Popular Science Monthly/Volume 32/February 1888/Animal Agency in Soil-Making
By Professor N. S. SHALER.
THE admirable studies of Mr. Darwin on the influence of earth-worms upon the soil has made it clear that these animals exercise a most important effect in its preparation for the use of plants. Mr. Darwin's luminous essay has served to call attention to the effect of organic life on the development of the soil-coating. In the following pages I propose to submit the results of some studies of a general nature, which serve to show that a number of other animals have a considerable influence on the preparation of soils.
Our soils, as is well known, depend upon a variety of actions which serve to break up the rocky matter of the earth, and to commingle that matter with organic materials more rapidly than the erosive agents can remove the detritus from the point at or near which it decays. For the formation of the soil two actions, at least, are essential. First, the bed-rock must be broken into fragments sufficiently separated from each other to permit the passage of roots between them; second, the rock fragments must be still further comminuted and commingled with organic waste to make the combination of organic and inorganic matter on which the utility of the soil absolutely depends. Although the earth-worms are undoubtedly very important agents in overturning and breaking up of soil, it appears to me that they are most effective in the tilled fields or in the natural and artificial grass-lands. So far as I have been able to observe, these creatures are rarely found in our ordinary forests where a thick layer of leaf-mold, commingled with branches, lies upon the earth. The character of this deposit is such that the creatures are not competent to make their way through it, and they therefore in the main avoid such situations. Moreover, wherever the soil is of a very sandy nature, earth-worms are scantily found if they are present at all. These worms are practically limited to the soils of a somewhat clayey character, which have no coating of decayed vegetation upon them.
As the greater portion of the existing soil has been produced in forest regions, I shall first examine the action of various animals upon the soils of wooded countries. The mammals are, of all our vertebrates, the most effective in their action upon the soil of forests. Twenty species or more of our American mammals are burrowers in the forest-bed. They either make their habitations beneath the ground, or resort to it in the pursuit of food. Of these, our burrowing rodents are perhaps the most effective, but a large number of other small mammals resort to the earth and make considerable excavations. In forming their burrows, or in the pursuit of other burrowing animals, these creatures often penetrate through the whole or greater portion of the soil-covering. The material which is withdrawn from the burrow is accumulated about its mouth. The result is the overturning of a considerable amount of the earth, and a consequent commingling of the material with vegetable matter. When brought to the surface and left exposed to the action of frost, the breaking up of the material is greatly favored, and thus the formation of the soil is facilitated.
Considerable as is the effect of burrowing mammals, the principal overturning of the earth in our primeval forests is accomplished by the invertebrate animals. Where the woods are not very dense, and particularly where the soil is somewhat sandy, our largest species of ants are very effective agents in working over the soil. Their burrows extend to the depth of some feet below the surface, and each hill brings to the air several cubic feet of excavated matter, which, as slight inspection shows, is much commingled with vegetable matter. Wherever these ant-hills abound they commonly exist to the number of a score or more on each acre, and the occupants of each hill, in many cases, bring as much as a cubic foot of matter to the surface in the course of a single year. The action of rain constantly operates to diffuse this material on every side of the hill. We may often observe a thin layer of sediment extending for a considerable distance from the elevation.
As is well known to all those who have inspected the soil within virgin forests, the earth is occupied by a host of larval insects, principally belonging to the group of beetles, but including also many orthopterous insects. These creatures, in the course of their life underground, displace a good deal of soil, a portion of which is thrown upon the surface, the greater part, however, being merely dislodged beneath the surface. The effect, however, is to commingle and to break up the soil, and thus favor its comminution. Although the roots of trees do by far the larger part of the rending which is accomplished in the soil-layer, they do not bring about much commingling of the soil. The thrusts which they apply to it shear the materials about, and so, to a certain extent, mix them, but by far the larger part of the commingling is effected by the animal life which dwells beneath the forest-bed.
Where the woods are wet and favor the development of the crayfish, the effect of this group of animals on the overturning of the soil is extremely great. It probably exceeds that which is accomplished in our ordinary fields by the action of the earth-worms. A single cray-fish will often bring in the course of a single season's activity not less than half a cubic foot of earthy matter to the surface. In certain districts where these animals abound, there appear to be not less than a thousand to each acre of surface. If such be their number, it is evident that not less than five hundred cubic feet of matter is brought to the surface from a considerable depth in the course of a year. As this matter is generally of a rather fine nature and easily dissolved in water, it rapidly washes away and forms a thin sheet on the surface. I am inclined to believe that large areas of our wet woods and the open border-lands along our streams are completely overturned to the depth of two feet or more in the course of half a century by the actions of these animals. It is not impossible, indeed, that the very fine division of the soil which-characterizes the regions inhabited by these creatures may be in good part due to their action. In this manner the creatures may have in part worked to bring about the very conditions which best serve their needs.
In open grounds, in natural prairies or grass-plains, the smaller species of ants are extremely effective agents in overturning the soils. Wherever the ground remains for some time unplowed it becomes occupied by these creatures. In the sandy soils of Eastern Massachusetts, the overturning accomplished by these creatures assumes a geological importance. For many years I have been puzzled by the fact that the glacial terraces and plains of this region were extensively covered to the depth of a foot or more by a coating of fine sand and very small pebbles, while below the depth of a foot pebbles of larger size are very numerous, and the spaces between them but imperfectly occupied with any material. It is obviously impossible to explain these conditions through the action of earth-worms, for the reason that these creatures are rarely found in soils of this description. From much observation I have become convinced that this coating of sandy material is, to a great extent, to be explained by the action of various species of ants in the forest condition by the work of the larger black ants, and in the condition of open plains by that of the smaller species.
The amount of material which these creatures bring to the surface in a single season is surprising. At several points in Eastern Massachusetts I have found the surface to contain at least one ant-hill to each square foot of area, or about forty thousand hills to the acre. This is, probably, an exceptionally great number; it will, perhaps, be safer to estimate the number at twenty thousand to the acre. The incoherent heaps of excavated matter which these creatures form are quickly washed away by the rain, or in many cases, are blown away by strong winds, and so scattered over the surface. As soon as destroyed they are, in most cases, rebuilded, the result being that a single hill is reconstructed at least half a dozen times during a season. I have estimated that the amount of material brought to the surface often exceeds three cubic inches to each square foot of surface in a single year, or about a fiftieth of an inch of the whole area each year. Thus, in the term of fifty years, the accumulation of material on the surface would amount to as much as an inch, and reckoning the soil as having an average depth of one foot, a total overturning would be accomplished in less than a thousand years. It is likely that in some cases, over considerable areas, a tolerably complete overturning is brought about in less than a quarter of this time.
The effect of this action of ants on the soil-material is peculiar. The tendency is like that noted by Mr. Darwin in the case of earthworms to bring the finer particles to the surface. I am inclined to think the ants accomplished this part of their work even more effectively than the earth-worms, for the reason that they penetrate more deeply between the stones than their less active associates. Like the earth-worms, but in larger measure, the ants convey considerable amounts of organic matter into the soil. Their winter store of food is deeply buried, and much of it remains unconsumed in the nether earth. There is thus a constant inhumation of vegetable matter beneath the materials which they bring to the surface.
Although the burrowing vertebrates operate most vigorously in the forest-covered regions, they also exercise a certain influence on the open country. The moles which work only here and there in the forest are conspicuous agents in overturning the soil in the grassed regions. Still, as this group is peculiarly limited in its distribution, and rarely penetrates to more than four or five inches below the surface, it exercises a relatively small effect. The field-mice are more potent agents in effecting the character of the soil. Their dwelling-chambers are at a considerable depth below the surface, and in forming them, they bring a certain amount of matter to the open air, moreover the remains of their food, as well as their excrements, are important contributions to the organic matter of the soil. Insects in their larval stage exercise a less effect in the open field than in the forest-covered regions; still, they are not to be left out of account in considering the process of soil-making in such areas. In Europe the rabbit, which has a habit of burrowing to a considerable depth, and in certain districts west of the Mississippi, the prairie-dog, overturn the soil on the areas they occupy with considerable rapidity. Still, as the number of these creatures in any given district is not great, their influence is mostly exercised in a very local way.
The foregoing considerations make it tolerably clear that our ants are, in some districts, by far the most important agents in overturning the soil and in commingling the superficial organic matter with the mineral material of which it is composed. Although on a field of a certain class those which are of a clayey nature, the earth-worms, are probably more efficient soil-makers than the ants, this latter group appears to be, at least in the eastern part of North America, on the whole, by far the most effective in the preparation of the soil for the needs of plants. They do not, it is true, take the soil into their bodies and thus disintegrate it, as the earth-worms do, but they accomplish what is perhaps the more important task of rapidly overturning the soil-material as well within the forests as in the open fields wherever that material is of a sandy nature.