SUGGESTIONS FROM THE DEPARTMENT OF AGRICULTURE
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.