Outlines of the Earth's History: A Popular Study in Physiography

by Nathaniel Southgate Shaler

Great as is the effect of plants on the soil, that influence is almost equalled by the action of the animals which have the habit of entering the earth, finding there a temporary abiding place. The number of these ground forms is surprisingly great. It includes, indeed, a host of creatures which are efficient agents in enriching the earth. The species of earthworms, some of which occupy forested districts as well as the fields, have the habit of passing the soil material through their bodies, extracting from the mass such nutriment as it may contain. In this manner the particles of mineral matter become pulverized, and in a measure affected by chemical changes in the bodies of the creatures, and are thus better fitted to afford plant food. Sometimes the amount of the earth which the creatures take in in moving through their burrows and void upon the surface is sufficient to form annually a layer on the surface of the ground having a depth of one twentieth of an inch or more. It thus may well happen that the soil to the depth of two or three feet is completely overturned in the course of a few hundred years. As the particles which the creatures devour are rather small, the tendency is to accumulate the finer portions of the soil near the surface of the earth, where by solution they may contribute to the needs of the lowly plants. It is probably due to the action of these creatures that small relics of ancient men, such as stone tools, are commonly found buried at a considerable depth beneath the earth, and rarely appear upon the surface except where it has been subjected to deep ploughing or to the action of running streams.

Along with the earthworms, the ants labour to overturn the soil; frequently they are the more effective of the two agents. The common species, though they make no permanent hillocks, have been observed by the writer to lay upon the surface each year as much as a quarter of an inch of sand and other fine materials which they have brought up from a considerable depth. In many regions, particularly in those occupied by glacial drift, and pebbly alluvium along the rivers, the effect of this action, like that of earthworms, is to bring to the surface the finer materials, leaving the coarser pebbles in the depths. In this way they have changed the superficial character of the soil over great areas; we may say, indeed, over a large part of the earth, and this in a way which fits it better to serve the needs of the wild plants as well as the uses of the farmer.

Many thousand species of insects, particularly the larger beetles, have the habit of passing their larval state in the under earth. Here they generally excavate burrows, and thus in a way delve the soil. As many of them die before reaching maturity, their store of organic matter is contributed to the mass, and serves to nourish the plants. If the student will carefully examine a section of the earth either in its natural or in its tilled state, he will be surprised to find how numerous the grubs are. They may often be found to the number of a score or more of each cubic foot of material. Many of the species which develop underground come from eggs which have carefully been encased in organic matter before their deposition in the earth. Thus some of the carrion beetles are in the habit of laying their eggs in the bodies of dead birds or field mice, which they then bury to the depth of some inches in the earth. In this way nearly all the small birds and mammals of our woods disappear from view in a few hours after they are dead. Other species make balls from the dung of cattle in which they lay their eggs, afterward rolling the little spheres, it may be for hundreds of feet, to the chambers in the soil which they have previously prepared. In this way a great deal of animal matter is introduced into the earth, and contributes to its fertility.

Many of our small mammals have the habit of making their dwelling places in the soil. Some of them, such as the moles, normally abide in the subterranean realm for all their lives. Others use the excavations as places of retreat. In any case, these excavations serve to move the particles of the soil about, and the materials which the animals drag into the earth, as well as the excrement of the creatures, act to enrich it. This habit of taking food underground is not limited to the mammals; it is common with the ants, and even the earthworms, as noted by Charles Darwin in his wonderful essay on these creatures, are accustomed to drag into their burrows bits of grass and the slender leaves of pines. It is not known what purpose they attain by these actions, but it is sufficiently common somewhat to affect the conditions of the soil.

The result of these complicated works done by animals and plants on the soil is that the material to a considerable depth are constantly being supplied with organic matter, which, along with the mineral material, constitutes that part of the earth which can support vegetation. Experiment will readily show that neither crushed rock nor pure vegetable mould will of itself serve to maintain any but the lowliest vegetation. It requires that the two materials be mixed in order that the earth may yield food for ordinary plants, particularly for those which are of use to man, as crops. On this account all the processes above noted whereby the waste of plant and animal life is carried below the surface are of the utmost importance in the creation and preservation of the soil. It has been found, indeed, in almost all cases, necessary for the farmer to maintain the fertility of his fields to plough-in quantities of such organic waste. By so doing he imitates the work which is effected in virgin soil by natural action. As the process is costly in time and material, it is often neglected or imperfectly done, with the result that the fields rapidly diminish in fertility.

The way in which the buried organic matter acts upon the soil is not yet thoroughly understood. In part it accomplishes the results by the materials which on its decay it contributes to the soil in a state in which they may readily be dissolved and taken up by the roots into their sap; in part, however, it is believed that they better the conditions by affording dwelling places for a host of lowly species, such as the forms which are known as bacteria. The organisms probably aid in the decomposition of the mineral matter, and in the conversion of nitrogen, which abounds in the air or the soil, into nitrates of potash and soda—substances which have a very great value as fertilizers. Some effect is produced by the decay of the foreign matter brought into the soil, which as it passes away leaves channels through which the soil water can more readily pass.

By far the most general and important effect arising from the decay of organic matter in the earth is to be found in the carbon dioxide which is formed as the oxygen of the air combines with the carbon which all organic material contains. As before noted, water thus charged has its capacity for taking other substances into solution vastly increased, and on this solvent action depends in large part the decay of the bed rocks and the solution of materials which are to be appropriated by the plants.

Having now sketched the general conditions which lead to the formation of soils, we must take account of certain important variations in their conditions due to differences in the ways in which they are formed and preserved. These matters are not only of interest to the geologist, but are of the utmost importance to the life of mankind, as well as all the lower creatures which dwell upon the lands. First, we should note that soils are divisible into three great groups, which, though not sharply parted from each other, are sufficiently peculiar for the purposes of classification. Where the earth material has been derived from the rocks which nearly or immediately underlie it, we have a group of soils which may be entitled those of immediate derivation—that is, derived from rocks near by, or from beds which once overlaid the level and have since been decayed away. Next, we have alluvial soils, those composed of materials which have been transported by streams, commonly from a great distance, and laid down on their flood plains. Third, the soils the mineral matters of which have been brought into their position by the action of glaciers; these in a way resemble those formed by rivers, but the materials are generally imperfectly sorted, coarse and fine being mingled together. Last of all, we have the soils due to the accumulation of blown dust or blown sand, which, unlike the others, occupy but a small part of the land surface. It would be possible, indeed, to make yet another division, including those areas which when emerging from the sea were covered with fine, uncemented detritus ready at once to serve the purposes of a soil. Only here and there, and but seldom, do we find soils of this nature.