SOIL AND SUBSOIL (_Continued_).
ORGANISMS INFLUENCING SOIL CONDITIONS; BACTERIA, ETC.
MICRO-ORGANISMS OF THE SOIL.
Intimately correlated with the humus-substances of the soil, as well
as with its temporary contents of the carbohydrates (cellulose, gums
and sugars) from which humus is formed, is the multitudinous flora of
micro-organisms always present and exercising important functions in
connection with the growth of the higher plants. Extended researches
by Adametz, Schloesing and Müntz, Miquel, Koch, Fraenkel, Winogradsky,
Frank and many others, have thrown light upon the immense numbers and
great variety of minute organisms, especially of the bacterial group,
present in soils, and upon their distribution and activities in the
same. It has been shown that their numbers are greatest near (although
usually not at) the surface, decreasing rapidly downward and generally
disappearing wholly at depths between seven and eight feet; the latter
depth varying of course according to the nature and porosity of the
soil, and both depth and numbers being greatest in summer.
_Numbers of Bacteria in Soils._—Adametz found in one gram of soil,
38,000 bacteria at the surface, 460,000 at ten inches depth; in a
loam soil at the surface 500,000, at ten inches 464,000 in each gram
of earth. Of mould and similar fungous germs there were only 40 to 50
in the same, 6 species being true molds, while four were ferments,
including the yeasts of wine and beer. Fraenkel found in virgin land
from near Potsdam, a sudden, marked decrease at depths of from three
to five feet; while in earth from inhabited places within the city of
Berlin, considerable numbers were still present at eight and even ten
feet, in some cases.
In the researches lately made by Hohl at the bacteriological station
at Liebefeld, near Bern, it was found that in cultivated soils the
number of bacteria greatly exceeds the figures given by Fraenkel.
He found a gram of moist soil to contain from three to fifteen
millions of bacteria. In the cultivated soil of Liebefeld he found
5,750,000, in meadow land 9,400,000, in a manure pile 44,500,000 per
cubic centimeter. These figures seem high for so small a quantity
of material, but taking the average size of a bacterium, a cubic
centimeter might readily contain six hundred millions. (Grandeau, Ann.
Sci. Agronomique, vol. 1, p. 461, 1905).
Mayo and Kinsley (Rep. Kansas Exp’t Station for 1902-3) have
made elaborate investigations of the numbers and kinds of
bacteria found in various soils in Kansas, in connection
with different crops. It is noteworthy that in most cases
their figures exceed considerably those given by European
observers, as they often reach high into the millions, in
one case to over fifty millions, per cubic centimeter.[56]
[56] The mode of statement in the paper is not always quite clear as
to the manner in which the averages given were calculated. It must be
remembered that these data refer to cubic centimeters of soil, or about
twice the amount (1 gram) used by European observers.
Five fields with different soils were investigated; the
land being described as follows: “Field No. 1 is a black
loam containing considerable humus; field No. 2 is similar
to field 1 but contains more humus; field No. 3 is a thin
soil with clay gumbo subsoil; fields Nos. 4 and 5 are black
loams, but not as rich in humus as either No. 1 or No. 2.”
The average bacterial contents of the several fields are
given as follows:
Field No. 1 33,931,747 per cubic centimeter.
“ No. 2 53,596,060 “ “ “
“ No. 3 78,534 “ “ “
“ No. 4 8,643,006 “ “ “
“ No. 5 3,192,131 “ “ “
“The crop records of these fields for the past ten years
indicate that the crop yield has been (more or less?)
directly proportional to the bacterial content of the soil
of each field; field 2 has produced the largest yield, field
3 the least.”
Unfortunately no chemical analyses of any of these soils are
communicated; but at the request of the writer samples of
the soils of the first three fields were sent from the
Kansas station for humus determinations (courteously made by
Dr. H. C. Myers), which gave the following results:
Field No. 1 2.19% of Humus.
“ No. 2 3.07% “ “
“ No. 3 1.85% “ “
While these humus-percentages are not directly proportional
to the bacterial content, a favoring effect of high
humus-content is clearly shown. The bacterial and the
humus-content of these soils are sensibly, even if not
directly, correlated; which might reasonably be expected,
since the organic matter and the humus are the bacterial
food.
The investigation also showed wide differences in the
bacterial content of the same soil when different crops
were growing on it. Thus in samples taken on Aug. 15, there
were found in the first twelve inches of a black loam soil
bearing timothy and clover, 1,380,000, in the same with
alfalfa and clover, 21,091,000, with maize from one to over
two millions. In soils from the western part of Kansas,
the bacterial content of the same crops was much less (as
doubtless is the humus-content), and it is noteworthy that
the prairie buffalo grass shows throughout a relatively high
bacterial content in the first foot of the soil, ranging
next to alfalfa. The root bacteria living on the legumes
will naturally increase the bacterial content of the soils
on which they grow, more than plants which, like maize, do
not directly utilize bacterial action.
_Multiplication of the Bacteria._—Marshall Ward and
Duclaux have made some special observations in regard to
the rapidity with which certain bacteria multiply. Duclaux
summarizes the final conclusion thus: taking as a basis
the time of 35 minutes for the subdivision into two, which
has been frequently observed by Ward, there would be four
millions of bacteria produced in twelve hours. The first
filaments had plenty of room in a drop culture of one cubic
millimeter; but at the end their total volume amounted to
the tenth part of the total volume of the drop. At the above
rate, making 48 generations in 24 hours, 281,500 billions
of organisms would be produced. (Grandeau, Ann. Sci. Agron.
Vol. 1, 1905, p. 456).
_Aerobic and Anaerobic Bacteria._—As may readily be inferred, the
cultural and other surface conditions exert a potent influence both
upon the kinds and abundance of the bacteria and molds; since the
life-functions of some are dependent upon the presence of free oxygen
(“aerobic”), while others flourish best, or only, in the absence of
air (“anaerobic”), or are able to avail themselves of the presence
of _combined_ oxygen, by reduction of oxids present. Their number is
found, in general, to be greatest in cultivated lands, and bacteria
are there by far predominant over the moulds. On the other hand, the
moulds gain precedence in woodlands and meadows, at least so far as air
can gain access; while in the deeper layers of the same, as well as in
peaty lands, bacterial life is always scanty. This holds particularly
in respect to the nitrifying organisms, and others whose life-functions
are dependent upon abundant access of oxygen (aerobic).
_Food Material Required._—All bacteria, like the fungi, are dependent
for their development upon the presence of adequate amounts of some
organic food-material, best apparently in water-soluble form. In the
soil it seems to be chiefly compounds of the carbohydrate group,
especially various gums derived from the decaying plant substance, or
from stable manure; in artificial cultures, glucose is mostly found
to be a highly available food. When the decaying substance reaches
the state of humus, the latter seems to be available as food only to
comparatively few bacteria. The very abundant development of bacterial
life seems to be among the most important effects produced by stable
manure upon the surface soil, in establishing good tilth (“Bodengare”
in German).
_Functions of the Bacteria._—While there is still much uncertainty as
to the exact functions performed by most of these bacteria in respect
to soil-formation and plant growth, there are several kinds whose
activity has been proved to be of the utmost importance in one or both
directions; it having been shown that when the soil is sterilized
either by heat or antiseptic agents, certain essential processes are
completely suppressed until the soil is re-infected and the conditions
of bacterial life restored.
Probably the chief in importance are those connected with the processes
of _nitrification_ and _denitrification_, bearing as they do upon the
supply to plants of the most costly of the three substances furnished
by fertilizers. These organisms have been first extensively studied by
Winogradsky, while the conditions of their activity have been largely
developed by R. Warington.
[Illustration: FIG. 15.—Nitrosomonas. (Winogradsky).]
[Illustration: FIG. 16.—Nitrobacterium. (Winogradsky).]
_Nitrifying Bacteria._—The conversion of ammonia into nitrates is
accomplished under proper conditions by two organisms, or groups of
organisms; the first stage being the formation of nitrites by the
round, often flagellate cells of _nitrosomonas_ (or nitrosococcus).
The second, the oxidation of the nitrites into nitrates by very minute
rod-shaped bacilli, named _nitrobacteria_. The conditions under which
these bacteria can act are quite definite in that, aside from a supply
of the nitrifiable substance, a fairly high temperature (24° C. or 75°
F.) and a moderate degree of moisture, there must be a free access of
oxygen (air); and there must be present a base (or its carbonate) with
which the acids formed by oxidation can immediately unite. In an acid
medium (“sour” soils) nitrification promptly ceases; as it also does
whenever the amount of base present has been fully neutralized. The
bases most favorable to nitrification are lime and magnesia in the form
of carbonates, an excess of which does no harm; while in the case of
the carbonates of potash and soda, the amount must be strictly limited.
_Conditions of Activity._—Dumont and Crochetelle found
that up to .25 per cent, potassic carbonate acted favorably
on the process; which was, however, completely stopped by
as much as .8 per ct. Warington has shown that ammonic
carbonate similarly prevents nitrification when exceeding
about .37 per ct. Ammonia salts in general appear to be
antagonistic to the transformation of nitrites into nitrates.
Aside from the carbonates, some neutral salts favor nitrification very
markedly; while others tend to depress it. Deherain found that .5 per
cent of common salt suffices to prevent nitrification altogether, while
smaller amounts retard it proportionally. According to Dumont and
Crochetelle, potassium chlorid acts favorably up to .3 per cent, but at
.8 per cent suppresses nitrification. Earthy and alkaline sulfates, on
the contrary, seem to act favorably throughout, at least up to .5 per
cent. This is especially true of gypsum, which, according to Pichard,
accelerates the process more than any other substance known. Taking
the effect of gypsum as the maximum, he found that, other things being
equal, the amounts of nitrates formed were as shown in the table below,
the effect of gypsum being taken as 100:
Gypsum 100
Sodic Sulfate 47.9
Potassic Sulfate 35.8
Calcic Carbonate 13.3
Magnesic Carbonate 12.5
The above estimates are markedly confirmed by the
observations of the writer in the alkali soils of
California. In these, nitrates exist most abundantly when
the salts contained in the soil are mainly sulfates; while
wherever common salt or sodic carbonate are present in
considerable amounts, the amounts of nitrate found are
notably less. In saline seashore lands nitrates are usually
present in traces only. Wollny has moreover shown that
the nitrates themselves exert a repressive influence on
nitrification.
_Effects of Aeration and Reduction._—While the fostering effect of
sulfates upon nitrification is very energetic in well aerated soils,
they become injurious whenever by a reductive process in ill-drained
lands, the sulfates are reduced to sulfids. Under such conditions
the process will in any case be much impaired. On the other hand,
the favoring effect of abundant aeration was strikingly shown in
the experiment made by Deherain, in which a cubic meter of soil was
left unmoved for several months, while a similar mass was thoroughly
agitated once a week during the same time. The proportion of nitrates
formed in the latter case was as 70 to 1 formed in the quiescent soil
mass. It follows that the intensity of nitrification is essentially
dependent upon the porosity of the soil; and that it is thus greatly
favored in the pervious soil-strata of the arid regions. It also
follows that thorough and frequent tillage and fallowing greatly favor
nitrification; thus explaining one of the beneficial results of these
operations. At the same time, it is true that we may thus in a short
time seriously diminish the reserve stock of nitrogen contained in the
soil in the form of humus-amids; and since nitrates are exceedingly
liable to be lost from the soil in several ways, such excessive
nitrification is to be avoided.
_Unhumified Organic Matter does not Nitrify._—There can be little doubt
that the formation of ammonia from the amido-compounds in humus is
also the work of bacteria; but this, really the initial phase of the
nitrogen-nutrition of plants, has not yet been fully elucidated. That,
however, it is essentially only the ready-formed humus and not the
unhumified debris of the soil which participate in nitrification was
shown by the experiments of the writer, see chapter 19.
[Illustration: FIG. 17.—Bacillus denitrificans I. (Burri.)]
_Denitrifying Bacteria._—Among the sources of _loss of nitrates_ in
the soil is the action of denitrifying bacteria; some of which cause
merely the reduction of nitrates to nitrites and progressively to
ammonia, while others cause gaseous nitrogen to be given off from
nitrites and nitrates, resulting in their complete loss to the soil.
While there are probably several kinds of the latter class, the most
rapidly effective is an organism contained abundantly in fresh horse
dung, and also on the surface of old straw. This can readily be shown
by subjecting a very dilute solution (1-3 per cent.) of Chile saltpeter
to the action of fresh horse dung in a close flask, when nitrogen and
carbonic dioxid gases are evolved, and in a few days the nitrate has
totally disappeared. In the course of time this power of horse-manure
disappears; so that “rotted manure” is practically free from it and
under proper conditions serves nitrification so effectively, that in
the past it has served extensively for the production of saltpeter in
the “niter-plantations” for the industrial purposes; the material
of which was loose earth, marl and manure, kept moist and frequently
forked over for better aeration. Saltpeter is similarly produced in
stables, corroding the mortar of brick foundations. Nevertheless, it
is necessary to avoid the use, either together or at short intervals
apart, of Chile saltpeter and fresh manure; the manure if used first
should be allowed to remain at least two months in the soil before
saltpeter is applied.
The reduction of nitrates to nitrites and ammonia is brought
about by quite a number of bacteria, mostly anaerobic,
and such as consume combined oxygen in their development.
Thus the butyric ferment, which in the absence of readily
reducible compounds evolves free hydrogen, will in presence
of nitrates reduce the latter to nitrites, or form ammonia
by addition of hydrogen to nitrogen just set free by
reduction. Such reductive processes of course occur chiefly
in soils rich in organic matter, or ill-aerated. The ammonia
so formed, while at first simply combining with any humus
acids present, may in the course of time be itself reduced
to the amidic condition, being thereby rendered relatively
inert, until again brought into action by ammonia-forming
bacteria.
_Ammonia-forming Bacteria._—A large number of different bacteria appear
to be concerned in the formation of ammonia from compounds of the
albuminoid group, (and probably from humus). Among these is one of the
most common in soils (_Bacillus mycoides_, root bacillus), which while
forming ammonia carbonate in solutions of albumen, is also capable of
reducing nitrates to nitrites and ammonia in presence of a nutritive
solution of sugar.
The “hay bacillus” (_B. subtilis_), so abundantly developed in hay
infusions, and one of the most abundant in cultivated soils, has
together with B. ellenbachensis, B. megatherium, B. mycoides, and
others, by some been credited with important action in favoring
vegetation; so that a fairly pure culture of B. ellenbachensis has
been brought out commercially in Germany under the name of “Alinit.”
Rigorous culture experiments made by Stutzer and others have,
however, failed to show any general benefit from the use of alinit in
infecting either land or seeds. But there is no doubt of the _Effects
of Bacterial Life on Physical Soil Conditions._—It is apparent that
all conditions favoring the life of aerobic (air-needing) bacteria
tend also to produce the loose, porous state (tilth) of the surface
soil so conducive to the welfare of culture plants, designated by
German agriculturists as “Bodengare.” Whether or not this condition is
directly due to bacterial processes, as is thought by Stutzer (Landw.
Presse, 1904, No. 11) it is assuredly a highly important point to
be gained, and is essentially connected with the presence of humus
in adequate amounts, which is also a favoring condition of abundant
bacterial life. It seems that the preference given to the shallow
putting-in, or even surface application of stable manure, existing in
Europe, is largely based upon the marked effect upon the looseness of
the surface soil, generally credited to the physical effect of the
manure substance itself, but apparently largely due to the intensity of
bacterial action thus brought about.
[Illustration: FIG. 18.—Bacillus subtilis. (Wollny, after Brefeld.)]
[Illustration: FIG. 19.—Bacteria producing ammoniacal fermentation:
_A_, _C. mycoides_: _B_, _B. stutzeri_. (From Conn, Agr. Bacteriology.)]
[Illustration: FIG. 20.—Bacillus megaterium. (From Migula.)]
ROOT-BACTERIA OR RHIZOBIA OF LEGUMES.—Among the most important
bacteria, agriculturally, is that which enables plants of the
leguminous order—(peas, beans, vetches, clovers, lupins, etc.),—to
obtain their supply of nitrogen from the air independently of those
contained in the soil. The source of nitrogen to plants was long a
disputed question; it was at first supposed (by de Saussure) that it
was obtained directly from the soil by the absorption of humus; but
this was disproved, and Liebig then contended that it was derived
directly from the atmosphere through the ammonia in rain water. This
was then shown to be wholly inadequate; and Boussingault proved
conclusively that plants do not take up nitrogen gas from the air.
This was subsequently denied by Ville; but investigation at the
Rothamstead agricultural station by Lawes and Gilbert definitely
confirmed Boussingault’s results. At the same time they also proved
very definitely that while grass and root crops deplete the soil
of nitrogen, clover and other leguminous crops leave in the soil
more nitrogen than was previously present, even when the entire,
itself highly nitrogenous, leguminous crop is removed from the land.
The improvement of lands for wheat production by rotation with
clover had long ago become a practical maxim; but the cause was not
understood until, in 1888, Hellriegel and Wilfarth announced that the
variously-shaped excrescences or tubercles which had long been observed
as frequently deforming the roots of legumes, are caused by the attacks
of bacilli capable of absorbing the free nitrogen of the air and thus
enabling the host-plant to acquire its needed supply by absorbing
the richly nitrogenous matter thus accumulated in the excrescences.
The minute rod-shaped organism was named _Bacillus radicicola_ by
Beyerinck; _Rhizobium leguminosarum_, by A. Frank, who has published an
extensive treatise on the subject.[57]
[Illustration: FIG. 21.—Microscopic section of cell tissue, from a
nodule of Square-pod pea, showing cells filled with Rhizobia.[58]]
[57] Uber die Pilzsymbcose der Leguminosen, Berlin, 1890.
[58] Original figure from drawing by O. Butler, Asst. in Agr. Dep’t
Univ. of California.
Microscopic examination of the nodules shows their tissues to contain
partly motile, free bacteria, partly others (bacteroids), which have
assumed a quiescent condition, and are of much greater dimensions
than those of the motile form. These relatively thick, and sometimes
forked, forms, differing somewhat in each of the group adaptations
mentioned below, constitute the bulk of the cell-contents of the
nodules, and ultimately serve for the nutrition of the host-plant
with nitrogen. When the growth of the excrescence is completed, the
swollen, quiescent bacteroids gradually collapse and become depleted
of their nitrogenous substance; and finally the apparently empty husk
remains or drops off, carrying with it the minute cocci which in the
soil become active bacteria again. The nodules are thus found mainly
on the actively-growing roots, and at the time when vegetation and
assimilation are most active in the plant. In autumn, or when the
plants are in fruit, the roots may be wholly destitute of nodules.
[Illustration: FIG. 25.—Square-pod pea.—Tetragonolobus purpureus. FIG.
26.—White Lupin.—Lupinus albus.]
The adhesion of the nodules to the roots is mostly very loose, and
their falling-off when the seedlings are carelessly transplanted,
doubtless accounts for much of the difficulty generally found in
transplanting legumes when once established.
The figures annexed show the various forms assumed by the nodules
in different plants, and with them also the corresponding forms of
the bacteroids of each. The latter, here shown magnified about 1000
times, are taken from the inaugural dissertation of D. Brock on this
subject, published at Leipzig in 1891. It appears that the forms of the
bacteroids are quite as much varied as are those of the nodules they
form.
[Illustration: FIG. 22.—Common Vetch.—Vicia sativa.
FIG. 23.—Bur clover.—Medicago denticulata.
FIG. 24.—Garden pea.—Pisum sativum.]
_Varieties of Forms._—While these bacilli seem to be normally present
in most soils, it seems to be necessary that they should adapt
themselves for this symbiosis[59] with each of several groups of the
legumes in order to exert their most beneficial effects. In many
soils there appears to exist a “neutral form”, which requires about
a season’s time or more to adapt itself specially to the several
leguminous groups so that a great advantage is gained by infecting
either the seeds or the soil with the forms already adapted, when no
similar plant has lately occupied the same ground. Thus the bacillus of
the clover root is of little or no benefit to beans, peas or alfalfa,
and the root-bacilli of each of the latter are relatively ineffectual
when used to infect either of the other groups. The same is true of the
bacilli of lupins and of acacias, as applied to leguminous plants of
any other groups.[60]
[59] “Living together” beneficially; in contradistinction to
parasitism, which is injurious to the host plant.
[60] It is asserted by some observers that the root-bacilli producing
differently-shaped excrescences upon different legumes are distinct
species; but this view is not sustained by the experiments of Nobbe and
Hiltner, and seems intrinsically improbable.
_Mode of Infection._—The infection is especially effectual when applied
to the seeds before sowing; and for that purpose there may be used
either the turbid water made by stirring up in it some earth of a
properly infected field, or else water charged with a pure culture of
the appropriate kind, commercially known under the name of nitragin,
now manufactured for the purpose. Or else, the field to be sown may be
infected by spreading on it broadcast, _and promptly harrowing in_, a
wagon-load of earth per acre from a properly infected field. Such earth
must not be allowed to dry, or to be long exposed to light.
Specially effective (“virulent”) and hardy forms of such
bacteria have been produced under artificial culture by Dr.
Geo. T. Moore of the U.S. Department of Agriculture. These
cultures can be sent by mail on cotton imbued with them, for
the infection of seeds.
It is very important that the bacillus should be present in the
_earliest_ stages of the growth of the seedlings; otherwise the latter
will undergo a longer or shorter period of starvation, unless the soil
contains, or is furnished with, a sufficiency of available nitrogen
to supply their immediate wants. When such a supply is very abundant,
the legume crop will sometimes develop no nodules at all; but the best
crops appear to be the result of a thorough infection, and abundant
formation of the excrescences.
_Cultural Results._—The marked results obtained in certain soils by
inoculation with the legume-root bacillus are exemplified in the
following table, showing results of experiments by J. F. Duggar, at the
Alabama Experiment station.[61]
TABLE SHOWING INCREASE OF PRODUCTION BY SOIL INOCULATION.
==============================+=======+========+================
PER ACRE. | TOPS. | ROOTS. | NITROGEN.
| lbs. | lbs. | lbs. | Value.
------------------------------+-------+--------+-------+--------
Hairy vetch, not inoculated | 194 | 387 | 7 | $ 1.05
“ “ inoculated | 3045 | 1452 | 106 | 15.90
Crimson clover not inoculated | 106 | 266 | 4.3 | .65
“ “ inoculated | 4840 | 1452 | 143.7 | 21.25
------------------------------+-------+--------+-------+--------
[61] Bull. Ala. Exp’t Station, No. 96, 1898.
Such marked _increases_ from soil inoculation cannot of course be
expected in cases where the soil has previously borne leguminous crops
of similar nature and therefore already contains the root bacteria.
Hence Duggar found no increase of production when inoculating for
cowpea, land that had borne that crop two years before and already
contained the root bacteria. In the arid region, where the almost
universally calcareous soils usually bear a natural growth largely
composed of various leguminous plants, inoculation is likely to be less
commonly effective than in the humid region east of the Mississippi,
where leguminous plants are much less generally present in the native
flora.
The distinctive agricultural function of supplying nitrogen to the
soils on which they grow, renders inexcusable the persistence of some
writers and teachers in designating all forage plants as “grasses.”
Whatever excuse there may have been for this practice so long as the
nitrogen-gathering function of the legumes was unknown, disappears
with this discovery, and the misleading misnomer should be banished
from agricultural publications and lectures, at the very least.
_Other Nitrogen-Absorbing Bacteria._—An increase in the
nitrogen-content of some soils, aside from the action of leguminous
root-bacteria, has long been observed. As already stated, this increase
was at first ascribed to certain green algæ often seen to develop on
the soil surface; but it has now been shown that the nitrogen-gathering
function belongs to at least two bacteria, one of which (_Clostridium
pastorianum_) was discovered by Winogradski, the other (_Azotobacter
chroococcum_) by Beyerinck, and has since been farther investigated by
Koch, Kröber, Gerlach and Vogel, and last by Lipman and Hugo Fischer.
According to the latter it seems likely that Azotobacter chroococcum
lives in symbiosis with the green algæ, all of which, like the
Azotobacter itself, develop with special luxuriance on calcareous soils.
Lipman (Rep. Agr. Exp’t Station, New Jersey, 1903 and 1904)
describes as _Azotobacter vinelandii_ a form somewhat
different from the A. chroococcum, the nitrogen-assimilating
power of which he tested quite elaborately. He exposed to
air pure cultures of _A. vinelandii_ in nutritive
solution containing the proper mineral ingredients, and
glucose 20 grams per liter. 100 cub. centimeters of this
solution was exposed in flasks of respectively 250, 500
and 1000 cc. content, therefore having greater surface in
the larger flasks. After ten days, the amounts of nitrogen
fixed were found to be respectively 1.67, 3.19 and 7.90
milligrams. When mannite solution was employed instead of
glucose, a similar fixation was observed; and it was also
shown that the presence of combined nitrogen in the forms of
nitrates or ammonium salts discouraged the fixation by the
bacillus.
It was thus clearly proved that _A. vinelandii_ at
least does not need symbiosis with algæ to fix atmospheric
nitrogen; but experiments with mixed cultures of the above
bacillus and another (designated as No. 30 by Lipman) proved
that when these two co-operate the absorption of atmospheric
nitrogen is nearly doubled. As it is probable that this is
the case also with other soil bacteria, the importance of
this source of nitrogen to plants is obvious; provided of
course that the proper nutritive ingredients are present
in available form. Lipman shows that among the organic
nutrients, besides the sugars, glycerine and the salts of
propionic and lactic acids, and probably also others of the
same groups, can serve as nourishment to the nitrogen-fixing
bacteria.
DISTRIBUTION OF THE HUMUS WITHIN THE SURFACE SOIL.
The uniform distribution of the humus-contents of the surface soil, as
shown in sections of the same, is by no means easily accounted for.
The roots from which its substance is so largely derived are not so
universally distributed as to account for it; but least of all can the
rapid disappearance of the leaf-fall and other vegetable offal from
the surface be accounted for without some outside agencies. Of these,
the action of fungous vegetation, and of insects and earthworms, are
doubtless the chief ones.
_Fungi._—When we examine a decaying root, we find radiating from it
a zone of deeper tint, as though from a colored solution penetrating
outward. But since under normal conditions humus is insoluble, this
explanation cannot stand. Microscopic examination, however, reveals
that the outside limit of this zone is also the limit to which the
fungous fibrils concerned in the process extend; and as these fibrils
are much more finely distributed and much more numerous than the
roots of any plant, it is natural that the humus resulting from
their decomposition should be more evenly distributed than the roots
themselves.[62]
[62] Kosticheff, Formation and Properties of Humus; in abstract Jour.
Chem. Soc., 1891, p. 611.
Such fungous growth is not, however, confined to dead and decaying
roots only. A large number of trees and shrubs, among them pines and
firs, beeches, aspen and many others, also the heaths, and woody plants
associated with them, appear to depend largely for their healthy
development, notably in northern latitudes, upon the co-operation
(“symbiosis”) of fungous fibrils that “infest” their roots, enabling
them to assimilate, indirectly, the decaying organic (and inorganic)
matter which would otherwise be unavailable, and at the same time
converting that matter into their own substance. Fungous growths thus
mediate both the decomposition and rehabilitation of the vegetable
debris.
The vegetative fibrils (mycelia) of several kinds of molds are
constantly present in the soil, and while consuming the dead tissue
of the higher plants, spread their own substance throughout the soil
mass. The same is true of the subterranean or “root” mycelia of the
larger fungi, toadstools, mushrooms, which are commonly found about
dead stumps and other deposits of decaying vegetable and animal offal.
All these being dependent upon the presence of air for their life
functions, remain within such distance from the surface as will afford
adequate aeration; the depth reached depending upon the perviousness of
the soil and subsoil. In the humid region this will usually be within
a foot of the surface, but in the arid may reach to several feet.
Ultimately these organisms contribute their substance to the store of
humus in the land.
On the surface of moist soils we frequently find a copious
growth of green fibrils, which may be either those of algæ,
such as Oscillaria, or the early stages (prothallia) of
moss vegetation. This vegetation has been credited with
absorption of nitrogen from the air, thus enriching the
soil; but later researches have shown this effect to be due
to symbiotic bacteria (see above p. 156).
_Animal Agencies._—Darwin first suggested that wherever the common
earthworm (_Lumbricus_) finds the conditions of existence, it exerts a
most important influence in the formation of the humous surface-soil
layer; and the limitation imposed upon these conditions by the
subsoil has doubtless a great deal to do with the sharp demarcation
we often find between it and the surface soil. Briefly stated, the
earthworm nourishes itself by swallowing, successively, portions of
the surrounding earth, digesting a part of its organic matter and then
ejecting the undigested earth in the form of “casts,” such as may
be seen by thousands on the surface of the ground during or after a
rain. Darwin (The Formation of Vegetable Mold, 1881), has calculated
from actual observation that in humid climates and in a ground fairly
stocked with these worms, the soil thus brought up may amount to from
one-tenth to two-tenths of an inch annually over the entire surface;
so that in half a century the entire surface foot might have been
thus worked over. Aside from the mechanical effect thus achieved in
loosening the soil, and the access of air and water permitted by their
burrows, the chemical effects resulting from their digestive process,
and the final return of their own substance to the soil mass; also
their habit of drawing after themselves into their burrows leafstalks,
blades of grass and other vegetable remains, renders their work of no
mean importance both from the physical and chemical point of view. The
uniformity, lack of structure and loose texture of the surface soil,
especially of forests, as compared with subsoil layers of corresponding
thickness, is doubtless largely due to the earthworms’ work. It has
frequently been observed that when an unusual overflow has drowned out
the earthworm population of a considerable area, the surface soil layer
remains compacted, and vegetation languishes, until new immigration
has restocked the soil with them. Again, the humus formed under their
influence is always neutral, never acid.
Wollny (Forsch, Agr., 1890, p. 382), has shown by
direct experimental cultures in boxes, with and without
earthworms, surprising differences between the cultural
results obtained, and this has been fully confirmed by the
subsequent researches of Djemil (Ber. Physiol. Lab. Vers.
Halle, 1898). In Wollny’s experiments, the ratio of higher
production in the presence of the worms, varied all the way
from 2.6 per cent in the case of oats, 93.9 in that of rye,
135.9 in that of potatoes, 300 in that of the field pea, and
140 in that of the vetch, to 733 per cent in the case of
rape. Wollny attributes these favorable effects in the main
to the increased looseness, and perviousness of the soil to
air, and diminished water-holding power. Djemil’s results
all point in the same direction; and he shows, moreover,
that the allegation that the roots penetrate more deeply in
the presence of the worms by following their burrows, is
unfounded, the descending roots often passing close to and
outside of these.
The work of earthworms is especially effective in loamy soils and in
the humid regions. In the arid region, and in sandy soils generally,
the life-conditions are unfavorable to the worm, and the perviousness
elsewhere brought about by its labors already exists naturally in most
cases. It is stated by E. T. Seton (Century Mag. for June, 1904) that
the earthworm is practically non-existent in the arid region between
the Rocky Mountains and the immediate Pacific coast, from Manitoba to
Texas. In the Pacific coast region, however, they are abundant, and do
their work effectually.
_Insects_ of various kinds are also instrumental in producing, not only
the uniform distribution of humus in the surface soil, but also the
looseness of texture which we see in forest soils especially. Ants,
wasps, many kinds of beetles, crickets, and particularly the larvæ
of these, and of other burrowing creatures, often form considerable
accumulations, due directly both to their mechanical activity, and to
their excrements.
The work of _ants_ is in some regions on so large a scale as to attract
the attention of the most casual observer. Especially is this the case
in portions of the arid region, from Texas to Montana, where at times
large areas are so thickly studded with hills from three to twelve feet
in diameter, and one to two feet high, that it is difficult to pass
without being attacked by the insects. The “mounds” studding a large
portion of the prairie country of Louisiana seem also to be due to the
work of ants, although not inhabited at present.
Larger burrowing animals also assist in the task of mixing uniformly
the surface soils, and aiding root-penetration, as well as, in many
cases, the conservation of moisture. Seton (loc. cit.) even claims that
the pocket gophers (Thomomys) in a great degree replace the activity of
the earthworms in the arid region, where they, together with the voles
(commonly known there as field mice), exist in great numbers. Of course
the work of these animals, as well as that of the prairie dogs, ground
squirrels, badgers, etc., is incompatible with cultivation. But the
effects of their burrows on the native vegetation, and the indications
they give of the nature of the subsoil, are eminently useful to the
land-seeker.
Thus in the rolling sediment-lands of the Great Bend of
the Columbia, the observer is surprised to see the “giant
rye grass,” usually at home in the moist lowlands, growing
preferably on the crests of the ridges bordering the
horizon. Examination shows that this is due to the burrowing
of badgers, whereby the roots of the grass are enabled to
reach moisture at all times, even in that extremely arid
region.