Earth and Sky Every Child Should Know Part 4

Sand mixed with clay makes a mellow soil, which lets water and air pa.s.s freely through. The roots are more comfortable, and the tiny root hairs can reach the particles of both kinds of mineral food. But the needful third element is decaying plant and animal substances, called "humus."

These enrich the soil, but they do a more important thing: their decay hastens the release of plant food from the earthy part of the soil, and they add to it a sticky element which has a wonderful power to attract and hold the water that soaks into the earth.

_What is the best garden soil?_ A mixture of sand, clay, and humus is called "loam." If sand predominates, it is a sandy loam--warm, mellow soil. If clay predominates, we have a clay loam--a heavy, rich, but cool soil. All gradations between the two extremes are suited to the needs of crops, from the melons on sandy soil, to celery that prefers deep, cool soil, and cranberries that demand muck--just old humus.

_How do plant roots feed in soil?_ By means of delicate root hairs which come into contact with particles of soil around which a film of soil water clings. This fluid dissolves the food, and the root absorbs the fluid. Plants can take no food in solid form. Hence it is of the greatest importance to have the soil pulverized and spongy, able to absorb and hold the greatest amount of water. The moisture-coated soil particles must have air-s.p.a.ces between them. Air is as necessary to the roots as to the tops of growing plants.

_Why does the farmer plough and harrow and roll the land?_ To pulverize the soil; to mellow and lighten it; to mix in thoroughly the manure he has spread on it, and to reach, if he can, the deeper layers that have plant food which the roots of his crops have not yet touched. Killing weeds is but a minor business, compared with tillage.

Later, ploughing or cultivating the surface lightly not only destroys the weeds, but it checks the loss of water by evaporation from the cracks that form in dry weather. Raking the garden once a day in dry weather does more good than watering it. The "dust mulch" acts as a cool sunguard over the roots.

_The process of soil-making._ If the man chopping wood in the Yosemite Valley looks about him he can see the soil-making forces at work on a grand scale. The bald, steep front of El Capitan is of the hardest granite, but it is slowly crumbling, and its fragments are acc.u.mulating at the bottom of the long slope. Rain and snow fill all crevices in the rocks. Frost is a wonderful force in widening these cracks, for water expands when it freezes. The loosened rock ma.s.ses plough their way down the steep, gathering, as they go, increasing power to tear away any rocks in their path.

Wind blows finer rock fragments along, and they lodge in cracks. Fine dust and the seeds of plants are lodged there. The rocky slopes of the Yosemite Valley are all more or less covered with trees and shrubs that have come from wind-sown seeds. These plants thrust their roots deeper each year into the rock crevices. The feeding tips of roots secrete acids that eat away lime and other substances that occur in rocks. Dead leaves and other discarded portions of the trees rot about their roots, and form soil of increasing depth. The largest trees grow on the rocky soil deposited at the base of the slope. The tree's roots prevent the river from carrying it off.

When granite crumbles, its different mineral elements are separated.

Clear, gla.s.sy particles of quartz we call sand. Dark particles of feldspar become clay, and may harden into slate. Sand may become sandstone. Exposed slate and sandstone are crumbled by exposure to wind and frost and moving water, and are deposited again as sand-bars and beds of clay.

The most interesting phase of soil study is the discovery of what a work the humble earthworm does in mellowing and enriching the soil.

THE WORK OF EARTHWORMS

The farmer and the gardener should expect very poor crops if they planted seed without first ploughing or spading the soil. Next, its fine particles must be separated by the breaking of the hard clods. A wise man ploughs heavy soil in the fall. It is caked into great clods which crumble before planting time. The water in the clods freezes in winter.

The expansion due to freezing makes this soil water a force that separates the fine particles. So the frost works for the farmer.

Just under the surface of the soil lives a host of workers which are our patient friends. They work for their living, and are perhaps unconscious of the fact that they are constantly increasing the fertility of the soil. They are the earthworms, also called fishworms, which are distributed all over the world. They are not generally known to farmers and gardeners as friendly, useful creatures, and their services are rarely noticed. We see robins pulling them out of the ground, and we are likely to think the birds are ridding us of a garden pest. What we need is to use our eyes, and to read the wonderful discoveries recorded in a book called "Vegetable Mould and Earthworms," written by Charles Darwin.

The benefits of ploughing and spading are the loosening and pulverizing of the packed earth; the mixing of dead leaves and other vegetation on and near the surface with the more solid earth farther down; the letting in of water and air; and the checking of loss of water through cracks the sun forms by baking the soil dry.

The earthworm is a creature of the dark. It cannot see, but it is sufficiently sensitive to light to avoid the sun, the rays of which would shrivel up its moist skin. Having no lungs or gills, the worm uses the skin as the breathing organ; and it must be kept moist in order to serve its important use. This is why earthworms are never seen above ground except on rainy days, and never in the top soil if it has become dry. In seasons of little rain, they go down where the earth is moist, and venture to the surface only at night, when dew makes their coming up possible.

Earthworms have no teeth, but they have a long snout that protrudes beyond the mouth. Their food is found on and in the surface soil. They will eat sc.r.a.ps of meat by sucking the juices, and sc.r.a.pe off the pulp of leaves and root vegetables in much the same way. Much of their subsistence is upon organic matter that can be extracted from the soil.

Quant.i.ties of earth are swallowed. It is rare that an earthworm is dug up that does not show earth pellets somewhere on their way through the long digestive ca.n.a.l. The rich juices of plant substance are absorbed from these pellets as they pa.s.s through the body.

Earthworms explore the surface of the soil by night, and pick up what they can find of fresh food. Nowhere have I heard of them as a nuisance in gardens, but they eagerly feed on bits of meat, especially fat, and on fresh leaves. They drag all such victuals into their burrows, and begin the digestion of the food by pouring on it from their mouths a secretion somewhat like pancreatic juice.

The worms honeycomb the earth with their burrows, which are long, winding tubes. In dry or cold weather these burrows may reach eight feet under ground. They run obliquely, as a rule, from the surface, and are lined with a layer of the smooth soil, like soft paste, cast from the body. The lining being spread, the burrow fits the worm's body closely.

This enables it to pa.s.s quickly from one end to the other, though it must wriggle backward or forward without turning around.

At the lower end of the burrow, an enlarged chamber is found, where hibernating worms coil and sleep together in winter. At the top, a lining of dead leaves extends downward for a few inches, and in day time a plug of the same material is the outside door. At night the worm comes to the surface, and casts out the pellets of earth swallowed. The burrow grows in length by the amount of earth sc.r.a.ped off by the long snout and swallowed. The daily amount of excavation done is fairly estimated by the castings observed each morning on the surface.

One earthworm's work for the farmer is not very much, but consider how many are at work, and what each one is doing. It is boring holes through the solid earth, and letting in the surface water and the air. It is carrying the lower soil up to the surface, often the stubborn subsoil, that no plough could reach. It is burying and thus hastening the decay of plant fibre, which lightens heavy soil and makes it rich because it is porous. Moreover, the earthworms are doing over and over again this work of fining and turning over the soil, which the plough does but seldom.

By the continuous carrying up of their castings, the earthworms gradually bury manures spread on the surface. The collapse of their burrows and the making of new ones keep the soil constantly in motion.

The particles are being loosened and brought into contact with the soil water, that dissolves, and thus frees for the use of feeding roots, the plant food stored in the rock particles that compose the mineral part of the soil.

The weight of earth brought to the surface by worms in the course of a year has been carefully estimated. Darwin gives seven to eighteen tons per acre as the lowest and highest reports, based on careful collecting of castings by four observers, working on small areas of totally different soils. In England, earthworms have done a great deal more toward burying boulders and ancient ruins than any other agency. They eagerly burrow under heavy objects, the weight of which causes them to crush the honeycombed earth. Undiscouraged, the earthworms repeat their work.

"Long before man existed, the land was regularly ploughed, and continues still to be ploughed by earthworms. It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organized creatures."

After years of study, Charles Darwin came to this conclusion. The more we study the lives of these earth-consuming creatures, the more fully do we believe what the great nature student said. The fertile soil is made of rock meal and decayed leaves and roots. Only recently have ploughs been invented. But the great forest crops have grown in soil made mellow by the earthworm's ploughing.

QUIET FORCES THAT DESTROY ROCKS

Wind and water are the bl.u.s.tering active agents we see at work tearing down rocks and carrying away their particles. They do the most of this work of levelling the land; but there are quiet forces at work which might not attract our attention at all, and yet, without their help, wind and running water would not accomplish half the work for which they take the credit.

The air contains certain destructive gases which by their chemical action separate the particles of the hardest rocks, causing them to crumble. Now the wind blows away these crumbling particles, and the solid unchanged rock beneath is again exposed to the crumbling agencies.

The changes in temperature between day and night cause rocks to contract and expand, and these changes put a strain upon the mineral particles that compose them. Much scaling of rock surfaces is due to these causes.

Building a fire on top of a rock, and then das.h.i.+ng water upon the heated ma.s.s, shatters it in many directions. This process merely intensifies the effect produced by the mild changes of winter and summer. Water is present in most rocks, in surprising quant.i.ties, often filling the s.p.a.ces in porous rocks like sandstones.

When winter brings the temperature down to the freezing point, the water near the surface of the rock first feels it. Ice forms, and every particle of water is swollen by the change. A strain is put upon the mineral particles against which the particles of ice crowd for more room. Frost is a very powerful agent in the crumbling of rocks, as well as of stubborn clods of earth. In warm climates, and in desert regions where there is little moisture in the rocks, this destructive action of freezing water is not known. In cold countries, and in high alt.i.tudes, where the air is heavy with moisture, its greatest work is done.

Some kinds of rock decay when they become dry, and resist crumbling better when they absorb a certain amount of moisture. Alternate wetting and drying is destructive to certain rocks.

One of the unnoticed agents of rock decay is the action of lowly plants.

Mosses grow upon the faces of rocks, thrusting their tiny root processes into pits they dig deeper by means of acids secreted by the delicate tips. You have seen shaded green patches of lichens, like little rugs, of different shapes, spread on the surface of rocks. But you cannot see so well the work these growths are doing in etching away the surface, and feeding upon the decaying mineral substance.

Mosses and lichens do a mighty work, with the help of water, in reducing rocks to their original elements, and thus forming soil. No plants but lichens and mosses can grow on the bare faces of rocks. As their root-like processes lengthen and go deeper into the rock face, particles are pried off, and the under-substance is attacked. Higher plants then find a footing. Have you not seen little trees growing on a patch of moss which gets its food from the air and the rock to which it clings? The spongy moss cus.h.i.+on soaks up the rain and holds it against the rock face. A streak of iron in the rock may cause the water to follow and rust it out, leaving a distinct crevice. Now the roots of any plant that happens to be growing on the moss may find a foot-hold in the crack. Streaks of lime in a rock readily absorb water, which gradually dissolves and absorbs its particles, inviting the roots to enter these new pa.s.sages and feed upon the disintegrating minerals. Dead leaves decay, and the acids the trickling water absorbs from them are especially active in disintegrating lime rocks.

From such small beginnings has resulted the shattering of great rock ma.s.ses by the growth of plants upon them. Tree roots that grow in rock crevices exert a power that is irresistible. The roots of smaller plants do the same great work in a quieter way.

When a hurricane or a flood tears down the mountain-side, sweeping everything before it, trees, torn out by the roots, drag great ma.s.ses of rock and soil into the air, and fling them down the slope. Wind and water thus finish the destruction which the humble mosses and lichens began. What seemed an impregnable fortress of granite has crumbled into fragments. Its particles are reduced to dust, or are on the way to this condition. The plant food locked up in granite boulders becomes available to hungry roots. Forests, grain-fields, and meadows cover the work of destructive agencies with a mantle of green.

HOW ROCKS ARE MADE

The granite shaft is made out of the original substance of the earth's crust. Its minerals are the elements out of which all of the rock ma.s.ses of the earth are formed, no matter how different they look from granite.

Sandstone is made of particles of quartz. Clay and slate are made out of feldspar and mica. Iron ore comes from the hornblende in granite. The mineral particles, rea.s.sembled in different proportions, form all of the different rocks that are known.

Here in my hand is a piece of pudding-stone. It is made of pebbles of different sizes, each made of different coloured minerals. The pebbles are cemented together with a paste that has hardened into stone. This kind of rock the geologists call _conglomerate_. Pudding-stone is the common name, for the pebbles in the pasty matrix certainly do suggest the currants and the raisins that are sprinkled through a Christmas pudding.

Under the seash.o.r.es there are forming to-day thick beds of sand. The rivers bring the rock material down from the hills, and it is sorted and laid down. The moving water drops the heaviest particles near sh.o.r.e, and carries the finer ones farther out before letting them fall.

[Ill.u.s.tration: The town of Cripple Creek, Colorado, which has grown up like magic since 1891, covers the richest gold and silver mines in the world]

[Ill.u.s.tration: The level valley is filled up with fine rock flour washed from the sides of the neighboring mountains]