It is often stated that leguminous plants, such as clover, have power to obtain free nitrogen from the air. This is not strictly true. Red clover, for example, has no power in itself to get nitrogen from the air. It is true, however, that the microscopic organisms[3] which commonly live in tubercles upon the roots of the clover plant do have the power to take free nitrogen from the air and cause it to unite with other elements to form compounds suitable for plant food. The clover plant then draws upon this combined nitrogen in the root tubercles, and makes use of it in its own growth, both in the tops and in the roots of the plant.
These nitrogen-gathering bacteria live in tubercles upon the roots of various leguminous plants,[4] such as red clover, white clover, alfalfa, sweet clover, cowpeas, soy beans, vetch, field-peas, garden-peas, field and garden beans, etc. These tubercles vary in size from a pinhead to a pea, varying with the different kinds of plants, being especially small upon some of the clovers, and very large upon cowpeas and soy beans. The tubercles are, of course, easily seen with the eye, but the tubercle is only the home of the bacteria, somewhat as the ball upon the willow twig is the home of the insects within. The bacteria themselves are far too small to be seen with the unaided eye, although they can be seen by means of the most powerful microscope. Several million bacteria may inhabit a single tubercle. It is not necessary to see the bacteria, because if we find the tubercles upon the roots of the plant, we know that the bacteria are present within, as otherwise the tubercle would not be formed.
Although the plant itself, as clover, for example, has no power to feed upon the free or uncombined nitrogen in the air, yet these nitrogen-gathering bacteria do have the power to absorb the free nitrogen and cause it to combine with other elements, forming nitrates or other compounds which are suitable forms of nitrogen for plant food.
It has also been demonstrated that, as a rule, there are different species of nitrogen-gathering bacteria for markedly different species of leguminous plants. Thus we have one kind of bacteria for red clover, another kind for cowpeas, another kind for soy beans, and still a different kind for alfalfa.[5]
The Red Clover Bacteria.
That clover has no power in itself to gather atmospheric nitrogen, and that the bacteria do have power to feed the clover plant with nitrogen gathered from the air is very easy to demonstrate. It is one of the regular laboratory practices of the students in soil fertility in the Agricultural College to make this demonstration. Plate 1 is an illustration of such student work. The two pots which are shown were provided with all elements of plant food, excepting the one element nitrogen. Thus far the two pots are exactly alike. Each contains no nitrogen, as indicated by the label “No N.” Each pot is planted with the same number of red clover seeds. To the right-hand pot, however, some bacteria (“Bac.”) were added, while none were added to the left-hand pot. These bacteria were obtained by taking about one pound of soil from a clover-field where abundance of tubercles were found on the clover roots, adding this soil to about one quart of pure water, shaking for a few minutes, allowing the soil to settle, then taking a small quantity of almost clear solution, and adding it to the pot which we wished to inoculate with the red clover bacteria. Aside from the addition of these microscopic bacteria to the right-hand pot, these two pots were treated exactly alike throughout the experiment. It will be plainly seen that where the bacteria were added the clover was furnished with sufficient nitrogen to make a strong and luxuriant growth, while without the bacteria the clover (in the left-hand pot) only germinated and made what little growth it could with the small amount of nitrogen contained in the seed. This result is the difference between success and failure of the clover crop.
Plate 1. Red Clover: Effect of Bacteria. No Nitrogen in the Soil of Either Pot.
In general the clover bacteria are well distributed over the northern and central part of Illinois, but we now have some very strong evidence that they are not well distributed in some soils of large area in southern Illinois. There is also some evidence that they were not originally present even in the soils where they are now found in great abundance; and, furthermore, it seems very probable that these bacteria may cease to live in a soil where they have once been present, provided clover is not grown on the land for several years.