Plate 2 is made from a photograph of a cowpea root with the tubercles
upon it. This illustration shows the cowpea tubercles at nearly natural
size, which is about as large as the seed of ordinary garden peas.
[Illustration: PLATE 2. COWPEA ROOT TUBERCLES, NATURAL SIZE.]
The cowpea bacteria are already quite widely distributed in southern
Illinois, especially where this crop has been grown for several years,
but they are not common in the soils of other parts of the state. It
is doubtful, however, if it is necessary or even worth while to take
the trouble to inoculate soil for cowpeas. Some few tubercles almost
invariably develop on cowpea roots the first year they are seeded,
even where they have never been grown before, and if seeded the second
year on the same land the plants are usually abundantly provided with
root tubercles. Just why the cowpea bacteria develop so rapidly even
without special inoculation is not definitely known. It may be that the
same bacteria also live on some other leguminous plant which is more or
less widely distributed over the state, but it seems more likely that
the bacteria are brought with the seed. As a matter of fact, the cowpea
harvest is usually dirty. This is an annual plant, and consequently the
crop is grown on recently plowed land and is sometimes cultivated during
the season. Cowpeas are commonly harvested with a mowing machine and then
raked up on the loose ground. When they are threshed more or less dirt
remains with the seed. Furthermore, the seed coats are not infrequently
cracked, thus providing an excellent place for the lodgment of particles
of soil.
Whether it would be profitable to inoculate the land for cowpeas would
depend very largely upon the difficulty or cost of obtaining the infected
material. If soil thoroughly infected with the cowpea bacteria can be
scattered over the land at the rate of about 2,000 pounds to the acre
at a cost of $1.00 or less per ton, it might prove profitable. It is
doubtful if a light application of 100 or 200 pounds would produce any
very marked effect in the yield the first season. After the soil becomes
well infected the cowpeas then obtain much nitrogen from the air, and the
yield of cowpeas is likely to be largely increased. Of course there is no
fixation of atmospheric nitrogen if there are no tubercles on the roots.
In 1902 several plots of cowpeas were seeded on the soil experiment
field at the university. One of those plots (404) had become thoroughly
infected with the cowpea bacteria because of its being so situated that
more or less surface drainage water flowed over it from an adjacent field
upon which cowpeas had been grown for three successive years. Another
plot (408), owing to a slightly different situation, had not become
infected. The two plots were seeded in July after a crop of oats had been
removed from the land. Within three weeks after seeding, numerous root
tubercles could be found on the plants on the infected plot. Later on,
ten average consecutive plants were taken up as completely as possible,
and 412 tubercles were found on the roots, making an average of more than
40 tubercles to the plant. On Plot 408 only an occasional plant was found
infected, and such plants would usually have only a single large tubercle
on their roots. Ten average plants not infected were collected from Plot
408 for comparison with the ten infected plants from Plot 404.
[Illustration: PLATE 3. COWPEAS; EFFECT OF BACTERIA IN ORDINARY ILLINOIS
BLACK PRAIRIE SOIL.]
Plate 3 shows these two bunches of plants, the infected plants with root
tubercles on the _right_, and the plants without tubercles on the _left_.
Four more sets of ten plants each were then collected, two sets from Plot
404 and two from Plot 408. Each set of infected plants was separated into
three parts, (1) tops, (2) roots, (3) tubercles; and each set of plants
not infected was separated into (1) tops, and (2) roots. All of these
samples were dried and analyzed for nitrogen. The results obtained are
shown in Table 1.
The results clearly show the very great value of the nitrogen-gathering
bacteria in growing cowpeas. In each of the separate trials A, B, and C,
the infected plants contained about twice as much total dry matter as the
plants not infected. The infected plants also contained a much higher
percent of nitrogen than the plants not infected, the infected plants
containing 4.09 to 4.33 percent in the tops and 1.45 to 1.53 percent in
the roots, while those not infected contained only 2.32 to 2.69 percent
in the tops and .88 percent in the roots. Besides this, the tubercles
on the infected plants contain 5.76 to 6.05 percent of nitrogen. In
these young and rapidly growing plants the tubercles are much richer in
nitrogen than any other part of the plant. It should be stated that as
the plants approach maturity the nitrogen is largely absorbed from the
tubercles and stored in the tops and roots. At the time these plants were
taken up the tubercles actually contained more nitrogen than the roots.
The infected plants contained nearly four times as much nitrogen as the
plants not infected, and about three-fourths of the total nitrogen in the
infected plants was obtained from the air. The roots and tubercles of
the infected plants contained six to seven times as much nitrogen as the
roots of the plants not infected.