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.
Table I.—Fixation of Nitrogen by Cowpeas.
| Cowpea Plants. | Dry matter, cgs. | Nitrogen content, percent. | Nitrogen amount, cgs. | Nitrogen fixed by bacteria, cgs. | ||
|---|---|---|---|---|---|---|
| No. | Part. | |||||
| A1— | Ten plants, with bacteria present. | Tops | 3580 | 4.09 | 146 | 125 |
| Roots | 620 | 1.45 | 9 | |||
| Tubercles | 190 | 5.97 | 11 | |||
| Total | 4390 | 166 | ||||
| A2— | Ten plants, without bacteria | Tops | 1560 | 2.42 | 38 | |
| Roots | 300 | .88 | 3 | |||
| Total | 1860 | 41 | ||||
| B1— | Ten plants, with bacteria present. | Tops | 3970 | 4.31 | 171 | 140 |
| Roots | 690 | 1.47 | 10 | |||
| Tubercles | 300 | 6.05 | 18 | |||
| Total | 4960 | 199 | ||||
| B2— | Ten plants, without bacteria | Tops | 2060 | 2.69 | 55 | |
| Roots | 430 | .88 | 4 | |||
| Total | 2490 | 59 | ||||
| C1— | Ten plants, with bacteria present. | Tops | 3300 | 4.33 | 143 | 124 |
| Roots | 520 | 1.53 | 8 | |||
| Tubercles | 290 | 5.76 | 17 | |||
| Total | 4110 | 168 | ||||
| C2— | Ten plants, without bacteria | Tops | 1730 | 2.32 | 40 | |
| Roots | 400 | .88 | 4 | |||
| Total | 2130 | 44 | ||||
The Soy Bean Bacteria.
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.