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.