth is sometimes very slow, and with some species at any rate it is greatly accelerated by the addition of a small quantity of certain organic compounds. The ability of the lower algæ to use organic food materials varies specifically, quite closely related forms often reacting very differently to the same substance, but there have been shown to be a considerable number of forms which can make use of organic compounds to such an extent that they can grow entirely independently of light. In such cases the nutrition of the organism becomes wholly saprophytic, and the chlorophyll may be completely lost; it has frequently been observed, however, that on suitable nutrient media, even in complete darkness, certain algæ continue to grow and retain their green colour, provided that a sufficient supply of a suitable nitrogenous compound is present.

Chlorella vulgaris, an alga frequently found in soil, has been shown to be extremely plastic in its relations to food substances. Given only a dilute mineral-salts solution as food source, it absorbs CO₂ from the air, and grows in sunlight with moderate rapidity. The addition of glucose to the medium in the light greatly increases the rate and amount of growth and the size of the cells, while in the dark the colonies not only remain green but have been shown to develop more vigorously than in full daylight. The organism is also able to use peptone as a source of nitrogen in place of nitrates.

Stichococcus bacillaris and Scenedesmus spp., also occurring in soils, have been shown to be almost equally adaptable, though in these cases the organisms grow more slowly in the dark than on the corresponding medium in the light. Liquefaction of gelatine by the secretion of proteolytic enzymes has been shown to be a further property of certain species, resulting in the formation of amino acids such as glycocoll, phenylalanine, dipeptides, etc. This property is, however, possessed by only a limited number of species and in varying degree.

Up to the present very little work of this kind has been done upon algæ actually taken from the soil, and our knowledge is therefore very scanty. Of the species so far examined all show considerable increase in growth on the addition to the medium of glucose and other sugars, and tend to be partially saprophytic; a few have been shown to liquefy gelatine to some extent.

Servettaz, Von Ubisch, and Robbins have also demonstrated that the protonema of some mosses can make use of certain organic substances, especially the sugars, and grow vigorously in the dark. It has been shown, however, that light is essential for the development of the moss plant.

It was thought at Rothamsted that some light might be thrown upon the activities of the soil-algæ by making counts of the numbers present in samples of soil taken periodically within a circumscribed area. A dilution method similar to that in use in the protozoological laboratory was adopted and applied to samples of arable soil taken from the surface, and at depths of 2, 4, 6 and 12 inches vertically beneath. A considerable number of samples were examined in this way from two plots on Broadbalk wheat-field, viz.: the unmanured plot and that receiving a heavy annual dressing of farmyard manure. The numbers in the unmanured soil were observed to fall far short of those in that containing a large amount of organic matter, while in both plots the numbers varied considerably at different times of the year. The chief species in both plots were identical, and their vertical distribution was fairly uniform, but it was observed that the numbers of individuals varied according to the depth of the sample. The 6th and 12th inch samples contained very few individuals of comparatively few species, but the 4th inch samples yielded numbers that were not significantly less than those in the top inch. The 2nd inch sample was usually much poorer in individuals than either the top or the 4th inch.

It is unfortunate that this method of counting is not really satisfactory for the algæ, chiefly because it takes no account of the blue-green forms. The gelatinous envelope which encloses the filaments of these algæ prevents their breaking up into measurable units. Assuming, as appears to be the case for the two plots investigated, that the blue-green algæ are at least as numerous as the green forms, the total numbers should probably be at least twice as great as those calculated. Taking 100,000 as a rough estimate of the number of algæ per gram of manured soil in a given sample, and assuming the cells to be spherical and of average diameter 10µ, it has been calculated that the volume of algal protoplasm present was at least 3 times that of the bacteria though only one-third of that of the protozoa. This is probably only a minimum figure for this sample.

A soil population of this magnitude can not be without effect on the fertility of the soil. When growing on the surface of the ground exposed to sunlight the algæ must, by photosynthesis, add considerably to the organic matter of the soil, but when they live within the soil itself their nutrition must be wholly saprophytic, and they can be adding nothing either to the energy or to the food-content of the soil. How these organisms fit into the general scheme of life in the soil is at present undetermined, and there is a wide field for research in this direction.

III. Relation of Algæ to the Nitrogen Cycle

Probably the most important limiting factor in British agriculture is the supply of nitrogen available for the growing crop, and it seems likely that the soil-algæ are intimately connected with this question in several ways.

Periodic efforts have been made during the last half century to establish the fact that a number of the lower organisms, including the green algæ, have the power of fixing atmospheric nitrogen and converting it into compounds which are then available for higher plants. This property has been definitely established for certain bacteria, and rather doubtfully for some of the fungi, but until recently no authentic proof had been produced that algæ by themselves could fix nitrogen. The subject is too wide to be discussed in much detail here.

Schramm in America, working with pure cultures of algæ, tried for ten years to establish the fact of nitrogen fixation, and failed completely; more recently Wann has extended Schramm’s work, and claims to have proved indisputably that, given media containing nitrates as a source of nitrogen and a small amount of glucose, the seven species of algæ tested by him fixed atmospheric nitrogen to the extent of 4-54 per cent. of the original nitrogen content of the medium. So important a result needed corroboration, and Wann’s experiment, with some slight improvements, was therefore repeated at Rothamsted last summer.

This work has not yet been published, but in the whole series of ninety-six cultures, with four different species, each growing on six different media, there is no evidence that nitrogen fixation has taken place; but there has been a total recovery at the end of the experiment of 98·93 per cent. of the original nitrogen supplied. On the other hand, a flaw has been detected in Wann’s method of analysing those media containing nitrates, sufficiently great to account for the differences he obtained between the initial and final nitrogen content of his cultures. Hence, though one hesitates to say that the algæ are unable, given suitable conditions, to fix atmospheric nitrogen, one must admit that no one has yet proved that they can do so.

It is far more likely, however, that the experiments of Kossowitsch and others throw more light on the relation of soil algæ to nitrogen fixation. They affirm that greater fixation of nitrogen is effected by mixtures of bacteria and certain gelatinous algæ than by nitrogen-fixing bacteria alone, and that the addition of algæ to cultures of bacteria produces a stimulating effect only slightly less than that of sugar. It is probable, therefore, that the algæ, in their gelatinous sheaths, provide easily available carbohydrates from which the bacteria derive the energy essential to their work, and that nitrogen fixation in nature is due to the combined working of a number of different organisms rather than to the individual action of single species.

Russell and Richards have shown that the rate of loss of nitrogen by leaching from uncropped soils is far less than would be expected from a purely chemical standpoint, and suggest that certain organisms are present in the soil which, by absorbing nitrates and ammonium salts as they are formed, remove them from the soil solution and so help to conserve the nitrogen of the soil. It is probable that the soil algæ act in this manner, though to what extent has not yet been determined.

IV. Relation of Algæ to Soil Moisture and to the Formation of Humus Substances.

In warmer countries than our own, especially those with an adequate rainfall, the significance of soil algæ is perhaps more obvious to a casual observer. Treub states that after the complete destruction of the island of Krakatoa by volcanic eruption in 1883, the first colonists to take possession of the island were six species of blue-green algæ, viz., Tolypothrix sp., Anabæna sp., Symploca sp., Lyngbya 3 spp. Three years after the eruption these organisms were observed to form an almost continuous gelatinous and hygroscopic layer over the surface of the cinders and stones constituting the soil, and by their death and decay they rapidly prepared it for the growth of seeds brought to the island by visiting birds. Hence the new flora which soon established itself upon the island can be said to have had its origin in the alga-flora which preceded it. Fritsch has also emphasised the importance of algæ in the colonisation of new ground in Ceylon.

Welwitsch ascribes the characteristic colour from which the “pedras negras” in Angola derive their name to the growth of a thick stratum of Scytonema myochrous, a blue-green alga, which gradually becomes black and completely covers the soil. At the close of the rainy season this gelatinous stratum dries up very slowly, enabling the underlying soil to retain its moisture for a longer period than would otherwise be the case.

The gelatinous soil algæ are probably very important in this respect, for their slow rate of loss of water is coupled with a capacity for rapid absorption, and they are therefore able to take full advantage of the dew that may be deposited upon them and increase the power of the soil to retain moisture.

V. Relation of Algæ to Gaseous Interchanges in the Soil.

In the cultivation of rice the algæ of the paddy field have been found to be of extreme importance. Brizi in Italy has shown that although rice is grown under swamp conditions yet the roots of the rice plant are typical of those of ordinary terrestrial plants and have none of the structural adaptations to aquatic life so characteristic of ordinary marsh plants. Hence the plants are entirely dependent for healthy growth upon an adequate supply of oxygen to their roots from the medium in which they are growing. A serious disease of the rice plant, characterised by the browning and dying off of the leaves, which was thought at first to be due to the attacks of fungi, was found to be the effect of the inadequate aeration of the roots, while the entry of the fungi was shown to be subsequent to the appearance of the physiological disease. The presence of algæ in the swamp water was found to prevent the appearance of this disease, in that they unite with other organisms to form a more or less continuous stratum over the surface of the ground, and add to the gases which accumulate there large quantities of oxygen evolved during photosynthesis. The concentration of dissolved oxygen in the water percolating through the soil is thereby raised to a maximum, and the healthy growth of the crop ensured.

This work has been corroborated by Harrison and Aiyer in India, and a sufficient supply of algæ in the swamp water is now regarded as one of the essentials for the production of a good rice crop.

From what has been said, it appears that, although our knowledge of the soil algæ is extremely limited, and our conception of the part they play is largely based on speculation, yet the subject is one of enormous interest and worthy of investigation in many directions. In its present undeveloped state, it is a little difficult to foresee which lines of study are likely to prove most profitable, but there is little doubt that eventually the soil algæ will be shown to play a significant part in the economy of the soil.

SELECTED BIBLIOGRAPHY.

* Papers giving extensive bibliographies.

I. General.

II. Relation of Algæ to Light and Carbon.

III. Relation of Algæ to Nitrogen.