THE ORGANIC CONSTITUENTS OF THE SOIL SOLUTION.
The organic substances in the soil are tissue remains, to a large
extent of plants, and to a less extent of animals; and it is to be
expected that there may be found also in the soil the substances which
were in the organisms at the time of their death, and degradation and
decomposition products derived from these. Moreover, there are to
be anticipated numerous products of bacterial origin, secretions of
algae, fungi, etc., so that the organic complex in the soil may contain
numerous substances of widely different chemical characteristics.
Degradation products of proteins, fats, and carbohydrates, as well
as decomposition products may be expected in almost any soil. But it
does not follow that any particular organic substance (excluding, of
course, carbon dioxide or nitrates) is to be found in every soil. No
generalization regarding the organic substances in the soil can be made
such as that formulated for the inorganic compounds. It is probable
that further investigation will show certain organic substances or
classes of substances to be common to most soils, but it is reasonably
certain that many other organic substances will be found in only a few
soils, or occasionally, and these latter will be often a prominent
factor characterizing the particular soil in which they may occur.
Although no broad generalization is justified regarding the composition
of the soil solution with respect to organic substances dissolved,
nevertheless the extension of the methods developed in the study of the
inorganic substances dissolved has led to a considerable knowledge of
the organic ones.
In view of the facts shown in the preceding chapters, and at the
same time recognizing that good and poor soils respectively must
show differences in the soil solution if the fundamental thesis is
valid as to the relation of soils to crop production, experiments
have been made to investigate in a comparative way solutions obtained
from good and poor soils of the same type, locality, and physical
characteristics. For this purpose two samples of soil were taken from
adjacent fields which had been under observation for two years. The
soils were of the same type, Cecil clay, and were so similar in their
physical characteristics as to be distinguished with difficulty in
the laboratory. On one field a good crop of wheat was grown, followed
by a good crop of clover and tame grasses. On the other field, the
corresponding crops had been quite poor. The field yielding the good
crops had been plowed somewhat deeper, and had previously received a
moderate application of stable manure. Otherwise, so far as could be
learned, the cultural history of the fields had been the same. For
convenience, the sample from the first field will be designated “good,”
and from the other “poor.”
Aqueous extracts from these soils were prepared, the same proportion
of distilled water to soil being taken in each case, and the time of
contact being the same. The solutions were freed from suspended matter
by being passed through Pasteur-Chamberland bougies under pressure.
Young wheat seedlings germinated at the same time, and selected
carefully for uniformity of size and apparent vigor, were grown in
these solutions for three days. At the expiration of this period the
seedlings in the extract from the good soil were about five inches in
height, and the roots were clear, clean and turgid. The plants in the
poor extract were scarcely three inches in height, and the roots were
assuming a slimy, unhealthy appearance and becoming flaccid at the
tips. The plants were then all removed, the roots washed carefully in
tap water; the plants which had been in the poor solution were placed
in the good solution, and those which had been in the good solution
were placed in the poor solution. At the end of four days further,
the poor plants had surpassed in height the ones which had previously
been in the good solution, and the roots had acquired the general
characteristics of healthy plants. These which had been originally in
the good solution and then transferred to the poor, had made little
additional growth, and the roots had become somewhat flaccid.[101]
[101] The success of this and of many of the following experiments
was due in large measure to the skill and patience of Mr. James E.
Breazeale.
This experiment was repeated several times, not only with the soils
cited but with samples from adjacent good and poor spots in fields
on several soil types from widely separated areas; for instance,
Cecil clay from near Statesville, North Carolina; Sassafras loam
from Maryland; Windsor sand from Delaware; and similar results were
obtained. In other words, these water cultures produced plants which
showed much the same differences, in kind and degree, as had been
observed in the field. This was recognized as an important step
forward, for it indicated that _whatever was making a difference in
the crop-producing power of these soils in the field was transmitted
to their aqueous extracts_, and methods for studying the chemical
properties of solutions are far in advance of methods for studying
mixtures of solids.
The soil extracts described above were subjected to a careful analysis
for their mineral constituents. They were found to be practically
identical in this respect. Further, the poor extract contained
decidedly more nitrates than the good—from three to four times as much.
It follows, therefore, that the difference in the soils which produced
a good and a poor crop respectively, was not due to a difference in
mineral plant nutrients, or other mineral differences probably, nor to
their respective content of nitrates. Consequently, the poor solution
was such, not because of the lack of anything, but because of the
presence of something inimical or “toxic” to plant growth; and further,
this something must be an organic substance or substances more or less
soluble in water. This conclusion was confirmed in the following way.
Samples of the poor solution from the soil obtained near Statesville,
N. C., were diluted twice, five times, and ten times, and wheat
seedlings were grown in these solutions, using a sample of the good
solution as a check. It was found after several days growth that the
plants in the solution diluted tenfold were about as good, or perhaps
slightly better, than those grown in the check solution. In every
case diluting the poor solution had improved it for plant growth,
and the higher the dilution the greater the improvement, in spite
of the consequent dilution of the mineral plant nutrients. The only
explanation of these results which has yet suggested itself is that
the toxic organic substances present were less effective on dilution
until the concentration reached a point where they actually became
stimulative, as is common with toxins of every character.
Another set of experiments confirmed the conclusion that the poor
solution contained some organic substance inhibitory to plant growth.
A number of water cultures was prepared from the aqueous extract of
the poor soil, and lime in various forms was added to the cultures. To
two of the cultures lime carbonate and lime sulphate respectively were
added in excess, so that there was in each case a powdered solid at
the bottom of the containing vessel. At the end of two days the wheat
seedlings which were growing in the vessels containing the powdered
solids had decidedly outstripped those growing in all the others, the
tops having the appearance of unusually good and healthy plants. The
roots were of a very remarkable character, being exceptionally long,
very turgid, clear, clean and translucent.
At once, new experiments were carried out in which there were added
to the poor solution, precipitated ferric hydroxide freed from all
adhering salts, precipitated alumina, shredded filter-paper, absorbent
cotton, or carbon black. In every case the same result was obtained
as before, a much improved growth of top and a vastly better root
development. Since, by no possibility could these various added
substances have increased the concentration with respect to mineral
nutrients, another explanation must be sought. Aside from their
insolubility, the one property common to these various substances
was the large amount of surface they brought into contact with the
solution. The one obvious explanation of their effects on the growth of
the wheat seedlings, therefore, is that they withdrew or absorbed from
the solution some substance or substances deleterious to plant growth.
As diluting with respect to mineral nutrients could not possibly be
expected to improve the cultural value of the solution, the conclusion
seems evident that the effect produced by these various absorbents
was due to more or less complete removal from the solution of organic
substances inhibitory to plant growth. These experiments were then
repeated in a modified form by shaking the poor solution with such
absorbents as precipitated ferric oxide or carbon black and filtering
before adding the seedling plants. The solutions thus prepared proved
very satisfactory nutrient media, although the decided elongation of
the roots, always observed when the absorbents were in contact with the
solutions, was not so noticeable with these filtered solutions.
The experiments just described were repeated with extracts from a
number of soils which were supporting or had recently supported poor
crops. The accumulated mass of evidence admits of no doubt that in many
cases the apparent lack of fertility of a soil is due to the presence
of some organic substance or substances soluble in soil water. This
point established, there was studied the effect of fertilizers when
added to aqueous extracts from poor soils.
A large amount of experimenting has been done on this subject. It has
been found that the common commercial fertilizers, as well as many
other substances, when added to the soil extract containing growing
plants, sometimes improve the plants, sometimes the contrary. But, in
general, those particular substances which improve any given soil for
a crop also improve the aqueous extract of the soil for the growth of
the same crop plant: _i. e._, should a soil be known to respond well
to the application of superphosphates when planted to wheat, then the
probability is great that the aqueous extract of the soil will be
improved as a culture medium for the wheat plant by addition of calcium
phosphate. Particularly important in this connection are certain
experiments with organic fertilizers.
A soil which had been found to be quite unproductive with regard to
wheat and ordinary tame grasses yielded, however, a much better growth
of plants if pyrogallol or better pyrogallol and lime were added to
the soil some days before planting. An aqueous extract of this soil
tested with young wheat seedlings produced but a poor growth, as did
the soil itself. But with the addition of pyrogallol or pyrogallol and
lime to the soil extract, and especially if the extract so treated
were allowed to stand for a few days with free access of air, there
was obtained a culture medium which yielded remarkably good results
with wheat seedlings. Not only was there an excellent and increased
development of tops, but the roots of the seedlings grown in the
solution treated with pyrogallol were unusually long, turgid, clear
and translucent. Here, then, there was obtained an increased amount
and improved character of growth by the addition of a substance which
contained only carbon, hydrogen and oxygen, and no recognized plant
food. Other organic substances, such for instance as tannin, gave
similar results.
With the recognition that the presence of organic dissolved substances
in the nutrient medium produced effects on a growing plant of as great
or even greater magnitude than those produced by inorganic dissolved
substances, there was carried out a number of experiments to test
more specifically such substances as might reasonably be expected to
be present naturally in soils. The results thus obtained suggested
experiments with other related substances. The first substance to
suggest itself is stable manure. Taking it all in all, this substance
is probably the most efficient as well as the most generally used soil
amendment in the experience of mankind. The good effects produced by
this substance have in the past been generally considered as due to the
readily “available” potash, phosphoric acid and nitrogen it contains,
but thoughtful experimenters and agriculturists have long doubted
that this explanation is sufficient, since, after all, the mineral
constituents of stable manure are usually small in amount, and out of
all proportion to the effects resulting from its use. That some of the
results are due to an improvement in the physical condition of the soil
when manure is used has quite rightly been generally assumed; but to
its content of nitrogenous components its value has in the main been
ascribed.
A well-fermented aqueous extract of stable manure was prepared, and
filtered free of suspended solids. Four equal volumes of this solution
were taken. Three of these portions were evaporated to dryness
in platinum dishes, and the residues incinerated. To the dishes
containing: the ash were added respectively nitric acid, sulphuric
acid, and hydrochloric acid in slight excess, and the dishes again
brought to dryness. Water cultures for wheat seedlings were then
prepared.[102] Into one was introduced the given volume of manure
extract; into another the ash from an equal volume of the extract which
had subsequently been treated with nitric acid; and cultures with the
ash which had been treated respectively with sulphuric and hydrochloric
acid were similarly prepared. After ten days growth, the plants from
the several cultures were compared. The plants from the cultures which
contained the sulphates and the chlorides were not materially different
from the plants grown in the check culture. The plants from the nitrate
culture had larger shoots, but shorter roots than the check plants.
But the plants grown in the culture to which the manure extract had
been added directly had by far larger and better shoots and the roots
were incomparably superior to those grown in any other culture, being
larger, thicker, better branched, clear, bright and translucent, and
very turgid, very like the roots obtained in cultures to which carbon
black or precipitated ferric oxide had been added.
[102] Further studies on the properties of unproductive soils, B. E.
Livingston _et al._, Bull. =36=, 1907, and =48=, 1908, Bureau of Soils,
U. S. Dept. Agriculture.
The results of this experiment, which has been repeated a number
of times, using manure extracts of various origins, leave no doubt
that it is the organic components of the manure which produce the
characteristic effects, for the ash culture contained all and even more
of the mineral constituents “available” in the original extract, and
the nitrate culture excluded any explanation based on the nitrogenous
content of the manure. This conclusion was supported by the results of
another experiment.
To a manure extract was added alcohol, which precipitated most of the
organic dissolved substances but very little of the inorganic ones.
The precipitated organic matter was filtered off, dried carefully in a
water oven to eliminate the alcohol, and then taken up in sufficient
water to equal the original volume of manure extract. The nitrate
containing the major part of the salts was boiled vigorously to
eliminate the alcohol and water was then added to restore the original
concentration. A third solution was prepared by bringing together the
organic and inorganic substances which had previously been separated as
above described. The three solutions were used as water cultures for
wheat seedlings, a solution of the original manure extract being taken
for a check culture. The original manure extract and the reconstructed
manure extract gave plants of about equal development. The culture
containing the organic dissolved substances only, gave plants of
nearly, but not quite, equal development to those grown in the check
culture. But the plants grown in the solution containing the dissolved
minerals only, while fine plants and making what would ordinarily be
considered a good development, were decidedly smaller as regards their
aerial parts, and the roots were in no wise comparable to the roots
of the plants grown in the cultures containing the dissolved organic
substances.
This last experiment has been repeated, with dissolved substances
prepared from another manure extract, but in this case the organic
and inorganic substances were separated by dialysis. This suggested
yet another experiment, in which it was sought to hasten the process
of dialysis, by introducing electrodes into the manure extract, each
electrode being surrounded by some porous membrane, either of parchment
paper, or unglazed porcelain. Not only were the mineral constituents
of the manure extract readily separated in this way, passing into
the electrode chambers, as did also to some slight extent organic
compounds, but also about the outer walls of the electrode chambers
there was marked segregation and deposition of organic materials. The
organic substances deposited at the cathode were found to stimulate
greatly the growth of wheat seedlings while those deposited at the
anode were found to retard the growth of seedlings. It seems probable,
therefore, that stable manure contains organic components which produce
as great or greater effects upon growing plants as do the inorganic
substances it contains: that on the whole these organic components
induce increased plant growth, but some of them, by themselves alone,
would retard plant growth.
In a similar way green manures have been examined. If fresh clover,
alfalfa, or cowpeas, be macerated and an aqueous extract thus prepared,
it will in general be quite toxic to plants such as wheat; and if this
extract be allowed to stand and ferment or sour the resulting solution
will be totally unfit for the growth of seedling plants. But if the
clover, alfalfa, or cowpea vines be allowed to wilt thoroughly before
being macerated and extracted, or if they be macerated and incorporated
with soil and allowed to remain thus for ten days or a fortnight
before being extracted; then, the resulting solution will be quite
stimulating to such plants as wheat, corn or the grasses, when added
either to water or soil cultures. It would seem, therefore, that the
mineral constituents of the legumes commonly employed as green manures
are less important than the organic, in affecting the growth of crops
subsequently planted, and the inhibitory or toxic action of fresh green
manure seems to be recognized in the common practice of waiting some
days after turning under a green manure crop before seeding to a new
crop.
The wilting of a green manure involves a darkening and some blackening
of the mass, with apparently some absorption of oxygen. This fact
has suggested a trial of other organic substances which show a
decided ability to absorb oxygen. Among such substances, pyrogallol
stands preëminent. It has been shown that when pyrogallol, or better
pyrogallol and lime, is added to certain soils, naturally low in
productive power, and allowed to stand for a few days, these soils are
readily brought into good condition and support good crops of wheat,
rye, or grasses. Pyrogallol in water cultures is rather toxic to wheat
plants, even in quite dilute solutions. But if the aqueous solution
of pyrogallol be allowed to stand exposed to the air, and better if
the solution be made slightly alkaline as by the addition of lime,
oxygen is absorbed, and a dark brown or blackened solution is soon
formed, which is stimulating to wheat seedlings. Many experiments have
indicated it to be a general rule that soluble organic substances
which are toxic to plant growth yield oxidation products which are
harmless or positively beneficial.
The suggestion has been made that the well-known infertility of
subsoils, when freshly turned up, is caused by the presence of
alkaloids of the purine or codeine type, due to the activities of
anaerobic bacteria. Water cultures and pot cultures show that while
these substances do have a marked effect on plant growth, it is,
frequently, quite beneficial; strychnine for example, in certain
concentrations, produces a very decided stimulation in the growth of
wheat seedlings. It is clear that some other explanation will have to
be sought for the lack of fertility of subsoils.
A number of the substances which may be expected for one reason
or another to be present in soils, have been investigated as to
their effect on plants. In this connection may be cited the work of
Livingston[103] and of Dachnowski,[104] who have studied the effect
on vegetation of the organic substances dissolved in bog waters. In
the following table are given the results obtained by growing wheat
seedlings in solutions containing some one of a number of substances
which might be expected to occur in a soil or to be derivatives of such
substances. It will be observed that in the case of these dissolved
organic substances, as has been repeatedly established with the
inorganic ones, in concentrations sufficiently dilute not to be toxic,
they generally show the opposite effect and appear to be stimulating.
[103] Physiological Properties of Bog Water, by B. E. Livingston, Bot.
gaz., =39=, 348-355, (1905).
[104] The toxic property of bog water and bog soil, by Alfred
Dachnowski, Bot. gaz., =46=, 130-143, (1908).
TABLE I.—EFFECT OF VARIOUS ORGANIC COMPOUNDS UPON THE GROWTH OF
WHEAT PLANTS, WITH ESPECIAL REFERENCE TO THEIR TOXIC PROPERTIES[105]
[105] Certain organic constituents of soils in relation to soil
fertility, by Oswald Schreiner and Howard S. Reed, assisted by J. J.
Skinner, Bull. No. =47=, Bureau of Soils, U. S. Dept. Agriculture, 1907.
LEGEND:
A = Duration of experiment
B = Lowest concentration causing death
C = Lowest concentration causing injury
D = Concentration causing greatest stimulation
=======================+====+======+======+======+===================
| | | | |
| | | | |
Compound | A | B | C | D | Remarks
| | | | |
-----------------------+----+------+------+------+-------------------
|days|p.p.m.|p.p.m.|p.p.m.|
| | | | |
_a_ Aspartic acid | 10 | 500 | 100| .... |Normal growth in
HOOC.CH₂.CH(NH₂).COOH | | | | |concentration
| | | | |below 100 p.p.m.
-----------------------+----+------+------+------+-------------------
_b_ Asparagine | 9 | | | |No injury below
NH₂.OC.CH₂.CH(NH₂).COOH| | | | |1,000 p.p.m.
-----------------------+----+------+------+------+-------------------
_c_ Glycocoll, | 9 | | | |Tops of all plants
CH₂(NH₂).COOH | | | | |good. Roots slightly
| | | | |injured at higher
| | | | |concentrations
-----------------------+----+------+------+------+-------------------
_d_ Alanine, | 10 | .... | 500 | 25 |Only roots were
CH₃.CH(NH₂).COOH | | | | |injured at
| | | | |500 p.p.m.
-----------------------+----+------+------+------+-------------------
_e_ Leucine | 9 | .... | .... | .... |No injurious action
CH₃.(CH₂)₃.CH(NH₂).COOH| | | | |
-----------------------+----+------+------+------+-------------------
_f_ Tyrosine, | 11 | .... | 10 | |
OH | | | | |
/ | | | | |
C₆ H₄ | | | | |
\ | | | | |
CH₂.CH(NH₂).COOH | | | | |
-----------------------+----+------+------+------+-------------------
_g_ Choline, | 10 | | 500 | 1 |Roots affected more
CH₂CH₂OH | | | | | than tops
/ | | | | |
(CH₃)₃N | | | | |
\ | | | | |
OH | | | | |
-----------------------+----+------+------+------+-------------------
| | | | |
_h_ Neurine, | 9 | 250 | 25 | |
CH:CH₂ | | | | |
/ | | | | |
(CH₃)₃N | | | | |
\ | | | | |
OH | | | | |
-----------------------+----+------+------+------+-------------------
Neurine (neutralized) | 8 | 250 | 25 | |
-----------------------+----+------+------+------+-------------------
_i_ Betaine, | 9 | ... | ... | |No injury
CH₂.CO | | | | |
/ / | | | | |
(CH₃)₃N / | | | | |
\ / | | | | |
O | | | | |
-----------------------+----+------+------+------+-------------------
_j_ Alloxan, | 10 |1,000 | 100 | |
NH.CO | | | | |
/ \ | | | | |
CO CO | | | | |
\ / | | | | |
NH.CO | | | | |
-----------------------+----+------+------+------+-------------------
_k_ Guanine, | 12 | | | |Insoluble above 40
NH.C.NH.CO.C.NH | | | | |p.p.m. No harmful
\\ || \ | | | | |effects.
\\ || CH | | | | |
\\ || // | | | | |
N————C. N | | | | |
-----------------------+----+------+------+------+-------------------
_l_ Xanthine | | | | |No injurious
| | | | |action.
CO.NH.CO.C.NH | | | | |
\ || \ | | | | |
\ || CH | | | | |
\ || // | | | | |
NH——C—N | | | | |
-----------------------+----+------+------+------+-------------------
_m_ Guanadine, | 9 | 100 | 1 | |
NH₂ | | | | |
/ | | | | |
HN : C | | | | |
\ | | | | |
NH₂ | | | | |
-----------------------+----+------+------+------+-------------------
_n_ Skatol, | 9 | 200 | 50 | |Roots injured more
C.CH₃ | | | | |than tops
/ \\ | | | | |
C₆H₄ CH | | | | |
\ / | | | | |
NH | | | | |
-----------------------+----+------+------+------+-------------------
| | | | |
_o_ Pyridine, C₅H₅N | 9 | .... | 50 | .... |In solutions of 50
| | | | |p.p.m. and less
| | | | |the root growth
| | | | |was normal.
-----------------------+----+------+-------+------+------------------
Picoline, C₅H₄N.CH₃ | 7 |1,000 | 500 | 100 |
-----------------------+----+------+-------+------+------------------
| | | | |
Piperidin | 7 | 250 | 25 | |
CH₂ | | | | |
H₂C / \ CH₂ | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
H₂C \ / CH₂ | | | | |
NH | | | | |
-----------------------+----+------+-------+------+------------------
Piperidine | 7 | 100 | 25 | 1 |
(neutralized) | | | | |
-----------------------+----+------+-------+------+------------------
/ \ / \ | | | | |
| | | | | | | |
Quinolin, | | | | 6 | 500 | 5 | |
| | | | | | | |
\ / \ / | | | | |
N | | | | |
-----------------------+----+------+-------+------+------------------
_p_ Ricin | 10 | | 40 | |Insoluble above 50
| | | | | p.p.m.
-----------------------+----+------+-------+------+------------------
_q_ Mucin | 10 | | 100 | |Not tested in
| | | | | concentrations
| | | | |higher than
| | | | |100 p.p.m.
-----------------------+----+------+-------+------+------------------
| | | | |
_r_ Pyrocatechin, | 12 | 500 | 25 | 1 |
C₆H₄(OH)₂(1,2) | | | | |
-----------------------+----+------+------+------+-------------------
_s_ Arbutin, C₁₂H₁₆O₇ | 12 | 500 | 25 | 1 |
-----------------------+----+------+------+------+-------------------
_t_ Phloroglucin, | 13 | 500 | 25 | 1 |
C₆H₃(OH)₃(1,3,5) | | | | |
-----------------------+----+------+------+------+-------------------
_u_ Vanillin, | 9 | 500 | 1 | |
CHO | | | | |
/ | | | | |
C₆H₃——O.CH₃ | | | | |
\ | | | | |
OH | | | | |
-----------------------+----+------+------+------+-------------------
Vanillic acid, | 7 | 100 | 25 | 5 |
COOH | | | | |
/ | | | | |
C₆H₃—O.CH₃ | | | | |
\ | | | | |
OH | | | | |
-----------------------+----+------+------+------+-------------------
_v_ Quinic acid, | 10 | 500 | 100 | |
C₆H₇(OH)₄.COOH | | | | |
-----------------------+----+------+------+------+-------------------
O | | | | |
/ | | | | | |
_w_ Quinone, C₆H₄ | | 9 | 100 | 1 | |
\ | | | | | |
O | | | | |
-----------------------+----+------+------+------+--------------------
_x_ Cinnamic acid, | 8 | 100 | 25 | |
C₆H₅CH : CH.COOH | | | | |
-----------------------+----+------+------+------+-------------------
Sodium cinnamate | 12 | ... | 100 | |Roots were
| | | | |stimulated
| | | | |in lower
| | | | |concentrations
-----------------------+----+------+------+------+-------------------
_y_ Cumarin, | 8 | 100 | 1 | |
CH:CH.CO | | | | |
/ / | | | | |
C₈H₄/ / | 8 | 100 | 1 | |
\ / | | | | |
O | | | | |
-----------------------+----+------+------+------+-------------------
| | | | |Insoluble above
_z_ Daphnetin | 12 | | 50 | |50 p.p.m. Roots
| | | | |somewhat injured
CH : CH.CO | | | | |
/ / | | | | |
C₆H₂ ——— O | | | | |
\\ | | | | |
(OH)₂ | | | | |
----------------------+----+------+------+------+-------------------
_aa_ Esculin, C₁₅H₁₆O₉ | 13 | 500 | 1 | |
----------------------+----+------+------+------+-------------------
_bb_ Piperonal | | | | |
(heliotropine)— | | | | |
CHO | | | | |
/ | | | | |
C₆H₅——O | | | | |
\ \ | | | | |
\ \ | | | | |
O——CH₂ | 7 | 100 | 1 | ... |
------------------------+----+------+------+------+-------------------
_cc_ Borneol, C₁₀H₁₇(OH)| 10 | 100 | 1 | ... |
_dd_ Camphor, C₁₀H₁₆O | 8 | 300 | 5 | ... |
_ee_ Turpentine, C₁₀H₁₆ | 8 | 500 | 10 | ... |
------------------------+----+------+------+------+-------------------
_a._ Aspartic acid has been found in young sugar-cane and in seedlings
of the bean and pumpkin.
_b._ Asparagine was first found in asparagus; but has since been shown
to be relatively abundant in many species.
_c._ Glycocoll is one of the simpler and more common degradation
products of proteins.
_d._ Alanine is a common degradation product of proteins and is related
chemically to phenylalanine, and to tyrosine, which has been found in
many plants.
_e._ Leucine, an amino-acid of a paraffine series and a decomposition
product of proteids, has been found in certain mushrooms, vetches,
lupine, gourds, potatoes, corn, etc.
_f._ Tyrosine is an important decomposition product of proteids, is
widely distributed and found in many plants and fungi.
_g._ Choline is a derivative of certain lecithins and is found in many
seeds and growing plants.
_h._ Neurine is a substance closely related to choline, and probably
formed from it.
_i._ Betaine is closely related to both choline and neurine, and is
found in many seeds and plants.
_j._ Alloxan is closely related chemically to convicine, which latter
is found in beets and certain beans.
_k._ Guanine is a widely distributed nitrogenous body, and has
been found in the seeds of vetch, alfalfa, clover, gourds, barley,
sugar-beets and sugar-cane.
_l._ Xanthine, a substance closely related to guanine, has been found
in a number of plants.
_m._ Guanidine, a substance chemically related to guanine, has been
found in a number of plants of different species.
_n._ Skatol is a derivative of proteids and is a common product of the
activities of some varieties of bacteria.
_o._ Pyridine has been shown to exist in soils, as such probably, by
Shorey, who obtained it from certain soils in Hawaii.
_p._ Ricin is found in the castor-oil plant.
_q._ Mucin has been found in yams.
_r._ Pyrocatechin has been found in the bark of various trees, the
berries of the Virginia creeper, the sap of sugar-beets and in several
varieties of willows.
_s._ Arbutin has been found in many plants, especially in some of the
grasses.
_t._ Phloroglucin is easily derived from a number of plant constituents.
_u._ Vanillin forms readily from a glucoside, which is very widely
distributed in many plants, and by some authorities is supposed to be a
product of the decomposition of wood tissues.
_v._ Quinic acid, which is found with quinine in the cinchona bark,
also occurs in beet leaves, certain hays, cranberry leaves, and
occasionally in other plants.
_w._ Quinone has been shown to result from the action of a certain
fungus, _Streptothrix chromogena_, common in soils.
_x._ Cinnamic acid is found in certain barks, and forms esters which
have been found in the leaves of various plants.
_y._ Cumarin has been found in a large number of plants, including the
grasses, beets, sweet clover, etc.
_z._ Daphnetin occurs in some species of _Daphne_ and is closely
related to cumarin.
_aa._ Esculin, as well as the corresponding esculetin, has been found
occasionally in a number of plants.
_bb._ Heliotropine, or piperonal, has the odor of heliotrope and is
found in flowers.
_cc._ Borneol occurs in needles of different varieties of pine, fir,
spruce and hemlock, golden rod and thyme.
_dd._ Camphor is closely related chemically to borneol and is secreted
by a number of plants; it is found in the wood of _Cinnamomum_,
cinnamon root, in the leaves of sassafras, spikenard, rosemary,
rosewood, etc.
_ee._ Turpentine is a constituent of many plants and coniferous trees.
Finally, a number of organic substances has been isolated from soils.
Their composition, and in several cases their constitutions have been
determined. The effects of these on plants, when they are present
in the cultural media have been studied. Thus, Shorey[106] was able
to isolate picoline carboxylic acid (C₇H₇NO₂) from certain soils in
Hawaii, and this same substance has since been found in several soils
of the United States. In aqueous solutions it is quite toxic to wheat
seedlings. Since then a number of other definite organic compounds have
been isolated from soils belonging to at least eight different classes
of organic substances, including:[107]
Hentriacontane, C₃₁H₆₄.
Monohydroxystearic acid, CH₃(CH₂)₆CHOH(CH₂)₉COOH.
Dihydroxystearic acid, CH₂(CH₂)₇CHOH.CHOH.(CH₂)₇ COOH.
Agroceric acid, C₂₁H₄₂O₃.
Paraffinic acid, C₂₄H₄₈O₂.
Lignoceric acid, C₂₄H₄₈O₂.
Phytosterol, C₂₆H₄₄O.H₂O.
Pentosan, C₅H₈O₄.
Agrosterol, C₂₆H₄₄O.H₂0.
Picoline carboxylic acid, C₇H₇O₂N.
Histidine, C₆H₉O₂N₃.
Arginine, C₆H₁₄O₂,N₄.
Cytosine, C₄H₅ON₃.H₂O.
Xanthine, C₅H₄O₂N₄.
Hypoxanthine, C₅H₄ON₄.
Glycerides, resin acids, etc.
[106] Organic nitrogen in Hawaiian soils, by E. C. Shorey, report of
Hawaii Experiment Station, 1906, 37-59.
[107] Chemical Nature of Soil Organic Matter, by Oswald Schreiner
and Edmund C. Shorey, Bull. 74, Bureau of Soils, U. S. Department of
Agriculture, 1910.
Some of these, picoline carboxylic acid, dihydroxystearic acid and
the pentosan just cited, are toxic to growing plants; others are not.
The origin and mode of production of these substances in the soil
is, generally speaking, uncertain and obscure, and is yet one of the
important fundamental problems confronting the soil chemist.
It is important to note that the organic substances thus far isolated
from soils are of widely varying types, and with very different
chemical characteristics. As pointed out above, almost any type of
organic substance is likely to be found in soils, and the effects of
any of them on growing plants can hardly be predicted from _a priori_
considerations.
It has been found that as a general rule the continued growth of
one crop in any soil results in a low crop production. Pot cultures
have given even more pronounced results in the same direction. The
explanation long accepted is that the soil has, as a result of
continued cropping, become deficient in some one or more of the
“available” mineral nutrients. Pot experiments, where the garnered crop
was returned to the soil and still a diminished yield was obtained,
throw doubt on this explanation. Still further doubt results from
water-cultures which, by growing a crop in them, become “poor” for
subsequent crops, although there is maintained in them an ample
supply of mineral plant nutrients, and they are easily renovated by
good absorbers. These facts find a more satisfactory explanation as
being due to the production in the nutrient medium of deleterious
organic substances originating in the growing plant itself. This idea
seems to have been advanced first by De Candolle, in 1832,[108] to
account for the beneficial results obtained by employing a rotation of
crops. It appears to have been held by Liebig at one time, although
he subsequently abandoned it in favor of the view that the benefits
of a crop rotation are due to the several crops requiring different
proportions of mineral nutrients, and that the disturbance of the
balance in the soil produced by one crop is not unfavorable to the
growth of some other crop. Although lacking direct experimental
confirmation, this latter view of Liebig’s has long prevailed among
agricultural investigators, partly by reason of his authority, partly
by reason of the dominance of the plant-food theory of fertilizers,
and partly by reason of the fact that the ideas of De Candolle as
originally advanced included certain errors soon detected. The trend of
recent investigations has been distinctly in favor of a modified form
of the view of De Candolle. It has been recognized that other factors
enter into crop rotations, such as the elimination of associated weeds,
various kinds of animal, insect and plant parasites, preparation of
the soil by a deep-rooted crop for a shallow-rooted following crop,
etc. It has come to be recognized that there are natural associations
of plants, and natural rotations of vegetation certainly determined
by other than plant food factors. Thus, in the eastern United States,
wheat is followed by ragweed naturally, while across the fence
cocklebur and wild sunflower come in after the corn, the difference
in vegetation being as sharply marked after the removal of the crops
as when they still occupied the land. Analyses of the ragweed, for
instance, although it is a shallower rooted crop than wheat, show
that it takes from the soil as much of the mineral nutrients as
does the preceding[109] wheat crop. The investigation of Lawes and
Gilbert[110] on fairy rings showed that the continual widening of the
rings can not be satisfactorily explained by the comparison of the
mineral constituents in the soil within and without the rings. Work
at Woburn[111] on the effect of grass on apple trees finds no other
plausible explanation than that the growing grass produces in the
soil organic substances detrimental to young apple trees. A number of
similar cases have been recorded.
[108] See in this connection, Further studies on the properties of
unproductive soils, by B. E. Livingston, Bull. No. =36=, Bureau of
soils, Dept. of Agric., 1907, p. 7-9.
[109] Mr. J. G. Smith has made a comparison between the potash and
phosphoric acid content of the wheat and following crop of ragweed
grown on a farm in Fairfax Co., Va. His unpublished results, with some
others found in the literature, are given in the following table:
======================+======+==========+===========================
|Potash|Phosphoric|
Material | K₂O |acid, P₂O₅| Analyst
| % | % |
----------------------+------+----------+---------------------------
Wheat | 0.76 | 0.52 |Smith
Young ragweed | 1.78 | 0.73 |Smith
Ragweed in seed | 1.28 | 0.35 |Smith
Ragweed in seed and | | |
accompanying plants | 1.18 | 0.39 |Smith
Winter wheat in flower| 1.796| 0.51 |Wolff’s tables in Johnson’s
| | | “How Crops Grow,” p. 376.
Ragweed | 1.79 | 0.41 |DeRoode,in Bull. 19, W. Va.
| | | Agr. Exp. Sta., 1891
Ragweed | 1.809| 0.54 |Burney, 2d. Ann. rept.
| | | S. C. Stat., 1889, p. 146
----------------------+------+----------+---------------------------
On the whole, ragweed seems to require and take from the soil about
as much mineral matter as does wheat. It is stated by some of the
dairy farmers near Washington, who cut the mixture of ragweed, other
weeds and grass following wheat, for a hay crop, that the weight of
the ragweed crop is generally heavier than that of the wheat crop.
Therefore the ragweed actually removes more mineral matter from the
field than does the wheat. These facts lend no support to the popular
notion that wheat “exhausts” the soil of its “available” mineral plant
nutrients. For analyses of a number of common American weeds, see
Analyses of the ashes of certain weeds, by Francis P. Dunnington: Am.
Chem. Jour., =2=, 24-27, (1880).
[110] Note on the occurrence of “fairy rings,” by J. H. Gilbert: Jour.
Linn. Soc, =15=, 17-24, (1875).
[111] Second, third and fifth reports of the Woburn Experimental Fruit
Farm, =1900=, =1903=, =1905=.
Finally, although less work has been done in this direction with higher
plants than with other organisms, it is now recognized as a general law
of all living organisms that they function less readily as the products
of their activities accumulate.[112] These products may, however, be
inimical, neutral or even stimulating to other organisms.
[112] It may not be amiss to point out here that this general law holds
for all dynamic phenomena. In chemistry, for instance, the general law
is well recognized that the rate of reaction diminishes with increase
in the active mass of the reaction products. It can be shown that the
principle applies to plant growth. Young plants will withdraw potassium
more rapidly than chlorine from solutions of potassium chloride; that
is, the solution soon contains free hydrochloric acid. Conversely
the plants cause a solution of sodium nitrate to become alkaline.
Therefore, if the above principle holds, then the initial addition of
small amounts of hydrochloric acid to a solution of potassium chloride
should slow up the absorption of potassium by seedling wheat plants,
or the addition of sodium hydroxide the absorption of nitrogen from a
solution of sodium nitrate. Mr. J. J. Skinner has tested this idea with
the following results, growing carefully selected wheat seedlings, for
3 days in solutions of pure potassium chloride, solutions of potassium
chloride containing initially enough excess of hydrochloric acid to
be of an N/₅,₀₀₀ concentration with respect to the acid, solutions of
sodium nitrate, and solutions of sodium nitrate containing initially an
excess of sodium hydroxide.
Solutions of KCl containing 80 p.p.m. K₂O.
1 K₂O absorbed 40.0 p.p.m. 2 K₂O absorbed 40.0 p.p.m. 3 K₂O absorbed
36.3 p.p.m.
Solutions of KCl (80 p.p.m. K₂O) and HCl (N/₅,₀₀₀).
4 K₂O absorbed 26.7 p.p.m. 5 K₂O absorbed 29.5 p.p.m. 6 K₂O absorbed
26.7 p.p.m.
Solutions of NaNO₃ containing 80 p.p.m. NH₃.
7 NH₃ absorbed 30.2 p.p.m. S NH₃ absorbed 30.2 p.p.m. 9 NH₃ absorbed
32.5 p.p.m.
Solutions of NaNO₃ (80 p.p.m. NH₃) and NaOH (N/₅,₀₀₀).
10 NH₃ absorbed 27.8 p.p.m. 11 NH₃ absorbed 34.3 p.p.m. 12 NH₃ absorbed
27.8 p.p.m.
This problem has been investigated critically by direct experimentation,
growing wheat, and other seedlings in water and agar cultures.[113]
It has been shown that wheat renders the culture media unsuitable
for subsequent wheat crops, though it can be reclaimed or renovated
by treatment with such absorbents as carbon black, or by other
methods.[114] Wheat did about as well when grown in a medium which had
previously supported a growth of cowpeas as when planted in a fresh
medium; poorer results were obtained after oats; no crop produced such
poor results in the succeeding wheat crop as did wheat itself.
[113] Some factors in soil fertility, by Oswald Schreiner and Howard S.
Reed, Bull. No. =40=, Bureau of Soils, U. S. Dept. Agriculture, 1907.
[114] Soil fatigue caused by organic compounds, by Oswald Schreiner and
M. X. Sullivan: Jour. Biol. Chem., =6=, 39-50, (1909).
It is yet a matter of dispute as to whether the substances thus added
to nutrient media are truly excretory products of the plant, sloughed
off or otherwise eliminated from the surface of the roots, or further
elaborated by bacterial or other agencies before becoming effective.
These are important problems for the plant physiologist and the soil
chemist alike. It is beyond dispute, however, by reason of a large and
increasing weight of evidence, much of it direct experiment, that, as
a result of the growing of plants, soils and the soil water do contain
organic substances; harmful to the plant or organism eliminating them;
harmful, innocuous, or even stimulating to other plants or organisms.
For the elimination from the soil of toxic or inhibitory organic
substances, whether excreted by roots or otherwise produced, several
methods are more or less effective. When, as is sometimes the case,
the substance is volatile, it may be removed by heating, distilling
with steam, or passing a current of air through the soil or cultural
medium. These methods, while effective in the laboratory and possibly
applicable to greenhouse conditions, are naturally inapplicable to
field conditions. In this last case the obvious procedure is to
increase as much as possible the absorptive powers of the soil; to
secure the best possible drainage; and with these, the best possible
aeration of the soil.
It has been found that, in general, a cultural medium which has
been rendered unfit for the continued growth of a crop, is readily
renovated by treatment with oxidizing agents, and is sometimes rendered
even better than ever by such treatment, which would suggest that
the oxidation products from plant effluvia may be even beneficial
to the plant. To this end the growing plant seems itself to be an
active agent, apparently attempting automatically to protect itself
against the products of its own activities. It has been pointed out by
Molisch[115] that root secretions have an oxidizing power, apparently
of an enzymotic character. Some doubt of the validity of Molisch’s
work has been raised by Czapek, Pfeffer, and others; nevertheless it
is now accepted that while intercellular autoxidation or reduction
processes may take place in living roots, the higher plants, such
as our common crop plants, also show a more or less well-developed
extracellular oxidizing power in the neighborhood of the root tips and
root hairs.[116] That this oxidizing power displayed by growing roots
is enzymotic is indicated by the fact that artificial culture media
frequently display it also after plants have been grown in them for a
short while.[117]
[115] Über Wurzelausscheidungen und deren Einwirkung auf organische
Substanzen, von Hans Molisch. Sitzungsber. Akad. Wiss. Wien, Math. nat.
K1., =96=, 84-109 (1888).
[116] The rôle of oxidation in soil fertility, by Oswald Schreiner
and Howard S. Reed: Bull. No. =56=, Bureau of Soils, U. S. Dept.
Agriculture, 1909.
[117] From considerations as yet highly speculative, a different type
of oxidation by roots might be anticipated. It is recognized that in
the absorption of mineral nutrients by plants a certain amount of
selection enters. For example, a plant with its roots in a solution of
potassium chloride, absorbs more potassium than chlorine, relatively,
and free hydrochloric acid is left in the solution. Obviously in the
absorption, work is done, and a possible explanation is that water is
decomposed at the absorbing surface of the root, with the liberation of
oxygen. Theoretically, it ought not to be difficult to investigate this
by a study of the energy changes during absorption, but growing plants
do not lend themselves readily to such experimentation.
It has been shown that the oxidizing action of growing roots is
generally promoted by having the cultural medium slightly alkaline
or neutral rather than acid. It is also promoted by the addition
of various mineral salts, notably by nitrates, phosphates, or
lime salts. Potassium salts promote the oxidation but slightly,
and in some experiments have even produced a slight decrease. The
corresponding sodium and ammonium salts are more favorable than those
of potassium.[118] It appears altogether probable, therefore, that the
mineral salts in commercial fertilizers may have some importance in
this connection.
Whatever may be the role of mineral fertilizers towards organic
substances toxic to growing plants, it is certain that they have
an importance and one that is probably specific, as indicated by
some recent investigations.[119] Culture solutions containing the
constituents potassium, nitric acid and phosphoric acid were prepared
in such manner that they covered the range of all possible ratios
of these constituents in intervals of ten per cent. in each. Into
one set of these solutions was introduced dihydroxystearic acid,
into another set cumarin, and into a third set, vanillin, and into a
fourth set, quinone. The growth of wheat seedlings in these several
sets showed indubitably that these several organic substances which
are all deterrent to the growth of wheat, were modified in their
influence by the presence of the mineral salts; but that nitrates
were more efficient than the other minerals in the case of the
solutions containing dihydroxystearic acid or vanillin; phosphates
were most efficient in the case of the solutions containing cumarin,
and potassium most efficient in solutions containing quinone. As the
organic substances used in these experiments, either in themselves or
as typifying classes of compounds, may be anticipated in soils under
natural conditions, it is again apparent that mineral fertilizers have
a function in addition to the traditional one of increasing the supply
of mineral nutrients.
[118] Action of fertilizing salts on plant enzymes, by M. X. Sullivan,
Jour. biol. chem., =6=, (1909), proceed. XLIV.
[119] Private communication by Dr. Oswald Schreiner and Mr. J. J.
Skinner.
The fact that the oxidizing power of roots is more marked when grown
in aqueous extracts of soils in good tilth than in extracts made from
soils in poor tilth, shows that cultural methods are no less important
in field practice than are fertilizers in promoting this important
activity of plants. There is little reason to doubt that oxidizing
agencies other than plant roots (bacterial for instance) are more or
less active in every arable soil, and numerous investigations, among
which Russell’s researches[120] are conspicuous, leave little doubt
that oxidation processes are promoted by good tilth. It is apparent,
therefore, that by the activities of the plant itself as well as other
agencies, the general tendency in soils is the destruction of or
rendering innocuous harmful plant effluvia or other organic substances,
and to this end are effective each of the three methods of soil control
generally practiced, namely, tillage, crop rotation and fertilizers.
Among the organic components of the soil none have greater importance
and interest than those containing nitrogen or as they are frequently
called the nitrogen carriers. Conspicuous among these are the nitrates.
While it is now generally conceded that ammonia and other nitrogen
compounds can be taken up by higher plants and elaborated by them under
special conditions, it nevertheless remains true that plants draw their
needed supplies of nitrogen from the soil solution, mainly in the
form of nitrates. The problems presented by these nitrogen carriers
are mainly bacterial[121] and physiological, but certain features
are of direct importance to the soil chemist and to a study of the
soil solution. It is now known generally that there are many kinds of
nitrifying and denitrifying bacteria in soils, and that probably every
arable soil contains several species, or varieties at least of both
kinds. With good tilth and consequent aerobic conditions, nitrifying
processes prevail, and with poor tilth or in subsoils, anaerobic
conditions and denitrifying processes prevail. Warmth, moisture, the
reaction of the soil, and perhaps other factors markedly affect the
activity of the organisms of the soil solution. Another important
factor is that the absorptive powers of the higher plants are markedly
affected by sunlight, so that, especially on bright and clear days,
there is generally a higher concentration of nitrates in the soil
solution in the morning than in the evening. This fact would seem to
affect seriously the value of some recent and extensive investigations
where it has been sought to classify soils by their content of
water-dissolved nitrates. Nitric acid is more readily leached from
soils than are most other acid radicals. Consequently nitrates, like
other organic components of the soil solution, and unlike inorganic
components, tend to vary greatly in concentration.
[120] Oxidation in soils, and its connection with fertility, by
Edward J. Russell: Jour. Agric. Sci., I, 261-279, (1905); Pt. II. The
influence of partial sterilization, by Francis V. Darbishire and Edward
J. Russell, =2=, 305-326, (1907).
[121] The fixation of atmospheric nitrogen by bacteria, by J. G.
Lipman, Bull. =81=, Bureau of Chemistry, U. S. Dept. of Agriculture,
1904, p. 146-160; A review of investigations in soil bacteriology,
by Edward B. Voorhees and Jacob G. Lipman, Bull, =194=, Office of
Experiment Stations, U. S. Dept. of Agriculture, 1907.