Soy bean bacteria are evidently much less likely to be carried with
the seed than are the cowpea bacteria. The soy bean plant grows more
erectly than the cowpea (see Circular No. 69, “The Cowpea and Soy Bean in
Illinois”), and the crop is quite commonly harvested with a self-binder
which keeps it quite free from dirt. The soy bean seed is nearly round
and smooth, and the seed coat is not commonly cracked. These facts may
explain why the soy bean seed carry so few bacteria as compared with
cowpeas.
On one of the soil experiment fields on the university farm at Urbana,
where soy beams have been grown for three years, no tubercles could
be found on the plants either the first or second year, and only an
occasional plant with tubercles could be found the third year. In 1902 a
series of plots, some of which had been treated in different ways with
applications of limestone, phosphorus, and potassium, were seeded with
soy beans. No tubercles could be found at any time during the season on
the soy beans growing on any of the different plots. In 1903 the same
plots were again seeded to soy beans, and at the same time part of each
plot was inoculated with infected soy bean soil drilled in with the seed
at the rate of about 500 pounds of infected soil to the acre. When the
plants were only a few weeks old tubercles were to be found upon many
plants growing where the infected soil had been applied, and before the
close of the season at least half of these plants in the inoculated part
of the field had one or more tubercles upon their roots, and some plants
could be found whose roots were abundantly provided with tubercles. (See
Plate 4.)
On the uninoculated part of the field soy bean plants were examined
probably fifty times during the season, several plants being taken
up each time, but not a single tubercle was found at any time,
notwithstanding that this was the second crop of soy beans upon this
soil. Of course the inoculated part of the field did not become
sufficiently infected to markedly benefit the 1903 crop, but it is
planned to grow soy beans upon this field again in 1904 when the bacteria
will doubtless have multiplied sufficiently to produce marked results in
the growth of the crop.
From these and from other somewhat similar experiments it is concluded
that as a rule soy beans should be inoculated when they are first seeded,
and that they should then be grown a second year upon the same land. If
soy beans are afterward grown upon this land once in every three or four
years, the soil will doubtless remain well infected with the soy bean
bacteria.
It is believed that 100 pounds of infected soy bean soil per acre will
be sufficient to produce a thorough infection the second year, and it
is improbable that one ton of infected soil per acre would produce a
thorough inoculation the first season. One ton is only twenty times 100
pounds, while one tubercle which will be produced during a single season
from a single bacterium may contain many million bacteria, thus it will
be seen that it will be more economical to inoculate rather lightly
and allow the bacteria to multiply themselves rather than to inoculate
heavily at great expense.
[Illustration: PLATE 4. SOY BEAN ROOT TUBERCLES, NATURAL SIZE.]
It may be stated that the infected soy bean soil used in these
experiments was obtained from Mr A. A. Hinkley of Du Bois, Illinois, who
has been growing soy beans on the same land for many years until it has
become well infected. Mr. Hinkley has consented to furnish infected soy
bean soil so far as he is able to do without serious interference with
his regular work, to any one who may desire it, at a price which will
cover his expense and loss. This will probably amount to about $1.00 for
the first 100 pounds and fifty cents for each additional 100 pounds, in
the shipment, including the cost of bags, the purchaser to pay freight
from Bois station, which is located in Washington County, Illinois, on
the Illinois Central Railroad.