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.