THE ORIGIN OF FERTILE SOIL
“Fertile or arable soil,” resumed Uncle Paul, “constitutes only the
surface layer of earth, that which is worked by the farmer’s implements
and yields nutriment to the roots of plants and promotes their
development. In one place you will see bare rocks and utter barrenness;
in another you find fertile soil to a depth of an inch or two, scantily
carpeted with grass; and again, in a third, you come upon rich earth so
deep as to maintain abundant vegetation. But nowhere does this fertile
layer have an indefinite thickness: at a depth never very considerable
a subsoil having the qualities of the neighboring mountains is sure to
be found. How then has there come to be formed this layer of earth
whence is derived all the nutriment required by plants, animals, and
men?
“Undermined all winter, and even the whole year round on high
mountains, by the ice that forms in their slightest fissures, rocks of
all kinds break into small fragments, divide into grains of sand, fall
into dust, and furnish the powdery mineral matter which the rain washes
away and deposits in the valleys. This as a rule is the origin of
broken stones, sand, clay, and fertile soil. Ice by its expansive force
has detached them from the tops of mountains and the waters have swept
them away and carried them further. One can form an idea of the action
of ice in crumbling rocks to make soil of them and enrich the valleys,
by examining the surface of a hard road at the moment of thawing.
Firm underfoot before freezing, this surface loses its firmness after a
thaw and is pushed up here and there in little finely-powdered clods.
At the moment of freezing, the humidity with which the soil was
impregnated turned into ice which, increasing in volume, reduced to
fine particles the surface layer of the road. When the thaw comes,
these particles which the ice no longer holds together form first mud,
then dust. In exactly this manner arable land was formed by the
disintegration of rocks of all kinds, which were reduced to particles
by the action of frost.
But soil suitable for agriculture contains not only powdery mineral
matter, but also a little mold from the decomposition of vegetable
matter. To give you an idea of the causes which from the very earliest
times have little by little fertilized this rock-dust with vegetable
mold, let us take the following example.
Geography has taught you what a volcano is. It is a mountain whose
summit is hollowed out in an immense funnel-shaped excavation called a
crater. From time to time the ground trembles near a volcano and
formidable noises similar to the rolling of thunder and the booming of
cannon are heard from the depths of the mountain. The crater throws up
into the air a lofty column of smoke, dark by day, fiery red at night.
All at once the mountain is rent and vomits up through the crevices a
stream of fire, a current of melted rock, or lava. Finally the volcano
quiets down; the source of the terrible flood dries up. The streams of
lava harden and cease running; and after a lapse of time which may be
years they become quite cold. Now what is to become of this enormous
bed of black stone similar in character to the slag from a forge? What
will this sheet of lava covering an area of several square miles
produce?
“This desolate, blasted expanse seems destined never to be clothed with
verdure. But in any such assumption one would be mistaken. After
centuries and centuries a vigorous growth of oaks, beeches, and other
large trees will have taken root there. In fact, you will see that air,
rain, snow, and, above all, frost attack in turn the hard surface of
the lava, detach fine particles from it, and slowly produce a little
dust at its expense. On this dust there will spring into being certain
strange and hardy plants, those white or yellow patches, those
vegetable incrustations, calculated to live on the surface of stone and
known as lichens. These lichens fasten themselves to the lava, gnaw it
still more, and in dying leave a little mold formed from their decaying
remains. On this precious mold, lodged in some cavity of the lava,
there is now a growth of mosses which perish in their turn and increase
the quantity of fertilizing material. Next come ferns, which require a
richer soil, and after that a few tufts of grass; then some brambles,
some meager shrubs; and thus with each succeeding year the fertile soil
is added to from the new remnants of lava and mold left by the
preceding generation of plants that have gone to decay. It is in this
way that gradually a lava-bed finally becomes covered with a forest.
“Our own arable land had a similar origin. Sterile rocks, hard as they
are, contributed the mineral part by being reduced to dust through the
combined action of water, air, and frost; and the successive
generations of plant-life, beginning with the simplest, furnished the
mold.
“Notice how admirably, in the processes of nature, the smallest of
created beings perform their part and contribute as best they can to
the general harmony. To produce fertile soil there is needed something
more than the frosts and thaws that crumble the hardest rock: there is
need of plants hardy enough to live on this sterile soil, such as tough
grasses, mosses, lichens, which gnaw the stone. It is through the
medium of these rudimentary plants, so pitiful in appearance and yet so
hardy, that the dust of the rocks is enriched with mold and converted
into a soil capable of bearing other and more delicate plants.
“It is not in cultivated fields that you will find those thick carpets
of mosses and lichens, valiant disintegrators of stone; it is on the
mountain-tops that they can be seen at their work of crusting over the
smooth rock in order to convert it into fertile soil. It is from these
heights that this fertile soil has descended, little by little, washed
down by the rain, until it has fertilized the valleys. This work is
going on all the time; in hilly regions plants of the lowest order are
constantly adding to the extent of arable land. The little threads of
rain-water that furrow these regions carry away with them some of this
humus and bear it to the plains below.
“What a worthy subject for our thoughtful study is this formation of
arable soil by these legions of inferior plants, obscure workers
indefatigably crumbling the rock! What immense results obtained by the
simplest means!”