THE SOIL.
The frequent mention which it has been necessary to make of soil
phenomena in the preceding chapters shows how intimately this feature
in the structure of the earth is blended with all the elements of its
physical history. It is now necessary for us to take up the phenomena
of soils in a consecutive manner.
The study of any considerable river basin enables us to trace the more
important steps which lead to the destructure and renovation of the
earth's detrital coating. In such an interpretation we note that
everywhere the rocks which were built on the sea bottom, and more or
less made over in the great laboratory of the earth's interior, are at
the surface, when exposed to the conditions of the atmosphere, in
process of being taken to pieces and returned to the sea. This action
goes on everywhere; every drop of rain helps it. It is aided by frost,
or even by the changes of expansion and contraction which occur in the
rocks from variations of heat. The result is that, except where the
slopes are steep, the surface is quickly covered with a layer of
fragments, all of which are in the process of decay, and ready to
afford some food to plants. Even where the rock appears bare, it is
generally covered with lichens, which, adhering to it, obtain a share
of nutriment from the decayed material which they help to hold on the
slope. When they have retained a thin sheet of the _débris_, mosses
and small flowering plants help the work of retaining the detritus.
Soon the strong-rooted bushes and trees win a foothold, and by sending
their rootlets, which are at first small but rapidly enlarge, into the
crevices, they hasten the disruption of the stones.
If the construction of soil goes on upon a steep cliff, the quantity
retained on the slope may be small, but at the base we find a talus,
composed of the fragments not held by the vegetation, which gradually
increases as the cliff wears down, until the original precipice may be
quite obliterated beneath a soil slope. At first this process is
rapid; it becomes gradually slower and slower as the talus mounts up
the cliff and as the cliff loses its steepness, until finally a gentle
slope takes the place of the steep.
From the highest points in any river valley to the sea level the
broken-up rock, which we term soil, is in process of continuous
motion. Everywhere the rain water, flowing over the surface or soaking
through the porous mass, is conveying portions of the material which
is taken into solution in a speedy manner to the sea. Everywhere the
expansion of the soil in freezing, or the movements imposed on it by
the growth of roots, by the overturning of trees, or by the
innumerable borings and burrowings which animals make in the mass, is
through the action of gravitation slowly working down the slope. Every
little disturbance of the grains or fragments of the soil which lifts
them up causes them when they fall to descend a little way farther
toward the sea level. Working toward the streams, the materials of the
soil are in time delivered to those flowing waters, and by them urged
speedily, though in most cases interruptedly, toward the ocean.
There is another element in the movement of the soils which, though
less appreciable, is still of great importance. The agents of decay
which produce and remove the detritus, the chemical changes of the bed
rock, and the mechanical action which roots apply to them, along with
the solutional processes, are constantly lowering the surface of the
mass. In this way we can often prove that a soil continuously
existing has worked downward through many thousand feet of strata. In
this process of downgoing the country on which the layer rests may
have greatly changed its form, but the deposit, under favourable
conditions, may continue to retain some trace of the materials which
it derived from beds which have long since disappeared, their position
having been far up in the spaces now occupied by the air. Where the
slopes are steep and streams abound, we rarely find detritus which
belonged in rock more than a hundred feet above the present surface of
the soil. Where, however, as on those isolated table-lands or buttes
which abound in certain portions of the Mississippi Valley, as well as
in many other countries, we find a patch of soil lying on a nearly
level surface, which for geologic ages has not felt the effect of
streams, we may discover, commingled in the _débris_, the harder
wreckage derived from the decay of a thousand feet or more of vanished
strata.
When we consider the effect of organic life on the processes which go
on in the soil, we first note the large fact that the development of
all land vegetation depends upon the existence of this detritus--in a
word, on the slow movement of the decaying rocky matter from the point
where it is disrupted to its field of rest in the depths of the sea.
The plants take their food from the portion of this rocky waste which
is brought into solution by the waters which penetrate the mass. On
the plants the animals feed, and so this vast assemblage of organisms
is maintained. Not only does the land life maintain itself on the
soil, and give much to the sea, but it serves in various ways to
protect this detrital coating from too rapid destruction, and to
improve its quality. To see the nature of this work we should visit a
region where primeval forests still lie upon the slopes of a hilly
region. In the body of such a wood we find next the surface a coating
of decayed vegetable matter, made up of the falling leaves, bark,
branches, and trunks which are constantly descending to the earth.
Ordinarily, this layer is a foot or more in thickness; at the top it
is almost altogether composed of vegetable matter; at the bottom it
verges into the true soil. An important effect of this decayed
vegetation is to restrain the movement of the surface water. Even in
the heaviest rains, provided the mass be not frozen, the water is
taken into it and delivered in the manner of springs to the larger
streams. We can better note the measure of this effect by observing
the difference in the ground covered by this primeval forest and that
which we find near by which has been converted into tilled fields.
With the same degree of rapidity in the flow, the distinct stream
channels on the tilled ground are likely to be from twenty to a
hundred times in length what they are on the forest bed. The result is
that while the brook which drains the forested area maintains a
tolerably constant flow of clean water, the other from the tilled
ground courses only in times of heavy rain, and then is heavily
charged with mud. In the virgin conditions of the soil the downwear is
very slow; in its artificial state this wearing goes on so rapidly
that the sloping fields are likely to be worn to below the soil level
in a few score years.
Not only does the natural coating of vegetation, such as our forests
impose upon the country, protect the soil from washing away, but the
roots of the larger plants are continually at work in various ways to
increase the fertility and depth of the stratum. In the form of
slender fibrils these underground branches enter the joints and bed
planes of the rock, and there growing they disrupt the materials,
giving them a larger surface on which decay may operate. These bits,
at first of considerable size, are in turn broken up by the same
action. Where the underlying rocks afford nutritious materials, the
branches of our tap-rooted trees sometimes find their way ten feet or
more below the base of the true soil. Not only do they thus break up
the stones, but the nutrition which they obtain in the depths is
brought up and deposited in the parts above the ground, as well as in
the roots which lie in the true soil, so that when the tree dies it
becomes available for other plants. Thus in the forest condition of a
country the amount of rock material contributed to the deposit in
general so far exceeds that which is taken away to the rivers by the
underground water as to insure the deepening of the soil bed to the
point where only the strongest roots--those belonging to our
tap-rooted trees--can penetrate through it to the bed rocks.
Almost all forests are from time to time visited by winds which uproot
the trees. When they are thus rent from the earth, the underground
branches often form a disk containing a thick tangle of stones and
earth, and having a diameter of ten or fifteen feet. The writer has
frequently observed a hundred cubic feet of soil matter, some of it
taken from the depth of a yard or more, thus uplifted into the air. In
the path of a hurricane or tornado we may sometimes find thousands of
acres which have been subjected to this rude overturning--a natural
ploughing. As the roots rot away, the _débris_ which they held falls
outside of the pit, thus forming a little hillock along the side of
the cavity. After a time the thrusting action of other roots and the
slow motion of the soil down the slope restore the surface from its
hillocky character to its original smoothness; but in many cases the
naturalist who has learned to discern with his feet may note these
irregularities long after it has been recovered with the forest.
Great as is the effect of plants on the soil, that influence is almost
equalled by the action of the animals which have the habit of entering
the earth, finding there a temporary abiding place. The number of
these ground forms is surprisingly great. It includes, indeed, a host
of creatures which are efficient agents in enriching the earth. The
species of earthworms, some of which occupy forested districts as well
as the fields, have the habit of passing the soil material through
their bodies, extracting from the mass such nutriment as it may
contain. In this manner the particles of mineral matter become
pulverized, and in a measure affected by chemical changes in the
bodies of the creatures, and are thus better fitted to afford plant
food. Sometimes the amount of the earth which the creatures take in in
moving through their burrows and void upon the surface is sufficient
to form annually a layer on the surface of the ground having a depth
of one twentieth of an inch or more. It thus may well happen that the
soil to the depth of two or three feet is completely overturned in the
course of a few hundred years. As the particles which the creatures
devour are rather small, the tendency is to accumulate the finer
portions of the soil near the surface of the earth, where by solution
they may contribute to the needs of the lowly plants. It is probably
due to the action of these creatures that small relics of ancient men,
such as stone tools, are commonly found buried at a considerable depth
beneath the earth, and rarely appear upon the surface except where it
has been subjected to deep ploughing or to the action of running
streams.
Along with the earthworms, the ants labour to overturn the soil;
frequently they are the more effective of the two agents. The common
species, though they make no permanent hillocks, have been observed by
the writer to lay upon the surface each year as much as a quarter of
an inch of sand and other fine materials which they have brought up
from a considerable depth. In many regions, particularly in those
occupied by glacial drift, and pebbly alluvium along the rivers, the
effect of this action, like that of earthworms, is to bring to the
surface the finer materials, leaving the coarser pebbles in the
depths. In this way they have changed the superficial character of the
soil over great areas; we may say, indeed, over a large part of the
earth, and this in a way which fits it better to serve the needs of
the wild plants as well as the uses of the farmer.
Many thousand species of insects, particularly the larger beetles,
have the habit of passing their larval state in the under earth. Here
they generally excavate burrows, and thus in a way delve the soil. As
many of them die before reaching maturity, their store of organic
matter is contributed to the mass, and serves to nourish the plants.
If the student will carefully examine a section of the earth either in
its natural or in its tilled state, he will be surprised to find how
numerous the grubs are. They may often be found to the number of a
score or more of each cubic foot of material. Many of the species
which develop underground come from eggs which have carefully been
encased in organic matter before their deposition in the earth. Thus
some of the carrion beetles are in the habit of laying their eggs in
the bodies of dead birds or field mice, which they then bury to the
depth of some inches in the earth. In this way nearly all the small
birds and mammals of our woods disappear from view in a few hours
after they are dead. Other species make balls from the dung of cattle
in which they lay their eggs, afterward rolling the little spheres, it
may be for hundreds of feet, to the chambers in the soil which they
have previously prepared. In this way a great deal of animal matter is
introduced into the earth, and contributes to its fertility.
Many of our small mammals have the habit of making their dwelling
places in the soil. Some of them, such as the moles, normally abide in
the subterranean realm for all their lives. Others use the excavations
as places of retreat. In any case, these excavations serve to move the
particles of the soil about, and the materials which the animals drag
into the earth, as well as the excrement of the creatures, act to
enrich it. This habit of taking food underground is not limited to the
mammals; it is common with the ants, and even the earthworms, as noted
by Charles Darwin in his wonderful essay on these creatures, are
accustomed to drag into their burrows bits of grass and the slender
leaves of pines. It is not known what purpose they attain by these
actions, but it is sufficiently common somewhat to affect the
conditions of the soil.
The result of these complicated works done by animals and plants on
the soil is that the material to a considerable depth are constantly
being supplied with organic matter, which, along with the mineral
material, constitutes that part of the earth which can support
vegetation. Experiment will readily show that neither crushed rock nor
pure vegetable mould will of itself serve to maintain any but the
lowliest vegetation. It requires that the two materials be mixed in
order that the earth may yield food for ordinary plants, particularly
for those which are of use to man, as crops. On this account all the
processes above noted whereby the waste of plant and animal life is
carried below the surface are of the utmost importance in the creation
and preservation of the soil. It has been found, indeed, in almost all
cases, necessary for the farmer to maintain the fertility of his
fields to plough-in quantities of such organic waste. By so doing he
imitates the work which is effected in virgin soil by natural action.
As the process is costly in time and material, it is often neglected
or imperfectly done, with the result that the fields rapidly diminish
in fertility.
The way in which the buried organic matter acts upon the soil is not
yet thoroughly understood. In part it accomplishes the results by the
materials which on its decay it contributes to the soil in a state in
which they may readily be dissolved and taken up by the roots into
their sap; in part, however, it is believed that they better the
conditions by affording dwelling places for a host of lowly species,
such as the forms which are known as bacteria. The organisms probably
aid in the decomposition of the mineral matter, and in the conversion
of nitrogen, which abounds in the air or the soil, into nitrates of
potash and soda--substances which have a very great value as
fertilizers. Some effect is produced by the decay of the foreign
matter brought into the soil, which as it passes away leaves channels
through which the soil water can more readily pass.
By far the most general and important effect arising from the decay
of organic matter in the earth is to be found in the carbon dioxide
which is formed as the oxygen of the air combines with the carbon
which all organic material contains. As before noted, water thus
charged has its capacity for taking other substances into solution
vastly increased, and on this solvent action depends in large part the
decay of the bed rocks and the solution of materials which are to be
appropriated by the plants.
Having now sketched the general conditions which lead to the formation
of soils, we must take account of certain important variations in
their conditions due to differences in the ways in which they are
formed and preserved. These matters are not only of interest to the
geologist, but are of the utmost importance to the life of mankind, as
well as all the lower creatures which dwell upon the lands. First, we
should note that soils are divisible into three great groups, which,
though not sharply parted from each other, are sufficiently peculiar
for the purposes of classification. Where the earth material has been
derived from the rocks which nearly or immediately underlie it, we
have a group of soils which may be entitled those of immediate
derivation--that is, derived from rocks near by, or from beds which
once overlaid the level and have since been decayed away. Next, we
have alluvial soils, those composed of materials which have been
transported by streams, commonly from a great distance, and laid down
on their flood plains. Third, the soils the mineral matters of which
have been brought into their position by the action of glaciers; these
in a way resemble those formed by rivers, but the materials are
generally imperfectly sorted, coarse and fine being mingled together.
Last of all, we have the soils due to the accumulation of blown dust
or blown sand, which, unlike the others, occupy but a small part of
the land surface. It would be possible, indeed, to make yet another
division, including those areas which when emerging from the sea were
covered with fine, uncemented detritus ready at once to serve the
purposes of a soil. Only here and there, and but seldom, do we find
soils of this nature.
It is characteristic of soils belonging to the group to which we have
given the title of immediate derivation that they have accumulated
slowly, that they move very gradually down the slopes on which they
lie, and that in all cases they represent, with a part of their mass
at least, levels of rock which have disappeared from the region which
they occupied. The additions made to their mass are from below, and
that mass is constantly shrinking, generally at a pretty rapid rate,
by the mineral matter which is dissolved and goes away with the spring
water. They also are characteristically thin on steep slopes,
thickening toward the base of the incline, where the diminished grade
permits the soil to move slowly, and therefore to accumulate.
In alluvial soils we find accumulations which are characterized by
growth on their upper surfaces, and by the distant transportation of
the materials of which they are composed. In these deposits the
outleaching removes vast amounts of the materials, but so long as the
floods from time to time visit their surfaces the growth of the
deposits is continued. This growth rarely takes place from the waste
of the bed rocks on which the alluvium lies. It is characteristic of
alluvial soils that they are generally made up of _débris_ derived
from fields where the materials have undergone the change which we
have noted in the last paragraph; therefore these latter deposits have
throughout the character which renders the mineral materials easily
dissolved. Moreover, the mass as it is constructed is commonly mingled
with a great deal of organic waste, which serves to promote its
fertility. On these accounts alluvial grounds, though they vary
considerably in fertility, commonly afford the most fruitful fields of
any region. They have, moreover, the signal advantage that they often
may be refreshed by allowing the flood waters to visit them, an
action which but for the interference of man commonly takes place once
each year. Thus in the valley of the Nile there are fields which have
been giving rich grain harvests probably for more than four thousand
years, without any other effective fertilizing than that derived from
the mud of the great river.
The group of glaciated soils differs in many ways from either of those
mentioned. In it we find the mineral matter to have been broken up,
transported, and accumulated without the influence of those conditions
which ordinarily serve to mix rock _débris_ with organic matter during
the process by which it is broken into bits. When vegetation came to
preoccupy the fields made desolate by glacial action, it found in most
places more than sufficient material to form soils, but the greater
part of the matter was in the condition of pebbles of very hard rock
and sand grains, fragments of silex. Fortunately, the broken-up state
of this material, by exposing a great surface of the rocky matter to
decay, has enabled the plants to convert a portion of the mass into
earth fit for the uses of their roots. But as the time which has
elapsed since the disappearance of the glaciers is much less than that
occupied in the formation of ordinary soil, this decay has in most
cases not yet gone very far, so that in a cubic foot of glaciated
waste the amount of material available for plants is often only a
fraction of that held in the soils of immediate derivation.
In the greater portion of the fields occupied by glacial waste the
processes which lead to the introduction of organic matter into the
earth have not gone far enough to set in effective work the great
laboratory which has to operate in order to give fertile soil. The
pebbles hinder the penetration of the roots as well as the movement of
insects and other animals. There has not been time enough for the
overturning of trees to bring about a certain admixture of vegetable
matter with the soil--in a word, the process of soil-making, though
the first condition, that of broken-up rock, has been accomplished,
is as yet very incomplete. It needs, indeed, care in the introduction
of organic matter for its completion.
It is characteristic of glacial soils that they are indefinitely deep.
This often is a disadvantageous feature, for the reason that the soil
water may pass so far down into the earth that the roots are often
deprived of the moisture which they need, and which in ordinary soils
is retained near the surface by the hard underlayer. On the other
hand, where the glacial waste is made up of pebbles formed from rocks
of varied chemical composition, which contain a considerable share of
lime, potash, soda, and other substances which are required by plants,
the very large surface which they expose to decay provides the soil
with a continuous enrichment. In a cubic foot of pebbly glacial earth
we often find that the mass offers several hundred times as much
surface to the action of decay as is afforded by the underlying solid
bed rock from which a soil of immediate derivation has to win its
mineral supply. Where the pebbly glacial waste is provided with a
mixture of vegetable matter, the process of decay commonly goes
forward with considerable rapidity. If the supply of such matter is
large, such as may be produced by ploughing in barnyard manure or
green crops, the nutritive value of the earth may be brought to a very
high point.
It is a familiar experience in regions where glacial soils exist that
the earth beneath the swamps when drained is found to be
extraordinarily well suited for farming purposes. On inspecting the
pebbles from such places, we observe that they are remarkably decayed.
Where the masses contain large quantities of feldspar, as is the case
in the greater part of our granitic and other crystalline rocks, this
material in its decomposition is converted into kaolin or feldspar
clay, and gives the stones a peculiar white appearance, which marks
the decomposition, and indicates the process by which a great variety
of valuable soil ingredients are brought into a state where they may
be available for plants.
In certain parts of the glacial areas, particularly in the region near
the margin of the ice sheet, where the glacier remained in one
position for a considerable time, we find extensive deposits of
silicious sand, formed of the materials which settled from the
under-ice stream, near where they escaped from the glacial cavern.
These kames and sand plains, because of the silicious nature of their
materials and the very porous nature of the soil which they afford,
are commonly sterile, or at most render a profit to the tiller by dint
of exceeding care. Thus in Massachusetts, although the first settlers
seized upon these grounds, and planted their villages upon them
because the forests there were scanty and the ground free from
encumbering boulders, were soon driven to betake themselves to those
areas where the drift was less silicious, and where the pebbles
afforded a share of clay. Very extensive fields of this sandy nature
in southeastern New England have never been brought under tillage.
Thus on the island of Martha's Vineyard there is a connected area
containing about thirty thousand acres which lies in a very favourable
position for tillage, but has been found substantially worthless for
such use. The farmers have found it more advantageous to clear away
the boulders from the coarser drift in order to win soil which would
give them fair returns.
Those areas which are occupied by soil materials which have been
brought into their position by the action of the wind may, as regards
their character, be divided into two very distinct groups--the dunes
and loess deposits. In the former group, where, as we have noted (see
page 123), the coarse sea sands or those from the shores of lakes are
driven forward as a marching hillock, the grains of the material are
almost always silicious. The fragments in the motion are not taken up
into the air, but are blown along the surface. Such dune accumulations
afford an earth which is even more sterile than that of the glacial
sand plains, where there is generally a certain admixture of pebbles
from rocks which by their decomposition may afford some elements of
fertility. Fortunately for the interests of man, these wind-borne
sands occupy but a small area; in North America, in the aggregate,
there probably are not more than one thousand square miles of such
deposits.
Where the rock material drifted by the winds is so fine that it may
rise into the air in the form of dust, the accumulations made of it
generally afford a fertile soil, and this for the reason that they are
composed of various kinds of rock, and not, as in the case of dunes,
of nearly pure silica. In some very rare cases, where the seashore is
bordered by coral reefs, as it is in parts of southern Florida, and
the strand is made up of limestone bits derived from the hard parts
which the polyps secrete, small dunes are made of limy material.
Owing, however, in part to the relatively heavy nature of this
substance, as well as to the rapid manner in which its grains become
cemented together, such limestone dunes never attain great size nor
travel any distance from their point of origin.
As before noted, dust accumulations form the soil in extended areas
which lie to the leeward of great deserts. Thus a considerable part of
western China and much of the United States to the west of the
Mississippi is covered by these wind-blown earths. Wherever the
rainfall is considerable these loess deposits have proved to have a
high agricultural value.
Where a region has an earth which has recently passed from beneath the
sea or a great lake, the surface is commonly covered by incoherent
detritus which has escaped consolidation into hard rock by the fact
that it has not been buried and thus brought into the laboratory of
the earth's crust. When such a region becomes dry land, the materials
are immediately ready to enter into the state of soil. They commonly
contain a good deal of waste derived from the organic life which
dwelt upon the sea bottom and was embedded in the strata as they were
formed. Where these accumulations are made in a lake, the land
vegetation at once possesses the field, even a single year being
sufficient for it to effect its establishment. Where the lands emerge
from the sea, it requires a few years for the salt water to drain away
so that the earth can be fit for the uses of plants. In a general way
these sea-bottom soils resemble those formed in the alluvial plains.
They are, however, commonly more sandy, and their substances less
penetrated by that decay which goes on very freely in the atmosphere
because of the abundant supply of oxygen, and but slowly on the sea
floor. Moreover, the marine deposits are generally made up in large
part of silicious sand, a material which is produced in large
quantities by the disruption of the rocks along the sea coast. The
largest single field of these ocean-bottom soils of North America is
found in the lowland region of the southern United States, a wide belt
of country extending along the coast from the Rio Grande to New York.
Although the streams have channelled shallow valleys in the beds of
this region, the larger part of its surface still has the peculiar
features of form and composition which were impressed upon it when it
lay below the surface of the sea.
Local variations in the character of the soil covering are exceedingly
numerous, and these differences of condition profoundly affect the
estate of man. We shall therefore consider some of the more important
of these conditions, with special reference to their origin.
The most important and distinctly marked variation in the fertility of
soils is that which is produced by differences in the rainfall. No
parts of the earth are entirely lacking in rain, but over considerable
areas the precipitation does not exceed half a foot a year. In such
realms the soil is sterile, and the natural coating of vegetation
limited to those plants which can subsist on dew or which can take on
an occasional growth at such times as moisture may come upon them.
With a slight increase in precipitation, the soil rapidly increases in
productivity, so that we may say that where as much as about ten
inches of water enters the earth during the summer half of the year,
it becomes in a considerable measure fit for agriculture. Observations
indicate that the conditions of fertility are not satisfied where the
rainfall is just sufficient to fill the pores of the soil; there must
be enough water entering the earth to bring about a certain amount of
outflow in the form of springs. The reason of this need becomes
apparent when we study the evident features of those soils which,
though from season to season charged with water, do not yield springs,
but send the moisture away through the atmosphere. Wherever these
conditions occur we observe that the soil in dry seasons becomes
coated with a deposit of mineral matter, which, because of its taste,
has received the name of alkali. The origin of this coating is as
follows: The pores of the soil, charged from year to year with
sufficient water to fill them, become stored with a fluid which
contains a very large amount of dissolved mineral matter--too much,
indeed, to permit the roots of plants, save a few species which have
become accustomed to the conditions, to do their appointed work. In
fact, this water is much like that of the sea, which the roots of only
a few of our higher plants can tolerate. When the dry season comes on,
the heat of the sun evaporates the water at the surface, leaving
behind a coating composed of the substances which the water contains.
The soil below acts in the manner of a lamp-wick to draw up fluid as
rapidly as the heat burns it away. When the soil water is as far as
possible exhausted, the alkali coating may represent a considerable
part of the soluble matter of the soil, and in the next rainy season
it may return in whole or in part to the under-earth, again to be
drawn in the manner before described to the upper level. It is
therefore only when a considerable share of the ground water goes
forth to the streams in each year that the alkaline materials are in
quantity kept down to the point where the roots of our crop-giving
plants can make due use of the soil. Where, in an arid region, the
ground can be watered from the enduring streams or from artificial
reservoirs, the main advantage arising from the process is commonly
found in the control which it gives the farmer in the amount of the
soil water. He can add to the rainfall sufficient to take away the
excess of mineral matter. When such soils are first brought under
tillage it is necessary to use a large amount of water from the
canals, in order to wash away the old store of alkali. After that a
comparatively small contribution will often keep the soil in excellent
condition for agriculture. It has been found, however, in the
irrigated lands beside the Nile that where too much saving is
practised in the irrigation, the alkaline coating will appear where it
has been unknown before, and with it an unfitness of the earth to bear
crops.
Although the crust of mineral matters formed in the manner above
described is characteristic of arid countries, and in general peculiar
to them, a similar deposit may under peculiar conditions be formed in
regions of great rainfall. Thus on the eastern coast of New England,
where the tidal marshes have here and there been diked from the sea
and brought under tillage, the dissolved mineral matters of the soil,
which are excessive in quantity, are drawn to the surface, forming a
coating essentially like that which is so common in arid regions. The
writer has observed this crust on such diked lands, having a thickness
of an eighth of an inch. In fact, this alkali coating represents
merely the extreme operation of a process which is going on in all
soils, and which contributes much to their fertility. When rain falls
and passes downward into the earth, it conveys the soluble matter to a
depth below the surface, often to beyond the point where our ordinary
crop plants, such as the small grains, can have access to it, and
this for the reason that their roots do not penetrate deeply. When dry
weather comes and evaporation takes place from the surface, the fluid
is drawn up to the upper soil layer, and there, in process of
evaporation, deposits the dissolved materials which it contains. Thus
the mineral matter which is fit for plant food is constantly set in
motion, and in its movement passes the rootlets of the plants. It is
probably on this account--at least in part--that very wet weather is
almost as unfavourable to the farmer as exceedingly dry, the normal
alternation in the conditions being, as is well known, best suited to
his needs.
So long as the earth is subjected to conditions in which the rainfall
may bring about a variable amount of water in the superficial detrital
layer, we find normal fruitful soils, though in their more arid
conditions they may be fit for but few species of plants. When, by
increasing aridity, we pass to conditions where there is no tolerably
permanent store of water in the _débris_, the material ceases to have
the qualities of a soil, and becomes mere rock waste. At the other
extreme of the scale we pass to conditions where the water is
steadfastly maintained in the interstices of the detritus, and there
again the characteristic of the soil and its fitness for the uses of
land vegetation likewise disappear. In a word, true soil conditions
demand the presence of moisture, but that in insufficient quantities,
to keep the pores of the earth continually filled; where they are thus
filled, we have the condition of swamps. Between these extremes the
level at which the water stands in the soil in average seasons is
continually varying. In rainy weather it may rise quite to the
surface; in a dry season it may sink far down. As this water rises and
falls, it not only moves, as before noted, the soluble mineral
materials, but it draws the air into and expels it from the earth with
each movement. This atmospheric circulation of the soil, as has been
proved by experiment, is of great importance in maintaining its
fertility; the successive charges of air supply the needs of the
microscopic underground creatures which play a large part in enriching
the soil, and the direct effect of the oxygen in promoting decay is
likewise considerable. A part of the work which is accomplished by
overturning the earth in tillage consists in this introduction of the
air into the pores of the soil, where it serves to advance the actions
which bring mineral matters into solution.
[Illustration: _Mountain gorge, Himalayas, India. Note the difference
in the slope of the eroded rocks and the effect of erosion upon them;
also the talus slopes at the base of the cliffs which the torrent is
cutting away. On the left of the foreground there is a little bench
showing a recent higher line of the water._]
In the original conditions of any country which is the seat of
considerable rainfall, and where the river system is not so far
developed as to provide channels for the ready exit of the waters, we
commonly find very extensive swamps; these conditions of bad drainage
almost invariably exist where a region has recently been elevated
above the level of the sea, and still retains the form of an irregular
rolling plain common to sea floors, and also in regions where the work
done by glaciers has confused the drainage which the antecedent
streams may have developed. In an old, well-elaborated river system
swamps are commonly absent, or, if they occur, are due to local
accidents of an unimportant nature.
For our purpose swamps may be divided into three groups--climbing
bogs, lake bogs, and marine marshes. The first two of these groups
depend on the movements of the rain water over the land; the third on
the action of the tides. Beginning our account with the first and most
exceptional of these groups, we note the following features in their
interesting history:
Wherever in a humid region, on a gentle slope--say with an inclination
not exceeding ten feet to the mile--the soil is possessed by any
species of plants whose stems grow closely together, so that from
their decayed parts a spongelike mass is produced, we have the
conditions which favour the development of climbing bogs. Beginning
usually in the shores of a pool, these plants, necessarily of a
water-loving species, retain so much moisture in the spongy mass
which they form that they gradually extend up the slope. Thus
extending the margin of their field, and at the same time thickening
the deposit which they form, these plants may build a climbing bog
over the surface until steeps are attained where the inclination is so
great that the necessary amount of water can not be held in the spongy
mass, or where, even if so held, the whole coating will in time slip
down in the manner of an avalanche.
The greater part of the climbing bogs of the world are limited to the
moist and cool regions of high latitudes, where species of moss
belonging to the genus _Sphagnum_ plentifully flourish. These plants
can only grow where they are continuously supplied with a bath of
water about their roots. They develop in lake bogs as far south as
Mexico, but in the climbing form they are hardly traceable south of
New England, and are nowhere extensively developed within the limits
of the United States. In more northern parts of this continent, and in
northwestern Europe, particularly in the moist climate of Ireland,
climbing bogs occupy great areas, and hold up their lakes of
interstitially contained water over the slopes of hills, where the
surface rises at the rate of thirty feet or more to the mile. So long
as the deposit of decayed vegetable matter which has accumulated in
this manner is thin, therefore everywhere penetrated by the fibrous
roots of the moss, it may continue to cling to its sloping bed; but
when it attains a considerable thickness, and the roots in the lower
part decay, the pulpy mass, water-laden in some time of heavy rain,
break away in a vast torrent of thick, black mud, which may inundate
the lower lands, causing widespread destruction.
In more southern countries, other water-loving plants lead to the
formation of climbing bogs. Of these, the commonest and most effective
are the species of reeds, of which our Indian cane is a familiar
example. Brakes of this vegetation, plentifully mingled with other
species of aquatic growth, form those remarkable climbing bogs known
as the Dismal and other swamps, which numerously occur along the coast
line of the United States from southern Maryland to eastern Texas.
Climbing bogs are particularly interesting, not only from the fact
that they are eminently peculiar effects of plant growth, but because
they give us a vivid picture of those ancient morasses in which grew
the plants that formed the beds of vegetable matter now appearing in
the state of coal. Each such bed of buried swamp material was, with
rare exceptions, where the accumulation took place in lakes, gathered
in climbing bogs such as we have described.
Lake bogs occur in all parts of the world, but in their best
development are limited to relatively high latitudes, and this for the
reason that the plants which form vegetable matter grow most
luxuriantly in cool climates and in regions where the level of the
basin is subject to less variation than occurs in the alternating wet
and dry seasons which exist in nearly all tropical regions. The
fittest conditions are found in glaciated regions, where, as before
noted, small lakes are usually very abundant. On the shores of one of
these pools, of size not so great that the waves may attain a
considerable height, or in the sheltered bay of a larger lake, various
aquatic plants, especially the species of pond lilies, take root upon
the bottom, and spread their expanded leaves on the surface of the
water. These flexible-leaved and elastic-stemmed plants can endure
waves which attain no more than a foot or two of height, and by the
friction which they afford make the swash on the shore very slight. In
the quiet water, rushes take root, and still further protect the
strand, so that the very delicate vegetation of the mosses, such as
the _Sphagnum_, can fix itself on the shore.
As soon as the _Sphagnum_ mat has begun its growth, the strength given
by its interlaced fibres enables it to extend off from the shore and
float upon the water. In this way it may rapidly enlarge, if not
broken up by the waves, so that its front advances into the lake at
the rate of several inches each year. While growing outwardly it
thickens, so that the bottom of the mass gradually works down toward
the floor of the basin. At the same time the lower part of the sheet,
decaying, contributes a shower of soft peat mud to the floor of the
lake. In this way, growing at its edge, deepening, and contributing to
an upgrowth from the bottom, a few centuries may serve entirely to
fill a deep basin with peaty accumulation. In general, however, the
surface of the bog closes over the lake before the accumulation has
completely filled the shoreward portions of the area. In these
conditions we have what is familiarly known as a quaking bog, which
can be swayed up and down by a person who quickly stoops and rises
while standing on the surface. In this state the tough and thick sheet
of growing plants is sufficient to uphold a considerable weight, but
so elastic that the underlying water can be thrown into waves. Long
before the bog has completely filled the lake with the peaty
accumulations the growth of trees is apt to take place on its surface,
which often reduces the area to the appearance of a very level wet
wood.
[Illustration: Fig. 17.--Diagram showing beginning of peat bog: A,
lake; B, lilies and rushes; C, lake bog; D, climbing bog.]
Climbing and lake bogs in the United States occupy a total area of
more than fifty thousand square miles. In all North America the total
area is probably more than twice as great. Similar deposits are
exceedingly common in the Eurasian continent and in southern
Patagonia. It is probable that the total amount of these fields in
different parts of the world exceeds half a million square miles.
These two groups of fresh-water swamps have an interest, for the
reason that when reduced to cultivation by drainage and by subsequent
removal of the excess of peaty matter, by burning or by natural decay,
afford very rich soil. The fairest fields of northern Europe,
particularly in Great Britain and Ireland, have been thus won to
tillage. In the first centuries of our era a large part of
England--perhaps as much as one tenth of the ground now tilled in that
country--was occupied by these lands, which retained water in such
measure as to make them unfit for tillage, the greater portion of this
area being in the condition of thin climbing bog. For many centuries
much of the energy of the people was devoted to the reclamation of
these valuable lands. This task of winning the swamp lands to
agriculture has been more completely accomplished in England than
elsewhere, but it has gone far on the continent of Europe,
particularly in Germany. In the United States, owing to the fact that
lands have been cheap, little of this work of swamp-draining has as
yet been accomplished. It is likely that the next great field of
improvement to be cultivated by the enterprising people will be found
in these excessively humid lands, from which the food-giving resources
for the support of many million people can be won.
[Illustration: Fig. 18.--Diagram showing development of swamp: A,
remains of lake; B, surface growth; c, peat.]
The group of marine marshes differs in many important regards from
those which are formed in fresh water. Where the tide visits any
coast line, and in sheltered positions along that shore, a number of
plants, mostly belonging to the group of grasses, species which have
become accustomed to having their roots bathed by salt water, begin
the formation of a spongy mat, which resembles that composed of
_Sphagnum_, only it is much more solid. This mat of the marine marshes
soon attains a thickness of a foot or more, the upper or growing
surface lying in a position where it is covered for two or three hours
at each visit of the tide. Growing rapidly outward from the shore, and
having a strength which enables it to resist in a tolerably effective
manner waves not more than two or three feet high, this accumulation
makes head against the sea. To a certain extent the waves undermine
the front of the sheet and break up masses of it, which they
distribute over the shallow bottom below the level at which these
plants can grow. In this deeper water, also, other marine animals and
plants are continually developing, and their remains are added to the
accumulations which are ever shallowing the water, thus permitting a
further extension of the level, higher-lying marsh. This process
continues until the growth has gone as far as the scouring action of
the tidal currents will permit. In the end the bay, originally of
wide-open water, is only such at high tide. For the greater part of
the time it appears as broad savannas, whose brilliant green gives
them the aspect of rare fertility.
Owing to the conditions of their growth, the deposits formed in marine
marshes contain no distinct peat, the nearest approach to that
substance being the tangle of wirelike roots which covers the upper
foot or so of the accumulation. The greater part of the mass is
composed of fine silt, brought in by the streams of land water which
discharge into the basin, and by the remains of animals which dwelt
upon the bottom or between the stalks of the plants that occupy the
surface of the marshes. These interspaces afford admirable shelter to
a host of small marine forms. The result is, that the tidal marshes,
as well as the lower-lying mud flats, which have been occupied by the
mat of vegetation, afford admirable earth for tillage. Unfortunately,
however, there are two disadvantages connected with the redemption of
such lands. In the first place, it is necessary to exclude the sea
from the area, which can only be accomplished by considerable
engineering work; in the second place, the exclusion of the tide
inevitably results in the silting up of the passage by which the water
found its way to the sea. As these openings are often used for
harbours, the effect arising from their destruction is often rather
serious. Nevertheless, in some parts of the world very extensive and
most fertile tracts of land have thus been won from the sea; a large
part of Holland and shore-land districts in northern Europe are made
up of fields which were originally covered by the tide. Near the mouth
of the Rhine, indeed, the people have found these sea-bottom soils so
profitable that they have gone beyond the zone of the marshes, and
have drained considerable seas which of old were permanently covered,
even at the lowest level of the waters.
On the coast of North America marine marshes have an extensive
development, and vary much in character. In the Bay of Fundy, where
the tides have an altitude of fifty feet or more, the energy of their
currents is such that the marsh mat rarely forms. Its place, however,
is taken by vast and ever-changing mud flats, the materials of which
are swept to and fro by the moving waters. The people of this region
have learned an art of a peculiar nature, by which they win broad
fields of excellent land from the sea. Selecting an area of the flats,
the surface of which has been brought to within a few feet of high
tide, they inclose it with a stout barrier or dike, which has openings
for the free admission of the tidal waters. Entering this basin, the
tide, moving with considerable velocity, bears in quantities of
sediment. In the basin, the motion being arrested, this sediment
falls to the bottom, and serves to raise its level. In a few months
the sheet of sediment is brought near the plane of the tidal movement,
then the gates are closed at times when the tide has attained half of
its height, so that the ground within the dike is not visited by the
sea water, and can be cultivated.
[Illustration: Fig. 19.--Map of Ipswich marshes, Massachusetts, formed
behind a barrier beach.]
Along the coast of New England the ordinary marine marshes attain an
extensive development in the form of broad-grassed savannas. With this
aspect, though with a considerable change in the plants which they
bear, the fringe of savannas continues southward along the coast to
northern Florida. In the region about the mouth of the Savannah River,
so named from the vast extent of the tidal marshes, these fields
attain their greatest development. In central and southern Florida,
however, where the seacoast is admirably suited for their development,
these coastal marshes of the grassy type disappear, their place being
taken by the peculiar morasses formed by the growth of the mangrove
tree.
In the mangrove marshes the tree which gives the areas their name
covers all the field which is visited by the tide. This tree grows
with its crown supported on stiltlike roots, at a level above high
tide. From its horizontal branches there grow off roots, which reach
downward into the water, and thence to the bottom. The seeds of the
mangrove are admirably devised so as to enable the plant to obtain a
foothold on the mud flats, even where they are covered at low tide
with a depth of two or three feet of water. They are several inches in
length, and arranged with booklets at their lower ends; floating near
the bottom, they thus catch upon it, and in a few weeks' growth push
the shoot to the level of the water, thus affording a foundation for a
new plantation. In this manner, extending the old forests out into the
shallow water of the bays, and forming new colonies wherever the water
is not too deep, these plants rapidly occupy all the region which
elsewhere would appear in the form of savannas.
[Illustration: Fig. 20.--Diagram showing mode of growth of mangroves.]
The tidal marshes of North America, which may be in time converted to
the uses of man, probably occupy an area exceeding twenty thousand
square miles. If the work of reclaiming such lands from the sea ever
attains the advance in this country that it has done in Holland, the
area added to the dry land by engineering devices may amount to as
much as fifty thousand square miles--a territory rather greater than
the surface of Kentucky, and with a food-yielding power at least five
times as great as is afforded by that fertile State. In fact, these
conquests from the sea are hereafter to be among the great works which
will attract the energies of mankind. In the arid region of the
Cordilleras, as well as in many other countries, the soil, though
destitute of those qualities which make it fit for the uses of man,
because of the absence of water in sufficient amount, is, as regards
its structure and depth, as well as its mineral contents, admirably
suited to the needs of agriculture. The development of soils in desert
regions is in almost all cases to be accounted for by the former
existence in the realms they occupy of a much greater rainfall than
now exists. Thus in the Rocky Mountain country, when the deep soils
of the ample valleys were formed, the lakes, as we have before noted,
were no longer dead seas, as is at present so generally the case, but
poured forth great streams to the sea. Here, as elsewhere, we find
evidence that certain portions of the earth which recently had an
abundant rainfall have now become starved for the lack of that supply.
All the soils of arid regions where the trial has been made have
proved very fertile when subjected to irrigation, which can often be
accomplished by storing the waters of the brief rainy season or by
diverting those of rivers which enter the deserts from well-watered
mountain fields. In fact, the soil of these arid realms yields
peculiarly ample returns to the husbandman, because of certain
conditions due to the exceeding dryness of the air. This leads to an
absence of cloudy weather, so that from the time the seed is planted
the growth is stimulated by uninterrupted and intense sunshine. The
same dryness of the air leads, as we have seen, to a rapid evaporation
from the surface, by which, in a manner before noted, the dissolved
mineral matter is brought near the top of the soil, where it can best
serve the greater part of our crop plants. On these accounts an acre
of irrigated soil can be made to yield a far greater return than can
be obtained from land of like chemical composition in humid regions.
In many parts of the world, particularly in the northern and western
portions of the Mississippi Valley, there are widespread areas, which,
though moderately well watered, were in their virgin state almost
without forests. In the prairie region the early settlers found the
country unwooded, except along the margins of the streams. On the
borders of the true prairies, however, they found considerable areas
of a prevailingly forested land, with here and there a tract of
prairie. There were several of these open fields south of the Ohio,
though the country there is in general forested; one of these prairie
areas, in the Green River district of Kentucky, was several thousand
square miles in extent. At first it was supposed that the absence of
trees in the open country of the Mississippi Valley was due to some
peculiarity of the soil, but experience shows that plantations
luxuriantly develop, and that the timber will spread rapidly in the
natural way. In fact, if the seeds of the trees which have been
planted since the settlement of the country were allowed to develop as
they seek to do, it would only be a few centuries before the region
would be forest-clad as far west as the rainfall would permit the
plants to develop. Probably the woods would attain to near the
hundredth meridian.
In the opinion of the writer, the treeless character of the Western
plains is mainly to be accounted for by the habit which our Indians
had of burning the herbage of a lowly sort each year, so that the
large game might obtain better pasturage. It is a well-known fact to
all those who have had to deal with cattle on fields which are in the
natural state that fire betters the pasturage. Beginning this method
of burning in the arid regions to the west of the original forests,
the natural action of the fire has been gradually to destroy these
woods. Although the older and larger trees, on account of their thick
bark and the height of their foliage above the ground, escaped
destruction, all the smaller and younger members of the species were
constantly swept away. Thus when the old trees died they left no
succession, and the country assumed its prairie character. That the
prairies were formed in this manner seems to be proved by the
testimony which we have concerning the open area before mentioned as
having existed in western Kentucky. It is said that around the
timberless fields there was a wide fringe of old fire-scarred trees,
with no undergrowth beneath their branches, and that as they died no
kind of large vegetation took their place. When the Indians who set
these fires were driven away, as was the case in the last decade of
the last century, the country at once began to resume its timbered
condition. From the margin and from every interior point where the
trees survived, their seeds spread so that before the open land was
all subjugated to the plough it was necessary in many places to clear
away a thick growth of the young forest-building trees.
The soils which develop on the lavas and ashes about an active volcano
afford interesting subjects for study, for the reason that they show
how far the development of the layer which supports vegetation may
depend upon the character of the rocks from which it is derived. Where
the materials ejected from a volcano lie in a rainy district, the
process of decay which converts the rock into soil is commonly very
rapid, a few years of exposure to the weather being sufficient to
bring about the formation of a fertile soil. This is due to the fact
that most lavas, as well as the so-called volcanic ashes, which are of
the same material as the lavas, only blown to pieces, are composed of
varied minerals, the most of which are readily attacked by the agents
of decay. Now and then, however, we find the materials ejected from a
particular volcano, or even the lavas and ashes of a single eruption,
in such a chemical state that soils form upon them with exceeding
slowness.
* * * * *
The foregoing incomplete considerations make it plain that the
soil-covering of the earth is the result of very delicate adjustments,
which determine the rate at which the broken-down rocks find their
path from their original bed places to the sea. The admirable way in
which this movement is controlled is indicated by the fact that almost
everywhere we find a soil-covering deep enough for the use of a varied
vegetation, but rarely averaging more than a dozen feet in depth. Only
here and there are the rocks bare or the earth swathed in a profound
mass of detritus. This indicates how steadfast and measured is the
march of the rock waste from the hills to the sea. Unhappily, man,
when by his needs he is forced to till the soil, is compelled to break
up this ancient and perfect order. He has to strip the living mantle
from the earth, replacing it with growth of those species which serve
his needs. Those plants which are most serviceable--which are, indeed,
indispensable in the higher civilization, the grains--require for
their cultivation that the earth be stripped bare and deeply stirred
during the rainy season, and thus subjected to the most destructive
effect of the rainfall. The result is, that in almost all grain fields
the rate of soil destruction vastly surpasses that at which the
accumulation is being made. We may say, indeed, that, except in
alluvial plains, where the soil grows by flood-made additions to its
upper surface, no field tilled in grain can without exceeding care
remain usable for a century. Even though the agriculturist returns to
the earth all the chemical substances which he takes away in his
crops, the loss of the soil by the washing away of its substance to
the stream will inevitably reduce the region to sterility.
It is not fanciful to say that the greatest misfortune which in a
large way man has had to meet in his agriculture arises from this
peculiar stress which grain crops put upon the soil. If these grains
grew upon perennial plants, in the manner of our larger fruits, the
problem of man's relation to the soil would be much simpler than it is
at present. He might then manage to till the earth without bringing
upon it the inevitable destruction which he now inflicts. As it is, he
should recognise that his needs imperil this ancient and precious
element in the earth's structure, and he should endeavour in every
possible way to minimize the damage which he brings about. This result
he may accomplish in certain simple ways.
First, as regards the fertility of the soil, as distinguished from the
thickness of the coating, it may be said that modern discoveries
enable us to see the ways whereby we may for an indefinite period
avoid the debasement of our great heritage, the food-giving earth. We
now know in various parts of the world extensive and practically
inexhaustible deposits, whence may be obtained the phosphates,
potash, soda, etc., which we take from the soil in our crops. We also
have learned ways in which the materials contained in our sewage may
be kept from the sea and restored to the fields. In fact, the recent
developments of agriculture have made it not only easy, but in most
cases profitable, to avoid this waste of materials which has reduced
so many regions to poverty. We may fairly look forward to the time,
not long distant, when the old progressive degradation in the
fertility of the soil coating will no longer occur. It is otherwise
with the mass of the soil, that body of commingled decayed rock and
vegetable matter which must possess a certain thickness in order to
serve its needs. As yet no considerable arrest has been made in the
processes which lead to the destruction of this earthy mass. In all
countries where tillage is general the rivers are flowing charged with
all they can bear away of soil material. Thus in the valley of the Po,
a region where, if the soil were forest-clad, the down-wearing of the
surface would probably be at no greater rate than one foot in five
thousand years, the river bears away the soil detritus so rapidly that
at the present time the downgoing is at the rate of one foot in eight
hundred years, and each decade sees the soil disappear from hillsides
which were once fertile, but are now reduced to bare rocks. All about
the Mediterranean the traveller notes extensive regions which were
once covered with luxuriant forests, and were afterward the seats of
prosperous agriculture, where the soil has utterly disappeared,
leaving only the bare rocks, which could not recover its natural
covering in thousands of years of the enforced fallow.
Within the limits of the United States the degradation of the soil,
owing to the peculiar conditions of the country, is in many districts
going forward with startling rapidity. It has been the habit of our
people--a habit favoured by the wide extent of fertile and easily
acquired frontier ground--recklessly to till their farms until the
fields were exhausted, and then to abandon them for new ground. By
shallow ploughing on steep hillsides, by neglect in the beginning of
those gulches which form in such places, it is easy in the hill
country of the eastern United States to have the soil washed away
within twenty years after the protecting forests have been destroyed.
The writer has estimated that in the States south of the Ohio and
James Rivers more than eight thousand square miles of originally
fertile ground have by neglect been brought into a condition where it
will no longer bear crops of any kind, and over fifteen hundred miles
of the area have been so worn down to the subsoil or the bed rock that
it may never be profitable to win it again to agricultural uses.
Hitherto, in our American agriculture, our people have been to a great
extent pioneers; they have been compelled to win what they could in
the cheapest possible way and with the rudest implements, and without
much regard to the future of those who were in subsequent generations
to occupy the fields which they were conquering from the wilderness
and the savages. The danger is now that this reckless tillage, in a
way justified of old, may be continued and become habitual with our
people. It is, indeed, already a fixed habit in many parts of the
country, particularly in the South, where a small farmer expects to
wear out two or three plantations in the course of his natural life.
Many of them manage to ruin from one to two hundred acres of land in
the course of half a century of uninterrupted labour. This system
deserves the reprobation of all good citizens; it would be well,
indeed, if it were possible to do so, to stamp it out by the law. The
same principle which makes it illegal for a man to burn his own
dwelling house may fairly be applied in restraining him from
destroying the land which he tills.
There are a few simple principles which, if properly applied, may
serve to correct this misuse of our American soil. The careful tiller
should note that all soils whatever which lie on declivities having a
slope of more than one foot in thirty inevitably and rapidly waste
when subject to plough tillage. This instrument tends to smear and
consolidate the layer of earth over which its heel runs, so that at a
depth of a few inches below the surface a layer tolerably impervious
to water is formed. The result is that the porous portion of the
deposit becomes excessively charged with water in times of heavy rain,
and moves down the hillside in a rapid manner. All such steep slopes
should be left in their wooded state, or, if brought into use, should
be retained as pasture lands.
Where, as is often the case with the farms in hilly countries, all the
fields are steeply inclined, it is an excellent precaution to leave
the upper part of the slope with a forest covering. In this condition
not only is the excessive flow of surface water diminished, but the
moisture which creeps down the slope from the wooded area tends to
keep the lower-lying fields in a better state for tillage, and
promotes the decay of the underlying rocks, and thus adds to the body
and richness of the earth.
On those soils which must be tilled, even where they tend to wash
away, the aim should be to keep the detritus open to such a depth that
it may take in as much as possible of the rainfall, yielding the water
to the streams through the springs. This end can generally be
accomplished by deep ploughing; it can, in almost all cases, be
attained by under-drainage. The effect of allowing the water to
penetrate is not only to diminish the superficial wearing, but to
maintain the process of subsoil and bed-rock decay by which the
detrital covering is naturally renewed. Where, as in many parts of the
country, the washing away of the soil can not otherwise be arrested,
the progress of the destruction can be delayed by forming with the
skilful use of the plough ditches of slight declivity leading along
the hillsides to the natural waterways. One of the most satisfactory
marks of the improvement which is now taking place in the agriculture
of the cotton-yielding States of this country is to be found in the
rapid increase in the use of the ditch system here mentioned. This
system, combined with ploughing in the manner where the earth is with
each overturning thrown uphill, will greatly reduce the destructive
effect of rainfall on steep-lying fields. But the only effective
protection, however, is accomplished by carefully terracing the
slopes, so that the tilled ground lies in level benches. This system
is extensively followed in the thickly settled portions of Europe, but
it may be a century before it will be much used in this country.
The duty of the soil-tiller by the earth with which he deals may be
briefly summed up: He should look upon himself as an agent necessarily
interfering with the operations which naturally form and preserve the
soil. He should see that his work brings two risks; he may impoverish
the accumulation of detrital material by taking out the plant food
more rapidly than it is prepared for use. This injurious result may be
at any time reparable by a proper use of manures. Not so, however,
with the other form of destruction, which results in the actual
removal of the soil materials. Where neglect has brought about this
disaster, it can only be repaired by leaving the area to recover
beneath the slowly formed forest coating. This process in almost all
cases requires many thousands of years for its accomplishment. The man
who has wrought such destruction has harmed the inheritance of life.