That clover has no power in itself to gather atmospheric nitrogen,
and that the bacteria do have power to feed the clover plant with
nitrogen gathered from the air is very easy to demonstrate. It is one
of the regular laboratory practices of the students in soil fertility
in the Agricultural College to make this demonstration. Plate 1 is an
illustration of such student work. The two pots which are shown were
provided with all elements of plant food, excepting the one element
nitrogen. Thus far the two pots are exactly alike. Each contains no
nitrogen, as indicated by the label “No N.” Each pot is planted with the
same number of red clover seeds. To the right-hand pot, however, some
bacteria (“Bac.”) were added, while none were added to the left-hand
pot. These bacteria were obtained by taking about one pound of soil from
a clover-field where abundance of tubercles were found on the clover
roots, adding this soil to about one quart of pure water, shaking for a
few minutes, allowing the soil to settle, then taking a small quantity
of almost clear solution, and adding it to the pot which we wished to
inoculate with the red clover bacteria. Aside from the addition of these
microscopic bacteria to the right-hand pot, these two pots were treated
exactly alike throughout the experiment. It will be plainly seen that
where the bacteria were added the clover was furnished with sufficient
nitrogen to make a strong and luxuriant growth, while without the
bacteria the clover (in the left-hand pot) only germinated and made what
little growth it could with the small amount of nitrogen contained in the
seed. This result is the difference between success and failure of the
clover crop.
[Illustration: PLATE 1. RED CLOVER: EFFECT OF BACTERIA. NO NITROGEN IN
THE SOIL OF EITHER POT.]
In general the clover bacteria are well distributed over the northern and
central part of Illinois, but we now have some very strong evidence that
they are not well distributed in some soils of large area in southern
Illinois. There is also some evidence that they were not originally
present even in the soils where they are now found in great abundance;
and, furthermore, it seems very probable that these bacteria may cease to
live in a soil where they have once been present, provided clover is not
grown on the land for several years.
It will help us to understand this matter if we bear in mind that the
home of these bacteria is the tubercle upon the clover root. It is quite
evident that they will continue to live upon the decaying tubercles or
roots for three or four years after the clover plant has been killed. On
the other hand, we have some notable evidence that the bacteria do not
continue to live in a soil after five or six years’ continuous cropping
with absolutely no clover growing on the land during those years. It
is a simple matter for any one to determine whether the bacteria are
present or not, for the tubercles which are formed if the bacteria are
present are plainly seen attached to small roots. They look somewhat like
miniature potatoes, varying in size from pinheads on clover to peas on
soy beans or cowpeas. (See Plates 2 and 4.) It is important to remember
that the bacteria live in the soil and not in the seed.
When clover is cut for seed, it is frequently left to lie upon the ground
until the straw becomes half rotten and very dirty; and, consequently
when it is threshed, it practically always happens that there is at least
some small amount of dust and dirt taken with the seed. This dirt is
almost sure to carry with it some bacteria from the soil. If these few
bacteria are scattered with the clover seed when it is sowed they will
inoculate at least a few plants, and if they are allowed to multiply on
these plants, and especially if the same field is repeatedly seeded with
clover, the soil will ultimately become thoroughly infected with the
clover bacteria. Of course they may be carried from one part of the farm
to another, or even from one farm to another, by various agencies, as
dust or wind storms, surface drainage or flood waters, manure made from
clover hay, implements used in cultivating the soil, etc., etc.
Many of the older farmers of Illinois have stated to the writer that
when this country was very new it was commonly found difficult to get a
“catch” of clover on new land. After a good “catch” was once gotten, then
it was easier to get clover to grow on that land the next time. There was
a saying among the farmers that clover would not do well until they got
the “wild nature” out of the land. Their final success was undoubtedly
due, not to getting anything out of the land, but rather to getting the
bacteria into the land. Several Illinois farmers have reported some quite
remarkable results from very light applications of the clover chaff or
straw (obtained in hulling clover) in its beneficial effect on clover
on land where it was otherwise difficult to get a “catch.” There is a
somewhat general belief among farmers of long experience that clover
straw or chaff has some special value in getting a catch of clover aside
from its value as manure or for the seed which it sometimes contains.
Manager F. A. Warner of the Sibley Estate, Ford County, recently stated
to the writer that they had had very great difficulty to get clover to
grow when they first began growing clover on that large estate, some six
or eight years ago, although, after a good crop was once secured, they
rarely had any further difficulty in getting a catch of clover on the
same land.
On the common gray prairie soil of the Lower Illinoisan Glaciation,
in southern Illinois, the commonest type of soil in more than twenty
counties, practically no red clover is grown. In the spring of 1903 we
seeded red clover on that type of soil in three places; namely, on the
University of Illinois soil experiment fields near Edgewood, Effingham
County, near Du Bois, Washington County, and near Cutler, Perry County.
On certain plots the soil acidity had been corrected with lime and an
abundant supply of phosphorus (in bone meal) had been provided, potassium
also having been added on some plots. These fields were carefully
examined the latter part of June, and at Cutler and Du Bois the clover
was found to be dead or dying, and no tubercles could be found upon the
clover roots, although on the clover which had been seeded at about the
same time on the University fields at Urbana the root tubercles were
found in great abundance. At Edgewood a few tubercles were found and the
clover appeared to be growing fairly well. Infected red clover soil was
at once procured and scattered over the fields at Edgewood, Du Bois,
and Cutler, but it was evidently too late to be of any marked benefit.
At Cutler and at Du Bois the clover was a complete failure. (It will be
tried again next year.) At Edgewood it continued to grow fairly well,
and its progress next season (1904) will be watched with much interest.
It should be stated that the Experiment Station has been growing clover
for several years with varying degrees of failure on land adjoining the
present clover field at Edgewood, and it is possible that this year’s
apparent success from the start is due in part at least to the bacteria
which have been incidentally introduced and multiplied year after year
and scattered over the adjoining land by wind and dust storms. Before the
close of the season the tubercles developed in abundance on the roots of
the clover at Edgewood.
An experience reported by Professor Herbert W. Mumford, of the Animal
Husbandry Department of this university, will be of interest and value
in this connection. Professor Mumford commonly grows clover in his
rotations on his own private farm, but he states that at one time one
particular field was cropped continuously with timothy, oats, and corn
for some six years or more without any clover whatever. It was then again
seeded to clover, but the crop made a complete failure, although on other
land where clover had been grown more recently a successful clover crop
was grown from the same kind of seed seeded at about the same time.
The following year this particular field was again seeded to clover.
This time the “catch” was not a total failure, but it was too poor to
save, and it was plowed up and the land again seeded to clover the next
year, and an excellent catch of clover resulted. After this, clover was
frequently grown on this field, and no special difficulty was had in
getting good crops.
While the failure of clover may often be due to drouth, and in some
places due to soil acidity (lack of lime), and sometimes even due to
an insufficient supply of available phosphorus or of potassium, we now
know with certainty that it sometimes fails because of the absence of
the nitrogen-gathering bacteria, especially on land which has never
grown clover, and probably also on land which has not grown it recently.
We should always remember that the bacteria do not thrive in strongly
acid soils. Even though they may sometimes live in such soils and
perhaps produce some tubercles upon the roots of certain hardy, strong
growing legumes, like cowpeas, nevertheless we are obtaining some strong
evidence that in such acid soils they have but little power to gather
nitrogen from the air. That ground limestone is the most economical
and satisfactory material to use in correcting the acidity of soils is
strongly indicated by the information we have thus far obtained. On
the upland prairie soils of the Lower Illinoisan Glaciation where red
clover has never been grown successfully, largely because of the acidity
of the soil, it will undoubtedly be helpful and profitable not only to
correct the acidity of the soil with ground limestone, but also to secure
infected soil from some field of timber land or bottom land where red
clover is growing, well provided with root tubercles, and inoculate the
field with it. This soil should be collected to a depth of three or four
inches and scattered over the prairie land at the rate of a few hundred
pounds per acre at the time the clover is seeded or before.