The inoculation of soil, like many other lately exploited theories, has no doubt been known for hundreds of years. There are evidences that it was practiced in England at least a century ago, and it is thought to be an old custom among the Chinese. Some space was given to “soil inoculation” in a preceding chapter, attention there being devoted to the simple methods of infecting soil with bacteria.
Some twenty years ago a German scientist, Nobbe, discovered that the small nodules found on the roots of the leguminous plants contained bacteria that took nitrogen from the air and transferred it to the plant. It had been known that cultivated soils were rapidly losing their original supply of nitrogen and there seemed no practicable way of restoring it in sufficient quantity. Commercial nitrogen costs fifteen cents per pound and the expense of applying it to the land to equalize the loss from an ordinary farm crop is almost equal to the value of a crop. Hence, the discovery that the legumes were nitrogen-gathering, by means of these bacteria, was hailed as one of the greatest of the age. With millions of pounds of nitrogen over his land there seemed now a method whereby the farmer could utilize some needed portions of it. The bacteria live in tubercles upon the roots of various leguminous plants, such as Red clover, Sweet clover, Bur clover, alfalfa, cowpeas, garden peas, vetches and beans. These tubercles are the home of the bacteria, minute forms of vegetable life, too small to be seen with the naked eye. The legumes have no power in themselves to draw nitrogen from the air, yet these bacteria seem to have the power to absorb the free nitrogen and cause it to combine with other elements, forming nitrates or other assimilable compounds suitable for plant food. It has also been demonstrated that, as a rule, there are different species of these bacteria for different species of legumes.
After German scientists had made their discoveries, one of the existence of these bacteria on the roots of the legumes and another that in the laboratory, these organisms could be bred and multiplied, they seemed unable to develop them of sufficient vigor to survive any general distribution. At this point Dr. George T. Moore of the United States Department of Agriculture, hit upon a plan of cultivating them in a way by which they could be preserved for many months, and he also gave them a much greater power. This nitrogen-fixing power was so developed that seeds soaked in the solution, it is claimed, sprouted and produced plants in sand that possessed no nitrates. He then found that these bacteria when grown upon nitrogen-free media will retain a high activity for a long time, if carefully dried out and then revived in a liquid medium. He also devised a plan by which they could be mailed to any part of the world and arrive in perfect condition.
Most experiment station men have given to Dr. Moore great credit for his discoveries and have tried, with rather marked success, his method of nitro-culture; on the other hand a few have been opposed to the method from the beginning and are still opposed. It would seem that the tests made prove that nitro-culture does introduce bacteria when directions are followed.
Of course if a farmer’s soil has the alfalfa bacteria, it is not necessary to use any system of inoculation. If Sweet clover or Bur clover is growing in the vicinity, or if alfalfa is thriftily growing near without the aid of inoculation, it is hardly necessary to make there further efforts for the introduction of bacteria.
In a preceding chapter the opinion was expressed that in most states, and in most localities in those states, inoculation was unnecessary because bacteria were already present; yet it is true that these bacteria are present, if at all, in varying quantity. If comparatively few are present, any method of inoculation that will introduce more into the soil, to hasten the growth of the tubercles and promote the gathering of nitrogen, will be helpful. The fact is, we know as yet comparatively little of all there is to be learned about this wonderful principle. It is altogether probable that the use of nitro-culture is as practicable and as free from objections as the use of inoculated soil, especially from distant and unknown fields. In one of its bulletins the United States Department of Agriculture has summarized its advice as to inoculation as follows:
When Inoculation is Necessary.—Inoculation is necessary—
1. On a soil low in organic matter that has not previously borne leguminous crops.
2. If the legumes previously grown on the same land were devoid of nodules, of “nitrogen knots,” thus showing the need of the nodule-forming bacteria.
3. When the legume to be sown belongs to a species not closely related to one previously grown on the same soil. For instance, soil in which Red clover forms nodules will often fail to produce nodules on alfalfa when sown with the latter crop for the first time.
When Inoculation May Prove Advantageous.—Inoculation may prove advantageous—
1. When the soil produces a sickly growth of legumes, even though their roots show some nodules.
If the cultures introduced are of the highest virility, their use will often result in a more vigorous growth.
2. When a leguminous crop already sown has made a stand, but shows signs of failing, owing to the absence of root nodules.
The use of the culture liquid as a spray or by mixture with soil and top-dressing may save the stand if other conditions are favorable.
When Inoculation is Unnecessary.—On the other hand, inoculation is unnecessary and offers little prospect of gain—
1. When the leguminous crops that are usually grown are producing up to the average, and the roots show nodules in normal abundance.
Cultures of nitrogen-fixing bacteria are not to be regarded in the light of fertilizers, or as capable of increasing the yield under average conditions. They do not contain nitrogen itself, but bacteria, which make it possible for the legumes to secure nitrogen from the air (through the formation of root nodules). Where the soil is already adequately supplied with these bacteria, it will not usually pay to practice artificial inoculation.
2. When the soil is already rich in nitrogen.
It is neither necessary nor profitable to inoculate a soil rich in nitrogen when sowing legumes. Not only does the available nitrogen in the soil render the formation of nodules less necessary, but the nitrogenous materials in the soil largely prevent the bacteria from forming nodules.
Any increased virility in nitrogen-fixing power possessed by any of the types of bacteria yet distributed, may be rapidly lost in a soil containing an abundance of nitrogen, because the bacteria are in a medium in which there is no demand for activity in securing atmospheric nitrogen.
When Failure is to be Expected.—Inoculation will fail where other conditions (aside from the need of bacteria) are not taken into account, among which are the following—
1. In soil that is acid and in need of lime.
Liming to correct acidity is as important for the proper acidity of the bacteria as for the growth of the plants.
2. In soil that is deficient in fertilizers, such as potash, phosphoric acid or lime.
The activity of the bacteria in securing nitrogen from the air and rendering it available to the legumes does not do away with the need for such fertilizing elements as potash and phosphorus.
3. It must also be remembered that inoculation does not “act like magic”; it will not overcome results due to bad seed, improper preparation and cultivation of the ground, and decidedly adverse conditions of weather and climate.
In the use of the cultures, also, failure is almost certain where the directions are not carefully studied and intelligently followed.
The discovery in Illinois that inoculation of the soil for alfalfa was necessary in certain places and not in others suggested the theory that bacteria living on some other, probably native, plant were identical with the alfalfa bacteria. Investigations led to satisfactory evidence that this was the case and that the native plant was the ordinary Sweet clover (Melilotus alba). The illustration facing page 230 shows the results of a series of pot culture experiments made at the University of Illinois and reported in Bulletin No. 94 by Prof. Cyril G. Hopkins. The four photographs were made five, six, seven and eight weeks, respectively, from the time of planting. Alfalfa seed were planted in each of the five pots, in carefully prepared soil practically free of combined nitrogen, and at the same time four of the five pots were inoculated as follows:
Pot No. 1.—Not inoculated.
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.
The results indicate that the same effect is produced upon the growth of the alfalfa by the nitrogen-gathering bacteria obtained from Sweet clover as by those from the older alfalfa, and seem to prove that infected Sweet clover soil can be used for the inoculation of alfalfa fields. Investigations have shown that 100 pounds of thoroughly infected soil to the acre is sufficient to produce a satisfactory inoculation within one year from the time it is applied.