SOIL BUILDERS
(1907)
MANY people who till the soil, either as a business or as a recreation, look upon it merely as dirt—cold, inert, lifeless, changeless. I have met farmers in New England who took it for granted that the land they till to-day is about the same as it was two hundred years ago, when their forefathers cleared it, except for being less fertile. They had not noticed, or at least had not interpreted, the soil-building and soil-changing agencies at work all about them—wearing away the uplands, enriching the meadows, reducing the rocks, filling the swamps; changing from year to year the contour of their farms and their agricultural value.
THE WEATHERING OF ROCKS
Every farm soil is a complex material and has an interesting history. Most soils are a mixture of ground rock, decayed plants and the remains of insects and animals. Some soils, as the sands, are almost entirely particles of rocks; others, as peat and muck land, are made almost entirely of decayed plants. Neither of these extremes makes a good farm soil, as a rule. The majority of the soils in which plants are cultivated are made mostly of ground rock, with the addition of a greater or less amount of decayed plants.
Rock has been, and is still being, ground by weathering—the action of air, rain, snow, frost, heat and ice. Ever since the surface of the earth cooled, making a crust of rock, these agencies have been constantly at work, breaking up this crust, wearing away fragments of rock and carrying them to lower levels. They are Nature's plows. All mountains and hills are slowly wearing away. We can no longer regard them as "firm and everlasting." "Whole mountain chains of geologic yesterday have disappeared from view," says Merrill, "and we read their history only in their ruins." The Appalachian Mountains have already lost by weathering and erosion as much material as now remains. Even within the memory of one man, a hill may become noticeably lower.
The whole earth is being leveled—very slowly, yet quite perceptibly. The face of every rock is roughened and chipped by the elements. Drops of rain wear away particles of it; water freezes m the crevices, expands, and chips off fragments. The air searches these crevices and corrodes them, as it does iron. Everywhere cliffs are lower, rocks are smaller and soils are finer than they used to be. The big rock that the farmer has plowed around for thirty years is smaller now than it was when he first "rode horse" for his father. The stones on the gravelly knoll pass between the cultivator teeth easier than they used to. All about us, in the wild and on the farm, are indisputable evidences that soil is being made by the weathering of rocks. Most farm soils are still incomplete—they contain rocks and stones that are slowly being made into soil. A few, as the clays and alluvial soils, are changing less; but even the finest clay soil is affected by weathering to some extent. The reducing and fining process is universal and ceaseless.
An interesting example of soil formation by weathering is the heaving of stones to the surface, especially in the clay soils of northern states. A vivid recollection of my boyhood is the thankless task of picking up stones from rocky New England fields. This had to be done every fall and every spring. Though we might pick up and cart off in the fall every stone to be seen, there would always be many on top of the ground by the time for spring plowing.
These stones were heaved up. The clay soil in which they were embedded became wet, froze and expanded, throwing the stones upon the surface, there being the least resistance in that direction. So many of our small fields of a few acres had immense piles of stones in each of the four corners, the accumulation of many years. When these stones are not picked up they lie upon the surface and are slowly reduced to soil.
Soil Becoming Rock.—The reverse process, of changing soil into rocks, is also taking place. Many of the common rocks and stones that we may pick up in our fields were once soil. Sandstone, which is now sought for trimming buildings, is sand that has been hardened into stone. "Pudding" stones, or conglomerates, are made of gravel. Sometimes these rocks may be broken up, by weathering or erosion, and the soil in them again become available for plant growth. Thus the materials of the earth's surface may be worked over and over during countless cycles of time. The soil that nourishes plants to-day may be the building stones of a future generation. The soil of every farm has an antiquity of no ordinary character.
Extreme Changes in Temperature Crack Rocks.— Weathering from changes in temperature is as
effective, though often not as noticeable, as weathering from other causes. The changes of temperature from summer to winter, and even from the heat of mid-day to evening, are sufficient to tear rocks to pieces. Rocks are made of several or many different minerals, each of which expands and contracts differently when subjected to heat or cold. The result is that the rocks are cracked and split from being pulled many ways. There are few parts of the world where surface temperatures are uniform for any length of time; hence nearly all surface rocks, even the smallest stones, and especially those in the North, are being slowly pushed and pulled to pieces by alternate expansion and contraction. According to Shaler, a change of temperature of 150° F., which is common in the North between the extremes of summer and winter, makes a granite rock 100 feet in diameter expand one inch.
In regions having great extremes of temperature daily, particularly in Texas, Montana, Arizona, and other parts of the West where rocks are sparsely protected by vegetation, the splitting of rocks is quite noticeable and is sometimes attended with gun-like reports and cracking sounds loud enough to be heard many rods. Livingstone states that in South Africa blocks of stone weighing 200 pounds are frequently split off during the night by the contraction due to the rapid fall of temperature. Many people have noticed how pieces are chipped off from the foundation stones of a building that has burned. In most parts of eastern United States, where the rocks are more or less protected by vegetation, the cracking of rocks from this cause is less noticeable; but it is certain that all rocks everywhere are being affected more or less. The simile—"immovable as a rock," is not perfect. Even the rock, our common symbol of stability, is subject to the universal law of change; it is broken down, re-created and broken down again, over and over, while it fills its place in the working out of the Great Design.
PLANTS AS SOIL BUILDERS
Broken rock alone, however, does not make a fertile soil, as the farmer defines fertility. There are plants that thrive on bare rock, but the plants that are grown as farm crops are of a higher order and cannot rough it like this. A fertile soil—one that will grow large crops of the higher plants, either wild or cultivated—must contain a considerable amount of humus, which is chiefly decayed vegetation. A soil made of rock alone may contain all the mineral plant food that farm crops need, but it is apt to lack nitrogen and has not the right texture.
The Evolution of a Soil.—Nothing in nature is more interesting than the gradual evolution of a fertile soil from a barren rock, and nothing is more significant of the illimitable wisdom of the Creator. The history of soil building reads something like this: In the beginning is a lofty cliff, mute witness of the eruptions and disturbances through which the earth passed in cooling. It is bare and desolate. No living thing finds nourishment upon it. For centuries the storms beat against it; ice, rain and sudden changes in temperature pry off great boulders, which crash into the valleys. In the course of time there come to be upon these boulders, and upon the rocks and stones split off from them, lichens and other humble plants that are able to send their minute root structures into the crevices and live upon the slight substances that are formed on the surface by weathering, together with what they can get from the air. These lichens are very acid and are able to etch the rocks. They die and decay, leaving the beginning of a fertile soil in the crevices and upon the ledges. The growth of higher plants is thus made possible; perhaps the mosses gain a foothold. These in turn elaborate more of the rock for their own use and in turn die, enriching the so\l with themselves. Now there is a pocket of soil upon the ledge which may be able to support such humble plants as ferns or saxifrage. Thus the process goes on from decade to decade and from century to century, the lower plants being succeeded by larger and more highly organized plants, as the rocks are made finer by weathering and are enriched by the decay of the plants that they nourish. Finally the soil can support mulleins, honeysuckles, or fir trees. Many years later it may be able to support a crop of corn, timothy, or apples. A fertile farm soil is the product of many agencies working through thousands of years.
How Plants are Making Soil To-day.—Plants are helping to make fertile soil to-day as they have for centuries. Each year the forest floor receives a fresh carpet of leaves, and the older generations of trees fall to the ground and slowly pass into mold. Each year the grass in the meadows and the weeds by the roadside add their substance to the soil from which they have sprung, thereby enabling it to nurture other and lustier plants in succeeding years. Lichens spread their thin substance over rocks, and mosses take up the battle where the lichens leave off, just as of old.
Swamp lands and meadows are the most conspicuous examples of soils built mostly of plants. Lakes, ponds, streams and swamps are being filled in, not only with soil washed from surrounding higher land, but also with plants. The little pond that I skated upon as a boy is reduced to a mere mud hole now; the lilies, sedges, reeds, cattails and other aquatic and semi-aquatic plants have encroached upon it from the edges year by year, until now hay is cut where I used to catch bullheads. Most of the rich valleys and meadows of northern United States were once water-courses or glacial lakes. The weedy water's edge of today may be a sphagnum bog a century hence and a cabbage field in another hundred years. The mangrove swamp of this century, reaching trunk-like roots into the sea, may be the tilled land of a future generation.
Stems and Roots Split Rocks.—Plants also aid in soil building, to a considerable extent, by the pressure they exert upon rocks. The roots of trees often follow the crevices of rocks to a considerable depth, and by the force of growth help to widen them. Even on top of the ground one may see many examples of rocks that have been rent by the growth of trees. Among greenhouse plants it is quite common to find pots that have been split apart by the growth of roots. But in many of the cases where rocks are split apart, and a tree is growing in the crevice, the rock was first split open by weathering and the tree then widened the crevice. The acids secreted by the roots of plants dissolve a small portion of plant food from the rocks that the roots embrace. Rocks that have been etched by root acids may be found in almost any tree-covered ledge. In these various ways plants are contributing to the up building of our agricultural soils.
The peculiar value of certain plants as soil binders must not be forgotten. One of the most efficient and certainly the most notorious of soil binders is "quack-grass," and its counterparts variously known as "Johnson-grass," "witch-grass," "couch-grass," and other aliases. The evil reputation of this grass is due to the fact that it is extremely difficult to kill, because the long underground stems may root at any point. The smaller the pieces into which the roots are chopped by the irate husbandman, the more widely and thoroughly is the pest scattered. This is just the reason why "quack" is such an excellent soil binder; the tough, white root-stalks thread the soil in every direction, soon making a network of fibers, which prevent light soils from washing badly. Steep banks or slopes are sometimes held by establishing quack grass upon them; the underground stems are chopped into small pieces and these are sown thickly. Several other grasses, notably Bermuda grass, are particularly serviceable in such cases.
In some sections, notably in Oregon, Eastern Massachusetts and Western Michigan, drifting sands are held by planting them with sedges or "beach grass." In Holland the dikes are planted with rushes to bind the soil. Willows and osiers planted on the banks of turbulent streams are effectual in preventing them from eating away their banks. Morning-glories and related plants are called bind-weeds, because the vines root at the joints and hold the soil tenaciously. A few horses tails planted in a wet place soon make a dense mat of roots which grasp the soil so firmly that it cannot wash away. These are only a few examples of plants that are particularly valuable for this purpose. All plants are soil binders to some extent, as well as soil makers; they not only enrich it with their herbage, but also hold it with their roots and, if they lie close to the ground, with their herbage also. In hilly sections some plants may be used to great advantage in checking erosion. This problem is discussed in Chapter XI.
HOW ICE HAS MADE SOIL
Once the northern part of North America was covered with a great sheet of ice, reaching as far south as Cape Cod, northern Pennsylvania, Ohio and westward to the Rockies. Geologists tell us that this immense glacier must have been several hundreds, and in some places several thousands of feet thick. It slowly bore down from the north, moving only a few inches to a few feet an hour, scraping the surface of the earth and carrying great quantities of rocks, stones and soil to the southward. According to some authorities certain parts of the glacier must have exerted a pressure of at least two hundred thousand pounds per square inch upon parts of the surface over which it passed. The bottom of the ice sheet became studded with huge boulders, which acted like teeth, tearing and grinding the rocks over which the ponderous mass passed. Some of these boulders, -scratched and worn, may be still seen in the hillside pastures of New England and other parts of the glaciated region. Some exposed ledges of rock still show the deep grooves that were cut in them by these boulder teeth.
When the ice melted a mass of soil material was dropped, perhaps many hundreds of miles away from the place where it was picked up. Rocks that could have come only from the mouth of Lake Huron are found in the drift or glacial soils in Ohio. Rocks from Ontario are found as far south as Kentucky. Great masses of ice were stranded here and there over the land. The streams of water resulting from the melting of the ice still further mixed the rocks, and the soils that the glacier had ground from the rocks.
The result of this ice sheet is the endless variety of soils that are found in the North. Most of the soils of that part of the northern United States that was covered by the great glacier were made by this agency. They are technically known as "drift" soils. Where parts of the ice sheet settled and melted away there were formed "morains" or "drumlins," the long, rounded knolls so common in northeastern United States. Since the time when this ice sheet covered our land, moving water has still further shifted and mixed soils, rounded the knolls and deepened the gullies. But most of the great variety of soil and diversity of contour in this region is due to the scouring, crushing, molding, transporting and distributing power of the great glacier. Small glaciers, performing exactly the same work, may be seen to-day in the Alps, Alaska, and other frigid regions.
ANIMALS AS SOIL BUILDERS
Animal life contributes much more to the building of soils than seems possible at first thought. Eventually every animal and insect returns to the soil, from which it came. The addition of animal matter to the soil is not nearly so evident as the addition of vegetable matter from decaying plants; yet, when we reflect upon it, the excrements and the remains of all creatures upon the earth must aggregate a considerable amount.
Of no small importance also, are the burrows, channels, holes, etc., in which animals live or by .which they feed. Ants, moles, gophers, wood-chucks, and the like are insignificant soil builders as individuals, but in the aggregate they have great influence. Ants are abundant on many of the lighter soils and often exercise a profound influence on their structure and agricultural value. Shaler has calculated that ants bring to the surface of a four-acre field, in Cambridge, Massachusetts, enough sand and fine soil to cover the entire area one-fifth inch deep each year. This is probably a larger amount of material than ants move in most places, although those of us who have had to fight ants in lawns are quite willing to accept these figures; but they call our attention to the insidious and far-reaching influence that these tiny creatures may exert. Since the material brought to the surface by the smaller ants is mostly fine sand and smaller particles of soil, they being unable to move the larger particles, it is evident that the texture of the surface soil must be greatly modified by their industry.
The mounds built by the large black and brown hill-building ants are often two feet in height and four feet in diameter. They are composed mostly of soil brought from below, mixed with bits of leaves and bark. They are being washed down constantly by rains and added to the surface soil. These ants usually build a new mound each year. Furthermore, the subterranean burrows and channels of ants, penetrating as they do from several inches to many feet, have a pronounced effect upon the texture of the soil, and upon its aeration.
The burying beetle, crayfish, woodchuck, chipmunk, mole, gopher, prairie dog, ground squirrel, badger, and other burrowing animals and insects, all contribute largely, in the aggregate, to the movement and aeration of soils, the Tatter four being especially abundant west of the Mississippi. Gophers have honeycombed millions of acres, and prairie dogs and ground squirrels have been no less industrious. However injurious these animals may be otherwise, and however difficult may be the task of exterminating them so that crops can be grown, they certainly serve a useful purpose in mixing the subsoil with the surface soil and promoting better drainage and aeration. Thousands of acres of land in the United States have been submerged by the erection of beaver dams and their value for agricultural purposes has been profoundly influenced thereby. The beaver is no longer an important factor in soil building with us, but he has contributed very largely in the past.
The Important Service of Angleworms.—The most important soil builder among animals is the angleworm or earthworm. Of these there are many kinds, from the big, snaky "night walker," that the fisherman with a torch finds crawling along the ground at night, to the tiny red ones beneath the pile of old manure. In South Africa some earthworms are two feet long. All of them are most industrious soil workers. After a rainy night, especially in early spring, the ground may be thickly strewn with their castings. On digging down in most moist soils a labyrinth of angleworm channels will be found. These burrows go more than five or six feet below the surface.
Angleworms benefit farm soils in several ways. The channels that they make loosen, aerate and drain the soil to a considerable depth, far deeper than the subsoil plow works. The small roots and rootlets that reach deep into the subsoil usually follow the worm burrows, This is particularly true of tenacious soils, in which angleworms most frequently work. They are rarely numerous in very light, sandy soils because these do not contain a sufficient quantity of vegetable matter upon which they may feed. Again, the soil is fined by being passed through the worms. In making these channels the worm swallows the soil for the purpose of using as food the decaying vegetable matter it contains. As it passes out through the worm this soil is ground, as grain is ground in a chicken's gizzard. Charles Darwin estimated that the angleworms in English soils passed through their bodies and ground over ten tons of soil per acre each year; that is, they deposited about one-fifth of an inch of castings over the entire surface each year. This is the richest kind of top-dressing. He estimated that there are about 50,000 earth worms in each acre of English garden land, and about 25,000 in each acre of meadow land. Our American soils are as full of "bait worms" as the English soils, and their influence on our agriculture must be fully as pronounced as that assigned to them by the great scientist.
THE ACTION OF MOVING WATER ON SOIL
No soil is ever at rest. It is constantly receiving and constantly losing. The additions come mostly from the weathering of rocks and the decay of plants and animals. The losses are mostly due to the action of moving water. Moving water has been given the gigantic task of world leveling, and is working at it industriously and successfully. The mountains, hills and knolls are worn away; water carries the particles down the valleys and deposits them as soil. Lakes and ponds are being filled with soil washed from higher land. The flat lands about the lakes and streams are made mostly of soil worn away from the surrounding highlands. The streams carry great quantities of soil and deposit it in the shallows and bends. The coarser and heavier materials, as gravel and sand, are deposited first and the finer material, as clay, is deposited only when the current becomes sluggish. At the mouths of streams, where the current is sluggish, a "delta" is often formed by the accumulation of soil carried down by the streams. It has been estimated that the amount of soil carried to the Gulf of Mexico every year by the Mississippi River would cover a square mile of territory 268 feet deep. At this rate, the American continent might be reduced to sea-level in four and one-half million years. This is but a small proportion, however, of the total amount of soil that these rivers carry, for most of it is left along their banks. According to reliable measurements, England is 550 square miles smaller now than at the time of the Norman Conquest, owing to the soft chalk and clay shores being crumbled away by waves.
Every stream is constantly changing its course; many a valley farmer has had the river take away a large slice of his farm and give it to his neighbor down stream. Brooks states that within a generation the Connecticut River has gradually taken several hundred acres of rich meadow land from the town of Hadley and bestowed it upon the town of Matfield. Smaller streams, even the tiniest rills, are transporting and building soil in a similar manner. Sometimes this action of water is beneficial, but usually it is injurious. The loss of farm soil by erosion is discussed in Chapter XI.
Alluvial Soils.—The flat lands near streams are often flooded each year and receive a top-dressing'1 of rich mud that keeps them extremely fertile. The Nile is a noted example, but many of our own rivers, including the Ohio and Mississippi, fertilize their meadows in the same way, much to the profit of man. The fertile plains of Egypt, once the "granary of the world, ' are not made of native soils, but of soil washed down from the mountains of Abyssinia, many hundreds of miles away. All the rich rice and cotton fields of southern Louisiana were built by the Mississippi River, of soil brought from the mountains three thousand miles away. In some places this soil is three hundred feet deep. These various kinds of alluvial or water-built soil are among the most valuable for agricultural purposes. In any hilly country one can observe this kind of soil building going on at a rapid rate.
Besides transporting soil from place to place, water also assists in soil building by wearing away the rock over which it passes. It would seem hardly possible that water should be capable of wearing away so rapidly the hardest of rocks, were it not that we can see the action going on all around us. Even a single drop, falling continuously year after year, will eat a deep hole in the hardest rock. When a volume of water is in motion, and especially when it is carrying along with it particles of soil, its grinding and filing effect is much more pronounced. The stones on the bottom of the brook at home are rounder and smaller now than when we first watched the tadpoles there. The spring that slaked our thirst twenty years ago has worn a deeper channel in the rock over which it flows. Each year the apex of the Horseshoe Falls of Niagara is four feet nearer Lake Erie. The Colorado River, which has already worn a channel half a mile deep in the solid rock of the Grand Canon, is cutting deeper every year. All water, even the purest spring water, has some minerals and gases dissolved in it, and these help it to dissolve the rock. Rain water contains small quantities of carbonic acid gas and other gases, which increase its power to dissolve rocks.
SOILS BUILT WHOLLY OB PARTLY BY THE WIND
Soils built wholly or in part by wind are not uncommon. In arid regions, along the sea coast and near the shores of the Great Lakes, the drifting sands often cover and ruin valuable soils. Some of the most productive farm soils in this country were made, and are still being made, by wind. A noted example is the Palouse region of eastern Washington, eastern Oregon and northern Idaho. Here the land is a succession of rounded knolls and hills, which are sometimes several hundred feet high and are a rich, black, basaltic ash to the bottom. The native Indians account for the hills in a legend. They say that at one time all this region was a level prairie of marvelous fertility. Wonderful crops of maize were raised upon it by the red men. One evil day they heard that the white men were coming. Knowing by repute the white man's greed, the Indians went to work to gather the precious soil into huge heaps, preparatory to carrying it away into the mountains, beyond the grasp of the avaricious whites. But the white men came before the soil could be carried away, took it for their grain fields, and it has been in heaps ever since. The more prosaic geologist, however, says that these fertile hills were made almost entirely by wind, assisted by erosion. In parts of California, Oregon, Washington, and Wyoming, the clayless "dust soil" becomes cracked and loosened in dry weather and is carried away by the wind in dense clouds, banking up like snow behind rocks and bushes. Recall, also, the stories of caravans in the desert being overwhelmed by sandstorms. There are numerous records of large quantities of soil being carried over a thousand miles by wind.
Even where the soil has been made mostly by other agencies, wind contributes something to it. Fine soil, leaves, chaff and dust are swept over the hill crest and deposited on the leeward slope. The amount of soil that is made and transported by wind, in the form of dust, must amount to an appreciable quantity in the course of a year. The slope opposite to that of the prevailing wind is usually less abrupt than the other, because so much soil material has been deposited there by the wind.
Still another way in which wind assists in making soil is by blowing fine particles of sharp sand and dust against the rocks and so wearing them away. At first thought it would seem that the result of this would be very insignificant, but in reality it is often quite important. In arid parts of the United States and elsewhere, the millions and millions of soil grains blown against cliffs and rocks leave a striking testimony to their abrasive power. In a surprisingly short time rough corners are worn smooth, great boulders are undermined, hollows are scoured out, and sometimes large, erect rocks are completely filed off near the base, where the wind-blown sand is thickest, and fall over. The "Mushroom Rocks" of Wyoming are a notable example. In humid sections, the filing of rocks by blown sand is less conspicuous, except near the sea-coast. The windows of houses near the coast are roughened and sometimes eaten through by the natural sandblast.
THE SOIL TEEMS WITH LIFE
There are other soil builders, more minute but not less active or influential than those that have been mentioned. The old idea was that the soil is dead; the fact is, it teems with life. It contains germs of decay, bacteria that influence in some mysterious way the palatability of plant foods, ferments of many kinds, moulds of diverse sorts—a fertile soil fairly hums with activity. Countless tiny organisms, visible only to the eye behind a microscope, are constantly at work, changing, breaking down, building up. Some are beneficial, some are harmful, some are harmless. How many kinds there are, and what part each plays in the complex operation of soil building, nobody knows, for the science of bacteriology is yet at its beginning.
Every farm presents many phases of soil building and soil wasting. The farmer should observe the various agencies at work upon his land, and turn them to his own profit. He should remember that the soil is not dead, but alive; that it is constantly swept by winds, worn and transported by waters, broken and refined by frost and air, loosened and enriched by plants and animals, and all the while creeping nearer and nearer to a level.