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.

Group.Colour.Pigments.
I.Flagellatæ.Euglenaceæ.
Cryptomonadineæ.		Green.Chlorophyll.
II.Algæ—
1.Myxophyceæ.Mostly filamentous, chiefly Oscillatoriaceæ and Nostocaceæ.Blue-green to violet or brown.Phycocyanin.
Chlorophyll.
Carotin.
2.Bacillariaceæ.Mostly pennate, chiefly Naviculoideæ.Golden-brown.Carotin.
Xanthophyll.
Chlorophyll.
3.Chlorophyceæ.(i)Protococcales, Ulotrichales, Conjugatæ, etc.Green.Chlorophyll.
(ii)Heterokontæ.Yellow-green.Chlorophyll.
Xanthophyll.
III.Bryophyta.Filamentous moss protonema.Green.Chlorophyll.

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.

Species.Percentage of Samples containing
given Alga.
Uncultivated
Damp Sandy Soil.		Cultivated Soils.
Shores
of
Elbe.		Shores
of
Lakes.		Sea-
shore.		Sandy.Clay.Marsh-
land.
Anabæna variabilis46  43   9  10·360  46  
Anabæna torulosa31  14·363·627·634·356·4
Cylindrospermum muscicola23  28·6 0  24  48·659  
Cylindrospermum majus 0  14·3 0  38  40  33·3
Nostoc Sp. III. 7·7 0   0  38  37  48·7

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.

Kind of Soil.Percentage
of Samples
Containing
Blue-green
Algæ.		Number
of Samples
Examined.
Cultivated marshland95  40
Cultivated clay soil94·637
Uncultivated moist sandy soils88·635
Cultivated sandy soil64·445
Uncultivated- Woodland12·540
Sandy heathland 9  34
Moorland 0  35

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.)

Group.Number
of Samples
Productive.		Number of Species.
Maximum
per
Sample.		Average
per
Sample.		Total.
per cent.
Diatoms 95·5 9 3·720
Blue-green algæ 77·3 7 2·524
Green algæ100   7 4·320
Moss protonema100  ———
Total—2010·5—

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.