The stage of the organism that infects the plant is not at present known. It may be supposed that it is the motile “swarmer.” The entry is normally effected through the root-hairs. The hair is attacked close to the tip, and an enzyme is apparently produced which causes the tip to bend over in a characteristic manner. The organisms multiply within the root hair and pass down it, producing a characteristic gelatinous thread filled with bacteria, in the rod form. This “infection thread” passes down into the cells of the root tissue, where it branches profusely. In young stages of nodule formation the branches can be seen penetrating cells in the pericycle layer. Rapid cell division of these root cells is induced. In the course of this cell division abnormal mitotic figures are sometimes found, such as occur in pathological growths. The cells push outward the root cortical layer, and so form a nodule.
Certain of the cells in the centre of the nodule become greatly enlarged, and in the fully grown nodule are seen to be filled with bacteria. Differences have been described in the morphology of the organisms in different parts of the nodule.[62] Whether the different stages of the organism are equally capable of fixing nitrogen, or what is the significance of these stages within the nodule, is not certainly known. It has been held that it is the irregular bacteroid forms that are chiefly concerned with nitrogen fixation. In older nodules the organisms become irregular and stain faintly, and the bacteroidal tissue breaks down, the nodule finally decaying. In the fixation of nitrogen that occurs in the nodules, the bacteria without doubt derive the necessary energy from the carbohydrates of the host-plant. There is evidence that the plant assists the process of fixation by removing soluble metabolic products from the neighbourhood of the bacteria. Golding[22] was able to obtain a greatly increased fixation of nitrogen in artificial cultures by arranging a filtering device so as to remove the products of metabolism.
The great practical importance of leguminous crops in agriculture has led to numerous attempts being made to increase their growth, and the fixation of nitrogen in them, by inoculating the seed or the soil with suitable nodule-bacteria. This inoculation can be effected either with soil in which the host-plant has been successfully grown, and which should consequently contain the organism in fair numbers, or else pure cultures of the organisms isolated from nodules may be used. Very varying results have been obtained with inoculation trials.
In farm practice a leguminous crop has often been introduced into a new area where it has never previously grown. In such soil it is very probable that varieties of the nodule organism capable of infecting the roots may not exist. In such cases inoculation with the right organism or with infected soil often produces good results.
The more difficult case, however, is that in which the legume crop has been grown for a long time in the locality, and where the soil is already infected with right organisms. This, the more fundamental problem, applies especially to this country. Here it would seem that inoculation with a culture of the organism will benefit the plant only (1) if the naturally occurring organisms are present in very small numbers; or (2) if the organisms in the culture added are more virulent than those already in the soil. The problem of successful inoculation would therefore seem to be bound up with that of grading up the infective virulence of the organism to a higher level.
Successful nodule development in a legume crop is also dependent to a large degree on the soil conditions. The effects of soil conditions on nodule development have been studied by numerous workers. Moisture has been found very greatly to affect the nodule development. Certain salts have a very definite effect on nodule formation.[64] Their effect on the number of nodules developing has been studied, but the reason for this effect is unusually difficult to decide. The action is usually a complex one. Thus phosphates are known to stimulate nodule formation. They probably act in several ways. In the first place, they may cause the nodule organisms to multiply in the soil; in the second place, they produce a greater root development in the plant, thus increasing the chances of infection; and in the third place, Bewley and Hutchinson[4] have found that phosphates cause the appearance of the motile stage of the organism in cultures. A real understanding of the influence of environment on nodule production will produce great improvements in our methods of legume cropping.
D. The Relation of Bacterial Activities to Soil Fertility.
The various activities of the soil bacteria have a vital importance to the growth of higher plants, which are dependent for their existence on certain of these processes. In the first place, as we have seen, bacteria decompose the tissues of higher plants and produce humus materials, which are essential to the maintenance of good physical properties in the soil. Then the nitrate supply on which most higher plants depend is produced by the decomposition of organic nitrogen compounds by bacteria in their search for energy. The depletion of the total nitrogen content of the soil through rain and through the removal of nitrogen in the crops, is to some extent compensated by the fixation of atmospheric nitrogen by certain bacteria. On the other hand, in the assimilation of nitrogen compounds to build up protein, the bacteria are competing with higher plants for one of their essential food constituents, and their action may, under certain conditions, cause a temporary nitrogen starvation. One must remember, however, that large quantities of nitrate are lost from field soils by washing-out through rain action, especially in winter. The assimilation of nitrate and ammonia by micro-organisms keeps some of this nitrogen in the soil, and at certain periods may thus be beneficial.
There is another important respect in which soil bacteria influence plant growth. Their activities result in the release of inorganic salts, such as potash and phosphates, in a form available for the use of plants. The release of phosphorus and potassium compounds takes place in two ways. In the first place, organic matter containing phosphorus and potassium, in an insoluble form, is attacked by bacteria, resulting in these elements being set free as inorganic salts available to the higher plant. Secondly, much of the phosphorus supplied to the soil from rock minerals is present as insoluble phosphates, such as apatite and iron phosphate. Much of the potassium, too, is derived from insoluble silicate minerals. In both cases the conversion of the insoluble minerals into soluble phosphates and potassium compounds is brought about to a large extent by solution in water containing carbonic and other acids. These acids are largely produced by micro-organisms, which, in addition to carbonic acid, produce organic acids, and in specialised cases, sulphuric and nitrous acids. It has been found, for example, that in a compost of soil with sulphur and insoluble phosphate, sufficient sulphuric acid may be produced by the oxidation of the sulphur by bacteria to convert an appreciable amount of phosphate into a soluble form. When we consider the functions performed by soil bacteria, therefore, it is not surprising to find that high bacterial activity in the soil is associated, as a rule, with fertility.