THE DYNAMIC NATURE OF SOIL PHENOMENA.
In soil investigations, until recently, the assumption has been made,
more or less explicitly, that any given soil mass, as for instance a
field, remains fixed or in place indefinitely. It has been admitted,
of course, that some physical, chemical and biological processes
might be taking place in the soil, but these have been regarded as
relatively unimportant in their effects upon the soil mass _in toto_.
It has been assumed that the only important change taking place in the
soil is a loss of mineral plant nutrients, partly by leaching, partly
by removal in the garnered crops. In other words, the soil has been
regarded as a static system. This is a fundamental error. In studying
the soil as a medium for crop production, we must consider the plant
itself, or at least that part of the plant which enters the soil,
namely, the root; the solid particles of the soil; the soil water,
or the aqueous solution from which the plant draws all the materials
for its sustenance, excepting the carbon dioxide absorbed by its
aerial portions; the soil atmosphere; the biological processes taking
place. The one common characteristic of all these things is that they
are continually in a state of change; therefore the soil problem is
essentially dynamic.
The root of a growing plant is always moving.[19] The amount of motion
may be small or large, depending upon the surrounding conditions or
attendant circumstances, but cessation of motion means the death of
the root. This becomes evident from a consideration of the mechanism of
root growth. The living root absorbs and excretes water and dissolved
substances through a restricted area just back of the root tip or the
tips of the root hairs. While absorption is taking place, however,
there is a deposition of denser material over the absorbing area,
or “root corking.” But coincident with the corking process, the tip
is pushed forward between the soil grains into the nutrient medium,
new cells are formed and a new absorbing surface continually brought
into functional activity. A failure of the plant root to move forward
in this way would mean a reabsorption of root effluvia with harmful
consequences to the plant, or a corking over of the root without
further formation of absorbing surface and with consequent cessation
of its functioning. This would mean the inevitable death of the root,
and, if general, of the whole plant. It is clear, therefore, that root
penetration and absorption of plant nutrients are essentially dynamic.
[19] In order to penetrate the soil, a living root must be capable
of exerting large pressures, and indeed, the magnitude of these
pressures has been determined for some cases. See, for citations of
the literature, Pfeffer, Plant Physiology, translated by Ewart, 1903,
Vol. 2, p. 124 _et seq._ But it can not be doubted that, in general,
root movement is much facilitated and perhaps directed by movements
among the soil particles. As the absorbing tip of the root removes film
water from the adjacent soil grains, there is a necessary rearrangement
of these grains with a shrinking away from the tip, which then moves
forward by taking advantage of the movements among the soil grains.
The solid components of the soil are always in motion. Every soil, no
matter how flat the area or how well protected by vegetal covering,
suffers some translocation of soil material through rains, as is
evidenced by suspended material in the run-off waters. On hillsides
this is shown by the soil accumulating on the “up” sides of fences,
especially stone fences. In the aggregate this movement is probably
quite large everywhere. It is manifestly so in the watersheds of many
of the world’s important rivers as shown by their muddy waters and
the formation of deltas, sometimes of great area and agricultural
importance.
With the saturation or approach to saturation of the surface soil the
particles are more easily moved among themselves by an extraneous
force. It is very rarely that the surface of a field is a dead level.
Consequently when the soil is wetted, the gravitational force on the
individual soil grains produces a more or less pronounced “creeping”
effect down hill. On decided slopes this soil creep is believed to be
of great importance in connection with soil erosion.[20]
As important as is the translocation of material by water, quite as
important probably is that produced by the winds. These are blowing all
the time, uphill as well as down, and their range of action is thus
far wider than is that of rain and flood. The effectiveness of the
wind as a translocating agency is seldom realized or even suspected by
the layman, although it is commonly known that the air always contains
some dust, and dust storms are familiar phenomena. That soil material
can be carried long distances is certain, however, as for instance
the sirocco dust, often carried from the Sahara over Europe.[21] Dust
carried high into the air by volcanic eruptions sometimes travels
enormous distances, as in the case of the eruption of Krakatoa, when
such material is reported to have traveled thousands of miles, and
volcanic debris from the eruptions at Soufrière fell upon ships several
hundred miles distant. Arctic explorers have reported the finding of
wind-borne soil materials over the polar ice, and mountaineers have
observed similar deposits on snow-capped peaks. Soil material on roofs
and similar inaccessible places has been observed many times, and
testifies to the continual activity of the wind. The burial of objects
even of considerable size by wind-borne soil gives like testimony.
[20] Soil erosion is undoubtedly one of the greatest economic problems
of the time, and yet there is scarcely any subject about which there
are current so many popular misconceptions. In the rivers and to those
who use the rivers the water-borne soil material is an unmitigated
nuisance, save possibly to a few cultivators of low-lying lands who
for one reason or another, may flood their fields for the sake of the
silt deposited. To the upland farmer, however, erosion is not only a
necessity of natural conditions which can not be avoided entirely, but
under proper control it may be even a blessing. The scalded and gullied
hillsides, a trial and unnecessary disgrace to the owner, are probably
not the main sources of the material which finds its way to the river.
On the contrary, what are regarded usually as well-tilled fields supply
the greater part of the suspended material in the rivers. The problem
of erosion on the farm is not merely to check gullying and scalding,
and deepening of stream heads, but to so adjust both cropping system
and cultural methods as to secure a reasonable translocation of surface
soil material with a minimum contamination of the neighborhood streams.
See, Man and the earth, by Nathaniel Southgate Shaler, 1905.
[21] For a comprehensive discussion of wind as a translocating agent,
see: The movement of soil material by the wind, by E. E. Free, Bureau
of Soils, Bull. No. 68, U. S. Dept. Agriculture.
Measurements of the amount of action of wind in translocating soil
material are rare and probably have a qualitative value only. But
Udden[22] in what appears to be a conservative calculation, finds “the
capacity of the atmosphere [over the Mississippi Valley] to transport
dust is 1000 times as great as that of the [Mississippi] River.” The
wind seldom is carrying anything like so great a load as it is capable
of carrying. That is, the wind in its attack upon the land surface
does not ordinarily obtain so large an amount of material capable of
being wind-borne as it is possible for the wind to carry when suitable
material is artificially provided. It should be remembered that,
speaking generally, the velocity of the wind is lower just at the
surface of the ground than at heights above, and it is necessary to get
the soil material above the surface before the wind can exercise its
full efficiency as a carrying agent.
[22] Erosion, transportation and sedimentation performed by the
atmosphere, by J. A. Udden, Jour. Geol., 2, 318-331 (1894).
Moreover, wind-borne material is constantly being deposited as well
as being removed from the land surface. It is evident, however, that
this movement of soil material by winds is very great, and there
is no reason to believe that it is of any less importance in other
areas than in the Mississippi Valley. It is also evident that the
individual grains in any surface soil of any particular field or area
are continually and more or less rapidly changing, and the farmer is
not dealing to-day with just the same soil complex he faced a few years
back, or will face a few years hence.
But besides the movements of the solid components of the soil by
translocating agencies, other movements are constantly taking place.
Whenever a moderately dry soil becomes wetted, it “swells up” until a
certain critical amount of moisture is present above which there is
a shrinking. But as a wet soil dries out again below the critical
amount, there is again a shrinking. As it is always either raining or
not raining, soils are always either getting wetted or are drying.
Consequently the individual grains are continually moving about among
themselves. A heavy object, such as stone, when left on the ground
gradually sinks into it.[23] Earthworms, burrowing animals and insects
are continually at work in most arable soils. The action of frost in
“heaving” a soil is familiar to everyone. Not so well known, however,
is the fact that the apparently superficial cracks which occur to a
greater or less extent in every soil, under drought conditions, are
in reality quite deep, extending well into the subsoil. By the edges
breaking off, and by wind- and water-borne material being carried in,
considerable surface soil is thus brought into the subsoil. Through
these various agencies, therefore, the solid components of the soil are
continually subject to much mixing; subsoil is becoming surface soil,
and to some extent _vice versa_. An important result of these various
processes is the bringing into the surface soil of degradation and
decomposition products from underlying rocks. The processes involved
are essentially dynamic.[24]
[23] On the small vertical movements of a stone laid on the surface
of the ground, by Horace Darwin, Proceedings of the Royal Society of
London, 68, 253-261, (1901). On the other hand, geological literature
would probably furnish numerous references to the heaving out of
boulders, probably as the result of successive freezings and thawings
of the soil. The shape of the stone as well as the specific nature
of the movements of the soil particles evidently has an important
influence in determining whether the stone sinks into the soil or _vice
versa_.
[24] It is clear that as the soil is continually changing through
physical agencies, the chemical analysis of it can not be expected to
furnish evidence as to the mineral constituents removed by crops or by
leaching.
The soil solution is also a dynamic problem. When the rain falls on
the soil, a part, the “run-off,” flows over the surface and finds its
way into the regional drainage; a part immediately evaporates into the
air, and is designated as the “fly-off;” a third part, the “cut-off,”
enters the soil.[25] The cut-off water penetrates the soil by way of
the larger openings and interstices, and mainly under the influence
of gravity. For convenience this downward-moving water is designated
as “gravitational” water. It moves through the soil with comparative
rapidity and a portion reappears elsewhere as seepage water, springs,
etc. But with the return of fair-weather conditions at the surface,
there is increased evaporation and augmentation of the fly-off, and
there is developed a drag or “capillary pull” on the water below.
A large portion of the cut-off thus returns to the surface, mainly
through films over the surface of the soil grains and in the finest
interstices.[26]
[25] This terminology has been suggested by Dr. W. J. McGee.
[26] Leather, however, thinks the water returns from only a limited
depth, some 5-7 feet; see, The loss of water from soil during
dry weather, by J. Walter Leather, Memoirs of the Department of
Agriculture, Agricultural Research Institute, Pusa, India, Chemical
series, I, 79-116, (1908). Dr. George N. Coffey has called the author’s
attention to some observations in Western Kansas, where a prolonged
drought had dried the soil to a considerable depth. A fairly heavy rain
wetted the soil to less than two feet from the surface, and practically
all of this moisture had returned to the surface and evaporated
within a few days. Such special cases as these, however interesting
in themselves, are even less so than the normal cases in humid areas,
where a part of the water passes through the soil as seepage, the
larger portion returning to the surface, sometimes through distances of
many feet.
The soil atmosphere is continually in motion, following with more or
less decided lag the barometric changes in the atmosphere above the
soil. Moreover, the chemical and physical processes continually taking
place in the soil involve the absorption or the formation of free
carbonic acid, and it seems probable that all rain water penetrating
the soil gives up some oxygen to the soil atmosphere. The bacteria
and lower life forms are necessarily undergoing changes continually.
In fact all components of the soil are continually undergoing, or are
involved in, changes of one kind or another.
It is certain that investigation of the various motions and changes
taking place in the soil is quite as important as investigation of the
soil components, and that no clear idea of the chemistry of the soil
can be obtained without it. The development of a rational practice of
soil control is possible only when the soil is regarded from a dynamic
viewpoint.