That soil inoculation with alfalfa bacteria is commonly of very great
value in growing alfalfa has been shown very conclusively by the
investigations reported in Bulletin No. 76,[6] “Alfalfa on Illinois
Soil.” In some places, however, inoculation was found to be unnecessary.
A careful and extensive investigation of alfalfa growing in different
parts of Illinois revealed the fact, as stated in Bulletin 76, “that the
alfalfa bacteria are certainly present in some places in the state while
in most other places they are certainly not present in sufficient number
to become of appreciable assistance to the alfalfa within three or four
years, and the question naturally arises how it happens that some fields
are already infected while others are not.” It was suggested in that
bulletin that the alfalfa bacteria may “live on some other plants besides
alfalfa and that one of these plants is native or has been introduced
in certain sections” of the state. It was also suggested “that a few
bacteria are always carried with alfalfa seed, and that if the alfalfa
is grown continuously or repeatedly in any place the soil will finally
become thoroughly infected, and the bacteria will then be carried by
flood waters, dust storms, etc., over adjoining fields, and possibly for
long distances, especially along river valleys.” This latter suggestion
was known to be a fact at the time it was written; and subsequent
investigations have furnished conclusive proof that the alfalfa bacteria
do live upon another plant; namely, the ordinary sweet clover (_melilotus
alba_). This is a rank-growing leguminous plant, frequently reaching a
height of four to six feet. When young it markedly resembles alfalfa, but
it can easily be distinguished by its characteristic odor when cut or
bruised, as by rubbing between the hands. As the sweet clover approaches
maturity it differs very much from alfalfa. The sweet clover grows very
tall, and usually branches from a main stem. It has white flowers (there
is also a less common yellow variety), and the seeds are borne in small
round pods (usually containing only one or two seeds each), arranged on
long slender spikes, each spike bearing many pods. The alfalfa commonly
grows about two and a half feet high, with many stems growing from the
crown of the root, especially after it is two or three years old. It
bears purple flowers and peculiar spiral-shaped seed pods. Sweet clover
is a biennial plant, dying after reaching maturity, which commonly
occurs the second year of its growth. Like many other biennial plants,
it probably often lives more than two years if not allowed to produce
seed. Alfalfa is a perennial plant, and it is said that there are alfalfa
fields which have been cut annually for more than fifty years without
reseeding. The similarity of alfalfa and sweet clover when young, and
also the similarity of the tubercles formed on the roots of each have
long been noticed, and the possibility of the same bacteria living upon
both plants has already been suggested in the agricultural press.
During the season of 1903 the writer spent some time in the northern
part of Illinois in connection with the general and detail surveys of
Illinois soil. Many new fields of alfalfa were observed, and they were
carefully examined for root tubercles. In Winnebago County, where sweet
clover is very prevalent along roadsides and in waste places, it was
noted that the abundance of root tubercles on the alfalfa plants seemed
to be closely related to the presence of sweet clover in the vicinity,
strongly indicating that the bacteria which live upon sweet clover were
also at home upon the alfalfa roots. These indications were strengthened
by further investigations in Lake County, especially upon the Fowler
farm, near Lake Villa, where a field of alfalfa seeded last spring
without artificial inoculation was found to be thoroughly infected with
the bacteria, and growing vigorously with a good dark green color. This
field had a few sweet clover plants growing in it, and the borders of the
field were covered with sweet clover. Other fields of alfalfa seeded in
the neighborhood at the same time, but upon soils where sweet clover had
not grown near by, were apparently complete failures, many of the plants
having died and most of those still living being only a few inches high,
very weak, and yellow or pale green in color.[7]
In order to obtain more absolute knowledge regarding this important
subject, a series of pot culture experiments has been carried on under
controlled conditions in the pot culture laboratory at the university.
Five pots were filled with sterilized sand which was practically devoid
of plant food. A supply of phosphorus, potassium, and all other mineral
elements necessary for the growth of plants was added to each of the five
pots, care being taken to keep the sand practically free of combined
nitrogen. Alfalfa seed were then planted in each of the five pots, and at
the same time four of the five pots were inoculated as follows:
Pot No. 1.—Not inoculated (check pot).
Pot No. 2.—Inoculated with bacteria obtained from infected alfalfa soil.
Pot No. 3.—Inoculated with bacteria obtained from alfalfa root tubercles.
Pot No. 4.—Inoculated with bacteria obtained from infected sweet clover
soil.
Pot No. 5.—Inoculated with bacteria obtained from sweet clover root
tubercles.
Plate 5 clearly shows the results obtained and certainly furnishes
conclusive proof that the same effect is produced upon the growth of the
alfalfa whether the nitrogen-gathering bacteria used for the inoculation
are obtained from alfalfa soil, from alfalfa tubercles, from sweet clover
soil, or from sweet clover tubercles. It also illustrates the importance
of bacteria in growing alfalfa as will be seen by comparing the four
inoculated pots with the uninoculated pot, which is No. 1, on the left in
each series of views. The upper view was taken when the alfalfa plants
were five weeks old; the next series when they were six weeks old; the
next, seven weeks old; and the lower series when they were eight weeks
old, from the time of seeding.
A duplicate series of pots prepared in exactly the same manner gave
similar results.
The infected alfalfa soil was obtained from a field of three-year-old
alfalfa, which was inoculated when first seeded, with infected alfalfa
soil obtained from an old alfalfa field in Kansas. About one pound of
this soil was shaken in a quart of water, the soil allowed to settle, and
some of the nearly clear solution used for the inoculation of Pot No. 2.
The alfalfa tubercles from which bacteria were obtained were carefully
washed in distilled water to free them from adhering soil particles, and
then rubbed up in distilled water, a small amount of this water being
then used for the inoculation of Pot No. 3. The infected sweet clover
soil was obtained from a place by the roadside where sweet clover was
growing luxuriantly and well provided with root tubercles. This place was
about two miles from the nearest field ever seeded to alfalfa, so far as
known. A water extract from this soil was used to inoculate Pot No. 4.
The bacteria from sweet clover tubercles were obtained in the same manner
as those from alfalfa tubercles, and were used to inoculate Pot No. 5.
[Illustration: PLATE 5. ALFALFA: EFFECT OF BACTERIA FROM ALFALFA AND FROM
SWEET CLOVER.
Pot 1.—No bacteria.
Pots 2 and 3.—Bacteria from alfalfa.
Pots 4 and 5.—Bacteria from sweet clover.
The four series of photographs were taken five, six, seven and eight
weeks from time of planting, respectively.]
From these investigations we thus have conclusive evidence that infected
sweet clover soil can be used for the inoculation of alfalfa fields,
the bacteria of the two plants acting the same. The infected soil may
be obtained from any place where the sweet clover is found growing with
abundance of tubercles on its roots. The soil may be collected to a depth
of three or four inches and scattered over the alfalfa field at the rate
of 100 pounds or more to the acre. It is well to scatter the infected
soil at about the time the alfalfa is seeded, and harrow it in with the
alfalfa seed, although it may be applied some days or even some weeks
before seeding time, and probably it would be all right to apply the
infected soil the fall before, for it is known that the bacteria will
live in soil for several months, even though the soil be placed in sacks
and allowed to become quite dry.
Investigations have shown that 100 pounds of thoroughly infected soil
to the acre is sufficient to produce a very satisfactory inoculation
within one year from the time it is applied. Of course, somewhat heavier
applications may well be made if it can be done at small expense. The
infected soil need not be applied with any high degree of uniformity, but
special care should be taken that the higher places and watersheds are
not missed in scattering it over the field. If a few square yards, or
even square rods, should be missed on the slopes or lower land, it would
make but little difference, as the bacteria will be washed over such
places from the higher land.
After the soil becomes somewhat dry it is easily scattered by hand from
the wagon or from a sack which one can carry. Sometimes it is applied by
means of an end gate seeder or a fertilizer drill, or it could be spread
by a manure spreader with an application of manure.
The question naturally arises whether there is not danger of getting
some sweet clover seed with the infected sweet clover soil, and thus of
getting sweet clover mixed with the alfalfa in the field.
In the writer’s opinion there is little or nothing to fear in this
matter. In the first place, the amount of sweet clover seed thus obtained
would be very small, probably none at all, if one were careful to scrape
off the vegetable matter, and perhaps a half inch of earth before
collecting the infected soil (most of the bacteria are probably between
one-half inch and six inches in depth, as most of the tubercles develop
and decay between those depths); second; it is doubtful if a small amount
of sweet clover hay would lessen the value of alfalfa hay in the least,
for stock frequently eat small amounts of sweet clover of their own
choice even when it is nearly mature, and if it is cut while still quite
immature and tender it makes quite satisfactory hay, so much so that in
some sections of the United States, particularly in the South, sweet
clover is regularly seeded on fields and cut for hay, and it is found to
be a valuable and very nutritious feed, the live stock eating it in large
quantities, and with apparent relish, after they have acquired a taste
for it; third, sweet clover is not known as a bad weed in the fields or
meadows, even where it has been a common roadside plant for many years,
and, being naturally a biennial plant, if it were cut down every five or
six weeks, as we commonly cut alfalfa during the season, it would almost
certainly die out after a few years while alfalfa, a perennial plant,
would continue to live.
Only one instance has come to the writer’s attention where alfalfa has
been growing for several years with sweet clover growing in the field or
fence rows beside it. This is on the farm of Mr. D. S. Mayhew, of Mercer
County, Illinois, who writes as follows regarding the matter:
“Will say that the sweet clover has made no headway in my
meadow, as it did not go to seed, on account of my cutting
it so often. The sweet clover got into the alfalfa in the
seed when I sowed it. I do not think it will do any harm in
a meadow, but I believe it would do harm in a pasture if it
wasn’t cut down as stock will not eat the sweet clover.”
Of course if sweet clover should get into the field and persist in
growing, and if it were found to injure the alfalfa appreciably or
markedly, we can always resort to plowing the ground up and growing corn
or other crops, thus obtaining some benefit from the leguminous crop for
its fertilizing value, and at the same time completely eradicating the
sweet clover, but leaving the soil well infected with alfalfa bacteria
ready to serve in case alfalfa should be again seeded within a few years.