ABSORPTION BY SOILS OF SOLIDS FROM SOLUTIONS. ABSORPTION OF GASES. AIR
OF THE SOILS.
ABSORPTION OF SOLIDS FROM THEIR SOLUTIONS.
Just as solids have the power of condensing gases upon their surfaces,
to an extent proportional to that surface, and therefore to the state
of fine division: so fine powders have the power of withdrawing from
solutions solids held in solution, to an extent varying with the
nature of the substance dissolved, and the absorbing solid. The most
commonly-known manifestation of this principle is that sea-water
filtering through the sands of the shore, will at a certain distance
become sensibly less brackish, and finally so nearly fresh as to be
capable of domestic use.[98] The extent to which this occurs is in a
measure proportional to the fineness of the sand, and to the amount of
clay present in it. This is a clearly physical effect, independent of
any chemical action whatever; for it occurs equally with quartz sand,
charcoal, glass, limestone, or other rock powders having no chemical
effect upon the substance dissolved or upon the liquid dissolving
it. Very large amounts of water are often required to remove all the
soluble matter thus “adsorbed.”
[98] In many cases this decrease of salinity is probably due to a slow
influx of fresh water from landward; but very often it cannot be thus
explained.
_Decolorizing Action._—One of the commonest applications of this
principle is the decolorization of colored solutions by means of finely
pulverized charcoal. This property of charcoal, as is well known, is
extensively utilized in the arts, and particularly in the refining of
sugar; the charcoal used in this case being preferably bone charcoal
(“bone black”), which on account of its state of extreme fineness, and
separation by the earthy particles with which it is associated, is more
effective than any other form. It is rendered still more effective,
however, by the extraction of these earthy particles (calcic carbonate
and phosphate) by means of acid; for by removal of the earthy
particles, the surface of the charcoal is greatly increased, and its
decolorizing as well as its absorbing power increases accordingly.
While in one and the same substance the decolorizing effect is more
or less directly proportional to the fineness of the particles,
corresponding to increased surface, it is nevertheless true that in
this case, as in that of the absorption of gases, there are specific
differences between different powders; so that for example no other
substance can replace charcoal in the decolorizing effect which it
produces upon colored solutions. It must not, however, be supposed that
there is any special reason why _coloring_ matters, as such, should be
taken up by preference. Coloring matters are of all kinds of chemical
composition, and have in common only the fact that a relatively small
amount produces a very strong coloring effect; hence their name, and
hence also the apparently extraordinarily strong effect produced upon
them by charcoal.
This effect is not, however, by any means greater than it is in the
case of many other compounds which are colorless.
_Complex Action of Soils._—The powdery ingredients of soils, of course,
share this power with all other powders. In the case of soils, however,
the action is almost always much more complex than in that of charcoal,
because solutions that are passed through the soil are apt to act
chemically upon one or the other of its ingredients, usually resulting
in a partial exchange of ingredients between the soil and the solution;
one or more of the constituents of the solution being retained by the
soil, while one or more of the (basic) soil constituents pass into the
solution, in combination with its acidic ingredients.
Thus when a very dilute (½ or 1%) solution of potassic
chlorid is filtered through almost any soil, the first
portions passing through will be practically free from
potash, but will contain the chlorids of calcium and
magnesium. But as more of the solution is passed through,
potash passes also ultimately without absorption. In
addition to the zeolitic and clay portions of the soil, the
humus is very effective in absorbing mineral ingredients
from solution, and retaining them in such manner as to be
readily available to plant growth. (See chap. 8, p. 124.)
In view of the almost invariable conjunction of physical and chemical
effects, it may be fairly said that no solution, at least of mineral
salts, can pass through the soil without being changed in its
concentration and chemical composition. It is sometimes difficult to
decide to which of the two classes of effects the several changes may
be due.
_Purifying Action of Soils._—The disinfecting action of dry soil,
absorbing offensive gases from manure piles and from earth closets, has
already been alluded to. Similarly it is a matter of common experience
that the colored and otherwise offensive drainage from manure piles,
tanneries, dyeworks, etc., is not only deodorized but also decolorized
when passed through a sufficiently thick layer of clay soil. The
filtration through fine sand by which the drinking waters of cities
are so commonly purified before delivery to the consumer are familiar
examples of the same effects.
Equally familiar, however, is the fact that this power of
decolorization and retention of offensive compounds is limited; that
after a while the filtering earth or sand becomes saturated, and
afterwards the water or drainage will pass through without any sensible
purification.
It is therefore clear that this purifying effect of earth cannot
be relied upon for the permanent protection of wells from the
surface-drainage from barnyard or house refuse. Even if fissures or
layers of sand or gravel should not intervene so as to permit of the
direct communication of surface-drainage with wells, it is certain that
in the course of a few years at most, the intervening earth will become
so far saturated with the noxious ingredients that the latter will pass
through unhindered, and may contaminate to a considerable extent the
domestic supply of drinking water.
_Waste of Fertilizers._—The same, of course, holds true in regard to
manure-water, or soluble fertilizers of any kind used on the soil of
a field. The soil will retain them to a certain extent; but beyond
that limit any surplus added will be quickly washed through into the
country drainage by the rains. Moreover, a soil once so saturated
will yield to rain water filtering through it, notable amounts of all
the ingredients absorbed in it; and, at least so far as the physically
condensed soluble ingredients are concerned, long-continued leaching
with pure water will inevitably result in the withdrawal of additional
amounts of absorbed ingredients, apparently dividing themselves up _pro
rata_ between the water and the soil.
It is obviously of the utmost importance to the farmer to know to what
extent the soil will retain manurial ingredients against the influence
of leaching rains; for unless this is taken into consideration, it may
readily happen that the fertilizer supplied before a rainy season will
be washed through beyond the reach of plant-roots, and so practically
become a dead loss.
_Absorptive Power Varies._—So far as the mere physical absorption is
concerned, it will readily be understood that a coarse sandy soil
exercises less retentive influence upon dissolved substances than clay
or humous soils. In the humid region, where sand is substantially
nothing but granular silica (see above, chap. 6, page 86), the same may
be measurably true as regards the chemical absorption also. In the arid
region, on the contrary, a great many sandy or silt soils, very poor in
clay, exert fully as much chemical absorption as clay soils, and are
no more liable to the washing-out of soluble fertilizers introduced
than are the latter. For the chemical absorption lies chiefly in the
zeolitic portion of the soil (see above chap. 3, p. 37), which in the
humid region accumulates in the clay, while in the arid it remains
encrusting the sand and silt grains.
_Generalities regarding Chemical Absorption and Exchange._—In regard to
the leaching-out and absorption or retention of substances important to
agriculture, the following general statement may be made:
The substances most likely to be leached out of soils are, of bases:
soda, magnesia and lime; of acidic constituents: chlorine, sulfuric
acid and nitric acid. Lime sometimes passes off with either of the
above acidic ingredients, and also in the form of carbonate.
Substances rather tenaciously retained in soils are: potash and ammonia
among the bases, and phosphoric acid among the acids.
Thus (as stated above) when a weak (one or two per cent) solution
of potassic chlorid or sulfate is poured upon a column of good soil
several inches thick, it will be found that the first portions passing
through are free from potash, but contain the chlorids or sulfates of
magnesium and calcium. If potassic nitrate be used, lime and magnesia
will pass off as nitrates; while in the case of potassic phosphate,
both ingredients will be retained. A solution of gypsum (calcic
sulfate) will usually cause the passing-off of some of the magnesia,
soda and potash contained in the soil, in the form of sulfates; but
the amount of potash thus dissolved soon diminishes to a mere trace.
Solutions of potassic or amnionic phosphates will be absorbed and
retained by the soil to a very considerable extent, before the soil
becomes saturated.
While it is true that the degree to which the soil retains the several
ingredients may serve in a very general way to indicate their richness
or poverty in the same, the attempt to make such experiments serve
to determine the agricultural needs of soils has met with but little
practical acceptance.
_Drain Waters._—The table on p. 22, chapter 2, illustrates forcibly the
working of the above principles, which are verified by the composition
of drain-waters. In all, the chief nutritive ingredients of plants,
except nitrogen, are present in traces only; chlorids, nitrates and
sulfates of sodium and magnesium form the bulk of the permanently
soluble matter, with usually a considerable proportion of calcic (and
magnesic) carbonate, depending upon the amount of the earth-carbonates
present in the soil, as well as upon that of oxidizable organic
matter from which carbonic acid can be formed. That calcic carbonate
filters readily through the soil has already been somewhat elaborately
discussed (see chap. 3, p. 41); one of the results being that the
surface soil is sometimes almost completely depleted of this important
substance, while it accumulates at a greater or less depth in the
subsoil, or in underdrains, as the case may be.
Of the ingredients appearing in the above list, the one of greatest
agricultural importance is nitric acid, since chlorine and sulfuric
acid, as well as soda, are required only in very small quantities
by most culture plants; so that they rarely need to be supplied
in fertilizers. Nitric acid, however, is not only one of the most
important fertilizers, but also the most expensive; hence the
passing-off of nitrates in drainage-water is of such serious concern
to the farmer, that the causes of its occurrence, and the means of
preventing such loss, should be fully understood. This subject will,
however, be more fully considered farther on.
_The above Distinctions not Absolute._—It should, however, be also
understood that while the above statements hold good in a general way,
yet the line drawn is by no means an absolute one. For just as in the
case of physical adsorption the long passing-through of distilled water
will gradually abstract the substances condensed on the surface of
the soil-grains, so an overwhelming amount of a solution of any one
kind will have a tendency to substitute its own ingredients for those
already present in the soil, removing the latter to a greater or less
extent, even in the case of potash and phosphoric acid.
As an example in point, may be cited the case of the natural
minerals Analcite and Leucite, which Lemberg was able to
reciprocally transform from their natural condition of
soda- and potash-alumina silicates merely by alternate
treatment with solutions of potassium and sodium chlorids
respectively. (See chap. 3, p. 37). The same is true in
the case of the zeolitic matter of the soil. There is
nevertheless a distinct preference in the direction of the
retention of potash as against soda; so that in the case
of alkali soils, a large excess of potash is found to be
present in the zeolitic form, notwithstanding the presence
of sometimes very large amounts of the chlorid, sulfate and
carbonate of soda. This preferable retention of potash is,
of course, of material advantage in the case of the use of
soluble potash-fertilizers, as well as in preventing the
waste of the potash of the soil itself.
ABSORPTION, OR CONDENSATION, OF GASES BY SOILS.
Like all bodies in a state of fine division, soils are capable of
absorbing a not inconsiderable amount of various gases. It may be said
that in general, other things being equal, the amount thus condensed on
the surface of the soil-grains is more or less directly proportional
to the facility with which the gas is condensed by either pressure or
cooling. Hence the very large amount of water-gas or vapor which may
be absorbed by soils, as shown in a preceding chapter. But excepting
perhaps the case of ammonia, moist soils are less absorbent of gases
than dry ones.
Oxygen and nitrogen, the main constituents of the atmosphere, being
difficultly condensable by either pressure or cold, are absorbed by
soils only to a relatively small, yet by no means unimportant extent.
The condensation of oxygen within the soil-mass is doubtless of
considerable importance in the processes of oxidation, as is shown by
its partial replacement by carbonic gas in the free air of the soil
(see chap. 2, p. 17). The intensifying of oxidizing action caused by
surface condensation is well illustrated in the case of finely divided
platinum, in which hydrogen is brought to rapid combustion when mixed
with oxygen; as well as by the effect of bedding tainted meat in
charcoal powder, when all odors of decay disappear, both by absorption
and oxidation, ammonia and carbonic gas alone ultimately escaping
through the powder.
_Carbonic dioxid and ammonia gases_, both normal constituents of the
atmosphere, and of high importance to plant nutrition, are more readily
condensable than either oxygen or nitrogen, and consequently may be
taken up by the soil in larger relative proportions. Especially is
this the case with ammonia gas, which is not only readily condensed by
pressure, but is also extremely soluble in water; so much so that it
rushes into a tube filled with this gas almost as quickly as though
it were a vacuum. Water will absorb at the ordinary temperature,
under normal pressure, about 700 times its volume of ammonia gas; but
inasmuch as the proportion of the latter in the atmosphere amounts
to only a few millionths, the actual amount taken up can only (as
in the case of all gases) be proportional to its proportion (or
“partial pressure”) multiplied into its coefficient of absorption.
Consequently, water exposed to the ordinary air can absorb at best only
a small fraction of a per cent of ammonia. Its presence in soils can
be readily demonstrated by passing through the warmed soil a current
of purified air, which is made to bubble through Nessler’s reagent
(potassio-mercuric iodid) solution.
_Absorption of gases by dry soils._—Perfectly dry soils are powerful
absorbers of ammonia, and their absorption of this gas, as well as of
carbonic gas, can readily be shown by the arrangement shown on the page
opposite.
The two tubes shown to the left are filled with carbonic
gas, those to the right with ammonia gas. After being
immersed in a mercurial trough, there are introduced into
each tube through the mercury small cylinders (conveniently
one cubic centimeter in volume) consisting respectively of
a very sandy soil or loose hardpan, a gray plastic clay,
a gray clay soil or adobe, a very black “adobe” clay, and
a highly ferruginous and humous soil (from Hawaii), which
gives the highest absorption of all; next brown peat, and
pine charcoal. The latter, and the ferruginous soil, were
also exposed for the absorption of carbonic gas. All the
absorbing cylinders are first heated for an hour to 110°C.
(218°F) for the purpose of expelling from them moisture,
air, and other absorbed gases. They are then quickly
introduced into the tubes through the mercury and allowed to
absorb the gases enclosed until the mercury columns cease to
show any farther rise; in which condition they are shown in
the figure.
It will be seen that this absorption is a different one, not only
for each of the different substances used, but is also differently
proportioned for the two gases. For it will be noted that while the
clay soil has absorbed a very much larger amount of ammonia than the
charcoal, and the sandy soil has remained far behind both: yet the
charcoal has absorbed a considerably larger _proportion_ of carbonic
gas than either the clay or the sandy soil, proving that charcoal has
a strong _specific_ absorptive power for carbonic gas, independently
of the relative size of clay and charcoal particles respectively.
The sandy soil shows, by its low absorption even of ammonia gas,
the coarseness of its particles and the scarcity of clay in its
composition. The highest absorption of all is shown by the ferruginous
soil from Hawaii, containing nearly 40% of ferric oxid together with
3⅓% of humus. The moisture-absorption of this soil at the ordinary
temperature is 19.7 per cent. The difference in the absorbing power
of the (non-humous) gray clay and gray adobe soil indicates the
strong influence of humus upon the absorption; which is still farther
emphasized by the difference between the gray and black adobe, the
latter containing 1.2% of humus. As to the peat, since its weight was
only .5 grams against an average of 2 grams for the soils employed, its
absorptive power _by weight_ doubtless exceeds all other substances.
[Illustration: FIG. 51.—Absorption of Carbonic and Ammonia Gases by
different Soils.]
While the experiment shown in the figure serves as a convenient and
striking demonstration for lecture purposes, it is of course not
adapted to a direct comparison of the absorbing powers of the several
substances, because of different heights of the mercurial columns
counteracting the atmospheric pressure. For direct comparative
measurement the tubes must be sunk in mercury so as to equalize the
levels inside and outside, since the corrected volumes obtained by
calculation would not serve the purpose.
According to special measurements made under normal atmospheric
pressure, the writer found that a black clay soil (“adobe”) absorbed
(at 60°F) over two hundred times its bulk of ammonia gas, while under
the pressure of one-fifth of an atmosphere (as shown in the photograph)
the absorption was one hundred and twenty-three times its bulk. This
energetic absorption of ammonia and related gases explains the marked
disinfecting effects which a covering of dry earth exerts in the case
of cemeteries, manure piles, and earth closets. But the difference
between the sandy soil and the clay soil in the amount of absorption
admonishes us that in all these cases, to secure disinfection the earth
to be used should contain as much clay as possible, and should not be
mere sand, as is sometimes the case. It also shows that the addition
of charcoal to such materials does not increase their efficacy, as has
been supposed, but that an equal bulk of clay would be more efficient.
Of course, so soon as the absorbing cylinders used for this experiment
are exposed to the atmosphere, the principle above stated in regard to
“partial pressure” asserts itself. The absorbed gases quickly begin
to be given off, and in some hours the equilibrium with the ordinary
conditions of the atmosphere is re-established. That the strong
absorptive power of soils for ammonia is to some extent effective
in maintaining the supply of this substance by absorption from the
atmosphere, cannot be doubted.
Boussingault, and later Stenhouse, determined the absorptive power of
wood charcoal for ammonia to be 90 and 98 volumes respectively.
THE COMPOSITION OF GASES ABSORBED FROM THE ATMOSPHERE BY VARIOUS SOLIDS.
In 1864 and 1865 Reichardt and Blumtritt[99] investigated elaborately
the composition of gases driven off by heat from various powders,
including soils, exposed to the atmosphere. All the substances examined
were therefore “air-dry,” therefore to a certain extent moist; and
the presence of this aqueous vapor of course modifies in a measure
the results that would have been obtained had the materials used been
exposed to dry air only. They found that, as had already been stated
by previous observers, the presence of capillary water _diminishes_
materially the absorption of gases, especially of those not as easily
absorbed by water as are carbonic gas and ammonia. Contrary to what
might have been expected from the more ready condensation of oxygen by
pressure or cold, in nearly all cases nitrogen is absorbed to a greater
extent than oxygen, and sometimes exclusively so; so that in some cases
the latter was found to be present only in traces, as will be perceived
from the subjoined table:
COMPOSITION OF GASES ABSORBED FROM THE ATMOSPHERE
BY VARIOUS POWDERS
================================+========+=======
| 100 | 100
| Grms | Vol’s
Substance. |gave cc.| gave
| Gas. | Vol’s.
| | Gas.
------------------------------=-+--------+-------
Charcoal, coniferous, air dry | 16.21 |
“ moistened and air-dried | 140.11 | 59.0
“ Lombardy Poplar | 466.95 | 195.4
Peat | 162.58 |
Garden Earth, moist | 13.70 | 19.9
“ “ air-dried | 30.28 | 53.6
River Silt, air-dried | 40.53 | 48.07
“ “ slightly moistened | 24.12 | 29.2
“ “ air-dried | 26.52 | 30.05
Clay, long exposed | 25.58 | 39.05
“ slightly moistened | 28.62 | 35.08
--------------------------------+--------+-------
Ferric Hydrate, commercial | 251.59 | 275.0
“ “ freshly | 375.54 | 308.6
precipitated, | |
air-dried | |
--------------------------------+--------+-------
Ferric Oxid, ignited | 39.4 | 52.4
Aluminic Hydrate, air-dried | 69.02 | 82.0
“ “ dried at 100°C. | 10.83 | 13.6
Prepared Chalk, 1864-65 | 43.48 | 52.4
“ “ 1865 | 38.98 | 48.0
--------------------------------+--------+-------
Calcic Carbonate, precipitated, | 65.09 |
1864-65 | |
“ “ precipitated, | 51.53 | 52.0
1865-66 | |
--------------------------------+--------+-------
Magnesic Carbonate | 729.21 | 124.9
Gypsum, finely powdered | 17.26 |
--------------------------------+--------+-------
================================+==================================
| 100 vol’s gas contained
+--------+------+--------+---------
Substance. | | | |
|Nitrogen|Oxygen|Carbonic| Carbon
| | | Dioxid |Monoxid
--------------------------------+--------+------+--------+---------
Charcoal, coniferous, air dry | 100.00 | 0.0 | 0.0 | 0.0
“ moistened and air-dried | 85.60 | 2.12 | 9.15 | 3.13
“ Lombardy Poplar | 83.60 | 0.0 | 16.50 | 0.0
Peat | 44.44 | 4.60 | 50.96 | 0.0
Garden Earth, moist | 64.34 | 2.85 | 24.06 | 8.75
“ “ air-dried | 64.70 | 2.04 | 33.26 | 0.0
River Silt, air-dried | 67.69 | 0.0 | 18.61 | 13.70
“ “ slightly moistened | 67.34 | 0.0 | 30.56 | 2.10
“ “ air-dried | 67.40 | 9.09 | 16.07 | 7.44
Clay, long exposed | 70.17 | 4.71 | 25.12 |
“ slightly moistened | 59.59 | 6.39 | 34.02 |
--------------------------------+--------+------+--------+---------
Ferric Hydrate, commercial | 33.26 | 1.43 | 65.31 | 0.00
“ “ freshly | 26.29 | 3.85 | 69.86 | 0.00
precipitated, | | | |
air-dried | | | |
--------------------------------+--------+------+--------+---------
Ferric Oxid, ignited | 82.87 |13.41 | 3.72 | 0.00
Aluminic Hydrate, air-dried | 40.60 | 0.00 | 59.40 |
“ “ dried at 100°C. | 83.09 |16.91 | 0.00 |
Prepared Chalk, 1864-65 | 100.00 | 0.00 | 0.00 |
“ “ 1865 | 74.49 |15.49 | 10.02 |
Calcic Carbonate, precipitated, | 80.81 |19.19 | 0.00 |
1864-65 | | | |
Calcic Carbonate, precipitated, | 77.37 |15.09 | 7.54 |
1865-66 | | | |
Magnesic Carbonate | 63.92 | 6.72 | 29.36 |
Gypsum, finely powdered | 80.95 |19.05 | 0.00 |
--------------------------------+--------+------+--------+---------
[99] Journal für praktische Chemie, Vol. 98, p. 167.
_Discussion of the Table._—It will be observed that
in this table, the largest amount of total gas given off
by equal weights of any one substance was in the case of
carbonate of magnesia; but it is quite probable that in
part, at least, this large amount of gas was due to the
evolution of carbonic gas from the easily decomposable
carbonate; the more as the analysis of the gases shows over
29% of carbonic gas. But the highest absorption by equal
_volumes_ of any substance is shown by the ferric
hydrate; next to this by the light poplar charcoal, and
next by the carbonate of magnesia. The high absorptive
power here shown by the ferric hydrate is of great interest
in connection with the facts already stated regarding the
absorption of moisture and ammonia by ferruginous soils
(see page 274, this chapter); and the fact that the larger
proportion of the gas—as much as 70% in one case—consisted
of carbonic gas, is particularly interesting in the same
connection. Both in the amount of gas contained, and in
the proportion of carbonic gas therein, the ferric hydrate
exceeds even peat, the representative of humus in soils.
It will, however, be noted that in the garden soil, also,
the proportion of carbonic gas is very large, while that of
oxygen is very low. It is curious to note that in very few
cases the proportion of oxygen to nitrogen is the same as
in the atmosphere; in most cases the nitrogen predominates
considerably beyond its normal proportion, and in two cases,
that of charcoal and of calcic carbonate (whiting), the gas
was found to consist of pure nitrogen.
We are forced to conclude that the substances here enumerated, as a
rule, condense oxygen in smaller proportions than they do nitrogen, or
carbonic gas. As regards the carbon monoxid mentioned in the table, it
is doubtful that it was contained as such in the substance originally
examined; it may readily have been formed under the influence of the
heat required in expelling the gases from the substances containing
organic matter. Among the important results shown in the table, is the
comparative determination of the gases in moist, and in dry garden
earth, showing that in the moist earth the amount of gas absorbed
ranged from less than one-half down to almost one-fourth that absorbed
by the dry. The importance of these differences in the case of the
fallow can readily be appreciated.
The changes in the absorptive power brought about by
wetting and drying, as shown in the above table, are very
insignificant. In the case of the charcoal, soil and silt
the diminution may fairly be assumed to be caused by the
deposition of soluble salts on the surface, partly clogging
the pores. In the case of the clay as well as in that of
the river silt, the inevitable content of organic matter
in process of decomposition has doubtless influenced the
result, as is suggested by the increase of carbonic gas.
That prepared chalk should in one case contain exclusively
nitrogen gas, in the other case mixed gases, seems to
indicate a difference in the air to which it is exposed,
or in the water employed in its preparation; the latter
case agreeing substantially with the results obtained from
the precipitated carbonate. In both (as well as in the
carbonates of barium and strontium), the absorption of
carbonic gas is very small, or _nil_.
It thus appears that for the condensation of carbonic dioxid gas,
ferric and aluminic hydrates are prepotent among mineral substances;
while clays, river silts and soils may always be expected to contain
relatively large proportions of this gas in absorption.
THE AIR OF SOILS.
_The Empty Space in Soils._—In dry soils the empty space, usually
amounting to from 35 to 50 per cent of its volume, is filled with
air;[100] in moist or wet soils the space unoccupied by water is
similarly filled. Hence when soils are in their best condition for
the support of vegetation (chap. 11, p. 202), about one half of their
interstices is filled with water, the other half with air. Actual
measurements of the amount of air contained in well-cultivated garden
soil have been shown by Boussingault and Levy to range between 10,000
and 12,000 cubic feet per acre, substantially agreeing, therefore, with
the above statement. In uncultivated forest soil, on the contrary, they
found only from somewhat less than 4000 to 6000 cubic feet of air per
acre. Extended observations since carried out by Wollny, Ebermayer, and
others have in general confirmed the earlier observations, while adding
greatly to their significance in respect to their relations to plant
growth, and to the process of humification and soil-formation.
[100] The normal composition of atmospheric air is given on p. 16,
chap. 2.
As a matter of course, when water evaporates from the soil in drying,
its place is taken by air so far as it is not filled by capillary water
drawn from below.
_Functions of Air in Soils._—That roots require for the performance of
their vegetative functions the presence of oxygen, has already been
discussed; but there can be no question that the higher productiveness
of well-cultivated soils is largely due to the greater and readier
access of air to the roots. Apart from this direct function, however,
the presence of oxygen in the soil serves other important purposes,
and among these doubtless the most dominant is the promotion of the
oxidation of the organic matter of the soil through the agency of
micro-organisms; and more particularly that of nitrification, which
chiefly governs the supply of nitrogen to non-leguminous plants. In
the case of leguminous plants, the presence of air as a furnisher of
nitrogen as well as oxygen is absolutely essential.
The injurious effects of insufficient aeration of the soil have been
repeatedly referred to already (pp. 45, 76). In water-logged soils
reductive fermentations are soon set up, and the nitrates of the
soils are reduced partly with the evolution of nitrogen gas, partly
to ammonia; while their oxygen is consumed to supply the demands of
the roots. Ferric oxid is reduced to ferrous carbonate, sulfates to
sulfids; thus deranging the whole process of plant-nutrition and
absorption of plant-food. If continued for any length of time these
conditions end in the death of the plant. Too much importance cannot
therefore be attached to the proper aeration of the soil and subsoil.
_Excessive Aeration; Compacting the Soil._—On the other hand, excessive
aeration of the soil may be injurious in causing a serious waste of
moisture; especially in arid climates, where the hot, dry winds may
readily destroy the germinating power of the swollen seed when the
seed-bed is too loose and open, and later may injure or destroy the
feeding roots. The abundant growth of grain often seen in the tracks
of a wagon carrying the centrifugal sower, when the stand in the
general surface is very scanty, is usually due to the consolidation of
the seed-bed, and suggests at once the well-known efficacy of light
rolling to insure quicker germination and a better stand. Similarly,
the rolling of grain fields in spring is often the saving clause for
a crop in dry years. But such needful consolidation must not, of
course, be carried to the extent of creating a surface crust which
would subsequently serve to waste the subsoil moisture. Hence, the
soil-surface should be rather dry when rolling is resorted to.
The pressing of the earth around transplanted plants, similarly, is
a needful precaution, not only with respect to the drying-out of the
soil, but also to insure close contact between the roots and the soil.
_The Composition of the Free Air of the Soil_ usually differs from
the air above, in that besides being saturated with moisture, its
nitrogen-content is slightly increased (by one-half to over one per
cent); the oxygen-content on the other hand, is diminished, being
in part (sometimes nearly to the extent of one-half of its volume)
replaced by carbonic gas, derived partly from its secretion by the
roots, partly from the oxidation of organic substances. It naturally
follows that the richer the soil in the latter, the more carbonic gas
will be formed under favoring conditions; so that in freshly-manured
land the amount of oxygen transformed into carbonic gas will be
greatest, while in the surface-soil of ordinary fields, carbonic gas
rarely reaches to as much as one per cent. In all cases, however, the
content of carbonic gas in the air of the soil is materially higher
than that of the air above it, and thus serves to intensify greatly
the solvent and disintegrating effect of the soil water upon the soil
materials (see chap. 2, p. 17). The soil-mass itself, however, retains
carbonic dioxid with considerable tenacity, so that it is not possible
to wash it out completely by filtering water through it. When water
containing carbonic gas in solution is filtered through the soil, the
gas is sometimes completely absorbed, the water passing off free from
gas.
The presence of free carbonic gas in soils is readily demonstrated by
passing through the warmed soil a current of air, which is then made to
bubble through lime water; a clouding of the latter, and the ultimate
formation of a precipitate of calcic carbonate, proves the presence of
the gas, and may also serve to measure its amount.
From the fact that the free air in normal soils may contain as much as
one-fortieth of its bulk of carbonic gas, besides what may be contained
in the condensed form, we may conclude that this gas is formed within
them with considerable rapidity; for otherwise, in view of the free
communication and diffusion with the outer air, such large amounts
could not be maintained in the surface-soil. Doubtless a considerable
proportion of the carbonic gas normally contained in the atmosphere is
thus supplied from within the soil itself.
_Relation of Carbonic Gas to Bacterial and Fungous Activity._—It has
been fully demonstrated by the researches of Koch, Miquel, Adametz,
Fuelles, Wollny and others, that the formation of carbonic gas in the
soil is not a purely chemical oxidation process, but is essentially
dependent upon the presence and life-activity of numerous kinds of
organisms, bacterial as well as fungous. The crucial proof of this fact
is that the presence of any antiseptic diminishes, and if exceeding
certain proportions completely suppresses, the formation of carbonic
gas; while on the other hand all conditions known to be favorable to
the life of such organisms, viz., the proper conditions of temperature
and moisture (varying with different kinds), increase the formation
of the gas. Such formation is of course, however, conditioned upon
the presence of oxygen. In the case of most bacteria, there is a
certain limit beyond which the presence of their own product exerts
an injurious or repressive effect upon their activity; so that if the
gas accumulates beyond that limit, the rate of its formation decreases
despite of otherwise favorable conditions.
It follows that the best life-conditions of these organisms (even
when anerobic) cannot be fulfilled below a certain limited depth in
the soil; and all observations show that their number decreases very
rapidly with increasing depth (see chap. 9, p. 142), varying with the
perviousness of the soil, but rarely exceeding four or five feet in
the humid regions; though doubtless found at greater depths in the
arid climates. It is also obvious that the use of any antiseptic or
poisonous materials on the field or in the manure pile will tend to
disturb and restrain the useful activity of these organisms.
_Putrefactive Processes._—Carbonic gas is formed also, but to a much
more limited extent, in _putrefactive processes_, occurring in the
absence, or with only limited access, of air or oxygen. These processes
likewise are conditioned upon the presence or activity of (largely
anerobic) bacteria; but they should not occur in normally constituted,
and especially in tilled soils, being as a rule inimical to the growth
of cultivated plants (see chap. 9, p. 145).