THE IMPROVEMENT OF THE SOIL BY MECHANICAL PROCESSES.
Comparatively few uncultivated soils possess the physical properties or
chemical composition required for the production of the most abundant
crops. Either one or more of the substances essential to the growth of
plants are absent, or, if present, they are deficient in quantity, or
exist in some state in which they cannot be absorbed. Such defects,
whether mechanical or chemical, admit of diminution, or even entire
removal, by certain methods of treatment, the adaptation of which to
particular cases is necessarily one of the most important branches of
agricultural practice, as the elucidation of their mode of action is of
its theory. The observations already made with regard to the characters
of fertile soils must have prepared the reader for the statement that
these defects may be removed, either by mechanical or chemical
processes. The former method of improvement may at first sight appear to
fall more strictly under the head of practical agriculture, of which the
mechanical treatment of the soil forms so important a part, and that
their improvement by chemical means should form the sole subject of our
consideration in a treatise on agricultural chemistry. But the line of
demarcation between the mechanical and the chemical, which seems so
marked, disappears on more minute observation, and we find that the
mechanical methods of improvement are frequently dependent on chemical
principles; and those which, at first sight, appear to be entirely
chemical, are also in reality partly mechanical. It will be necessary
for us, therefore, to consider shortly the mechanical methods of
improving the soil.
_Draining._--By far the most important method of mechanically improving
the soil is by draining--a practice the beneficial action of which is
dependent on a great variety of circumstances. It is unnecessary to
insist on the advantage derived from the rapid removal of moisture,
which enables the soil to be worked at times when this used to be almost
impossible, and other direct practical benefits. Of its more strictly
chemical effects, the most important is probably that which it produces
on the temperature of the soil. It has been already remarked that the
germination of a seed is dependent on the soil in which it is sown
acquiring a certain temperature, and the rapidity of the after-growth of
the plant is, in part at least, dependent on the same circumstance. The
necessary temperature is speedily attained by the heating action of the
sun's rays, when the soil is dry; but when it is wet, the heat is
expended in evaporating the moisture with which it is saturated; and it
is only after this has been effected that it acquires a sufficiently
high temperature to produce the rapid growth of the seeds committed to
it.
The extent to which this effect occurs may be best illustrated by
reference to some experiments made by Schuebler, in which he determined
the temperature attained by different soils, in the wet and dry state,
when exposed to the sun's rays, from 11 till 3 o'clock, in the latter
part of August, when the temperature in the shade varied from 73 deg. to
77 deg..
+-----------------------+-------+-------+
| Description of Soil. | Wet. | Dry. |
+-----------------------+-------+-------+
| | Degs. | Degs. |
| Siliceous sand | 99.1 | 112.6 |
| Calcareous sand | 99.3 | 112.1 |
| Sandy clay | 98.2 | 111.4 |
| Loamy clay | 99.1 | 112.1 |
| Stiff clay | 99.3 | 112.3 |
| Fine bluish-grey clay | 99.5 | 113.0 |
| Garden mould | 99.5 | 113.5 |
| Arable soil | 97.7 | 111.7 |
| Slaty marl | 101.8 | 115.3 |
+-----------------------+-------+-------+
In a soil which is naturally dry or has been drained, the superfluous
moisture escapes by the drains, and only that comparatively small
quantity which is retained by capillary attraction is evaporated, and
hence the soil is more frequently and for a longer period in a condition
to take advantage of the heating effect of the sun's rays, and in this
way the period of germination, and, by consequence also, that of
ripening is advanced. The extent of this influence is necessarily
variable, but it is generally considerable, and in some districts of
Scotland the extensive introduction of draining has made the harvest, on
the average of years, from ten to fourteen days earlier than it was
before. It is unnecessary to insist on the importance of such a change,
which in upland districts may make cultivation successful when it was
previously almost impossible. The removal of moisture by drainage
affects the physical characters of the soil in another manner; it makes
it lighter, more friable, and more easily worked; and this change is
occasioned by the downward flow of the water carrying with it to the
lower part of the soil the finer argillaceous particles, leaving the
coarser and sandy matters above, and in this way a marked improvement is
produced on heavy and retentive clays. The access of air to the soil is
also greatly promoted by draining. In wet soils the pores are filled
with water, and hence the air, which is so important an agent in their
amelioration, is excluded; but so soon as this is removed, the air is
enabled to reach and act upon the organic matters and other decomposable
constituents present. In this way also provision is made for the
frequent change of the air which permeates the soil; for every shower
that falls expels from it a quantity of that which it contains, and as
the moisture flows off by the drains, a new supply enters to take its
place, and thus the important changes which the atmospheric oxygen
produces on the soil are promoted in a high degree. The air which thus
enters acts on the organic matters of the soil, producing carbonic acid,
which we have already seen is so intimately connected with many of its
chemical changes. In its absence the organic matters undergo different
decompositions, and pass into states in which they are slowly acted on,
and are incapable of supplying a sufficient quantity of carbonic acid to
the soil; and they thus exercise an action on the peroxide of iron,
contained in all soils, reduce it to the state of protoxide, or, with
the simultaneous reduction of the sulphuric acid, they produce sulphuret
of iron, forms of combination which are well known to be most injurious
to vegetation.
The removal of water from the lower part of the soil, and the admission
of air, which is the consequence of draining, submits that part of it to
the same changes which take place in its upper portion, and has the
effect of practically deepening the soil to the extent to which it is
thus laid dry. The roots of the plants growing on the soil, which stop
as soon as they reach the moist part, now descend to a lower level, and
derive from that part of it supplies of nourishment formerly
unavailable. The deepening of the soil has further the effect of making
the plants which grow upon it less liable to be burned up in seasons of
drought, a somewhat unexpected result of making a soil drier, but which
manifestly depends on its permitting the roots to penetrate to a greater
depth, and so to get beyond the surface portion, which is rapidly dried
up, and to which they were formerly confined.
It may be added also that the abundant escape of water from the drains
acts chemically by removing any noxious matters the soil may contain,
and by diminishing the amount of soluble saline matters, which sometimes
produce injurious effects. It thus prevents the saline incrustation
frequently seen in dry seasons on soils which are naturally wet, and
which is produced by the water rising to the surface by capillary
attraction, and, as it evaporates, depositing the soluble substances it
contained, as a hard crust which prevents the access of air to the
interior of the soil.
It is thus obvious that the drainage of the soil modifies its properties
both mechanically and chemically. It exerts also various other actions
in particular cases which we cannot here stop to particularize. It
ameliorates the climate of districts in which it is extensively carried
out, and even affects the health of the population in a favourable
manner. The sum of its effects must necessarily differ greatly in
different soils, and in different districts; but a competent
authority[J] has estimated, that, on the average, land which has been
drained produces a quarter more grain per acre than that which is
undrained. But this by no means exhausts the benefits derived from it,
draining being merely the precursor of further improvement. It is only
after it has been carried out that the farmer derives the full benefit
of the manures which he applies. He gains also by the increased facility
of working the soil, and by the rapidity with which it dries after
continued rain, thus enabling him to proceed at their proper season with
agricultural operations, which would otherwise have to be postponed for
a considerable time.
It would be out of place to enlarge here upon the mode in which draining
ought to be carried out; it may be remarked, however, that much
inconvenience and loss has occasionally been produced by too close
adherence to particular systems. No rules can be laid down as to the
depth or distance between the drains which can be universally
applicable, but the intelligent drainer will seek to modify his practice
according to the circumstances of the case. As a general rule, the
drains ought to be as deep as possible, but in numerous instances it may
be more advantageous to curtail their depth and increase their number.
If, for instance, a thick impervious pan resting on a clay were found at
the depth of three feet below the surface, it would serve no good
purpose to make the drains deeper; but if the pan were thin, and the
subjacent layer readily permeable by water, it might be advantageous to
go down to the depth of four feet, trusting to the possible action of
the air which would thus be admitted, gradually to disintegrate the pan,
and increase the depth of soil above it. It is a common opinion that if
we reach, at a moderate depth, a tenacious and little permeable clay, no
advantage is obtained by sinking the drains into it; but this is an
opinion which should be adopted with caution, both because no clay is
absolutely impermeable, even the most tenacious permitting to a certain
extent the passage of water, and because the clay may have been brought
down by water from the upper part of the soil, and may have stopped
there merely for want of some deeper escape for the water, and which
drains at a lower level might supply. In some cases it may even be
advisable to vary the depth of the drains in different parts of the same
field, and the judicious drainer may sometimes save a considerable sum
by a careful observation of the peculiarities of the different parts of
the ground to be drained.
_Subsoil and Deep Ploughing._--It frequently happens, when a soil is
drained, that the subsoil is so stiff as to permit the passage of water
imperfectly, and to prevent the tender roots of the plant from
penetrating it, and reaching the new supplies of nourishment which are
laid open to them. In such cases the benefits of subsoil ploughing and
deep ploughing are conspicuous. The mode of action of these two methods
of treatment is similar but not identical. The subsoil plough merely
stirs and opens the subsoil, and permits the more ready passage of water
and the access of air and of the roots of plants--the former to effect
the necessary decompositions, the latter to avail themselves of the
valuable matters set free. But deep ploughing produces more extensive
changes; it raises new soil to the surface, mixes it with the original
soil, and thus not only brings up fresh supplies of valuable matters to
it, but frequently changes its chemical and mechanical characters,
rendering a heavy soil lighter by the admixture of a light subsoil, and
_vice versa_. Both are operations which are useless unless they are
combined with draining, for it must manifestly serve no good purpose to
attempt to open up a soil unless the water which lies in it be
previously removed. In fact, subsoiling is useless unless the subsoil
has been made thoroughly dry; and it has been found by experience that
no good effects are obtained if it be attempted immediately after
draining, but that a sufficient time must elapse, in order to permit the
escape of the accumulated moisture, which often takes place very slowly.
Without this precaution, the subsoil, after being opened by the plough,
soon sinks together, and the good effects anticipated are not realized.
The necessity for allowing some time to elapse between draining and
further operations is still more apparent in deep ploughing, when the
soil is actually brought to the surface. In that case it requires to be
left for a longer period after draining, in order that the air may
produce the necessary changes on the subsoil; for if it be brought up
after having been for a long time saturated with moisture, and
containing its iron as protoxide, and the organic matter in a state in
which it is not readily acted upon by the air, the immediate effect of
the operation is frequently injurious in place of being advantageous.
One of the best methods of treating a soil in this way is to make the
operation a gradual one, and by deepening an inch or two every year
gradually to mix the soil and subsoil; as in this way from a small
quantity being brought up at a time no injurious effects are produced.
Deep ploughing may be said to act in two ways, _firstly_, by again
bringing to the surface the manures which have a tendency to sink to the
lower part of the soil, and, _secondly_, by bringing up a soil which has
not been exhausted by previous cropping--in fact a virgin soil.
The success which attends the operation of subsoiling or deep ploughing
must manifestly be greatly dependent on the character of the subsoil,
and good effects can only be obtained when its chemical composition is
such as to supply in increased quantity the essential constituents of
the plant; and it is no doubt owing to this that the opinions
entertained by practical men, each of whom speaks from the results of
his own experience, are so varied. The effects produced by deep
ploughing on the estates of the Marquis of Tweeddale, are familiarly
known to most Scottish agriculturists, and they are at once explained by
the analyses of the soil and subsoil here given, which show that the
latter, though poor in some important constituents, contains more than
twice as much potash as the soil.
Soil. Subsoil.
Insoluble silicates 87.623 82.72
Soluble silica 0.393 0.12
Alumina and peroxide of iron 4.129 8.60
Lime 0.341 0.18
Magnesia 0.290 0.24
Sulphuric acid 0.027 0.03
Phosphoric acid 0.240 trace
Potash 0.052 0.12
Soda 0.050 0.04
Water 1.956 3.26
Organic matter 5.220 4.02
------ -----
100.321 99.33
In addition to the difference in the amount of potash, something is
probably due to the large proportion of alumina and oxide of iron in the
subsoil, which for this reason must be more tenacious than the soil
itself, which appears to be rather light. In other instances, the use of
the subsoil plough has occasioned much disappointment, and has led to
its being decried by many practical men; but of late years its use
having become better understood, its merits are more generally
admitted. We believe, that in all cases in which the soil is deep, more
or less marked good effects must be produced by its use, but of course
there must be cases in which, from the defective composition of the
subsoil or other causes, it must fail. It may sometimes be possible _a
priori_ to detect these cases, but in a large majority of them our
knowledge is still too limited to admit of satisfactory conclusions
being arrived at.
_Improving the Soil by Paring and Burning._--It has long been familiarly
known, that a decided improvement has been produced on some soils by
burning. Its advantages have chiefly been observed on two sorts, heavy
clays and peat soils, on both of which it has been practised to a great
extent. The action of heat on the heavy clays appears to be of a twofold
character, depending partly on the change effected in its physical
properties, and partly on a chemical decomposition produced by the heat.
The operation of burning is effected by mixing the clay with brushwood
and vegetable refuse, and allowing it to smoulder in small heaps for
some time. It is a process of some nicety, and its success is greatly
dependent on the care which has been taken to keep the temperature as
low as possible during the whole course of the burning.
Experience has shown that burning is by no means equally advantageous to
all clays, but is most beneficial on those containing a considerable
quantity of calcareous matter, and of silicates of potash. In such clays
heat operates by causing the lime to decompose the alkaline silicates,
and liberate a quantity of the potash which was previously in an
unavailable state. Its effect may be best illustrated by the following
analyses by Dr. Voelcker of a soil, and the red ash produced in burning
it.
Soil. Red Ash.
Water 0.93 1.18
Organic matter 10.67 3.32
Oxides of iron and alumina 13.40 18.42
Carbonate of lime 23.90 8.83
Sulphate of lime trace 1.15
Carbonate of magnesia 1.10 "
Magnesia " 1.76
Phosphoric acid trace 0.71
Potash 0.38 1.08
Soda 0.13 "
Chloride of sodium " 1.03
Insoluble matter, chiefly clay 49.66 62.52
----- -----
100.17 100.00
In this instance the quantity of burned soil amounted to about fifteen
tons per acre, and it is obvious that the quantity of potash which had
been liberated from the insoluble clay and the phosphoric acid are equal
to that contained in a considerable manuring. In order to obtain these
results, it is necessary, as has been already observed, to keep the
temperature as low as possible during the process of burning, direct
experiment having shown that when this precaution is not observed
another change occurs, whereby the potash, which at low temperatures
becomes soluble, passes again into an insoluble state. A part of the
beneficial effect is no doubt also due to the change produced in the
physical characters of the clay by burning, which makes it lighter and
more friable, and by mixture with the unburnt clay ameliorates the
whole. This improvement in the physical characters of the clay also
requires that it shall be burnt with as low a heat as possible; for if
it rises too high, the clay coheres into hard masses which cannot again
be reduced to powder, and the success of the operation of burning may
always be judged of by the readiness with which it falls into a uniform
friable powder.
The improvement of peat by burning has been practised to some extent in
Scotland, though less frequently of late years than formerly; but it is
still the principal method of reclaiming peat soils in many countries,
and particularly in Finland, where large breadths of land have been
brought into profitable cultivation by means of it. The _modus operandi_
of burning peat is very simple; it acts by diminishing the superabundant
quantity of humus or other organic matters, which, in the previous
section we have seen to be so injurious to the fertility of the soil. It
_may_ act also in the same way as it does on clay, by making part of the
inorganic constituents more really soluble, although it is not probable
that its effect in this way can be very marked. Its chief action is
certainly by destroying the organic matters, and by thus improving the
physical character of the peat, and causing it to absorb and retain a
smaller quantity of water than it naturally does. For this reason it is
that it proves successful only on thin peat bogs, for if they be deep,
the inorganic matters soon sink into the lower part, and the surface
relapses into its old state of infertility. It is probably for this
reason that the practice has been so much abandoned in Scotland, more
especially as other and more economical modes of treating peat soils
have come into use.
_Warping._--This name has been given to a method of improving soils by
causing the water of rivers to deposit the mud it carries in suspension
upon them, and which has been largely practised in the low lying lands
of Lincoln and Yorkshire, where it was introduced about a century ago.
It is most beneficial on sandy or peaty soil, and by its means large
tracts of worthless land have been brought under profitable cultivation.
It requires that the land to be so treated shall be under the level of
the river at full tide, and it is managed by providing a sluice through
which the river water is allowed to flood the land at high tide, and
again to escape at ebb, leaving a layer of mud generally about a tenth
of an inch in thickness, which it brought along with it. By the
repetition of this process, a layer of several feet in thickness, of an
excellent soil, is accumulated on the surface. Herapath, who has
carefully examined this subject chemically, has shown that in one
experiment where the water used contained 233 grains of mud per gallon,
210 were deposited during the warping. The following analyses will show
the general nature of the matters deposited, and the change which they
produce on the soil:--No. 1 is the mud from the Humber in its natural
state, No. 2 a specimen of warp of average quality artificially dried,
No. 3 a sandy soil before warping, and No. 4, the same fifteen years
after having received a coating of 11 inches of mud.
+---------------------------------+---------+--------+---------+--------+
| | 1 | 2 | 3 | 4 |
| +---------+--------+---------+--------+
| Water | 47.49 | ... | 1.06 | 2.00 |
| Organic matter | 5.94 | 6.93 | 2.20 | 7.61 |
| Chloride of Calcium | ... | ... | } ... | ... |
| Magnesium | } ... | 0.10 | } ... | ... |
| Sodium | } 1.66 | } 0.94 | } 0.14 | 0.16 |
| Potassium | } | } | } | |
| Sulphate of Soda | } ... | 0.31 | } ... | ... |
| Magnesia | } ... | 1.18 | } ... | ... |
| Lime | trace | 1.10 | trace | trace |
| Carbonate of Magnesia | 2.60 | 0.31 | trace | 0.29 |
| Lime | 3.59 | 8.18 | trace | 0.46 |
| Potash and Soda | 0.18 | 0.47 | trace | 0.17 |
| Magnesia | 1.69 | 2.60 | trace | 0.26 |
| Lime | 0.39 | 0.68 | trace | 0.14 |
| Peroxide of Iron | } 6.63 | 5.05 | 0.08 | 1.17 |
| Alumina | } | 8.18 | 0.39 | 0.41 |
| Phosphate of Iron | 0.58 | 1.04 | trace | 0.28 |
| Silica | ... | 9.05 | 0.14 | 2.77 |
| Sand and Stones | 29.15 | 55.87 | 95.91 | 84.97 |
| |---------|--------|---------|--------|
| | 100.00 | 100.00 | 100.00 | 100.00 |
+---------------------------------+---------+--------+---------+--------+
It is easy to understand the importance of the effects produced by
adding to any soil large quantities of a mud containing upwards of one
per cent of phosphate of iron; and in point of fact, Herapath has
calculated that in one particular instance the quantity of phosphoric
acid brought by warping upon an acre of land, exceeded seven tons per
acre. As, moreover, the matters are all in a high state of division,
they must exist in a condition peculiarly favourable to the plant. The
overflow of the Nile is only an instance of warping on the large scale,
with this difference, that it is repeated once only in every year,
whereas, in this country, the operation is repeated at every tide until
a deposit sometimes of several feet in thickness is obtained, after
which it is stopped, and the soil brought under ordinary cultivation.
An operation which is, in some respects, the converse of warping, has
been carried out on Blair-Drummond Moss, where the peat has been
dislodged and carried off by the action of water, leaving the subjacent
soil in a state fitted for cropping. Of course both this and warping are
restricted to special localities, but they are most important means of
ameliorating the soil when circumstances admit of their being carried
out.
_Mixing of Soils._--When soils possess conspicuous defects in their
physical, and even in their chemical properties, great advantages may,
in some instances, be derived from their proper admixture. A light sandy
soil, for instance, is greatly improved by the addition of clay, and
_vice versa_; so that, when two soils of opposite properties occur near
to one another, both may be improved by mixture. It has been applied to
the improvement of heavy clay soil and of peat, the former being mixed
with sand or marl so as to diminish its tenacity; the latter with clay
or gravel to add to its inorganic matters, and in both instances it has
proved successful.
The process of chalking, which has been carried out on a large scale in
some parts of England, and which consists in bringing up the chalk from
pits, penetrating through the overlying tenacious clay, and mixing it
with the soil, operates, to some extent, in a similar manner, though no
doubt the lime also exercises a strictly chemical action. It is probable
that the mixing of soils might be advantageously extended, and it merits
more minute study than it has yet obtained. Its use is obviously limited
by the expense, because, of course, where good effects are to be
obtained, it is necessary to remove large quantities of soil, in some
instances as much as 50 or 100 tons per acre, but the expense might be
much diminished if it were carried out methodically, and on a
considerable scale. The admixture of highly fertile soils with others of
inferior quality is also worthy of attention; indeed, it is understood
that this has been done, to some extent, with the rich trap soils of
some parts of Scotland, but the extent of the benefit derived from it
has not been made public.
FOOTNOTES:
[Footnote J: Mr. Dudgeon, Spylaw. Transactions of the Highland Society,
vol. cxxix., p. 505.]