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
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
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
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
Next: Tillage Of The Soil