Part 2
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 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 |Nitrogen|Nitrogen|Nitrogen -----------------+-----------+ matter,|content,|amount, |fixed by No. | Part | cgs. |percent.| cgs. |bacteria, | | | | | cgs. -----------------+-----------+--------+--------+--------+--------- A1—Ten plants, | Tops | 3580 | 4.09 | 146 | with bacteria | Roots | 620 | 1.45 | 9 | present. | Tubercles | 190 | 5.97 | 11 | | | ---- | | --- | | Total | 4390 | | 166 | 125 | +--------+--------+--------+ A2—Ten plants, | Tops | 1560 | 2.42 | 38 | without bacteria | Roots | 300 | .88 | 3 | | | ---- | | --- | | Total | 1860 | | 41 | -----------------+-----------+--------+--------+--------+--------- B1—Ten plants, | Tops | 3970 | 4.31 | 171 | with bacteria | Roots | 690 | 1.47 | 10 | present. | Tubercles | 300 | 6.05 | 18 | | | ---- | | --- | | Total | 4960 | | 199 | 140 | +--------+--------+--------+ B2—Ten plants, | Tops | 2060 | 2.69 | 55 | without bacteria | Roots | 430 | .88 | 4 | | | ---- | | --- | | Total | 2490 | | 59 | -----------------+-----------+--------+--------+--------+--------- C1—Ten plants, | Tops | 3300 | 4.33 | 143 | with bacteria | Roots | 520 | 1.53 | 8 | present. | Tubercles | 290 | 5.76 | 17 | | | ---- | | --- | | Total | 4110 | | 168 | 124 | +--------+--------+--------+ C2—Ten plants, | Tops | 1730 | 2.32 | 40 | without bacteria | 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.
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.
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.
THE ALFALFA AND SWEET CLOVER BACTERIA.
That soil inoculation with alfalfa bacteria is commonly of very great value in growing alfalfa has been shown very conclusively by the investigations reported in Bulletin No. 76,[6] “Alfalfa on Illinois Soil.” In some places, however, inoculation was found to be unnecessary. A careful and extensive investigation of alfalfa growing in different parts of Illinois revealed the fact, as stated in Bulletin 76, “that the alfalfa bacteria are certainly present in some places in the state while in most other places they are certainly not present in sufficient number to become of appreciable assistance to the alfalfa within three or four years, and the question naturally arises how it happens that some fields are already infected while others are not.” It was suggested in that bulletin that the alfalfa bacteria may “live on some other plants besides alfalfa and that one of these plants is native or has been introduced in certain sections” of the state. It was also suggested “that a few bacteria are always carried with alfalfa seed, and that if the alfalfa is grown continuously or repeatedly in any place the soil will finally become thoroughly infected, and the bacteria will then be carried by flood waters, dust storms, etc., over adjoining fields, and possibly for long distances, especially along river valleys.” This latter suggestion was known to be a fact at the time it was written; and subsequent investigations have furnished conclusive proof that the alfalfa bacteria do live upon another plant; namely, the ordinary sweet clover (_melilotus alba_). This is a rank-growing leguminous plant, frequently reaching a height of four to six feet. When young it markedly resembles alfalfa, but it can easily be distinguished by its characteristic odor when cut or bruised, as by rubbing between the hands. As the sweet clover approaches maturity it differs very much from alfalfa. The sweet clover grows very tall, and usually branches from a main stem. It has white flowers (there is also a less common yellow variety), and the seeds are borne in small round pods (usually containing only one or two seeds each), arranged on long slender spikes, each spike bearing many pods. The alfalfa commonly grows about two and a half feet high, with many stems growing from the crown of the root, especially after it is two or three years old. It bears purple flowers and peculiar spiral-shaped seed pods. Sweet clover is a biennial plant, dying after reaching maturity, which commonly occurs the second year of its growth. Like many other biennial plants, it probably often lives more than two years if not allowed to produce seed. Alfalfa is a perennial plant, and it is said that there are alfalfa fields which have been cut annually for more than fifty years without reseeding. The similarity of alfalfa and sweet clover when young, and also the similarity of the tubercles formed on the roots of each have long been noticed, and the possibility of the same bacteria living upon both plants has already been suggested in the agricultural press.
During the season of 1903 the writer spent some time in the northern part of Illinois in connection with the general and detail surveys of Illinois soil. Many new fields of alfalfa were observed, and they were carefully examined for root tubercles. In Winnebago County, where sweet clover is very prevalent along roadsides and in waste places, it was noted that the abundance of root tubercles on the alfalfa plants seemed to be closely related to the presence of sweet clover in the vicinity, strongly indicating that the bacteria which live upon sweet clover were also at home upon the alfalfa roots. These indications were strengthened by further investigations in Lake County, especially upon the Fowler farm, near Lake Villa, where a field of alfalfa seeded last spring without artificial inoculation was found to be thoroughly infected with the bacteria, and growing vigorously with a good dark green color. This field had a few sweet clover plants growing in it, and the borders of the field were covered with sweet clover. Other fields of alfalfa seeded in the neighborhood at the same time, but upon soils where sweet clover had not grown near by, were apparently complete failures, many of the plants having died and most of those still living being only a few inches high, very weak, and yellow or pale green in color.[7]
In order to obtain more absolute knowledge regarding this important subject, a series of pot culture experiments has been carried on under controlled conditions in the pot culture laboratory at the university. Five pots were filled with sterilized sand which was practically devoid of plant food. A supply of phosphorus, potassium, and all other mineral elements necessary for the growth of plants was added to each of the five pots, care being taken to keep the sand practically free of combined nitrogen. Alfalfa seed were then planted in each of the five pots, and at the same time four of the five pots were inoculated as follows:
Pot No. 1.—Not inoculated (check pot).
Pot No. 2.—Inoculated with bacteria obtained from infected alfalfa soil.
Pot No. 3.—Inoculated with bacteria obtained from alfalfa root tubercles.
Pot No. 4.—Inoculated with bacteria obtained from infected sweet clover soil.
Pot No. 5.—Inoculated with bacteria obtained from sweet clover root tubercles.
Plate 5 clearly shows the results obtained and certainly furnishes conclusive proof that the same effect is produced upon the growth of the alfalfa whether the nitrogen-gathering bacteria used for the inoculation are obtained from alfalfa soil, from alfalfa tubercles, from sweet clover soil, or from sweet clover tubercles. It also illustrates the importance of bacteria in growing alfalfa as will be seen by comparing the four inoculated pots with the uninoculated pot, which is No. 1, on the left in each series of views. The upper view was taken when the alfalfa plants were five weeks old; the next series when they were six weeks old; the next, seven weeks old; and the lower series when they were eight weeks old, from the time of seeding.
A duplicate series of pots prepared in exactly the same manner gave similar results.
The infected alfalfa soil was obtained from a field of three-year-old alfalfa, which was inoculated when first seeded, with infected alfalfa soil obtained from an old alfalfa field in Kansas. About one pound of this soil was shaken in a quart of water, the soil allowed to settle, and some of the nearly clear solution used for the inoculation of Pot No. 2. The alfalfa tubercles from which bacteria were obtained were carefully washed in distilled water to free them from adhering soil particles, and then rubbed up in distilled water, a small amount of this water being then used for the inoculation of Pot No. 3. The infected sweet clover soil was obtained from a place by the roadside where sweet clover was growing luxuriantly and well provided with root tubercles. This place was about two miles from the nearest field ever seeded to alfalfa, so far as known. A water extract from this soil was used to inoculate Pot No. 4. The bacteria from sweet clover tubercles were obtained in the same manner as those from alfalfa tubercles, and were used to inoculate Pot No. 5.
From these investigations we thus have conclusive evidence that infected sweet clover soil can be used for the inoculation of alfalfa fields, the bacteria of the two plants acting the same. The infected soil may be obtained from any place where the sweet clover is found growing with abundance of tubercles on its roots. The soil may be collected to a depth of three or four inches and scattered over the alfalfa field at the rate of 100 pounds or more to the acre. It is well to scatter the infected soil at about the time the alfalfa is seeded, and harrow it in with the alfalfa seed, although it may be applied some days or even some weeks before seeding time, and probably it would be all right to apply the infected soil the fall before, for it is known that the bacteria will live in soil for several months, even though the soil be placed in sacks and allowed to become quite dry.
Investigations have shown that 100 pounds of thoroughly infected soil to the acre is sufficient to produce a very satisfactory inoculation within one year from the time it is applied. Of course, somewhat heavier applications may well be made if it can be done at small expense. The infected soil need not be applied with any high degree of uniformity, but special care should be taken that the higher places and watersheds are not missed in scattering it over the field. If a few square yards, or even square rods, should be missed on the slopes or lower land, it would make but little difference, as the bacteria will be washed over such places from the higher land.
After the soil becomes somewhat dry it is easily scattered by hand from the wagon or from a sack which one can carry. Sometimes it is applied by means of an end gate seeder or a fertilizer drill, or it could be spread by a manure spreader with an application of manure.
The question naturally arises whether there is not danger of getting some sweet clover seed with the infected sweet clover soil, and thus of getting sweet clover mixed with the alfalfa in the field.
In the writer’s opinion there is little or nothing to fear in this matter. In the first place, the amount of sweet clover seed thus obtained would be very small, probably none at all, if one were careful to scrape off the vegetable matter, and perhaps a half inch of earth before collecting the infected soil (most of the bacteria are probably between one-half inch and six inches in depth, as most of the tubercles develop and decay between those depths); second; it is doubtful if a small amount of sweet clover hay would lessen the value of alfalfa hay in the least, for stock frequently eat small amounts of sweet clover of their own choice even when it is nearly mature, and if it is cut while still quite immature and tender it makes quite satisfactory hay, so much so that in some sections of the United States, particularly in the South, sweet clover is regularly seeded on fields and cut for hay, and it is found to be a valuable and very nutritious feed, the live stock eating it in large quantities, and with apparent relish, after they have acquired a taste for it; third, sweet clover is not known as a bad weed in the fields or meadows, even where it has been a common roadside plant for many years, and, being naturally a biennial plant, if it were cut down every five or six weeks, as we commonly cut alfalfa during the season, it would almost certainly die out after a few years while alfalfa, a perennial plant, would continue to live.
Only one instance has come to the writer’s attention where alfalfa has been growing for several years with sweet clover growing in the field or fence rows beside it. This is on the farm of Mr. D. S. Mayhew, of Mercer County, Illinois, who writes as follows regarding the matter:
“Will say that the sweet clover has made no headway in my meadow, as it did not go to seed, on account of my cutting it so often. The sweet clover got into the alfalfa in the seed when I sowed it. I do not think it will do any harm in a meadow, but I believe it would do harm in a pasture if it wasn’t cut down as stock will not eat the sweet clover.”
Of course if sweet clover should get into the field and persist in growing, and if it were found to injure the alfalfa appreciably or markedly, we can always resort to plowing the ground up and growing corn or other crops, thus obtaining some benefit from the leguminous crop for its fertilizing value, and at the same time completely eradicating the sweet clover, but leaving the soil well infected with alfalfa bacteria ready to serve in case alfalfa should be again seeded within a few years.
CONCLUSIONS.