Of the Analysis of Soils, and of the agricultural relations
between soils and plants.
We have seen that the earths have a threefold capacity; that they
receive and lodge the roots of plants and support their stems; that
they absorb and hold air, water and mucilage--aliments necessary to
vegetable life; and that they even yield a portion of themselves to
these aliments. But we have also seen, that they are not equally
adapted to these offices; that their parts, texture and qualities
are different; that they are cold or warm, wet or dry, porous or
compact, barren or productive, in proportion as one or other may
predominate in the soil; and that to fit them for discharging the
various functions to which they are destined, each must contribute
its share, and all be minutely divided and intimately mixed. In this
great work nature has performed her part, but as is usual with her,
she has wisely and benevolently left something for man to do.
This necessary march of human industry, obviously begins by
ascertaining the _nature of the soil_. But neither the touch, nor
the eye, however practised or acute, can in all cases determine
this. _Clay_, when wet, is cold and tenacious--a description that
belongs also to magnesian earths: _sand_ and _gravel_ are hard
and granular; but so also are some of the modifications of lime:
_vegetable mould_ is black and friable, but not exclusively so; for
schistous and carbonaceous earths have the same properties.
It is here, then, that chemistry offers herself to obviate
difficulties, and remove doubts; but neither the apparatus nor
process of this science, are within the reach of all who are
interested in the inquiry, and we accordingly subjoin a method,
less comprehensive, but more simple and sufficiently exact, for
agricultural purposes, and which calls only for two vases, a pair of
scales, clean water and a little sulphuric acid.
"1st. Take a small quantity of earth from different parts of the
field, the soil of which you wish to ascertain, mix them well
together and weigh them; put them in an oven, heated for baking
bread, and after they are dried, weigh them again; the difference
will show the _absorbent power of the earth_. When the loss of
weight in 400 grains, amounts to 50, this power is great, and
indicates the presence of much animal or vegetable matter; but when
it does not exceed twenty, the absorbent power is small, and the
vegetable matter deficient.[2]
[2] See Davy's Elements.
"2d. Put the dried mass into a vase with one fourth of its own
weight of clear water; mix them well together: pour off the dirty
water into a second vase, and pour on as much clean water as before;
stir the contents, and continue this process until the water poured
off, is as clear as that poured on the earth. What remains in the
first employed vase is _sand_, _silicious_ or _calcareous_.
"3d. The dirty water, collected in the second vase, will form a
deposit, which (after pouring off the wa-ter) must be dried,
weighed and _calcined_. On weighing it _after_ this process, the
quantity lost will show the portion of _animal_ and _vegetable mould
contained in the soil_; and,
"4th. This calcined matter must then be carefully pulverised and
weighed, as also the first deposit of sand, but without mixing
them. To these, apply (separately) sulphuric acid, and what they
respectively lose in weight, is the portion of _calcareous_ or
_aluminous earths_ contained in them. These last may be separated
from the mass by soap lie, which dissolves them."[3]
[3] This manner of analysing soils is that described by M. Rose,
member of the institute of France, &c. and recommended to French
agriculturists.
Here is the light we wanted. In knowing the disease, we find the
cure. Clay and sand qualify each other; either of these will
correct an excess of lime; and magnesian earth, when saturated with
_carbonic acid_, becomes fertile.
But entirely to alter the constitution of a soil, whether by
mechanical or other means, is a work of time, labour and expense,
and little adapted to the pecuniary circumstances of farmers in
general. Fortunately, a remedy, cheaper, more accessible and
less difficult, is found in that _great diversity_ of habits and
character, which mark the vegetable races. We shall, therefore, in
what remains of this section, indicate the principal of these, as
furnishing the basis of all rational agriculture.
1st. _Plants have different systems of roots, stems and leaves,
and adapt themselves accordingly to different kinds of soils:_
the Tussilago prefers clay, the Spergula sand; Asparagus will
not flourish on a bed of granite nor Musus Islandicus on one of
alluvion. It is obvious, that _fibrous rooted_ plants, which occupy
only the surface of the earth, can subsist on comparatively stiff
and compact soils in which those of the leguminous and cruciform
families would perish, from inability to penetrate and divide.
2d. _Plans of the same, or of a similar kind, do not follow each
other advantageously in the same soil._ Every careful observer must
have seen how grasses alternate in meadows or pastures, where nature
is left to herself. At one time, timothy, at another clover, at a
third red-top, and at a fourth blue grass prevails. The same remark
applies to forest trees; the original growth of wood, is rarely
succeeded by a second of the same kind; pine is followed by oak, oak
by chesnut, chesnut by hickory. A young apple tree will not live
in a place where an old one has died; even the pear tree does not
thrive in succession to an apple tree, but stone fruit will follow
either with advantage. "In the Gautinois (says Bosc,) saffron is not
resumed but after a lapse of twenty years; and in the Netherlands,
flax and colzat require an interval of six years. Peas, when they
follow beans, give a lighter crop than when they succeed plants of
another family."[4]
[4] The ill effect of a succession of crops of the same kind was
not unknown to the Romans. We have proof of this in the following
passage of Festus: "Resistilibus ager fit qui continuo biennio
seseritur farreo spico id est aristato, quad ne fiat _solent, qui
pradia locant, excipere_."
3d. _Vegetables, whether of the name family or not, having a similar
structure of roots, should not succeed each other._ It has been
observed, that trees suffer considerably by the neighbourhood of
sainfoin and lucern, on account of the great depth to which the
roots of these plants penetrate--whereas culmiferous grasses do them
no harm.
4th. _Annual or biennial trefoils, prevent the escape of moisture by
evaporation, or filtration, from sandy and arid soils_, and should
constantly cover them in the absence of other plants;[5] while
_drying and dividing crops_, as beans, cabbages, chickory, &c. &c.
_are best fitted to correct the faults of stiff and wet clays_.
[5] The "Sterilis tellus medio versatur in æstu" of Virgil, shows
the opinion he entertained of a husbandry that left the fields
without vegetation.
5th. _When plants, are cultivated in rows or hills, and the ground
between them is thoroughly worked, the earth is kept open, divided
and permeable to air, heat and water, and accordingly receives from
the atmosphere nearly as much alimentary provision as it gives to
the plant._ This principle is the basis of the drill husbandry.
6th. _All plants permitted to go through the phases of vegetation
(and of course to give their seeds) exhaust the ground in a greater
or less degree; but if cut green, and before seeding, they take
little from the principle of fertility._
7th. _Plants are exhausters in proportion to the length of time
they occupy the soil._ Those of the culmiferous kinds (wheat, rye,
&c.) do not ripen under ten months, and during this period, forbid
the earth from being stirred: while, on the other hand, leguminous
plants occupy it but six months, and permit frequent ploughings.
This is one reason why culmiferous crops are greater exhausters than
leguminous; another is, that the stems of culmiferous plants become
hard and flinty, and their leaves dry and yellow, from the time of
flowering till the ripening of the seed--losing their inhaling or
absorbing faculties--circulating no juices, and living altogether
in their roots, and on aliments exclusively derived from the earth,
whereas leguminous or cruciferous plants, as cabbages, turnips, &c.
&c. have succulent stems, and broad and porous leaves, and draw
their principal nourishment from the atmosphere. The remains of
culmiferous crops, also are fewer, and less easily decomposed, than
those of the leguminous family.
8th. _Meadows, natural and artificial, yield the food necessary to
cattle, and, in proportion as these are multiplied, manures are
increased and the soil made better._ Another circumstance that
recommends them is, that so long as they last, they exact but little
labour, and leave the whole force of the farmer to be directed to
his arable grounds.[6]
[6] The good effect of these mixtures was known to the ancients,
from whom the practice has descended to us.
9th. _Grasses are either fibrous or tap-rooted, or both. The remarks
already made in articles 1, 2 and 3, apply also to them._ Timothy,
red-top, oat-grass and rye-grass, succeed best in stiff, wet soils.
Sainfoin does well on soils the most bare, mountainous and arid;
lucern and the trefoils, (or clovers,) only attain the perfection of
which they are susceptible, in warm, dry, calcareous earth.
10th. _The ameliorating quality of tap-rooted plants is supposed
to be in proportion to their natural duration_; annual clover,
(lupinella) has less of this property than biennial, (Dutch clover,)
biennial less than sainfoin, and sainfoin less than lucern.
11th. _Any green crop, ploughed into the soil, has an effect highly
improving_; but for this purpose, lupins and buckwheat (cut when in
flower) are most proper.
12. _Mixed crops_ (as Indian corn, pumpkins, and peas and oats,)
_are much and profitably employed_, and _with less injury to the
soil than either corn or oats alone_.