our comforts and luxuries come in the first place, as we have seen, from
the soil. The more crowded people become upon the earth, and the greater
the number that engage in manufacturing and trade, the more important
becomes the care and cultivation of the soil. If we do not take the best
of care of the soil, there may come a time when there will not be food
enough for us all.
CHAPTER NINE
THINGS OF WHICH SOIL IS MADE
Let us take a spadeful of soft, dark earth from the garden and see if we
can find of what it is made.
We will first put the earth in a dish of water and stir it thoroughly.
We notice that the water at once becomes muddy and that little particles
of a dark substance rise to the surface. These particles appear to be
pieces of stems and leaves.
This crumbling vegetation is _peat_, a substance which fills many swamps
and, when cut into blocks and dried, is used for fuel. When scattered
through the earth peat has a very different use. As the leaves and stems
of plants die and slowly mingle with the earth, they give it the dark
color, which usually extends down for two or three feet. As this
vegetation changes, or decays, as we usually say, it furnishes a number
of substances which supply food to the roots of growing plants. One of
the most important of these is _nitrogen_, an invisible gas.
The decaying vegetation which we find mixed with the soil has other
uses. It holds water and so helps to keep the soil moist. It makes the
soil loose and more easy to cultivate. It absorbs heat from the sun and
so helps to warm the soil. This vegetable matter, when it is completely
decayed, we call _humus_. Soils that are rich in humus are usually very
fertile.
We will now turn the muddy water into another dish, pour more clear
water upon the material that remains in the bottom of the dish, and wash
it again, repeating the work until the water is no longer muddied. We
will set aside the dish containing the muddy water and examine what
remains in the bottom of the dish that once contained the earth or soil.
This is mostly sand, but with it are rough fragments of rock which can
be crumbled in the hand. The greater number of the little sand grains
are _quartz_. Some of them are clear like glass, others are reddish. In
this quartz sand are a few grains of _iron_ which the magnet picks out,
and a number of scales of yellow _mica_.
After standing a few hours the muddy water has become clear, and a
deposit of a yellowish substance has collected in the bottom of the
dish. We will carefully pour off the water and examine what remains.
This fine soft mud we call _clay_. As it dries and becomes hard it
shrinks and cracks, and thus breaks up into little pieces. Clay forms a
greater or lesser part of all soil. Clay soil is very sticky when it is
wet, as you will be sure to remember if you have tried to walk over it.
When soil is formed largely of clay we speak of it as a _heavy soil_. In
the West it is called _adobe_ and is sometimes used in making houses.
When adobe soil dries, great cracks form in it. These cracks are
sometimes large enough for small animals to fall into. When there is a
large amount of sand, we speak of the soil as _light_ or _sandy_. A soil
composed of sand and clay is sometimes called _loam_. If it is nearly
all clay it is a _clay loam_; if there is much sand it is a _sandy
loam_.
Soils found in low, swampy places are sometimes formed almost wholly of
decaying vegetable matter. Such soils are known as _peat soils_. They
are usually very fertile.
We have now learned about three things that the soil contains that are
bulky and easy to discover: decaying vegetation, sand, and clay. These
are, however, far from being all that compose the soil. There are still
many other things, some of which are invisible to the unaided eye and
difficult to find.
We will next take the clear water that remained after the mud settled.
We will pour it into a dish, place the dish over a fire, and let the
water boil slowly until it has all evaporated. There will remain in the
bottom of the dish a thin white coating. Moisten this with a drop of
vinegar or other weak acid and it will disappear in a mass of little
bubbles. Such behavior teaches us that the white substance is probably a
mixture of _lime_ and _soda_. Besides these there are tiny particles of
_potash_ and _phosphorus_, which we cannot distinguish by the means we
have used.
Some soils contain a great deal of lime, and because they have been
formed from limestone, are called _limestone soils_. Plants need a
little soda, but when there is much in the soil it will kill them. Soils
rich in soda are known as _alkali soils_. They were formed in the bottom
of lakes the waters of which contained soda. Salt is another harmful
thing found in the soil. You can sometimes see faint whitish deposits of
soda and other salts on the soil in flower pots.
There is one more thing that the soil contains that we must not forget,
for it is one of the most important of them all. This is a living
organism so small that we cannot see it with the unaided eye. Many
thousands of these organisms are contained in a bit of earth such as you
could take up on the point of a small knife blade. We have named them
_bacteria_.
Plants cannot make use of most of the substances in the soil without the
aid of these organisms. The bacteria live upon the materials of the
soil and change them into such form that plants can digest them.
Soil may be supplied with all kinds of plant food in just the right
amount and yet, if it is packed hard and is not watered, no living thing
can take root in it and grow. Plants drink their food and so we must
supply water. They also require oxygen, as do other living things. For
this reason we must leave the soil loose, so that the air can enter it
and the roots get the oxygen which it contains.
Thus we learn how wonderfully the soil is made. We learn that it
contains many things required by plants. In order that the plants may be
thrifty, there must be enough but not too much of these different
things.
CHAPTER TEN
HOW THE SOIL IS MADE
The substances which we found in the soil teach us that it was formed
from the rocks. If we could take the sand, clay, potash, soda, lime, and
iron that we found in the soil and put them together as Nature knows how
to do, we should have rock again.
But if we should take a piece of rock and crush it to a fine sand, that
would not be soil, because soil cannot be made in that way. It takes
Nature many, many years, as the rocks slowly crumble and decay, to
change the materials of which they are composed into true soil with its
swarms of bacteria and its plant food.
If we should dig down through the soft earth under our feet, we would at
last come to solid rock. This is the rough and jagged crust of the earth
on which rests the carpet of soil. In the mountains where the slopes are
steep the rocks stick up through the soil. The outer parts of this solid
rock are, however, always crumbling. Little particles, as soon as they
become loosened, either fall by their own weight or are washed away.
Some of the rock fragments collect upon the gentler slopes and finally
turn to soil. This soil is not rich and it dries out quickly, because it
is shallow. The soil in the valleys, as we have already learned from the
muddy rivulet, is deep and rich.
Nature is slowly spreading her mantle of soil over the earth. In some
parts of the earth one can travel for hundreds of miles and see no
rocks. One might think that in time Nature's work would be finished. But
before the mountains in one place have crumbled and been washed away,
she raises up new ones somewhere else so that the tearing-down work
begins again.
[Illustration: _H. W. Fairbanks_
Little by little the great rocks break in pieces and crumble finally to
form soil.]
Let us, in imagination, sit down by the side of a rock, prepared to stay
there many years, that we may learn just how Nature makes the soil. It
will be a long, long time before we can see any change in the rock. Each
bright day the sun warms the cold rock and makes it expand a very
little. At night the rock grows cold and shrinks. In this way minute
crevices are finally formed between the grains of the different minerals
that make up the rock.
When it rains, water creeps into the tiny crevices. The water carries
with it a little carbonic acid which the raindrops took from the air.
This substance aids in dissolving some of the rock materials. If the
nights are very cold, the water in the crevices freezes and opens them a
little wider, for ice, as you know, takes up a little more room than it
did when it was water.
Plants also aid in breaking the rock. Often seeds are dropped by the
wind, and the rootlets of some of these seeds, when they sprout, may
find a crevice large enough and deep enough for them to push their way
into the rock. In these crevices they find a little food and slowly grow
larger and stronger. By and by some of the roots are strong enough to
push apart large pieces of rock.
If the rock which we are studying is granite, we shall after a time be
able to pick out the different minerals of which it is composed. We can
tell the grains of quartz, because they look glassy and remain very
hard. Other grains, which we call _feldspar_, soften and change into
clay, which makes the water muddy as it runs over the rocks. We see also
little scales of yellow mica, sometimes called "fool's gold," and a few
grains of iron. There are tiny quantities of other things which we shall
not be able to see, for the rainwater dissolves them and carries them
away.
As the rock slowly crumbles to sand and clay, the bacteria begin to make
their home in it. Hardy plants, that are not particular about what they
grow in, get a foothold, and when they die their stems and leaves decay
and mix with the rock particles until at last this material begins to
look like soil. It has become dark in color and rich in plant food.
Then, many other plants that require a good soil take root there. The
rock has at last completely disappeared under the layer of soil and its
carpet of vegetation.
Suppose, now, that we dig down and find how deep the soil is and what
lies below it. When we have gone down two feet the soil is harder and of
a lighter color, for there are fewer plant remains in it. This poorer,
lighter-colored soil we call _subsoil_. If we dig a little deeper, we
shall find pieces of rock in the subsoil. Below these we come to soft,
crumbling rock and last of all the solid rock.
The soil that is found resting on the rocks from which it was formed is
known as _residual soil_. This name is given to such soil, because it is
what remains after long years of rock decay during which the rains have
washed away a part of the finer material.
What has become of the soft earth that the water washed away? The muddy
rivulet has already told us its interesting story. We have learned that
a part of this earth (or soil) is borne to the distant ocean. There it
is forever lost unless the sea bottom should some day become dry land.
Stranger things than that have happened on this ancient earth of ours.
The part of the soil which the water carried away to form the rich
valley lands and deltas is known as _alluvial soil_.
[Illustration: _U. S. Department of Agriculture_
A flood plain, where alluvial soil has been deposited by the river.]
Long ago the northern part of our country was covered with a sheet of
ice. This ice crept slowly southward, and as it moved along it tore off
all the soil and loose rocks on the surface of the earth over which it
passed. When it melted it left them spread roughly over the country.
Such material forms _glacial soil_. It is often deep but not very rich.
[Illustration: _U. S. Geological Survey_
Soil brought by a glacier and deposited as the ice melted.]
There is another kind of soil, formed by the wind. If you have ever been
in a dust storm you have seen the fine, powdery substance that settles
over everything and creeps into the smallest cracks. In some countries
where there are strong winds and not much rain there is little
vegetation on the surface to hold the soil. Year after year the winds
pick up particles of the dusty soil, whirl them high in the air, and do
not let them down again until they have been carried many miles. In some
far-off land where the winds go down the dust particles settle again to
the earth. After a long, long time, enough dust collects to form a
thick layer of the richest soil. This is called aeolian soil, from the
word _AEolus_, meaning the "wind."
There is one more kind of soil which we ought to know about; that is
_peat soil_. It is found in marshy or swampy lowlands and is formed
largely of plant remains. When lands with such soil are drained, they
prove very rich.
CHAPTER ELEVEN
HOW VEGETATION HOLDS THE SOIL
[Illustration: _H. W. Fairbanks_
What the rivulets did to the hillside pastures where the grass was
destroyed.]
A walk up the mountains on a rainy day is not a pleasant one. There are
mud and water under our feet, and overhead are the dripping branches
which, if touched, send down a shower of drops. But if we keep our eyes
open we shall learn something which will be of great value to us. We
shall learn how it is that Nature holds the soil on the slopes--the
wonderful soil which it takes her so long a time to make and which is
the source of all our wealth.
Our way up the mountains is by a winding road. We first pass the
foothills upon which there are scattered oaks. The rain is steadily
pouring down and rivulets loaded with mud are eating little gullies all
over the slopes. Along the roadside, where they have united, the
rivulets form a torrent which is making a deep ditch that threatens to
render the road impassable.
These slopes were once covered with grass and the rivulets ran down them
without doing any harm. But so many sheep were pastured here that the
grass was killed. The roots, which once formed a thick protecting sod,
are now decaying. How quickly the rivulets have taken advantage of the
unprotected slopes!
The road leads still upward until it brings us to where there were once
pine forests. The lumbermen cut off all the trees, and then fire came
and burned the decaying vegetation which once lay spread over the
ground. Now all that remains is bare earth and blackened stumps.
What are the raindrops doing here? They gather in rivulets just as they
do on the once grassy hillside; but because there are so many roots
still remaining in the ground they have not done much work. They are not
loitering, however, and by and by, when the roots have rotted, they will
seize their chance and begin tearing away the soil from the mountain
side.
But this is not the end of the road. Farther up we come to the primeval
forests, where the giant trees stand just as they did before men came.
Here we can see how the slopes are protected, for in making the road the
workmen cut deep into the hillside. They first removed a layer of pine
needles and decaying branches. Then they cut through a layer of soil
about two feet thick which was completely filled with little roots of
trees and bushes. Below this they came to the soft subsoil, which
contained only a few roots, and at the bottom they reached the solid
rock.
The layer of roots and soil at the top of the bank, you can see from the
picture, now overhangs the road, because the raindrops which beat
against the bank have washed away all that they could reach of the
unprotected earth at the bottom. How plainly we can see the network of
roots. What a hard task it must be for the water to get at the soil in
which these roots are growing.
[Illustration: _H. W. Fairbanks_
The layer of roots holds the soil on the mountain side.]
We will now leave the road and, although it is still raining hard, we
will walk a distance through the forest and see if there is anything
more that we can learn. We are soon in the deep woods where, perhaps, no
one has ever been before. Around us are trees of all ages and sizes,
from little seedlings to great giants six feet through. Among them are
the crumbling stumps of trees long dead. Their trunks lie on the ground,
and many are so soft and rotten that we can kick them to pieces with our
feet.
As we walk our feet never touch the real earth. It is always on the
soft, yielding leaves and crumbling branches that we step. These leaves
and branches form a thick layer completely hiding the soil. But the
strangest thing is that, although the rain is still falling, we can
discover no rivulets. What, then, becomes of the water? The soft,
decaying vegetation on which we are walking and the rotting stumps and
logs act like a great sponge. As long as this sponge can take up the
falling drops, none have a chance to run away. If it rains a very long
time and the sponge becomes saturated, the drops that creep away and
finally unite in rivulets in the hollows do no harm to the soil, for
they cannot get at it.
[Illustration: _H. W. Fairbanks_
The roots of the tree grip the soil like the fingers of a great hand.]
Long after the storm has passed, the earth underneath the trees remains
wet, while the ground out in the open has become dry. A part of the
water held by the decaying vegetation evaporates. Another part creeps
down through the earth to the crevices in the rocks and feeds the
springs.
Let us now put aside our storm clothes and journey, in imagination, far
away to where it seldom rains--to that land which we call the desert.
Here the bare rocks of the mountain slopes are burned brown by the hot
sun. Here there is little soil and only a few little bushes that somehow
manage to live. Why does not the soil gather over the rocks as it does
in other places? The rocks are surely crumbling, for we can crush some
of the pieces in our hands.
Once in a long time it rains in this desert. Then the drops descend
furiously. The water gathers in rivulets and these turn to torrents
which sweep down the slopes. They carry away the particles of sand and
clay which would in time, if there were plant roots to hold them, turn
to soil.
The winds also help keep the desert rocks bare and free of soil. Have
you ever been in a dust storm or have you read of caravans caught in
such storms in the Sahara Desert? The fierce wind picks up the particles
of sand and clay from the bare earth and sweeps them along as it does
the snow in winter, or it whirls them in clouds high in the air. The
dust clouds are often so dense that they hide the sun and all landmarks
by which the traveler can guide his way. But have any of us ever seen
the winds pick up much dust from the green fields where the vegetation
protects the surface?
[Illustration: _H. W. Fairbanks_
The vegetation prevents the wind from blowing the sand away, so that
wherever the roots obtain a hold there a little mound is formed.]
If we turn now to a very wet country, such as that upon our northwest
coast, where often nearly eight feet of rain falls in a year, we shall
find the vegetation so dense that it hides both soil and rocks. Here
water can do little in wearing away the soil, even upon the steepest
slopes, while the wind cannot get a peep at the earth.
Does it not seem strange that where little rain falls the earth washes a
great deal faster than where it rains very heavily? The reason is that
the more it rains the more dense becomes the carpet of vegetation. If we
wish to preserve the soil, we must preserve the natural growth on the
hillsides.
CHAPTER TWELVE
WHAT HAPPENS WHERE THERE IS NO PROTECTING CARPET OF VEGETATION
Not all of the muddy streams are due to the carelessness of men. It is
the business of some of the servants of Nature, as we have already
learned, to tear down the mountains and fill up the hollows in the
earth. It is the business of others to spread a carpet of vegetation
over the surface, and wherever they have already succeeded in their work
the waters run clear most of the time.
Where it is dry so much of the time that few plants can live, the
destructive servants have their own way when the occasional rains come.
Where there is a warm sun and frequent rains, a green carpet is spread
over all the slopes. But when men destroy the carpet and take no care of
the soil underneath, the raindrops are able to do as much damage as they
do during the cloudbursts in the deserts.
The Colorado is one of those rivers in the basin of which few people
live. Much of its journey is through a land in which there is little
vegetation. Here, the waters from the melting snows upon the lofty
mountains about the basin and those of the occasional heavy rains have
things their own way. They are always yellow with mud. The amount of mud
which this river carries has been measured. You will hardly believe me
when I tell you that it amounts to sixty-one million tons every year.
This is enough to cover 164 square miles one foot deep. We might call
this the cream of the soil from all the slopes of the great basin of the
Colorado River.
In other parts of our land, where abundant rains fall, the streams tell
a different story. Before men came the water of these streams was clear
throughout the greater part of the year. It was only when the rains were
very heavy that the soil washed away, for the vegetation held it well.
Now the gullies on the hillsides and along the roads tell us as plainly
as though they could speak that our country is losing wealth here.
[Illustration: _H. W. Fairbanks_
The roots of the tree form a wonderful network underground from which
the water cannot tear the soil.]
The soil is our most valuable possession. The people of many lands are
suffering from poverty today because their forefathers did not take care
of the soil as they should. In such lands the people who live on the
mountain sides are poor, because the best of their soil has been washed
away. Those who live in the valleys are often poor because of the sands
and gravels which floods have spread over their fertile fields.
While it is raining, let us fill a bottle from some muddy stream and
allow it to stand until the water settles. In the bottom will then
appear a layer of fine mud, or _silt_ as it is usually called. How much
soil do you suppose the rivulets washed from my garden and from yours
during the last severe storm? How much do you suppose all the rivulets
which make up the rivers of your state washed from all the gardens and
fields during the same storm? Make a guess and then multiply your answer
by the number of storms in one year and that by fifty years, and you
will get a quantity greater than you would believe possible.
This is the way Nature takes her toll for our carelessness. So quietly
does she do it that often the farmer does not have any idea of what is
happening. She is like a thief that comes and steals his goods while he
is sleeping.
[Illustration: _Bailey Willis_
The soil on the hillsides of China is being washed away because of the
thoughtlessness of the people.]
When the farmer finally awakes and begins to wonder why his crops grow
smaller each year, he has already lost the cream of his soil. He must
at once stop plowing the steep hillsides and leaving the ground bare for
the winter rains to wash it away. To save the slopes he can either
terrace them or he can sow grass or clover, which will form a sod and
hold the soil. If the farmer can get peas, beans, alfalfa, or clover to
grow upon his wasted lands, they will make it fertile again, for these
plants have the wonderful power of taking nitrogen from the air and
storing it in the soil.
[Illustration: _American Forestry Association_
The farmer who owns this land will soon be made poor because of his
carelessness in destroying the covering of the soil.]
More earth has been washed from the hillsides of our country during the
last fifty years than during thousands of years before white people
came. The farm lands have been injured, the bays have been made
shallower, and many river channels have been so filled up that it is
more difficult to navigate them now than it was in the early days.
The farmer, the stockman, the lumberman, and the miner has each been
selfishly doing his share in the destruction of the soil. Each one has
thought only of how he could make the most money in the shortest time.
It has not occurred to them that they are making it difficult for their
children and grandchildren to live.
In the Southern states thousands of acres are being gullied by the
rains, and the soil destroyed. The floods of spring have become worse in
late years, because of the destruction of the forest cover in the
Appalachian Mountains. Buildings and bridges are frequently carried
away, and gravel and boulders are washed over the rich bottom lands.