On the surface of moist soils we frequently find a copious growth of green fibrils, which may be either those of algæ, such as Oscillaria, or the early stages (prothallia) of moss vegetation. This vegetation has been credited with absorption of nitrogen from the air, thus enriching the soil; but later researches have shown this effect to be due to symbiotic bacteria ([see above p. 156]).
Animal Agencies.—Darwin first suggested that wherever the common earthworm (Lumbricus) finds the conditions of existence, it exerts a most important influence in the formation of the humous surface-soil layer; and the limitation imposed upon these conditions by the subsoil has doubtless a great deal to do with the sharp demarcation we often find between it and the surface soil. Briefly stated, the earthworm nourishes itself by swallowing, successively, portions of the surrounding earth, digesting a part of its organic matter and then ejecting the undigested earth in the form of “casts,” such as may be seen by thousands on the surface of the ground during or after a rain. Darwin (The Formation of Vegetable Mold, 1881), has calculated from actual observation that in humid climates and in a ground fairly stocked with these worms, the soil thus brought up may amount to from one-tenth to two-tenths of an inch annually over the entire surface; so that in half a century the entire surface foot might have been thus worked over. Aside from the mechanical effect thus achieved in loosening the soil, and the access of air and water permitted by their burrows, the chemical effects resulting from their digestive process, and the final return of their own substance to the soil mass; also their habit of drawing after themselves into their burrows leafstalks, blades of grass and other vegetable remains, renders their work of no mean importance both from the physical and chemical point of view. The uniformity, lack of structure and loose texture of the surface soil, especially of forests, as compared with subsoil layers of corresponding thickness, is doubtless largely due to the earthworms’ work. It has frequently been observed that when an unusual overflow has drowned out the earthworm population of a considerable area, the surface soil layer remains compacted, and vegetation languishes, until new immigration has restocked the soil with them. Again, the humus formed under their influence is always neutral, never acid.
Wollny (Forsch, Agr., 1890, p. 382), has shown by direct experimental cultures in boxes, with and without earthworms, surprising differences between the cultural results obtained, and this has been fully confirmed by the subsequent researches of Djemil (Ber. Physiol. Lab. Vers. Halle, 1898). In Wollny’s experiments, the ratio of higher production in the presence of the worms, varied all the way from 2.6 per cent in the case of oats, 93.9 in that of rye, 135.9 in that of potatoes, 300 in that of the field pea, and 140 in that of the vetch, to 733 per cent in the case of rape. Wollny attributes these favorable effects in the main to the increased looseness, and perviousness of the soil to air, and diminished water-holding power. Djemil’s results all point in the same direction; and he shows, moreover, that the allegation that the roots penetrate more deeply in the presence of the worms by following their burrows, is unfounded, the descending roots often passing close to and outside of these.
The work of earthworms is especially effective in loamy soils and in the humid regions. In the arid region, and in sandy soils generally, the life-conditions are unfavorable to the worm, and the perviousness elsewhere brought about by its labors already exists naturally in most cases. It is stated by E. T. Seton (Century Mag. for June, 1904) that the earthworm is practically non-existent in the arid region between the Rocky Mountains and the immediate Pacific coast, from Manitoba to Texas. In the Pacific coast region, however, they are abundant, and do their work effectually.
Insects of various kinds are also instrumental in producing, not only the uniform distribution of humus in the surface soil, but also the looseness of texture which we see in forest soils especially. Ants, wasps, many kinds of beetles, crickets, and particularly the larvæ of these, and of other burrowing creatures, often form considerable accumulations, due directly both to their mechanical activity, and to their excrements.
The work of ants is in some regions on so large a scale as to attract the attention of the most casual observer. Especially is this the case in portions of the arid region, from Texas to Montana, where at times large areas are so thickly studded with hills from three to twelve feet in diameter, and one to two feet high, that it is difficult to pass without being attacked by the insects. The “mounds” studding a large portion of the prairie country of Louisiana seem also to be due to the work of ants, although not inhabited at present.
Larger burrowing animals also assist in the task of mixing uniformly the surface soils, and aiding root-penetration, as well as, in many cases, the conservation of moisture. Seton (loc. cit.) even claims that the pocket gophers (Thomomys) in a great degree replace the activity of the earthworms in the arid region, where they, together with the voles (commonly known there as field mice), exist in great numbers. Of course the work of these animals, as well as that of the prairie dogs, ground squirrels, badgers, etc., is incompatible with cultivation. But the effects of their burrows on the native vegetation, and the indications they give of the nature of the subsoil, are eminently useful to the land-seeker.
Thus in the rolling sediment-lands of the Great Bend of the Columbia, the observer is surprised to see the “giant rye grass,” usually at home in the moist lowlands, growing preferably on the crests of the ridges bordering the horizon. Examination shows that this is due to the burrowing of badgers, whereby the roots of the grass are enabled to reach moisture at all times, even in that extremely arid region.
CHAPTER X.
SOIL AND SUBSOIL (Continued)
THEIR RELATIONS TO VEGETATION.