Origin Of The Soil





The word _soil_ occurs many times in this little book. In agriculture

this word is used to describe the thin layer of surface earth that, like

some great blanket, is tucked around the wrinkled and age-beaten form of

our globe. The harder and colder earth under this surface layer is

called the _subsoil_. It should be noted, however, that in waterless and

sun-dried regions there seems little difference between the soil and the

subsoil.



Plants, insects, birds, beasts, men,--all alike are fed on what grows in

this thin layer of soil. If some wild flood in sudden wrath could sweep

into the ocean this earth-wrapping soil, food would soon become as

scarce as it was in Samaria when mothers ate their sons. The face of the

earth as we now see it, daintily robed in grass, or uplifting waving

acres of corn, or even naked, water-scarred, and disfigured by man's

neglect, is very different from what it was in its earliest days. How

was it then? How was the soil formed?



Learned men think that at first the surface of the earth was solid rock.

How was this rock changed into workable soil? Occasionally a curious boy

picks up a rotten stone, squeezes it, and finds his hands filled with

dirt, or soil. Now, just as the boy crumbled with his fingers this

single stone, the great forces of nature with boundless patience

crumbled, or, as it is called, disintegrated, the early rock mass. The

simple but giant-strong agents that beat the rocks into powder with a

clublike force a millionfold more powerful than the club force of

Hercules were chiefly (1) heat and cold; (2) water, frost, and ice; (3)

a very low form of vegetable life; and (4) tiny animals--if such minute

bodies can be called animals. In some cases these forces acted singly;

in others, all acted together to rend and crumble the unbroken stretch

of rock. Let us glance at some of the methods used by these skilled

soil-makers.



Heat and cold are working partners. You already know that most hot

bodies shrink, or contract, on cooling. The early rocks were hot. As the

outside shell of rock cooled from exposure to air and moisture it

contracted. This shrinkage of the rigid rim of course broke many of the

rocks, and here and there left cracks, or fissures. In these fissures

water collected and froze. As freezing water expands with irresistible

power, the expansion still further broke the rocks to pieces. The

smaller pieces again, in the same way, were acted on by frost and ice

and again crumbled. This process is still a means of soil-formation.



Running water was another giant soil-former. If you would understand its

action, observe some usually sparkling stream just after a washing rain.

The clear waters are discolored by mud washed in from the surrounding

hills. As though disliking their muddy burden, the waters strive to

throw it off. Here, as low banks offer chance, they run out into

shallows and drop some of it. Here, as they pass a quiet pool, they

deposit more. At last they reach the still water at the mouth of the

stream, and there they leave behind the last of their mud load, and

often form of it little three-sided islands called _deltas_. In the same

way mighty rivers like the Amazon, the Mississippi, and the Hudson, when

they are swollen by rain, bear great quantities of soil in their sweep

to the seas. Some of the soil they scatter over the lowlands as they

whirl seaward; the rest they deposit in deltas at their mouths. It is

estimated that the Mississippi carries to the ocean each year enough

soil to cover a square mile of surface to a depth of two hundred and

sixty-eight feet.






The early brooks and rivers, instead of bearing mud, ran oceanward

either bearing ground stone that they themselves had worn from the rocks

by ceaseless fretting, or bearing stones that other forces had already

dislodged. The large pieces were whirled from side to side and beaten

against one another or against bedrock until they were ground into

smaller and smaller pieces. The rivers distributed this rock soil just

as the later rivers distribute muddy soil. For ages the moving waters

ground against the rocks. Vast were the waters; vast the number of

years; vast the results.



Glaciers were another soil-producing agent. Glaciers are streams "frozen

and moving slowly but irresistibly onwards, down well-defined valleys,

grinding and pulverizing the rock masses detached by the force and

weight of their attack." Where and how were these glaciers formed?



Once a great part of upper North America was a vast sheet of ice.

Whatever moisture fell from the sky fell as snow. No one knows what made

this long winter of snow, but we do know that snows piled on snows until

mountains of white were built up. The lower snow was by the pressure of

that above it packed into ice masses. By and by some change of climate

caused the masses of ice to break up somewhat and to move south and

west. These moving masses, carrying rock and frozen earth, ground them

to powder. King thus describes the stately movement of these snow

mountains: "Beneath the bottom of this slowly moving sheet of ice, which

with more or less difficulty kept itself conformable with the face of

the land over which it was riding, the sharper outstanding points were

cut away and the deeper river canons filled in. Desolate and rugged

rocky wastes were thrown down and spread over with rich soil."



The joint action of air, moisture, and frost was still another agent of

soil-making. This action is called _weathering_. Whenever you have

noticed the outside stones of a spring-house, you have noticed that tiny

bits are crumbling from the face of the stones, and adding little by

little to the soil. This is a slow way of making additions to the soil.

It is estimated that it would take 728,000 years to wear away limestone

rock to a depth of thirty-nine inches. But when you recall the

countless years through which the weather has striven against the rocks,

you can readily understand that its never-wearying activity has added

immensely to the soil.



In the rock soil formed in these various ways, and indeed on the rocks

themselves, tiny plants that live on food taken from the air began to

grow. They grew just as you now see mosses and lichens grow on the

surface of rocks. The decay of these plants added some fertility to the

newly formed soil. The life and death of each succeeding generation of

these lowly plants added to the soil matter accumulating on the rocks.

Slowly but unceasingly the soil increased in depth until higher

vegetable forms could flourish and add their dead bodies to it. This

vegetable addition to the soil is generally known as _humus_.






In due course of time low forms of animal life came to live on these

plants, and in turn by their work and their death to aid in making a

soil fit for the plowman.



Thus with a deliberation that fills man with awe, the powerful forces of

nature splintered the rocks, crumbled them, filled them with plant food,

and turned their flinty grains into a soft, snug home for vegetable

life.





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