are filled in a different way. The lysimeters are first constructed and
placed over vertical columns of soil _in situ_, which are obtained by
digging away all the surrounding soil and leaving the columns standing.
The shorter lysimeter is sunk in this way to within two inches of its
entire length. It is then tipped over carrying the column of soil with
it. Six inches of the subsoil are then removed, when the drainage tube
and sand are put in, as in the first pair, and the bottom of the tube
soldered in place. The lysimeter is thus filled with the natural soil in
place. The longer box is in the same way filled, as far as possible,
with the soil in place, but a gravelly nature of the soil may render it
impossible to do the filling with a single column unbroken, so the
gravel and sand from the lower portion of the soil are to be filled in
separately. The drainage tube and bottom of sand are placed in the
longer lysimeter in the same way as in the shorter.
[Illustration:
FIGURE 23.
GROUND PLAN AND VERTICAL SECTION OF LYSIMETERS AND VAULTS SHOWING
POSITION OF THE APPARATUS.
1, 1, 1, 1, Lysimeters.
2, 2, 2, 2, Receiving bottles.
3, 3, Supplying apparatus.
4, 4, Skylights.
5, 5, 5, 5, Wall of vault.
6, 6, Brick walls.
7, Entrance Steps.
8, Vault.
]
The purpose of placing sand at the bottom of each lysimeter is to offer
a porous stratum in which free water may collect and rise to the level
of the perforated copper tube, which would prevent any further rise by
conveying the surplus above into the vault as drainage water. The soil
above the tube will therefore be constantly drained and the sand below
constantly saturated, unless the water be drawn up by the capillary
action of the soil as the result of evaporation from the surface.
By means of a proper arrangement within the vault, of a kind of
Mariotte’s bottle, the water may be caused to flow back through the
drainage tube into the lysimeter to take the place of that lost by
evaporation, and thus maintain the level of free water just below the
drainage tube. The water flowing back to the lysimeter, and the amount
of drainage water, are carefully measured by a system of graduated
tubes.
The lysimeters thus constructed represent tile-drained land; in one case
the tile being three feet below the surface and in the other six feet
below. The drainage waters collected in the receiving bottles can be
measured and analyzed from time to time, as occasion may require, to
determine the amount of plant food which is removed.
=179. Improved Method of Deherain.=[120]—Deherain’s earlier experiments
were made in pots containing about sixty kilos of soil. These vases
serve very well for some kinds of plants, but there are other kinds
which do not grow at all normally when their roots are imprisoned. For
instance, in pots, even of the largest size, wheat is always poor, beets
irregular, maize never acquires its full development, and the
conclusions which can be drawn from the experiments can not be
predicated of the action of the plant under conditions entirely normal.
It is necessary therefore to carry on the work in an entirely different
way, and to construct boxes so large as to make the conditions of growth
entirely normal. The arrangement of these boxes is shown in Fig. 24.
They are placed in a large trench, two meters wide, one meter deep, and
forty meters long. There are twenty boxes in this trench, the upper
surface of each containing four square meters area. The boxes are one
meter deep, and therefore can contain four cubic meters of soil. The
sides and bottoms of the boxes are made of iron lattice work, covered
with a cement which renders them impervious to water.
The bottom inclines from the sides towards the middle, and from the back
to the front, thus forming a gutter which permits of the easy collection
of the drainage. The drainage water is conveyed, by means of a pipe and
a funnel, into a demijohn placed in the ditch in front of the apparatus,
as shown in the figure. These receptacles stand in niches under the
front of the cases, and are separated by the brick foundations. Access
to them is gained by means of the inclined plane shown in the figure,
and this plane permits the demijohns in which the drainage water is
collected, to be removed with a wheelbarrow for the purpose of weighing.
This apparatus is especially suitable for a study of the distribution of
the nitrogen to the crop, the soil and the drainage waters. The loss in
drainage waters of potash and phosphoric acid is insignificant in
comparison with the loss in nitrogen.
The cases having been placed in position they are filled with the
natural soil, which is taken to the depth of one meter, in such a way
that the relative positions of the soil and subsoil are not changed.
While the soil is transferring to the cases it is carefully sampled in
order to have a portion representing accurately the composition of both
the soil and subsoil. These samples are subjected to analysis and the
quantities of nitrogen, phosphoric acid, and potash contained therein
carefully noted.
One or two cases should be left without crop or fertilizer to determine
the relations of the soil and subsoil to the rain-fall. Three or four
cases should be kept free of vegetation and receive treatment with
different fertilizer, in order to determine the influences of these on
the deportment of the soil to rain-fall. The rest of the cases should be
seeded with plants representing the predominant field culture of the
locality, and some of them should be fertilized with the usual manures
used in farm culture.
[Illustration:
FIGURE 24.
DEHERAIN’S APPARATUS FOR COLLECTING DRAINAGE WATER.
]
AUTHORITIES CITED IN PART THIRD.
Footnote 70:
Comptes rendus, Tome 112, p. 598.
Footnote 71:
Stockbridge, Rocks and Soils, p. 153.
Footnote 72:
Die Landwirtschaftlichen Versuchs-Stationen, Band 8, S. 40.
Footnote 73:
König, Untersuchung Landwirtschaftlich und Gewerblich Wichtiger
Stoffe, S. 48.
Footnote 74:
Methods of Swedish Agricultural Chemists, translated for author by F.
W. Woll.
Footnote 75:
Poggendorff’s Annalen, Fifth Series, Band 9, Ss. 102, et seq.
Footnote 76:
Pennsylvania Agricultural Experiment Station Report, for 1891, pp.
194, et seq.
Footnote 77:
Agricultural Science, Vol. 8, pp. 28, et seq. (Correction. For Fig.
13, second line from bottom of page 112, read Fig. 14.)
Footnote 78:
Haberland. Forschungen auf der Gebiete der Agricultur-Physik, 1878, S.
148.
Footnote 79:
Grundlagen zur Beurteilung der Ackerkrume, Weimar, 1882.
Footnote 80:
Vid. supra, 10.
Footnote 81:
These general principles are taken chiefly from a résumé of the
subject by Prof. H. A. Huston. Indiana Agricultural Experiment
Station, Bulletin 33, pp. 46, et seq.
Footnote 82:
Knop’s Agricultur Chemie, Abteil II.
Footnote 83:
Beiträge zur Frage der Bodenabsorption.
Footnote 84:
Henneberg’s Journal, 1859, S. 35.
Footnote 85:
Die Landwirtschaftlichen Versuchs-Stationen, Band 27, S. 107.
Footnote 86:
Die Bonitirung der Ackererde, S. 49.
Footnote 87:
Journal Chemical Society of London, 1868.
Footnote 88:
Landw. Central-Blatt, Band 11, S. 169.
Footnote 89:
bis Die Landwirtschaftlichen Versuchs-Stationen, Band 12, Ss. 21–50.
Footnote 90:
Jour. f. Landw., 1862, Band 3, Ss. 49–67.
Footnote 91:
Ann. d. Landw., Band 34, S. 319.
Footnote 92:
American Journal of Science, Vol. 14, p. 25.
Footnote 93:
bis (p. 122). Ms. communication to author.
Footnote 94:
Maryland Agricultural Experiment Station, Fourth Annual Report, p.
282.
Footnote 95:
Bulletin No. 4, U. S. Weather Bureau, p. 80.
Footnote 96:
bis (p. 125), Beiträge zur Agronomische Bodenuntersuchung, S. 31.
Footnote 97:
Zeitschrift für angewandte Chemie, 1889, S. 501.
Footnote 98:
Die Landwirtschaftlichen Versuchs-Stationen, Band 17, S. 85.
Footnote 99:
Ms. communication to author.
Footnote 100:
Proceedings of the Ninth Meeting of the Society for the Promotion of
Agricultural Science, p. 51.
Footnote 101:
Rocks and Soils, pp. 155 et. seq.
Footnote 102:
Anleitung zur Wissenschaftlichen Bodenuntersuchung, S. 137.
Footnote 103:
Analyse du Sol, p. 13.
Footnote 104:
Landwirtschaftliche Jahrbücher, Band 3, Ss. 771.
Footnote 105:
Forschungen auf dem Gebiete der Agricultur-Physik, 1885, Ss. 177, et
seq.
Footnote 106:
Vid. supra, S. 259.
Footnote 107:
Poggendorf, Annalen, Band 129, Ss. 437, et seq.
Footnote 108:
König, Untersuchung Landwirtschaftlich und Gewerblich Wichtiger
Stoffe, S. 59.
Footnote 109:
Vid. 37, S. 60.
Footnote 110:
Landwirtschaftliche Jahrbücher, Band 2, S. 383.
Footnote 111:
Forschungen auf dem Gebiete der Agricultur-Physik, 1880, S. 218.
Footnote 112:
Beurteilung der Ackerkrume, S. 222.
Footnote 113:
Wisconsin Agricultural Experiment Station, Seventh Annual Report, pp.
134, et seq.
Footnote 114:
Vid. supra, pp. 139, et seq.
Footnote 115:
Wisconsin Agricultural Experiment Station, Seventh Annual Report, p.
145.
Footnote 116:
Bulletin No. 4, Weather Bureau, pp. 13, et seq.
Footnote 117:
Philosophical Magazine, 1878.
Footnote 118:
Weather Bureau, Bulletin No. 4.
Footnote 119:
Forschungen auf dem Gebiete der Agricultur-Physik, 1891, S. 11.
Footnote 120:
Annales Agronomiques, Tome 16, p. 337; Tome 17, p. 49; Tome 18, p.
237; Tome 19, p. 69.
PART FOURTH.
MECHANICAL ANALYSIS OF SOILS.
THE FLOCCULATION OF SOIL PARTICLES.
=180. Relation of Flocculation to Mechanical Analysis.=—The tendency of
the fine particles of silt to form aggregates, which act as distinct
particles of matter, is the chief difficulty connected with the
separation of the soil into portions of equal hydraulic value by the
silt method of analysis. This tendency has been discussed fully by
Johnson[121] and Hilgard.[122]
=181. Illustration of Flocculation.=—A sediment, consisting of particles
of a hydraulic value, equal to one millimeter per second, is introduced
into an ordinary conical elutriating tube placed vertically, in which
the current of water entering below performs all the stirring which the
particles receive.
A current of water corresponding to a velocity below one millimeter per
second will, of course, not carry any of the particles out at the top of
the cylindrical tube, but will keep them moving through the conical
portion of the tube. If now the current be increased until its velocity
is greater than one millimeter per second after having run at the slower
velocity for fifteen or twenty minutes, very little of the sediment will
pass over, although theoretically the whole of it should. Even at a
velocity of five millimeters per second, much of the sediment will
remain in the tube. This, of course, is due to the coagulation of the
particles into molecular aggregates having a higher hydraulic value even
than five millimeters per second. These aggregates can be broken up by
violent stirring or moderate boiling, and the sediment reduced again to
its proper value. The conclusions which Hilgard derives from a study of
the above phenomena are as follows:
1. The tendency to coagulation is, roughly, in an inverse ratio to the
size of the particles. With quartz grains it practically ceases when
their diameter exceeds about two-tenths of a millimeter having a
hydraulic value of eight millimeters per second. The size of the
aggregates formed follows practically the same law as above. Sediment of
0.25 millimeter hydraulic value will sometimes form large masses like
snow-flakes on the sides of the elutriator tube.
2. The degree of agitation which will resolve the aggregates into single
grains is inversely as the size of the particles; or, more properly
perhaps, inversely as their hydraulic value.
3. The tendency to flocculation varies inversely as the temperature. So
much so is this the case that Hilgard at one time contemplated the use
of water at the boiling point in the mechanical analysis of soils, in
place of mechanical stirring.
4. The presence of alcohol, ether, and of caustic or carbonated
alkalies, diminishes the tendency to flocculation, while the presence of
acids and neutral salts increases it.
5. As between sediments of equal hydraulic value, but different
densities, the tendency to flocculation seems to be greater with the
less dense particles.
In regard to the mechanical actions which take place between the
particles, Hilgard considers them as irregular spheroids, each of which
can at best come in contact at three points with any other particle. The
cause of aggregation cannot therefore be mere surface adhesion
independent of the liquid, and the particles being submerged there is no
meniscus to create an adhesive tension.
Since experiment shows that the flocculative tendency is measurably
increased by the cohesion coefficient of the liquid, it seems necessary
to assume that capillary films of the latter interposed between the
surfaces of solids create a considerable adhesive tension even in the
absence of a meniscus.
=182. Effect of Potential of Surface Particles.=—Whitney suggests that
this is due to the potential of the surface particles of solids and
liquids.[123] The potential of a single water particle is the work which
would be required to pull it away from the surrounding water particles
and remove it beyond their sphere of attraction. For simplicity, it may
be described as the total force of attraction between a single particle
and all other particles which surround it. With this definition, it will
be seen that the potential of a particle on an exposed surface of water
is only one-half of the potential in the interior of the mass, as half
of the particles which formerly surrounded and attracted it were removed
when the other exposed surface of water was separated from it. A
particle on an exposed surface of water, being under a low potential,
will therefore tend to move toward the center of the mass where the
potential, _i. e._, the total attraction, is greater, and the surface
will tend to contract so as to leave the fewest possible number of
particles on the surface. This is surface tension.
If, instead of air, there is a solid substance in contact with the
water, the potential will be greater than on an exposed surface of the
liquid, for the much greater number of solid particles will have a
greater attraction for the water particles than the air particles had.
They may have so great an attraction that the water particle on this
surface, separating the solid and liquid, may be under greater potential
than prevails in the interior of the liquid mass. Then the surface will
tend to expand as much as possible, for the particles in the interior of
the mass of liquid will try to get out on the surface. This is the
reverse of surface tension. It is surface pressure, which may exist on a
surface separating a solid and liquid.
Muddy water may remain turbid for an indefinite time, but if a trace of
lime or salt be added to the water the grains of clay flocculate, that
is, they come together in loose, light flocks, like curdled milk, and
settle quickly to the bottom, leaving the liquid above them clear.
Ammonia and some other substances tend to prevent this and to keep the
grains apart if flocculation has already taken place.
If two small grains of clay, suspended in water, come close together
they may be attracted to each other or not, according to the potential
of the water particles on the surface of the clay. If the potential of
the surface particle of water is less than that of the particle in the
interior of the mass of liquid, there will be surface tension, and the
two grains will come together and be held with some force, as their
close contact will diminish the number of surface particles in the
liquid. If, on the other hand, the potential of the particle on the
surface of the liquid is greater than of the particle in the interior of
the mass, the water surface around the grains will tend to enlarge, as
there will be greater attraction for the water particles there than in
the interior of the mass of liquid, and the grains of clay will not come
close together and will even be held apart, as their close contact would
diminish the number of surface particles in the liquid around them.
=183. Influence of Surface Tension.=—Hilgard supposes that the surface
tension which is assumed to exist between two liquid surfaces must exert
a corresponding influence between the surfaces of solids and liquids,
apart from any meniscal action.
It is then to be expected that the adhesion of the particles
constituting one of these floccules will be very materially increased
whenever the formation of menisci between them becomes possible by the
removal of the general liquid mass. Suppose one of the floccules to be
stranded, it will, in the first place, remain immersed in a sensibly
spherical drop of liquid. As this liquid evaporates, the spherical
surface will become pitted with menisci forming between the single
projecting particles, and as these menisci diminish their radius by
still further evaporation, the force with which they hold the particles
together will increase until it reaches a maximum. As the evaporation
progresses beyond this point of maximum, the adhesion of the constituent
particles must diminish by reason of the disappearance of the smaller
menisci, and when finally the point is reached when liquid water ceases
to exist between the surfaces, the slightest touch, or sometimes even
the weight of the particles themselves, will cause a complete
dissolution of the floccule, which then flattens down into a pile of
single granules.
In regard to natural deposits from water, Hilgard supposes that they are
always precipitated in a flocculated state. The particles of less than
two-tenths millimeter diameter are carried down with those of a larger
diameter having much higher hydraulic value. Thus the deposition of a
pure clay can take place under only very exceptionable circumstances.
Whitney, on the other hand, suggests that grains of sand and clay carry
down mechanically the particles of fine silt and clay as they settle in
a turbid liquid in a beaker; and it is often difficult to wash out a
trace of fine material from a large amount of coarse particles, for this
reason, although there may be no trace whatever of flocculation.
=184. Destruction of Floccules.=—The destruction of the natural
floccules is seen in the ordinary process of puddling earth or clay. It
is also the result of violent agitation of water or of kneading or
boiling, or, finally, to a certain extent, of freezing. All these
agencies are employed by the workers in clay for the purpose of
increasing the plasticity which depends essentially upon the finest
possible condition of the material to be worked. As an illustration of
this, Hilgard cites the fact that any clay or soil which is worked into
a plastic paste with water, and dried, will form a mass of almost stony
hardness. If, however, to such a substance one-half per cent of caustic
lime be added, a substance which possesses in an eminent degree the
property of coagulating clay, the diminution of plasticity will be
obvious at once, even when in a wet condition. If now the mass be dried,
as in the previous case, it is easily pulverized. This is an
illustration of the effect of lime upon stiff lands, rendering them more
readily pulverulent and tillable. The conversion of the lime into a
carbonate in the above experiment by passing bubbles of carbonic acid
through the mass while still suspended in water does not restore the
original plasticity, thus illustrating experimentally the fact known to
all farmers that the effect of lime on stiff soil lasts for many years,
although the whole of the lime in that time has been converted into
carbonate.
=185. Practical Applications.=—The practical application of this is,
according to Hilgard, that the loosely flocculated aggregation of the
soil particles is what constitutes good tilth. For this reason the
perfect rest of a soil, if it is protected from the tamping influence of
rains and the tramping of cattle, may produce a condition of tilth which
cannot be secured by any mechanical cultivation. As an illustration of
this, the pulverulent condition of virgin soils protected in a forest by
the heavy coating of leaves may be cited. On the contrary, as pointed
out by Hilgard, there are some kinds of soil in which a condition of
rest may produce the same effect as tamping. These are soils which
consist of siliceous silt without enough clay to maintain them in
position after drying. In such a case, the masses of floccules collapse
by their own weight or by the least shaking, and fall closely together,
producing an impaction of the soil. This takes place in some river
sediment soils in which the curious phenomenon is presented of injurious
effects produced by plowing when too dry, which is the direct opposite
of soils containing a sufficient amount of clay and which are injured by
plowing too wet.
It is further observed that the longer a soil has been maintained in
good tilth, the less it is injured by wet plowing. This is doubtless,
according to Hilgard, due to the gradual cementation of the floccules by
the soil water which fixes them more or less permanently.
Whitney believes that the arrangement of the grains, or the condition of
flocculation in the soil, or the distance apart of the soil grains, is
determined, to a large extent, by the potential on the surface of the
grains; and he suggests that by changing this the exceedingly fine
grains of silt and clay can be pulled together or can be pushed further
apart, and so alter the whole texture of the land.
The action of alkaline carbonates in preventing flocculation, and thus
rendering tillage difficult or impossible, is pointed out by Hilgard in
the case of certain alkali soils of California. The soils which are
impregnated with alkaline carbonates are recognized by their extreme
compactness. The suggestion of Hilgard to use gypsum on such soils has
been followed by the happiest results. This gypsum renders any
phosphates present insoluble, and thus prevents loss by drainage, and
yet leaves the plant food in a sufficiently fine state as to be
perfectly available for vegetation.
=186. Suspension of Clay in Water.=—The suspension of clay in water and
the methods of producing or retarding flocculation and precipitation
have also been studied by Durham.[124] His experiment is made as
follows:
In a number of tall glass jars fine clay is stirred with water, and the
results of precipitation watched. In all cases it will be noticed that
the clay rapidly separates into two portions, the greater part quickly
settling down to the bottom of the jars, and the smaller part remaining
suspended for a greater or less length of time.
The power which water possesses of sustaining clay is gradually
destroyed by the addition of an acid or salt; a very small quantity, for
instance, of sulfuric acid, is sufficient to precipitate the clay with
great rapidity. In solutions of sulfuric acid and sodium chlorid of
varying strengths, suspended clay is precipitated in the order of the
specific gravity of the solutions, the densest solutions being the last
to clear up. This may be due to the greater viscosity of the denser
liquids.
The power which water possesses of sustaining clay is gradually
decreased by the addition of small quantities of certain salts and of
lime.
=187. Effect of Chemical Action=—Brewer[125] emphasizes the importance
of chemical action in the flocculation of clays. As expressed by him the
chemical aspects of the phenomena of sedimentation have either been
lightly considered or entirely ignored. Brewer is led to believe that
the action of clay thus suspended is analogous to that of a colloidal
body. Like a colloid, when diffused in water, the bulk of the mass is
very great, shrinking enormously on drying. He therefore concludes that
clays probably exist in suspension as a series of hydrous silicates
feebly holding different proportions of water in combination and having
different properties so far as their behavior to water is concerned.
Some of them he supposes swell up in water much as boiled starch does,
and are diffusible in it with different degrees of facility, and that
the strata observed on long standing of jars of suspended clay represent
different members of this series of chemical compounds which hold their
different proportions of combined water very feebly and are stable under
a very limited range of conditions.