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.