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.