however, should determine for himself whether his soil needs inoculation. The test is the formation of nodules on the roots. Nodules are more likely to occur on poor soil, especially if it he of a sandy character. If the roots of clover are found to possess nodules, then there is no need of inoculation for that form which grows upon clover. It is similarly true with peas, vetches, alfalfa and the like. The farmer can decide the matter for himself before he makes any outlay for material for inoculation purposes.
It should be said, however, that there is much experimental work yet to be done to show in how far such plants as soy beans enrich the ground for succeeding crops. Some contend that the results are disappointing, especially in the case of the soy bean which produces so large tubercles and in such quantity.
With respect to the ecological aspect of the question it may be said that the growing of mixed crops, cover crops, rotation of crops, and the problem of weeds enter largely into the subject. This involves the relationship existing among different species and the soil conditions. This is comparatively a new aspect of crop conditions, for very little has been done to establish any definite results, although for many years it has been suspected that one plant might exert some injurious or beneficial effects upon others in the same soil, apart from the physical conditions arising out of the relationship. It is quite commonly known that on the ground under trees certain plants will not thrive, for some reason or other, aside from the injury produced by the shading. The tree seems to produce an effect upon the soil injurious to that particular plant. Just how different species of plants react upon one another when growing in the same soil, whether in an injurious manner or not, and just how they affect the soil for a succeeding crop are questions full of promise.
It has recently been asserted that certain parasitic Mucors (molds) are capable of doing what the bacteria of the root-tubercles do for those leguminous plants upon which they work. And the suggestion has also been offered that some Mycorrhizas (root fungi) may serve the office of adding to the available nitrogen supply of the soil. And this may account for the immense growth of pine on sandy barren soil. The Mycorrhizas do serve the function of root hairs, and they may also take the place of the bacteria of the root-tubercles in supplying nitrogen to the host plant.
Soil fertility, then, is not so much a chemical as it is a physical bacteriological and ecological problem. So, whether the soil contain phosphorus, nitrogen, sulphur, potassium, calcium and other elements is not of so much importance as how it contains them, and what relationship they have to water, air, to one another, to the soil particles and to the bacterial content of the soil.
