SOIL AND SUBSOIL.
CAUSES AND PROCESS OF DIFFERENTIATION. HUMUS.
_Soil and Subsoil Ill-defined._—While the general mass of rock
debris formed by the action of the agencies heretofore discussed as
soil-material, may under proper conditions become soil capable of
supporting useful plant growth, universal experience has long ago
recognized and established the distinction between soil and subsoil:
by which are ordinarily meant, respectively, the portion of the
soil-material usually subjected to tillage, and what lies beneath.
There can be no question about the practical importance of this
distinction; but the definition of the two terms, as commonly given in
some works of agriculture, is both incomplete and, in its application
to many cases, partly misleading.
The differentiation of soil and subsoil is due partly to the action of
organic matter and micro-organisms, partly to physico-chemical causes,
now to be discussed in detail.
THE ORGANIC AND ORGANIZED CONSTITUENTS OF SOILS.
_Humus in the Surface soil._—The most obvious mark of distinction
between soil and subsoil is, usually, the darker tint of the former,
due to the presence of humus or vegetable mold, which becomes most
apparent by darkening of the tint when the soil is moistened. Thus
soils having a gray tint when dry, may become almost black when wetted.
When no such deepening of color occurs in wetting, the absence or great
deficiency of humus may safely be inferred. The only other substance
whose presence may invalidate the conclusions based upon the darkening
of the soil tint, is ferric hydrate (iron rust), which itself possesses
the property of darkening on wetting, and may effectually cover either
the presence or the absence of humus.
Since the formation of the humus depends upon the decomposition of
organic matter (mostly of the cellulose group) derived partly from
the roots, partly from the leaves and stems of plants growing and
dying on the soil, its accumulation near the surface is natural. But
since the depth to which roots penetrate varies greatly not only with
different plants, but very essentially in conformity with the greater
or less penetrability of the soil and subsoil, the depth to which the
dark humus tint may reach vertically varies correspondingly, from two
or three inches to several feet. In the case of soils that have been
formed by the gradual filling-up of swamps or marshes, the humus-tint
may reach to several yards depth.
_Surface Soil, and Subsoil._—It is thus apparent that the term “surface
soil,” while commonly confined by the farmer to the portion turned by
the plow or usually reached in cultivation by any implements, may or
may not belong, functionally, to layers of greatly varying thickness.
Similarly the term _subsoil_ may or may not refer, in individual
cases, to parts of the soil mass materially different from the surface
soil. Yet this distinction is of no mean practical importance, because
the efficacy of one of the most common measures of soil improvement,
viz., subsoil plowing or “_subsoiling_,” depends materially upon the
differences between soil and subsoil in each particular case. Most of
the diversity of opinion regarding the merits of this operation is
simply the result of a corresponding diversity in the natural facts and
cultural practice of each case.
_Causes of the Differentiation of Soil and Subsoil._—One of the
prominent points of difference between surface soils and subsoils has
already been mentioned in the usual predominance of root-mass in the
upper layers; to which is added a part at least of the substance of
fallen leaves and stems of its vegetation. How much of this vegetable
mass ultimately becomes converted into humus, as well as the nature of
the product formed, depends upon a great variety of circumstances; some
of which have already been mentioned in connection with the general
discussion of humification (chapt. 2, p. 20). Briefly stated, the main
controlling conditions are: the amount of water or moisture present,
the access of air (oxygen), a proper temperature, and the presence of
the several organisms which in the course of time take part in the
process of soil-formation.
_Ulmin Substances; Sour Humus_ (Germ. Rohhumus).—In the presence of
so much moisture or liquid water as will materially impede the access
of air, and with the concurrence of reasonably low temperatures, the
organisms that at first take the chief role in the transformation of
the vegetable tissues into humus-like substances are bacteria. But the
antiseptic nature of the compounds thus formed[36] soon puts an end
to their activity, and thereafter the process seems to be a purely
chemical one, and very slow. In peat bogs, the transition from the
fresh, dead stems and roots to brown peat is easily followed downward,
white cellulose fibers remaining apparently unchanged to some depth;
so that such fiber has been used for tissues and paper. The solid
decomposition-products are brown substances, partly soluble in water
and imparting to it a brown or coffee color (frequently seen in the
drains of marshes) and an acid reaction; the latter due to ulmic (as
well as apocrenic) acid, readily soluble in caustic and carbonated
alkalies, and forming insoluble salts with the earths and metals; while
another portion, ulmin, is insoluble in the same, but gradually becomes
soluble by oxidation.
[36] The antiseptic properties of sour humus are well exemplified in
the perfect state of preservation in which the remains of animals,
wood implements, etc., are found in bogs into which they have sunk in
prehistoric times.
The gaseous products formed under these conditions are carbonic dioxid
and “marsh gas” (methan, CH₄), the former predominating in the early
stages; while later, the carburetted hydrogen predominates, rendering
the gas readily inflammable.
_Sour Soils._—The “sour” soils thus produced in nature in presence of
excess of water bear only “sour” growth, such as sedges and rushes, of
little agricultural value; they usually require reclamation processes
before becoming adapted to ordinary crops. In old forests of northern
climates a peaty and more or less acid layer is sometimes formed on the
surface, above the black woods-earth, and retards somewhat the full
production of such land when taken into cultivation.[37]
[37] See Müller, Natürliche Humusformen.
Marshes and swamps, both fresh and salt, as above stated usually
show coffee-colored waters, which are also characteristic of the
streams that drain them, until by intermixture with waters containing
lime salts, the ulmic substances are neutralized and precipitated.
Such neutralization, preferably by means of lime, is the first step
towards the reclamation of lands bearing “sour” vegetation. The acid
reaction characterizing the ulmic substances is also characteristic
of many woodlands, notably in the United States of the soils of
the “Long-leaf-pine” region of the Cotton States, both upland and
lowland, as well as of many deciduous forests in northern climates.
Hence liming, whether artificial or natural, effects a most notable
improvement, together with a marked change of vegetation, in these
lands.
It has been long known that after long-continued
cultivation, soils originally of neutral or slightly basic
reaction become acid: and the liming of such lands is an
ancient practice in Europe. The matter, however, received
but scant attention until Wheeler and Hartwell, of the
Rhode Island Experiment Station, demonstrated the almost
universal acid condition of the older lands of that State,
and the excellent effects produced by neutralization with
lime, or even with the alkali carbonates.[38] The current
neutralization of the humus-acids is unquestionably one of
the cardinal advantages of calcareous lands; for such as
contain only small amounts of lime carbonate will of course
become acid more quickly under cultivation.
[38] Reports of the Rhode Island Exp’t Station, 1895, and ff.
_Humin Substances._—In the presence of only a moderate amount of
moisture, therefore under the influence of a more or less rapid
circulation of air, and in the presence of earthy carbonates
(especially that of lime) to prevent the formation of acids, or to
neutralize them as formed, the normal process of humification occurs;
mainly under the influence of fungous instead of bacterial growths. The
various molds take a prominent part in the conversion of the vegetable
substance into black, neutral, insoluble humus compounds. Such fungous
vegetation is always accompanied by the evolution of carbonic gas,
and the resulting fungous tissues are markedly richer in nitrogen and
carbon than the substance of the higher plants from which they were
derived (see chapt. 9). Comparative analyses show that in the normal
process of humification of vegetable substances, oxygen and hydrogen
are eliminated in the form of water and carbonic dioxid, while at
the same time there is an increase in the percentage of carbon, and
generally also of nitrogen; the latter more particularly in the case of
vegetable matter not very rich in that element. When once humification
is complete, oxidation, especially under arid conditions, bears mainly
upon the carbon and hydrogen, so that the nitrogen content may rise to
very high figures; while another portion is ultimately wholly oxidized,
with the formation of nitrates, under the influence of the nitrifying
bacteria, this being the process chiefly efficient in the nutrition of
vegetation with nitrogen.
As a matter of course, the several organic compounds
contained in plants may continue to exist in soils for
some time, varying according to conditions of temperature
and moisture. Thus dextrin, glucose, and even lecithin and
nuclein have been reported to be found. The activity of
the numerous fungous and bacterial ferments under favoring
conditions will, of course, limit the continued existence of
such compounds somewhat narrowly, so that they can hardly be
considered as active soil ingredients save in so far as they
favor the development of the bacterial flora.
_Porosity of Humus._—One of the essential features of natural humus is
its great porosity, whereby it not only becomes highly absorbent of
water and gases, but is also gradually oxidized, probably under the
influence of bacteria. For this oxidation, as measured by the evolution
of carbonic gas, progresses most rapidly under the same conditions
as to moisture, temperature and access of air, that are known to be
most favorable to fungous and bacterial growth. Hence the formation
of carbonic dioxid in the soil is assumed to be the measure of the
intensity of such activity.
_Physical and Chemical Nature of the Humus Substances._—The humus
substances are gelatinous when moist, but are neither markedly adhesive
or plastic. Like the other colloidal substances of the soil, they serve
to retain both gases and vapors, including moisture, liquid water,
and its dissolved solids. In the natural, porous condition they are
powerfully absorbent of gases, including especially aqueous vapor.
Dry humus swells up visibly when wetted, the volume-weight increasing
to the extent of two to eight times; so that humus stands foremost in
this respect among the soil constituents. The _density_ of natural
humus is about 1.4, being the lightest of the soil constituents. Hence
soils rich in humus are “light” not only in the farmer’s sense of being
easily tilled when not too wet, but also of light weight for equal
volumes when compared with clayey and sandy soils. Some data bearing
upon these points are given in the table[39] below, for the substances
moderately and uniformly packed:
VOLUME-WEIGHTS OF
Humus.[40] Clay. Quartz Sand.
.3349 1.0108 1.4485
When saturated with water, the same substances gave the following
figures:
Air-dry. Saturated Increase.
with water. %
Humus[41] .3565 1.1024 209.2
Clay 1.0395 1.6268 55.9
Quartz sand 1.4508 1.8270 25.9
[39] Wollny, Zersetzung der Organischen Stoffe, pp. 242, 243.
[40] Peat pulverized and extracted with alcohol and ether to remove
resinous substances.
[41] Peat pulverized and extracted with alcohol and ether to remove
resinous substances.
These data show strikingly the effects produced by the several physical
soil constituents upon some of its physical properties.
_Chemical Nature._—While humus artificially produced by the action of
caustic alkalies upon sugar or cellulose is free from nitrogen, all
naturally occurring humus contains the latter.
It is not, however, present in the form of ammonia, as it
cannot be set free by treatment in the cold with lime or
alkalies. When, however, natural humus is _boiled_
with these substances, ammonia is slowly given off, but the
process continues indefinitely and it seems to be impossible
to expel all the nitrogen in this manner. This behavior
being characteristic of amido-compounds, it is presumable,
in view of the slightly acid nature of the humus substances,
that natural humus is largely of an amidic constitution.
Artificial humic acid, formed by the action of caustic
alkalies upon sugar, gums or cellulose, combines with
ammonia as with other bases, and at first the ammonia
can be readily expelled from this as from other ammonia
salts. But after the lapse of some time it seems that the
amidic condition is assumed, so that caustic lye acts but
very slowly and cannot expel the whole of the nitrogen
present. This is very important in connection with the
practice of fertilization, as any ammonia taken up by or
generated in the soil is thus in the course of time rendered
comparatively inert, and unavailable to vegetation until
nitrified.
_Progressive Changes._—The natural neutral humin and ulmin, as found,
_e. g._, in the lower portions of peat beds, are in the course of time
by oxidation converted into ulmic and humic acids, capable of combining
with bases; by still farther oxidation they form apocrenic and crenic
acids, readily soluble in water and in part forming soluble salts with
lime, magnesia and other bases. These acids act strongly upon the more
readily decomposable silicates of the soil, and in the course of time
may dissolve out, and aid in the removal by leaching, of most of the
plant-food ingredients as well as the ferric hydrate of a soil. Thus
red or rust-colored soils may be rendered almost white by continued
“swamping” with stagnant water, and be greatly impoverished; and it is
doubtless largely through this agency that the underclays of coal beds
and the lower portions of peat beds, as well as peat and coal ashes,
are almost wholly destitute of mineral plant food.
_The Phases of Humification._—The progressive changes involved in
the process of humification of vegetable matter are illustrated in
the table below,[42] together with the farther changes by which such
matter may ultimately be transformed into the several varieties of
coal, and finally into anthracite, which already represents nearly pure
carbon, but in nature has sometimes been still farther transformed into
graphite (black-lead) and diamond.
[42] Data recalculated, omitting ash.
PROGRESS OF HUMIFICATION, AND FORMATION OF COAL.
(MOISTURE AND ASH OMITTED FROM CALCULATIONS.)
========+==========+======================+============+
| | Oak Wood. | |
| +------+--------+------+ Humin |
|Cellulose.|Fresh.|Decayed.| | and |
| | +--------+------+ Humic |
| | | Light | Dark | Acid. |
| | | Brown. |Brown.| |
--------+----------+------+--------+------+------------|
Carbon | 44.44 | 50.60| 53.60 | 56.20|49.4 to 59.7|
Hydrogen| 6.17 | 6.00| 5.20 | 4.90| 2.5 “ 4.5|
Oxygen | 49.38 | | | |35.8 “ 47.3|
| | 43.40| 41.20 | 38.90| |
Nitrogen| | | | | .3 “ 18.7|
--------+----------+------+--------+------+------------+
========+======================+================================
| Peat.[43] | Coals.
|--------+-------------+--------+-----------+-----------
| Brown | Black. | Lignite| Scotch | Penn’a
|Surface.+------+------+ Brown | Splint |Anthracite.
|(Ulmin.)|40 in.|80 in.| Coal. |Bituminous.|
| | | |(Bovey).| |
--------+--------+------+------+--------+-----------+-----------
Carbon | 57.80 |62.00 |64.10 | 69.50 | 84.20 | 94.80
Hydrogen| 5.40 | 5.20 | 5.00 | 5.90 | 5.80 | 2.60
Oxygen | 36.00 |30.70 |26.80 | 24.00 | 8.80 | }
| | | | | | } 2.60
Nitrogen| .80 | 2.10 | 4.10 | .60 | 1.20 | }
+-------+--------+------+------+--------+-----------+-----------
[43] Detmer, Landw. Versuchst., Vol. 14, 1871.
The steady increase of carbon and nitrogen, together with a
corresponding decrease of oxygen, are well illustrated in the analyses,
especially in the strictly comparable series of peat samples from
various depths. In this case there is also a steady decrease of
hydrogen, and an increase of ash from 2.72% in the surface layer, to
9.16 at 80 inches depth. This increase is due in the main, of course,
to the progressive volatilization of the organic matter in the forms of
carbonic dioxid and marsh gas (methan, CH₄).
In considering this table it should not be forgotten that while normal
humus stands very close to peat, and the latter when compressed in
certain stages would be undistinguishable from lignite or brown coal;
yet both peat and lignite are known to be formed under conditions
permitting much less access of air or oxygen than occurs in the
formation of normal black soil-humus. Hence even black peat cannot at
once stand in place of soil-humus when removed from its watery bed, but
requires considerable time and aeration (oxidation), and in most cases
neutralization with lime or marl, before it can serve the purposes of
humus in the soil.
Lignite and the progressively more carbonaceous coals
are and have been formed under the conjoined action of
submergence and pressure, sometimes also aided by heat; and
thus they cannot perform the function of soil-humus, any
more than the fire-clays or shales underlying them can resume
their original soil-functions without prolonged weathering.
_Amounts of Humus and Coal Formed from Vegetable
Matter._—Only very general and indefinite estimates
can be given of the amount of humus or coal formed from a
given quantity of vegetable matter, since these must vary
according to the conditions under which the transformation
occurs. The greater or less access of air and of moisture,
the temperature and pressure under which the process occurs,
will modify very materially the quantitative as well as
the qualitative result. In the hot arid regions the fallen
leaves may wholly disappear by oxidation on the surface of
the ground, while under humid conditions they are mostly
incorporated with the surface soil. If we assume that in
the humification of plant debris (estimating their average
nitrogen content at 1%), no nitrogen is lost, it would seem
that in the humid region one part of normal soil-humus might
be formed from 5 to 6 parts of (dry) plant debris; while
in the extreme regime of the arid regions, from 18 to 20
parts of the same would be required. But as most probably
some nitrogen also is lost in the process of humification, a
considerably larger proportion of original substance may be
actually required.
[Illustration: FIG. 14.—Section of lignitized log showing contraction
into solid lignite on drying.]
As to coal, it is usually assumed that it requires about
8 parts of vegetable matter for one of bituminous coal.
Much higher estimates are made by some, and an observation
made by the writer at the Port Hudson bluff, Mississippi,
in 1869, would seem to justify such estimates. The
above figure, from a sketch made at the time, shows the
proportions to which a pine log about eight inches in
diameter had shrunk in drying into a small sheet of
lignitized wood; the original trunk, projecting from a bed
of sand some forty feet below the surface, being so porous
and spongy that when wet it flattened somewhat by its own
weight; it was connected with the little sheet of lignite by
a spirally twisted, tapering stipe.
Here evidently the proportion of lignite formed was a very
minute one, doubtless because of the long leaching to which
the trunk had been subjected. It thus seems impossible, as
in the case of humus, to assign any definite proportion as
between woody matter and coal formed from it.
_Normal humification_ takes place only under the influence of moderate
temperature. When the temperature is too low, bacterial and fungous
growth are repressed or arrested; when too high, the fungous vegetation
assumes a different phase, the result of which is the almost total
oxidation of the organic matter, sometimes so accelerated as to
initiate rapid combustion “fire-fanging” of dung; leaving in any case
but a trifling organic residue of very high ash contents.[44]
[44] A striking illustration of this is afforded by Naegeli’s
experiment of enclosing several loaves of bread in a loosely closed
tin-box. After eighteen months there remained only seventeen per cent
of air-dry mouldy matter, totally destitute of starch.
_Eremacausis._—In the absence of a sufficient degree of moisture to
co-operate with the other agencies of humification, the final result
in the soil is practically the same as in the “fire-fanging” of dung.
The organic matter is almost wholly destroyed by direct oxidation
(eremacausis) with or without the aid of minute organisms; leaving
essentially only the ash behind to be reincorporated with the soil.
This is to a very great extent the predominant process in the arid
regions of the Globe; most of the soils formed in these climates being,
therefore, very poor in humus-substances, and deriving it almost
entirely from the decay of roots only.
The extent to which the humus of a soil may be derived from the
vegetable debris falling or growing upon the surface, varies greatly
with the climatic conditions as well with the nature of the soil. In
the forests of humid climates with loamy soils, not only does the
autumnal leaf-fall, as well as decaying twigs and trunks, become
obviously incorporated with the surface soil as decay progresses on the
lower surface, but active animal agencies (see below) carry the organic
remnants bodily down. But where heavy clay soils prevail, these animal
agencies are much restricted by the compactness of the material; only a
light surface-layer of mold would be formed, and the humus of the lower
soil layers must of necessity be derived from the decay of the roots
only. This origin is claimed by Kosticheff[45] for the high content
of black humus in the tchernozem or black earth of Russia. Following
Hellriegel in determining the weight of roots contained in successive
equal layers of soil from the surface downwards, Kosticheff gives for
each six inches the following data as found in the tchernozem, taking
as 100 the root-content of the surface layer:
===========+======+======+======+======+======+======
Number. | 1 | 1 | 2 | 2 | 3 | 3
Depth. |Roots.|Humus.|Roots.|Humus.|Roots.|Humus.
-----------+------+------+------+------+------+------
6 inches. |100. | 5.42 |100. | 8.11 |100. | 9.64
12 “ | 89.1 | 4.83 | 63.9 | 5.19 | 80.3 | 7.77
18 “ | 66.9 | 3.62 | 48.3 | 3.92 | 70.0 | 6.71
24 “ | 47.3 | 2.56 | 35.0 | 2.84 | 58.4 | 5.61
30 “ | 47.3 | 2.59 | 26.0 | 2.11 | 38.2 | 3.57
36 “ | 34.6 | 1.88 | 18.1 | 1.47 | 33.0 | 3.18
42 “ | 23.9 | 1.29 | 6.3 | .51 | 16.2 | 1.56
48 “ | 14.4 | .78 | | .70 | |
54 “ | 6.7 | .36 | | | |
-----------+------+------+------+------+------+------
[45] Abstract in Ann. de la Science Agronomique, Tome 2, 1887.
It will be seen that there is a very close correspondence of
the humus content with the root development in the several
layers, and it seems as if though but little of the humus
could be derived from the surface growth, which is that of
the grasses of the steppe.
The climate of the black-earth country of Russia is, though
not properly arid, yet one of rather deficient and uncertain
rainfall. But as a consequence of extremely arid conditions,
and in sandy lands, it may even happen that the immediate
surface soil contains _less_ humus than what, in the
farmers’ habitual parlance, would be called the subsoil;
because of the penetration of slow combustion for some
distance into the porous soils. It will then be lower down
that, in the presence of a favorable degree of moisture and
lower temperature, the conditions of normal humification are
fulfilled.
It is not always, then, that the commonly recognized distinction
between surface soil and subsoil based upon humus content can be
maintained. But the observation of everything bearing upon this point
is of the utmost importance in determining both the agricultural value
and the mode of treatment of the land.
_Losses of Humus from Cultivation and Fallowing._—The fact that humus
accumulates in woodlands and meadows, where no cultivation is given,
would naturally lead to the converse conclusion, viz., that cultivation
causes loss of humus and of its constituents. That this is actually
the case is recognized and widely acted upon in practice, and there is
no question that the general acceptance of stable manure as the most
widely useful fertilizer, despite its usually low content of plant-food
ingredients, is based upon the fact that it supplies vegetable matter,
in a condition highly favorable to its conversion into humus. The most
direct and cogent proof of the depletion of the soil of both humus and
nitrogen by continuous cultivation of cereal grains has been given
by Snyder,[46] who determined the loss both of humus and of nitrogen
suffered by a Minnesota soil during eight years’ continuous cultivation
of wheat. The total loss of nitrogen was 1700 pounds per acre, while
only 350 pounds were utilized by the crop; about 1400 pounds being
dissipated as gas or leached out as nitrates. A conservative estimate
of the loss of humus suffered during the same period was about a ton
per acre annually, and this loss seriously decreased not only the
nitrogen-content, but rendered the soil more compact and less retentive
of moisture. But by rotation of the wheat with clover in alternate
years, very nearly an equilibrium of both humus and nitrogen-content
was obtained. In addition, the amount of available mineral plant-food
was decreased by continuous grain culture. Ladd has made similar
observations in North Dakota, with similar results.
[46] Bull. No. 70 Minn. Exp’t Station, 1905.
That excessive aeration results in serious losses of humus as well as
of nitrogen, is very obvious in the arid region, where it is the habit
to maintain on the surface of orchards and vineyards during the dry,
hot summers, a thick mulch of well-tilled soil, thus preventing loss of
water by evaporation. In the course of years this surface soil becomes
so badly depleted of humus that good tilth becomes impossible, the soil
becoming light-colored and compacted; while the loss of nitrogen is
indicated by the small size of the orchard fruits. Similar losses are
of course sustained in the practice of bare summer-fallow, which at one
time was almost universal in portions of the arid region. The complete
extirpation of weed growth thus brought about, at first considered an
unmixed benefit, has ultimately had to be made up for by the practice
of green-manuring; since in the arid region the use of stable manure
encounters many difficulties.
_Estimation of Humus in Soils._ It has been usual to
determine the amount of humus in soils by means of (dry or
wet) combustion, calculating the humus from the carbonic
dioxid so formed, while measuring the nitrogen gas directly.
But in this process the entire organic matter of the soil,
humified and unhumified, is indiscriminately included;
and it is wholly uncertain to what extent the latter will
ultimately become humus, from the nitrification of which
plants are presumed to chiefly derive their nitrogen.[47] In
order to obtain definite results, the actual, functional
humus must be extracted from the soil mass by some solvent
which discriminates between the humified and unhumified
organic matter. This cannot be done by direct extraction
with caustic soda or potash, which inevitably dissolve
unhumified matters and tend to expel ammonia from the humus;
besides themselves acting as humifiers (see this chapter, p.
125.)
_Grandeau Method: Matière Noire._—The only method
now known which accomplishes this separation, practically
excluding the unhumified while fully dissolving the humified
matter—is that of _Grandeau_: the extraction of the
soil, first with dilute acid, in order to set the humic
substances free from their combinations with lime and
magnesia; and their subsequent extraction with moderately
dilute solutions of ammonia (or other alkali hydrates). Upon
the evaporation of the ammonia solution the humus is left
behind in the form of a black lustrous substance (“matière
noire” of Grandeau) much resembling the crust of soot formed
in flues from wood fires. As it contains a variable amount
of ash, it must be burnt and the ash subtracted from the
first weight.
[47] The humus determinations thus made, which include nearly all
those made by German chemists, give the humus-content from 40 to 50%
too high. The French determinations are mostly made by the method of
Grandeau.
_Amounts of Humus in Soils._—While in peat, marsh and muck lands the
humus-content may rise above twenty per cent, in ordinary cultivated
lands it rarely exceeds about five per cent, and very commonly falls
below three per cent, even in the humid regions. In properly arid soils
we find a very much lower average, rarely exceeding one per cent,
and frequently falling to .30 and even less. This scarcity of humus
manifests itself plainly in the prevalently light gray tint of the arid
soils.
Meadows and woodlands generally show the highest humus-content in their
surface soils, gradually increasing while in that condition; while when
taken into cultivation the humus-content gradually decreases, owing
to the free aeration and consequent “burning-out” caused by tillage.
Hence the humus must be from time to time replaced by the use of stable
manure, or green-manure crops, to prevent injurious changes in the
tilling qualities of the land. Not only humus as such, but according to
Schloesing also the insoluble colloid humates, produce in the soil a
loosening effect or tilth (Germ. Bodengare), which apparently cannot be
brought about by any other substance.[48]
[48] The decrease of humus from wheat culture in the soils of
Minnesota and North Dakota has been studied by H. Snyder and E. F.
Ladd, respectively. In the prairie lands of the latter State the total
organic matter in the first six inches of soil ranges from 15 to as
much as 26%, and the humus alone from 4 to 7.8%.
_Humates and Ulmates._—That the insoluble humates of lime, magnesia,
iron, manganese and alumina are present in most soils is conclusively
shown by the composition of the solution obtained by the extraction
of soils with weak acid, as above mentioned in connection with the
quantitative determination of humus according to Grandeau; since
these bases are almost always extracted by the weak acid. When the
brown solution of alkali humate obtained in this process is carefully
neutralized with sulfuric or hydrochloric acid, or is mixed with
solutions of the above bases, flocculent, insoluble precipitates are
formed, while the solution is discolored. Similar precipitates may be
obtained with other metallic solutions, notably with that of copper,
which precipitates the humus-acids most completely. Doubtless these
compounds contribute greatly to the conservation of the humus-content
of soils, protecting it to a certain extent from oxidation, and also
preventing excessive acidity. The brown tint of certain subsoils in
the northern humid regions have been shown by Tollens and others to
be due not to ferric hydrate, as had been supposed, but to calcic,
magnesic and aluminic humates. _None of the mineral bases or acids
present can be detected in the humic solution by the usual reagents._
_Mineral Ingredients in Humus._—That the mineral plant-food ingredients
present in the humus extracted by the Grandeau process, and which
remain as ash when the matière noire is burned, are capable of
nourishing plant growth, was directly shown by Grandeau, Snyder and
others. The former was inclined to consider that those substances were
mainly thus taken up by plants, under natural conditions. This theory,
however, has not been sustained by subsequent investigations; the
mineral plant-food thus extracted is not a measure of the immediate
productiveness of the soils, as demonstrated by Snyder, and the
residual soils are not sterile. It is also still doubtful to what
extent the mineral bases and acids are _naturally_ combined with the
humus-substances, it being contended by some that they are brought
into organic combination by the acid and ammonia extraction. The
investigations of Snyder and Ladd, above referred to, prove however to
some extent at least that the humus-substances are naturally combined
with them, and that probably they are largely made available to plants
through the direct and indirect action of the humus compounds. This
subject is farther considered in chapter 19.
The nature and amounts of these mineral substances are well exemplified
in the subjoined full analysis by Snyder, of the ash of the humus and
humates extracted from a compound sample of prairie soils of Minnesota,
which had been thrown down from the ammonia solution by simply
neutralizing the liquid:[49]
ASH OF HUMUS FROM MINNESOTA PRAIRIE SOILS.
Insoluble matter[50] 61.97
Potash (K₂O) 7.50
Soda (Na₂O) 8.13
Lime (CaO) 0.09
Magnesia (MgO) 0.36
Peroxid of Iron (Fe₂O₃) 3.12
Alumina (Al₂O₃) 3.48
Phosphoric acid (P₂O₅) 12.37
Sulfuric acid (SO₃) .98
Carbonic acid (CO₂) 1.64
[49] Precipitation with an excess of acid does not greatly change the
results.
[50] In California soils this is mostly silica soluble in carbonate of
soda.
The large amounts of the soluble alkalies potash and soda thrown
down with the humic matters are very striking, as is the very large
proportion of phosphoric acid. Lime and magnesia had, of course, been
mainly eliminated by the preliminary acid treatment.
_Functions of the Unhumified Organic Matter._—The unhumified plant
debris in the soil are not to be regarded as useless, even aside
from their potential conversion into active humus. Not only do these
remnants of vegetation lighten the soil, rendering it more pervious
to air and water, but in their progressive decay they give off
carbonic gas, which is active in soil-decomposition; and they serve as
nourishment to the soil bacteria upon which its thriftiness so greatly
depends. See below, chapter 9.
_The Nitrogen-Content of Humus._—Since soil-humus is doubtless the
chief depository of soil-nitrogen, and the main source from which,
through the process of nitrification, the nitrogen-supply to plants
is usually derived, its content of that element is a matter of
great interest. It has been customary to estimate approximately the
nitrogen-content of soils by the proportion of humus-substance present;
and as the light tints of the soils of the arid region indicate a small
humus-content, a scarcity of nitrogen seemed to be also indicated
for these lands. As this in a number of cases did not seem to accord
with actual experience, an investigation of the subject was made at
the California experiment station,[51] with the results shown in the
subjoined table. In considering these results it must be kept in mind
that while arid conditions can rarely be fulfilled in the humid region,
humid conditions are quite frequently locally represented in the arid,
in lowlands and on high mountains; while moderately moist benchlands
represent the semi-arid regime.
[51] _Hilgard and Jaffa._ On the Nitrogen-content of Soil-humus in the
Humid and Arid regions. Rep. Cal. Exp’t Station for 1892-4; Agric.
Science, April, 1894; Wollny’s Forsch. Geb. Agr. Phys., 1894.
HUMUS PERCENTAGE AND NITROGEN CONTENT IN
SOILS OF THE ARID AND HUMID REGIONS.
=====+==============================================+=====+========+========
| |Humus|Nitrogen|Nitrogen
Station| Soils arranged in order of nitrogen | in | in | in
Number| percentages in humus. |soil,| Humus, |soil,
| | per | per | per
| |cent.| cent. |cent.
-----+----------------------------------------------+-----+--------+--------
| SOILS OF THE ARID REGION (California). | | |
| | | |
2061 |Dark clay loam, Arroyo Grande Valley, | 3.06| 22.00 | .670
| San Luis County | | |
2291 |Red soil, Orland, Glenn Co. | .71| 21.10 | .150
1904 |Sediment Soil, Porterville, Tulare Co. | .90| 19.50 | .180
1901 |Sandy soil near Ceres, Stanislaus Co. | .64| 18.75 | .120
704 |Sandy soil of plains, near Fresno, Fresno Co. | .60| 18.66 | .112
6 |Black adobe soil, Stockton, San Joaquin Co. | 1.05| 18.66 | .196
1679 |Black adobe soil, Berkeley, Alameda Co. | 1.20| 18.58 | .203
2324 |Clay soil of desert, Imperial, San Diego Co. | .38| 18.40 | .070
1167 |Black clay loam soil, near Tulare, Tulare Co. | 1.66| 18.19 | .302
1536 |Brown loam soil, Windsor Tract, Riverside, | .20| 18.00 | .036
| Riverside County | | |
1126 |Sandy loam soil, Paso Robles, | .55| 17.27 | .095
| San Luis Obispo Co. | | |
2301 |Red hill soil, Upper Lake, Lake Co. | .81| 16.90 | .137
1607 |Plateau soil of desert, Lancaster, | .25| 16.80 | .042
| Los Angeles Co. | | |
1159 |Sandy plains soil, Tulare, Tulare Co. | .37| 16.75 | .062
1900 |Sandy soil, near Modesto, Stanislaus Co. | .84| 16.65 | .140
1113 |Clay loam soil (slate), Jackson, Amador Co. | .54| 16.60 | .090
1149 |Adobe clay soil, near Paso Robles, | .47| 16.18 | .074
| San Luis Obispo County | | |
1538 |Mesa soil, Chino, San Bernardino Co. | .65| 16.08 | .105
1147 |Sandy loam soil, Paso Robles, | .66| 16.06 | .106
| San Luis Obispo Co. | | |
2403 |Valley Soil, Wheatland, Yuba Co. | 1.50| 16.00 | .240
1281 |Red Mesa soil, Pomona, San Bernardino Co. | .58| 15.50 | .090
1117 |Sandy granitic soil, near Jackson, Amador Co. | .80| 15.27 | .123
1406 |Red loam soil, Arlington Heights, Riverside, | .30| 15.00 | .045
| Riverside County | | |
1172 |Red clay loam soil, east of Tulare, Tulare Co.| .72| 14.75 | .106
1958 |Sandy Mesa soil, Nipomo, San Luis Obispo Co. | .85| 14.45 | .122
1423 |Chocolate-red soil, Carisa plain, | .39| 14.36 | .056
| San Luis Obispo County | | |
1291 |Sandy hill land, near Jackson, Amador Co. | .76| 14.34 | .109
585 |Wire-grass loam soil, Visalia, Tulare Co. | 1.00| 14.10 | .146
863 |Red ridge loam soil, Grass Valley, Nevada Co. | 2.89| 13.91 | .402
1907 |Dark loam soil, near Chino, San Bernardino Co.| .92| 13.26 | .121
1115 |Sandy granitic soil, near Jackson, Amador Co. | .85| 13.20 | .112
332 |Plateau desert soil, Mojave, Los Angeles Co. | .28| 12.50 | .035
2126 |Gravelly soil, East Highlands, | .62| 11.75 | .070
| San Bernardino Co. | | |
1910 |Ojai Valley soil, Nordhoff, Ventura Co. | 1.64| 11.21 | .183
2187 |Sandy loam soil, Soledad, Monterey Co. | .97| 11.10 | .110
1759 |Sandy soil, Perris Valley, Riverside Co. | .53| 11.04 | .059
774 |Bench slope soil, Ontario, San Bernardino Co. | 1.29| 10.85 | .140
1984 |Red soil, East Highlands “ “ “ | .58| 10.50 | .060
2325 |Silt soil of desert, Imperial, San Diego Co. | .65| 10.70 | .070
1906 |Light sandy soil, Pomona, San Bernardino Co. | .95| 9.80 | .093
2430 |Hillside adobe, Berkeley, Alameda Co. | 1.85| 8.70 | .160
| +-----+--------+--------
| Average of arid uplands | .91| 15.23 | .135
| | | |
| SUB-IRRIGATED ARID SOILS (California). | | |
| | | |
586 |Sandy plains soil, Tulare, Tulare Co. | 1.14| 10.79 | .123
1466 |Loam soil, Miramonte, Kern Co. | .60| 10.66 | .064
1284 |Moist land loam soil, Chino, | 1.99| 10.20 | .203
| San Bernardino Co. | | |
1148 |Swale soil, near Paso Robles, | 1.16| 9.65 | .112
| San Luis Obispo Co. | | |
1714 |Bench soil, Santa Clara River, Piru, | .78| 9.56 | .074
| Ventura Co. | | |
77 |Alluvial soil, Tulare Lake bed, Tulare Co. | .47| 9.37 | .045
1880 |Creek bench soil, Niles, Alameda Co. | 1.19| 8.90 | .109
1903 |Sediment soil, Porterville, Tulare Co. | 1.12| 8.50 | .140
168 |Alluvial soil, Santa Clara river, Santa Paula,| .84| 7.99 | .067
| Ventura Co. | | |
1760 |Green-sage land, Perris Valley, Riverside Co. | .91| 7.70 | .070
506 |Alluvial soil, Colorado River, Yuma, | .75| 7.47 | .050
| San Diego Co. | | |
1636 |Red soil, Manton, Tehama Co. | 2.00| 6.86 | .137
1758 |Alkali soil, Perris Valley, Riverside Co. | .60| 6.83 | .071
1963 |Sandy loam soil, Willows, Glenn Co. | .36| 6.05 | .022
2080 |Sandy soil, Santa Maria Valley, | 1.64| 5.36 | .090
| Santa Barbara Co. | | |
| | | |
| Average of sub-irrigated arid soils | 1.06| 8.38 | .099
| | | |
| HUMID SOILS FROM ARID AND HUMID REGIONS | | |
| (California). | | |
| | | |
207 |Eel River Alluvial soil, Ferndale, | 1.25| 6.96 | .085
| Humboldt Co. | | |
2319 |Alluvial soil, Hupa Valley, Humboldt Co. | 7.83| 6.70 | .514
213 |Marsh soil, Novato, Meadows, Marin Co. | 1.54| 6.36 | .089
1704 |Valley soil, Hollister, San Benito Co. | .94| 5.21 | .049
2295 |Tule soil, Upper Lake, Lake Co. | 1.70| 4.50 | .077
110 |Alluvial soil, Putah Creek, Dixon, Solano Co. | 1.71| 4.25 | .072
37 |Redwood Valley soil, Pescadero, San Mateo Co. | 2.28| 3.07 | .070
| | | |
| Average for California | 2.45| 5.29 | .135
| | | |
| OTHER STATES. | | |
| | | |
26 |Bog soil, Michigan[52] |33.02| 6.08 | 2.012
|Back-land clay loam, Houma, Louisiana | 5.07| 4.20 | .218
|Duff soil, Oregon |13.84| 3.49 | .483
|Sandy prairie soil, Harris Co., Texas | 2.13| 3.66 | .184
| | | |
| Average for other States | 7.01| 3.78 | .295
| | | |
23 |Red soil, Oahu Island, Hawaii (maximum) | 1.57| 5.07 | .078
27 |Guava soil, Hawaii Island (minimum) | 9.95| 1.71 | .170
|Average of 5 soils, Oahu Island | 3.01| 6.07 | .237
|Average of 2 soils, Maui Island | 9.07| 2.13 | .286
|Average of 4 soils, Hawaii Island | 6.17| 2.54 | .146
| | | |
| Average for Hawaiian Islands | 5.26| 3.69 | .169
| | | |
| Total for Humid soils, average | 4.58| 4.23 | .166
-------+----------------------------------------------+-----+--------+-------
[52] Introduced only for comparison of the nitrogen percentage in Humus
and not included in the average.
It thus appears that on the average the humus of the arid soils
contains about three and a half times as much nitrogen as that of the
humid; that in the extreme cases, the difference goes as high as over
six to one (see Nos. 37 and 704); and that in the latter cases, the
nitrogen-percentage in the arid humus considerably exceeds that of the
albuminoid group, the flesh-forming substances.
It thus becomes intelligible that in the arid region a humus-percentage
which under humid conditions would justly be considered entirely
inadequate for the success of normal crops, may nevertheless suffice
even for the more exacting ones. This is more clearly seen on
inspection of the figures in the third column, which represent the
product resulting from the multiplication of the humus-percentage of
the soil into the nitrogen-percentage of its humus; as appears in
comparing the respective averages, or Nos. 1167 and 110 and others. An
additional consideration is the probable greater ease with which the
nitrifying bacteria can act upon a material so rich in nitrogen.
We must not, then, be misled by the smallness of many humus-percentages
in the arid region, into an assumption of a deficiency in the supply of
soil-nitrogen.
_Decrease of Nitrogen-Content in Humus with
Depth._—Since the oxidation of the carbon and hydrogen
in the humus-substance, and the consequent increase of its
relative nitrogen-content, are manifestly dependent upon the
presence of air and heat, it is reasonably to be expected
that the nitrogen-percentage of the humus should decrease
with the depth of the soil. That this is really the case
is plainly shown in the subjoined table, which gives the
humus-percentages and the nitrogen-content of the humus from
the surface foot down to twelve feet, in a soil on the bench
of the Russian River, Cal., which is sub-irrigated, and
liable to more or less rainfall during the summer. It will
be seen that not only does the absolute humus-percentage
decrease quite regularly down to seven feet, at which point
there evidently was at one time a strong root development,
causing a notable increase of the humus-content; from which
again there is a regular decrease down to the twelfth
foot. It will be noted that the nitrogen-percentage in the
humus, while not decreasing with the same regularity as the
humus-content itself, yet exhibits a general recession from
5.30 to 1.15 in the ninth foot, to which direct oxidation
doubtless never penetrates.
HUMUS AND NITROGEN-CONTENT OF RUSSIAN RIVER SOIL.
==============+==============+=================+================
Depth in feet.| Per cent | Per cent | Per cent
| Humus | Nitrogen | Humus-Nitrogen.
| in soil. | in Humus. | in soil.
--------------+--------------+-----------------+----------------
1 | 1.21 | 5.30 | .064
2 | 1.16 | 4.32 | .054
3 | 1.14 | 3.87 | .044
4 | 1.17 | 3.76 | .044
5 | .74 | 2.16 | .016
6 | .60 | 2.66 | .016
7 | .47 | 2.54 | .012
8 | .78 | 1.54 | .012
9 | .54 | 2.24 | .012
10 | .52 | 1.15 | .006
11 | .53 | 1.51 | .008
12 | .44 | 1.81 | .008
--------------+--------------+-----------------+----------------
_Influence of the Original Materials on the composition of Humus._—The
great variability of the composition of humus formed from different
substances is well shown in the subjoined table, representing the
results of experiments made by Snyder,[53] who caused various
substances to humify by mixing the pulverized material intimately with
a soil poor in humus, and allowing the process to continue for a year.
At the end of that time the humus formed was extracted by the method of
Grandeau, outlined above, and analyzed, with the following results.
========+======+======+=======+======+========+=======+==========
|Sugar.| Oat | Green |Wheat |Sawdust.| Meat | Cow
| |Straw.|Clover.|Flour.| |Scraps.|Manure.[54]
--------+------+------+-------+------+--------+-------+----------
Carbon | 57.84| 54.30| 54.22| 51.02| 49.28 | 48.77| 41.93
Hydrogen| 3.04| 2.48| 3.40| 3.82| 3.33 | 4.30| 6.26
Nitrogen| 0.08| 2.50| 8.24| 5.02| 0.32 | 10.96| 6.16
Oxygen | 39.04| 40.72| 34.14| 40.14| 47.07 | 35.97| 45.63
--------+------+------+-------+------+--------+-------+----------
|100.00|100.00| 100.00|100.00| 100.00 | 100.00| 100.00
--------+------+------+-------+------+--------+-------+----------
[53] Bull. No. 53, Minn. Exp’t Station, p. 12, Chem. of Soils and
Fertilizers, p. 94.
[54] The figures for cow manure are so far out of range with any others
thus far observed, that it seems reasonable to suppose that they are
influenced by unchanged substances present in the excreta.
While it may be questioned whether the process of humification had in
these materials really reached the point of sensible completion in
all cases (notably in those of sawdust and cow manure), the great
variability of the products from different materials is very striking.
When the nitrogen-content is deducted the percentage composition of
the products agrees more nearly. Considering that the nitrogen is
probably present in the amid form, it is natural that hydrogen should
in a measure vary with it, as in the case of the clover, flour and
meat humus. Nitrogen being the most variable ingredient of humus, it
seems probable that the variation of the proportion of the humus-amids
present is the most potent factor in the variability of the composition
of natural soil-humus.
Arranging these results in the order of their nitrogen-content as in
the table below, we see that the latter approximately corresponds to
the original protein-content of the humified substances.
Humus from meat scraps 10.96 % Nitrogen.
“ “ green clover 8.24
“ “ cow manure 6.16
“ “ wheat flour 5.05
“ “ oat straw 2.50
“ “ sawdust .32
While the above data prove the correlation between the first products
of humification and the original substance, it must be remembered that
subsequently, under proper conditions, the nitrogen-percentage in humus
may, in the course of time, increase very greatly, even to a proportion
considerably above that contained in flesh itself. When we consider
that ordinarily, the latter, and the albuminoid substances generally,
decompose in contact with air with an abundant evolution of ammonia
compounds, sometimes leaving only a little fat (adipocere) behind, it
is surprising that the decomposition _within_ the soil should have
exactly the opposite result, viz., an _accumulation_ of the nitrogen.
The causes of this marked difference are not yet well understood, but
it is probably due to the differences in the kinds of bacteria that are
active in the two cases.
Snyder has also shown that the richer the organic matter humified is
in nitrogen, the more energetically it acts in rendering available
the mineral matters of the soil for plant nutrition. Correspondingly,
Ladd[55] has shown that with the increase of humus in the soil, there
is also a corresponding increase in the amounts of mineral plant-food
extracted from the soil by a four per cent solution of ammonia, such as
is employed in the Grandeau method of humus-determination.
[55] Bull., S. Dakota Station, Nos. 24-32, 35, 47.