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.