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.
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.
Plate 3. Cowpeas; Effect of Bacteria in Ordinary Illinois Black Prairie Soil.