up the superficial roots and the overground parts of the plants, which parts, when ploughed down, added this material to the surface soil. There is no doubt that plant food is removed from the subsoil and stored in the upper parts of the plants and that the above ex-planation should be considered. But the soil-enriching faculty of leguminous plants is connected with phenomena that render this explanation insufficient. Generally Alfalfa will not thrive on soil where it has never been grown before. The plants soon stop growth, turn yellow and finally die. If, however, some soil from an old Alfalfa field is sown on the land, a crop will be produced without any trouble. If the plants are examined, it will be found that the roots of those grown on old Alfalfa soil are provided with numerous nodules, whereas the roots of weak plants on virgin soil are destitute of them. Only quite recently have the origin and significance of these tubercles been understood. It has been proved that they are a kind of gall produced by certain bacteria. These bacteria live in the ground, attack the root hairs, break through their thin walls, and make their way to the interior of the root branches. There they propagate rapidly, forming masses within the nodules. Later on, most of the bacteria decompose and are used by the plants, which thus obtain additional food. As the bacteria are very rich in nitrogenous substances, the source of which is the air contained in the porous soil, leguminous plants are able to secure, indirectly through the bacteria, their nitrogen from the air. They are therefore able to accumulate nitrogen without robbing the soil and, when dying, to leave a supply of nitrogenous substances for succeeding crops.
When soil from land where Alfalfa, for instance, has been successfully grown is put on a field, that field is supplied with the bacteria necessary for the development of Alfalfa. The amount needed is not large, two hundred pounds being sufficient for an acre. Instead of soil from old fields, artificial cultures of bacteria are now available at many botanical laboratories. These cultures, with directions for their use, are on sale in bottles at a low price.
Nodule-forming bacteria are necessary for the proper development of all kinds of leguminous plants. But this does not mean that bacteria which will serve for a certain plant will satisfy another kind. On the contrary, there are different species and races of nodule-forming bacteria, and each species or race is able to produce nodules only on a certain kind of leguminous plant. Thus the bacteria which work on the roots of Red Clover are different from those which produce nodules on the roots of Alfalfa and are quite unable to benefit the latter plant. In using artificial cultures of nodule-bacteria therefore, care should be taken to procure the right kind.