SOIL AND SUBSOIL (_Continued_)
THEIR RELATIONS TO VEGETATION.
_Physical Effects of the Percolation of Surface Waters._—The muddy
water formed by the beating of rains on the soil surface will, in
penetrating the soil, carry with it the diffused colloidal clay to a
certain depth into the subsoil. We should therefore expect that as
a rule every subsoil will be more clayey than its surface soil; and
this is found to be almost universally the case in the humid region.
Subsoils are therefore almost always less precious and more retentive
of moisture, as well as of plant-food substances in solution, than
their surface soils, unless these are very rich in humus; and as the
finest particles are usually those richest in available plant-food,
it follows that subsoils will as a rule be found to contain larger
supplies of the latter than the surface soil. Common experience as well
as comparative analysis confirm both of these inferences so thoroughly,
that it becomes unnecessary to adduce examples in this place.
On the other hand, the reverse, upward movement of moisture caused
by surface evaporation tends constantly to bring any soluble salts
contained in the soil mass nearer to the surface, thus increasing the
stock of easily available plant-food in the surface soil. In extreme
cases, especially in the arid region, this accumulation of salts may
become excessive, and seriously injurious to plant growth. (See “Alkali
Soils”, chapters 21, 22.)
_Chemical effects of Water-Percolation._—The accumulation of plant-food
in the subsoil is not, however, due only to the mechanically-carried
particles, but also to the ingredients carried in solution from the
surface soil and redeposited in the more retentive subsoils. Especially
is this true of _lime carbonate_, which is dissolved by the carbonic
acid formed chiefly within the humic surface soil, and is often carried
off in amounts sufficient to obstruct drain tiles by its deposition
in contact with air (see chapt. 3). In the case of moderate rains,
however, it is carried no farther than the subsoil, and is there
redeposited, in consequence of the penetration of air, following the
water, and causing the carbonic gas to diffuse upward; thus leaving the
lime carbonate behind. In the majority of cases this results simply in
a gradual enriching of the subsoil in this substance; while the surface
soil may become so depleted as to require its artificial replacement by
liming or marling. The same general process occurs to a less extent, in
the case of magnesia.
_Calcareous Subsoils._—The fact that subsoils are more calcareous than
the corresponding surface soils is often of great practical importance,
in enabling the farmer to enrich his depleted surface soil in lime by
subsoil plowing. The accumulation of lime carbonate in the subsoil also
tends in a measure to offset the extreme heaviness sometimes resulting
from the accumulation of clay.
_Calcareous Subsoils and Hardpans._—When soils are very rich in lime,
and rains occur in limited showers rather than continuously, the
lime carbonate dissolved from the surface soil may accumulate in the
subsoil so as to either form calcareous “hardpan” by the cementing of
the subsoil mass; or it may accumulate and partly crystallize around
certain centers and thus form white concretions, known to farmers as
“white gravel.” The latter is the form usually assumed in the regions
of summer rains; while in the arid regions the deficient rainfall
causes this substance to accumulate, and calcareous hardpan to form, at
definite depths depending upon the maximum penetration of the annual
rainfall; sometimes in crystalline masses of veritable limestone
(“kankar” of India), or sometimes merely as crystalline incrustations
loosely cementing the subsoil.
_“Rawness” of Subsoils in Humid Climates._—From the greater compactness
of the subsoil which is almost universal in the humid regions, the
absence of humus and of the resulting formation of carbonic and humic
acids, it follows that its minerals are less subject to the weathering
process than are those of the surface soil. In the farmer’s parlance,
the subsoil is “raw” as compared with the surface soil; it is not so
suitable for plant-nutrition, and therefore must not be brought to the
surface to form the seed-bed, or be incorporated with the surface soil
to any considerable extent _at any one time_, if crop-nutrition is to
be normal. It is only in the course of time, by exposure to atmospheric
action as well as to that of the humus, and of plant roots, that it
becomes properly adapted to perform the functions of the surface soil.
_Soils and Subsoils in the Arid Region._—But however pronounced and
important are these distinctions and differences in the humid region,
they are found to be profoundly modified in the arid; where, as before
stated, the formation of colloidal clay is very much diminished,
so that most soils formed under arid conditions are of a sandy or
pulverulent type. There is then little or no clay to be washed down
into the subsoil, hence there is no compacting of the latter; the air
consequently circulates freely down to the depth of many feet.
Thus one of the most important distinctions between soil and subsoil
is to a great extent practically non-existent in the arid region, at
least within the depths to which tillage can be made to reach; so that
the limitations attached to subsoil-plowing in the countries of summer
rains do not apply to the characteristic soils of the arid regions.
Even the distinction in regard to humus is here largely obliterated
by the circumstance, already alluded to, that most of that substance
must, in the arid regions, be derived from the decay of roots, which
moreover reach to much greater depth in these soils. Hence even in the
uplands of the arid region it is common to find no change of tint from
the surface down to three feet, and even more. This, like the free
circulation of the air in consequence of porosity, tends to render the
distinction of soil and subsoil practically useless; since it disposes
of the objection to “subsoiling” based upon the inert condition of the
subsoil, which in humid climates so effectually interferes with the
welfare of crops unless subsoiling is restricted to a fraction of an
inch at a time.
These fundamental differences in the soils of the two regions are
illustrated schematically in the subjoined diagram, which shows on the
left the contrast between clay or clay loam soils, in which the depth
of the surface soil-sample to be taken is prescribed as nine inches by
the rules of the Association of Am. Official Chemists (in the writer’s
experience it is more nearly six inches as a rule). Alongside of the
Eastern soil thus characterized is placed a typical “adobe” soil from
the grounds of the California Experiment station, of which a sample
showing uniform blackness to three feet depth was exhibited at the
World’s Fair at Chicago in 1893. At the right is a profile of the noted
hop soil on the bench lands of the Russian river, Cal., in which the
humus-content was determined down to twelve feet, the humus-percentage
being .44% at that depth against 1.21% in the surface foot (see chapt.
8, p. 139). In this and similar soils the roots of hops reach down to
as much as fourteen feet without much lateral expansion; as shown in
plate No. 31 of this chapter. Similar conditions prevail in the sandy
uplands, as, _e. g._, in the wheat lands of Stanislaus county, Cal.,
mentioned above.
Taking the clay soils as a fair type for comparison, it would seem that
the farmer in the arid region owns from three to four farms, one above
another, as compared with the same acreage in the Eastern states.
_Subsoils and Deep-plowing in the Arid Region._—Up to the present time
this advantage is but little appreciated and acted upon by the farmers
of the arid region. They still instinctively cling to the practice
taught them by their fathers, and which is still promulgated as the
only correct practice, in most books on agriculture. There are of
course in the arid as well as in the humid region, cases in which deep
plowing is inadvisable; viz, that of marsh or swamp lands, as well as
sometimes in very sandy, porous soils, the cultural value of which
often depends essentially upon the presence of a somewhat consolidated,
and more retentive subsoil, which should not be broken up. But in most
soils not of extreme physical character, it is in the arid region not
only permissible, but eminently advisable to plow, for preparation, as
deeply as circumstances permit, in order to facilitate the penetration
of the roots beyond the reach of harm from the summer’s drought; while
for the same reason, subsequent cultivation should be to a moderate
depth only, for the better conservation of moisture, and the formation
of a protective surface mulch (see chapter 13).
[Illustration:
|TYPE OF EASTERN SOILS| TYPES OF CALIFORNIA SOILS.
---+---------------------+--------------------+-----------
Ft.| Upland. | Up land clay-loam. | Beach land.
---+---------------------+--------------------+-----------
FIG. 27.—Soil Profiles illustrating differences in Soils of Humid and
Arid Region.]
It must not be forgotten that there are in the lowlands of the arid
region (river swamps or tules, sea-coast marshes, etc.,) soils in which
surface soil and subsoil are differentiated as fully as in the humid
countries; at least so long as they have not been fully drained for a
considerable length of time. In swamp areas that have been elevated
above the reach of overflow or shallow bottom-water by geological
agencies, even the heavy swamp clays are fully aerated down to great
depths, and roots penetrate accordingly.
_Examples of Plant-growth on Arid Subsoils._—The fact that in the arid
region the surface-soil conditions reach to so much greater depths
than in the East and in Europe, is so important for farming practice
in that region that experimental evidence of the same should not be
withheld. Of such, some cases well established as typical of California
experience are therefore cited.
It is well known that in the Sierra Nevada of California
the placer mines of the Foothills, worked in the early
times, have long disappeared from sight, having been quickly
covered by a growth of the bull pine (_P. ponderosa_).
Much of this timber growth has for a number of years past
been of sufficient size to be used for timbering in mines,
and a second young forest is springing up on what was
originally the red earth of the placer mines, which appears
to the eye as hopelessly barren as the sands of the desert.
In this same red sandy earth not unfrequently cellars and
house foundations are dug, and the material removed, even to
the depth of eight feet, is fearlessly put on the garden and
there serves as a new soil, on which vegetables and small
fruits grow, the first year, as well as ever. In preparing
such land for irrigation by leveling or terracing no heed is
taken of the surface soil as against the subsoil, even where
the latter must be removed to the depth of several feet, so
long as a sufficient depth of soil material remains above
the bedrock.
The same is generally true of the benchlands; the irrigator
levels, slopes or terraces his land for irrigation with
no thought of discrimination between soil and subsoil,
and the cultural result as a rule justifies his apparent
carelessness. It is only where from special causes a
consolidated or hardpan subsoil is brought to the surface,
that the land when leveled shows “spotted” crops. Such
is the case in some of the “hog-wallow” areas of the San
Joaquin valley of California, and in some cases where by
long cultivation and plowing to the same depth, a compact
soil-layer or plowsole has been formed, and the land is
then leveled for the introduction of irrigation. In these
cases a section of the soil mass will usually show a marked
difference in color and texture. But, as a rule, in taking
soil samples, no noticeable difference can be perceived
between the first and the second, and oftentimes as far
down as the third and fourth foot. The extraordinary
root-penetration of trees, shrubs and taprooted herbs, whose
fibrous feeding-roots are found deep in the subsoil and
are sometimes wholly absent from the surface soil, fully
corroborate the conclusion reached by the eye. The roots
of grape vines have been found by the writer at the depth
of twenty-two feet below the surface, in a gravelly clay
loam varying but little the entire distance. In a similarly
uniform and pervious material, the loess of Nebraska,
Aughey[63] reports the roots of the native Shepherdia to have
been found at the depth of fifty feet.
[63] See Merrill, Rocks and Rock weathering.
_Resistance to Drought._—These peculiarities of the soils of the arid
region explain without any resort to violent hypotheses, the fact
that many culture plants which in the regions of summer rains are
found to be dependent upon frequent and abundant rainfall, will in
California, and in the country west of the Rocky Mountains generally,
thrive and complete their growth and fruiting during periods of four
to six months of practically absolute cessation of rainfall; when east
of the Mississippi a similar cessation for as many _weeks_ will ruin
the crops, if not kill the plants. In continental Europe, in 1892,
a six weeks’ drought caused almost all the fruit crops to drop from
the trees, and many trees failed to revive the next season; while at
the very same time, the same deciduous fruits gave a bountiful crop
in California, during the prevalence of the usual five or six months’
drought. This was without irrigation, or any aid beyond careful and
thorough surface tillage following the cessation of rains in April
or May, so as to leave the soil to the depth of five or six inches
in a condition of looseness perfectly adapted to the prevention of
evaporation from the moist subsoil, and of the conduction of the
excessive heat of the summer sun. This surface mulch will contain
practically no feeding-roots, the paralysis or death of which by heat
and drought would influence sensibly the welfare of the growing plant.
[Illustration: FIG. 28.—Root of an Eastern (Wisconsin) Fruit Tree.
(Photograph by Prof F. H. King.)]
_Root-system in the Humid Region._—It is quite otherwise where a dense
subsoil not only obstructs mechanically the deep penetration of any
but the strongest roots, but at the same time is itself too inert
to provide sufficiently abundant nourishment apart from the surface
soil, which is there the portion containing, alongside of humus, the
bulk of the available plant-food, and in which alone the processes of
absorption and nutrition find the proper conditions; such as access
of air and the ready and minute penetration of even the most delicate
rootlets and root-hairs. The largest and most active portion of the
root-system being thus accumulated in the surface soil, it follows
that unless the latter is constantly kept in a fair condition of
moistness, the plant must suffer material injury very quickly; hence
the often fatal effects of even a few weeks’ drought. The same occurs
in the arid region when often-repeated shallow plowing has resulted in
the formation of a “plowsole” which prevents the deep penetration of
roots; when a hot “norther” will often in a short time not only dry the
plowed soil, but will heat it to such extent as to actually bake the
roots it harbors. Under the same weather-conditions an adjoining field,
properly plowed, may almost wholly escape injury.
[Illustration: FIG. 29.—Prune Tree on Peach Root, at Niles, Cal.]
_Comparison of root development in the arid and humid regions._—Figures
28, 29 given here show the differences as actually seen in the case of
fruit trees as grown in Wisconsin and California, respectively, both in
the absence of artificial water-supply.
_Adaptation of humid species to arid conditions._—Figures, in No. 30,
show the root systems respectively of the riverside grape (_Vitis
riparia_) as grown in the Mississippi Valley states, and the natural
development as found in the Rock grape of Missouri and also in the wild
grape vine of California. It will be noted at once that the latter
directs its cord-like roots almost vertically from the first, until
it reaches a depth varying from 12 to 18 inches, where it begins to
branch more freely, but still with a strong downward tendency in all.
The roots of the riverside grape, on the contrary, tend to spread
almost horizontally, branching freely at the depth of a few inches and
manifestly deriving its supply both of plant-food and moisture mainly
from the surface soil. It is curious to observe the behavior of this
vine when cuttings are planted in California vineyards as a resistant
grafting-stock. Its first roots are sent out horizontally, very much as
is its habit in the East, so long as the soil moisture is maintained
near the surface. But as the season advances, the more superficial
rootlets are first thrown out of action by the advancing dryness and
heat of the surface soil, and many finally die the first year.
[Illustration: FIG. 30.—Root Growth of Resistant Grape Vines.]
Not unfrequently the entire root system developed by the uppermost bud
perishes; but usually its main roots soon begin to recede from the
threatening drought and heat of the surface, curving, or branching
downward in the direction of the moisture supply, and without detriment
to their nutrition because of the practical identity of the surface
soil and subsoil. As the portions of the roots near the surface thicken
and mature, their corky rind soon prevents their being injured by the
arid conditions to which they are subjected; while the root-ends,
finding congenial conditions of nutriment and aeration in the moist
depths, develop without difficulty as they would in their humid home.
Practically the same process of adaptation takes place in every one of
the trees, shrubs, or perennials belonging to the humid climates, until
their root system has assumed nearly the habit of the corresponding
native vegetation.
The photograph of the roots of a hop plant, grown on bench lands of the
Sacramento river, shows the roots extending to 8 feet depth, but where
broken off the main root is still nearly two millimeters in thickness,
proving that it penetrated at least two feet beyond the depth shown in
figure 31.
In the case of native annuals, either the duration of their vegetation
is extremely short, ending with or shortly after the cessation of
rains; or else their tap roots descend so low, and the nutritive
rootlets are developed at such depth, as to be beyond reach of the
summer’s heat and drought. For while it is true that rootlets immersed
in air-dry soil may absorb plant-food, this absorption is very slow
and can only be auxiliary to the main root system which, instead of
terminating in the surface soil as in the humid region, will be found
to begin to branch off at depths of 15 and 18 inches, and may then in
sandy lands descend to from 4 to 7 feet even in the case of annual
fibrous-rooted plants like wheat and barley.[64] In the case of maize
the roots of a late-planted crop may sometimes be found descending
along the walls of the sun-cracks in heavy clay land poorly cultivated;
and it frequently matures a crop without the aid of a single shower
after planting. See figures 33, 34.
[64] Shaler (Origin and Nature of Soils; 12th Rept. U. S. Geol.
Survey, p. 311) says: “Annual plants cannot in their brief period of
growth push their roots more than six to twelve inches below their
root-crowns”—a generalization measurably true for the humid region
only. According to F. J. Alway, the roots of cereals penetrate to 5-7
feet in Saskatchewan, also.
[Illustration: FIG. 31.—Hop Root from Sacramento Bench-land.]
The annexed plate (No. 32) shows the main roots of two native perennial
weeds of California, the goosefoot (_Chenopodium californicum_) and the
figwort (_Scrophularia californica_), common on the lower slopes of the
coast ranges. The soil was a heavy clay loam or “black adobe” resulting
from the weathering of the clay shale bedrock, fragments of which are
so abundantly intermixed with the substrata that excavation of the
roots became very difficult. Yet the main root of the goosefoot went
down below the depth of eleven feet.
The main root of the figwort, also, was followed below the depth of ten
feet without reaching the extreme end. This proves clearly that the
great penetration of the goosefoot was not, as might be supposed, due
to its bulbous root. Yet such thickening of the root just below the
crown is a rather common feature in arid-region plants, and can here be
noted even in the figwort, within whose botanical relationship bulbous
roots are almost unknown.
Any one accustomed to the cornfields of the Middle West, where in the
after-cultivation of maize it is necessary to restrict very carefully
the depth of tillage to avoid bringing up a mat of white, fibrous
roots, will be at once impressed with the remarkable adaptability of
maize to different climatic conditions, as exhibited in such cases and
shown in figures 33, 34. In southern California, in the deep mesa or
bench soils, corn stalks so tall that a man standing on horseback can
barely reach the tassel, and with two or three large ears, are quite
commonly grown under similar rainfall-conditions.
_Importance of proper Substrata in the Arid Region._—The paramount need
of deep penetration of roots in the arid region renders the substrata
below the range of what is usually understood by subsoil in the humid
climates, of exceptional importance. A good farmer anywhere will
examine the subsoil to the depth of two feet before investing in land;
but more than this is necessary in the arid region, where the surface
soil is often almost thrown out of action during the greater part of
the growing season, while the needful moisture and nourishment must be
wholly drawn from the subsoil and substrata; an examination of which
should therefore precede every purchase of land, or planting of crops.
[Illustration: FIG. 32.—Deep-Rooting of Native California Goosefoot and
Figwort.]
[Illustration: FIG. 33.—Kentucky Maize, grown in region of Summer
Rains. (Photography by A. M. Peter.)]
[Illustration: FIG. 34.—California Maize, Grown Without Rain or
Irrigation.]
Such examinations are most quickly made by means of a probe
consisting of a pointed, square steel rod five or six feet
long, provided at one end with a loop for the insertion of
a cross-handle like that of a carpenter’s auger. The handle
being grasped with both hands, the probe is forced into the
soil with a slight reciprocating motion, by the weight of
the operator; who soon learns how to interpret the varying
kinds of resistance, and on withdrawing the probe carefully
will generally be able to determine if bottom water has
been reached. Should this easy method of examination not
convey all the needful information, the post-hole auger may
be resorted to; and it is desirable that extra (three-foot)
rods or gaspipe joints be provided for the purpose of
lengthening the probe or auger, when necessary, to nine
or twelve feet. It will rarely be necessary to go to the
trouble of digging a pit for such examinations; but even
this is to be recommended rather than “buying a cat in a
bag” in the guise of an unexplored subsoil.
_Faulty Substrata._—A number of examples of “faulty lands,” _i. e._,
such as are underlaid by faulty substrata, are given in the annexed
diagram Fig. 35; the examples being taken from California localities
because of their having been most thoroughly investigated. Similar
cases, as well as others not here illustrated, of course occur more or
less all over the world.
No. 1 shows a case which, though at first sight an aggravated one of a
rocky substratum, is in reality that of some of the best fruit lands
in the State. The limited surface-soil is very rich, and is directly
derived (as a “sedentary” soil) from the underlying bedrock slate.
But this it will be noted stands _on edge_, and the roots of trees
and vines wedge their way along the cleavage planes of the slate to
considerable depth, deriving from them both nourishment and moisture.
Under similar conditions the California laurel, usually found on the
banks of streams, grows on the summits of rocky ridges in the Coast
Ranges.
The case of No. 2 is quite otherwise. Here the shale lies horizontally,
and though much softer than the slate of the first column, obstinately
resists the penetration of roots; so that the land, though fairly
provided with plant-food, is almost wholly useless for cultivation. It
is naturally covered with low, stunted shrubs or chaparral; only here
and there, where a cleft has been caused by earthquakes or subsidence,
a large pine tree indicates that nourishment and moisture exists within
the refractory clay stratum, and suggests blasting as a means of
rendering the land fit for trees at least.
[Illustration: FAULTY LANDS. CALIFORNIA
FIG. 35.—Faulty Lands, California.]
No. 3 is a case similar to that of No. 2, only there is here a dense
unstratified mass of red clay, of good native fertility. It is here
that the expedient of blasting the tree holes with dynamite was first
successfully employed, in central California. For lack of this,
extensive tracts of similar land in southern California, planted to
orchards, have completely failed of useful results after three years of
culture.
No. 4 shows a typical case of calcareous hardpan obstructing the
penetration of roots, even though usually interrupted at intervals,
because of the formation occurring mostly in swales, along which
the sheets lie more or less continuously. Here also, blasting will
generally permit the successful growth of trees and vines, whose roots
frequently will, in time, wholly disintegrate the hardpan and thus
render the land fit for field cultures. The depth at which such hardpan
is formed usually depends upon the depth to which the annual rainfall
penetrates. (See below, page 183).
Nos. 4, 5, and 6 all illustrate cases of intrinsically fertile, very
deep soils, shallowed by obstructions which in the case of No. 4 are
hardpan sheets, while in No. 5 the intervention of bottom water limits
root penetration, hence restricts the use of the land to relatively
shallow-rooted crops, and the use of only a few feet of the profusely
fertile soil. Such is the case where bottom water has been allowed to
rise too high, through the use of leaky irrigation ditches.
No. 6 illustrates a case not uncommon in sedimentary lands, where
bottom water is quite within reach of most plants, but is prevented
from being utilized by the intervention of layers of coarse sand or
gravel, through which the water will not rise; and the roots, while
they would be able to penetrate, are not near enough to feel the
presence of water underneath and therefore spread on the surface of
the gravel, suffering from drought within easy reach of abundance of
water. The “going-back” of large portions of orange orchards in the
San Bernardino Valley of California has been thus brought about; and
unfortunately this state of things is almost beyond the possibility of
remedy.
[Illustration: FIG. 36.—Almond Tree on Hardpan. Paso Robles Substation,
Cal.]
_Injury from Impervious Substrata._—The injurious effects of a
difficultly penetrable subsoil have already been discussed and are
self-evident. When the substratum is a dense clay, the rise of moisture
from below being very slow, it can easily happen that the roots cannot
penetrate deep enough in time for the coming of the dry season, and
that thus the crop will suffer. The case will be still worse when
hardpan cemented by lime or silex limits root-penetration, as well as
proper drainage. In such cases the culture of field crops often becomes
impracticable, even with irrigation, as its frequent repetition,
besides being costly, can rarely be commanded. In the case of trees,
the limitation of root-penetration results in the spreading-out of the
roots on the surface of the impenetrable layer; as shown in figure 36,
which exhibits a root-development that would be quite normal in the
regions of summer rains, but is wholly abnormal in the arid region, and
results in the unprofitableness or death of the trees. It has often
been attempted in such cases to plant trees in large holes dug deep
into the subsoil and refilled with surface earth and manure. All such
attempts result in failure, if only because the excavation inevitably
fills with water, which will soak away but very slowly into the dense
substrata, and will thus injure or drown out the roots. Besides, the
latter will remain bunched in the loose earth, and will thus be unable
to draw either moisture or nourishment from the surrounding land. It
is absolutely necessary to remedy this by loosening the substrata, if
success is to be attained.
_Shattering of Dense Substrata by Dynamite._—The permanent loosening
of dense substrata is best accomplished by moderate charges (½ to
¾ pounds) of “No. 2” dynamite at a sufficient depth (3 to 5 feet).
The shattering effect of the explosive will be sensible to the depth
of eight feet or more, and will fissure the clay or hardpan to a
corresponding extent sidewise. If properly proportioned the charge
will hardly disturb the surface; but if this be desired, from 1½ to
2½ pounds of black powder placed above the dynamite will throw out
sufficient earth to plant the tree without farther digging. Where labor
is high-priced this proves the cheapest as well as the best way to
prepare such ground for tree planting; and it has often been found that
in the course of time, the loosening begun by the powder has extended
through the mass of the land so as to permit the roots to utilize it
fully, and even to permit, in after years, of the planting of field
crops where formerly they would not succeed.
_Leachy Substrata._—While we may thus overcome the disadvantages of a
dense subsoil or hardpan, there is another difficulty not uncommonly
met with in alluvial lands, which cannot be so readily remedied. It
is the occurrence, at from two to six feet depth, of coarse sand or
gravel, through which capillary moisture will not ascend, but through
which irrigation water will waste rapidly, leaving the overlying soil
dry. Then unless very frequent irrigation can be given, the crop will
suffer from drought, unless indeed the gravel itself is filled with
bottom water upon which the root-ends can draw.
This case is a common one in the larger valleys of the arid region, and
in time of unusual drought the sloughs originally existing, but since
filled up, will be clearly outlined by the dying crops, while outside
of the old channels there may be no suffering.
_“Going-back” of Orchards._ On such land as this, and on such as has a
shallow soil underlaid by an impervious subsoil, trees will often grow
finely for three to five years; then suddenly languish, or turn yellow
and die, as the demand of their larger growth exceeds what moisture or
plant-food the shallow soil and subsoil can supply. Enormous losses
have arisen from this cause in many portions of the arid region, but
more especially in California, owing to the implicit confidence reposed
even by old settlers, and still more by newcomers, in the excellence
of the lands, as illustrated by farms perhaps a short distance away,
but differently situated with respect to the country drainage and the
geological formations. All such disappointments could have been avoided
by an intelligent observation of the substrata, either by probing or
digging. Important as is such preliminary examination in the region of
summer rains, it is a _vitally_ needful precaution in the arid region,
where the margin between adequate and inadequate depth of soil and
moisture-supply is much smaller.
When farmers note such distress in the orchard, the first
idea usually is that fertilization is needed. This in the
almost universally very rich lands of the arid region
is rarely the case until after many years of exhaustive
cultivation, and is scarcely ever of more than passing
benefit in such cases. The first suggestion should always
be an _examination of the substrata_, and especially
of the deeper roots; in the diseased or thirsty condition
of which the cause of the “die-back” or yellowing will
commonly be found. Of course no amount of fertilization can
permanently remedy such a state of things, arising from
impervious substrata, coarse gravel, or shallow bottom water.
_Hardpan._—By “hardpan” is understood a dense and more or less hardened
layer in the subsoil, which obstructs the penetration of both roots
and water, thus materially limiting the range of the former both
for plant-food and moisture, and giving rise to the disadvantages
following such limitation, as described in the case of dense subsoils.
The hardpans proper differ from the latter, however, in being usually
of limited thickness only; the direct consequence of their mode of
formation, which is not direct deposition by water or other agencies,
but the infiltration of cementing solutions into a pre-existing
material originally quite similar to that of the surface soil. Such
solutions usually come from above, more rarely from below, and are of
very various composition. The solutions of lime carbonate in carbonated
water have already been referred to in this connection; as has also the
fact that corresponding solutions of silica, associated more or less
with other products of rock decomposition (see chapters 2 and 4) are
constantly circulating in soils. The surface soil being the portion
where rock-weathering and other soil-forming processes are most active,
these solutions are chiefly formed there; and according as their
descent into the substrata is unchecked, or is liable to be arrested
at some particular level, whether by pre-existing close-grained layers
or by the cessation of rains, the subsequent penetration of air, and
evaporation of the water alone by shallow-rooted plants, may cause the
accumulation of the dissolved matter at a certain level, year after
year. Finally there is formed a subsoil-mass more or less firmly
cemented by the dissolved matters, sometimes to the extent of stony
hardness (lime carbonate in the arid regions, kankar of India), more
usually soft enough to be penetrated by the pick or grubbing hoe, and
sometimes by the stronger roots of certain plants; but resisting both
the penetration and the assimilation of plant food by the more delicate
feeding roots.
_Nature of the Cements._—The nature of the cements that serve to
consolidate the hardpan mass is substantially the same as those already
mentioned in the discussion of sandstones (chapt. 4, p. 55); with
the addition of those formed, usually in connection with siliceous
solutions, by the acids of the humus group. The latter class of
hardpans is especially conspicuous in the case of swampy ground and
damp forests, where “moorbedpan” and reddish “ortstein” (the latter
particularly developed in the forests of northern Europe, where it has
been studied in detail by Müller and Tuxen[65], are characteristic.)
The latter gives for a characteristic sample of the reddish hardpan
underlying a beech forest in Denmark a content of from 2.20 to 4.40%
of ulmic compounds, and shows that the color is due to these and not,
as had been supposed, to ferric oxid, which is present only in minute
quantities.
[65] See “Studien über die natürlichen Humusformen,” by Dr. P. E.
Müller.
_Bog ore, Moorbedpan, Ortstein._—It is otherwise with moorbedpan, which
often consists of a mass of bog iron ore permeated more or less with
humous substances, which impart to it the dark brown tint so often seen
also in the “black gravel” spots of badly-drained land. On the whole,
however, ferric cements are much less frequently found in hardpans than
in sandstones formed above ground.
_Clay substance_ washed from the surface into the subsoil by rains
(chapter 10, p. 161) always helps materially to render the hardpan
impervious when afterwards cemented, a much smaller proportion of
the cementing material sufficing in that case to form a solid layer.
In such cases however the cement is rarely of a calcareous nature,
since lime prevents the diffusion and washing-down of the clay. It is
mostly siliceous or zeolitic; if the former, acid will have little or
no effect upon the solidity of the hardpan; while if zeolitic, acid
will pretty promptly disintegrate it. The presence of humus acids in
the cements, if not apparent to the eye, is readily demonstrated by
immersing the hardpan fragment in ammonia water or a weak solution of
caustic soda; when if humus acids are the main cementing substance the
fragment will fall to crumbs, or be softened to an extent corresponding
to the amount of the humus present. Calcareous hardpan is, of course,
readily recognized by its quick disintegration by dilute acid, with
evolution of carbonic gas.
In “alkali” soils containing sodic carbonate (“black alkali”) there
is commonly found at the depth of two or three feet an exceedingly
refractory hardpan resulting from the accumulation of puddled clay (see
above chapt. 4, p. 62) in the subsoil, or sometimes even on the surface
of depressed spots. This hardpan, easily destroyed by the use of gypsum
and water, is described more in detail in chapter 22, on alkali soils;
it blues red litmus paper instantly.
_The Causes of Hardpan._—The recognition of the _cause_ of hardpan is
of considerable importance to the farmer, because of the influence
of the nature of the cement and the causes of its formation upon the
possibility and methods of its destruction, for the improvement of the
land.
It may be said in general that inasmuch as the cause of the formation
of hardpan is a stoppage of the water in its downward penetration, the
re-establishment of that penetration will tend to prevent additional
induration; moreover, experience proves that whenever this is
accomplished even locally, as around a fruit tree in an orchard, the
hardpan gradually softens and disappears before the frequent changes in
moisture-conditions and the attack of roots. The use of dynamite for
this purpose in California has already been referred to; it seems to
be the only resort when the hardpan lies at a considerable depth. When
it is within reach of the plow, it may be turned up on the surface by
the aid of a subsoiler and will then gradually disintegrate under the
influence of air, rain and sun. But when the hardpan is of the nature
of moorbedpan, containing much humic acid and perhaps underlaid by bog
iron ore, the use of lime on the land is indicated, and will in the
course of time destroy the hardpan layer. This is the more desirable
as in such cases the surface soil is usually completely leached of its
lime content, and is consequently extremely unthrifty.
Woodlands of northern countries bearing beech and oak are especially
apt to be benefited by the action of lime on the “raw,” acid humous
soil and underlying hardpan, which is commonly underlaid by a
leaden-blue sandy subsoil (“Bleisand” of the Germans, “Podzol” of
the Russians) colored brown by earth humates and mostly too moist in
its natural condition to permit of adequate aeration. These soils
are usually of but moderate fertility, and are best suited to forest
growth unless somewhat expensive methods of improvement can be put into
practice.
“_Plowsole._”—An artificial hardpan is very commonly formed under the
practice of plowing to the same depth for many consecutive years. The
consolidated layer thus created by the action of the plow (hence known
as plowsole) acts precisely like a natural hardpan, and is sometimes
the cause of the formation of a cemented subsoil crust simulating
the natural product. This is most apt to occur in clayey lands, and
greatly increases the difficulty of working them, while detracting
materially from the higher productiveness commonly attributed to them
as compared with sandy lands. Of course it is perfectly easy to prevent
this trouble by plowing to different depths in consecutive years, and
running a subsoil plow from time to time. In this case, also, lime
will generally be very useful and be found to aid materially in the
disintegration of the “plowsole.”
It is hardly necessary to insist farther upon the need of the
examination of land to be occupied, for the existence of hardpan or
other faulty subsoil, which may totally defeat for the time being the
farmer’s efforts, or make him lose his investment in plantations after
a few years. Probing by means of the steel rod described above (p. 177)
or boring with a post-hole auger; or finally, if necessary, digging a
pit to the proper depth (from four to six feet in the arid region),
should precede every purchase of new or unexplored agricultural land.
_Marly Substrata._—Among the causes of failure occasionally found in
the case of the “going-back” of orchards, is the occurrence of strongly
calcareous or marly substrata, at depths which in the humid region
would not be reached by the roots, but in the course of a few years are
inevitably penetrated by the roots of trees in the arid region. Then
there appears a stunting of the growth, and sometimes a yellowing of
leaves, or chlorosis, due to the influence of excessive calcareousness
at the depth of four or five feet. For this of course there is no
remedy except the planting of crops which, like the mulberry, Texas
grapes, Chickasaw plum and others, are at home on such lands; which in
the Eastern states are naturally occupied by the crab apple, honey
locust and wild plums.