Popular Science Monthly/Volume 67/November 1905/Soil Fertility
MICHIGAN AGRICULTURAL COLLEGE.
PROBABLY the most important single problem which confronts the farmer to-day is that of the conservation of the fertility of the soil. Fertility may be defined as that condition of the soil which yields the maximum of that material which the plant is capable of using for the best development of those qualities which the farmer desires. Strange as it may seem, just wherein this fertility consists is not yet fully known. Some years ago it was well known, that is to say, it was thought, that all that could be said had been said about it. At the present time, the more thoughtful and cautious among those studying the question of plant growth from a scientific standpoint are by no means settled upon the point. They recognize that there is yet much to be cleared up in regard to it, especially from the physical and the bacteriological sides. The question may be examined from the chemical, physical, bacteriological and ecological standpoints.
Much has been done in the chemical laboratory—too much, in fact, judging by the results obtained. It was thought that a knowledge of the chemical composition of the soil was the key to the solution of the problem. But it has never been decided just what the chemical composition should be to produce the best results, for the simple reason that it is not known exactly what the requirements of the plants are. Nor does a chemical analysis of the plant itself answer the question. There are certain elements which are no doubt necessary for proper growth, but the analysis of the content of the plant and of the soil does not give a very complete notion of the proper conditions under which certain substances should exist when in the soil. The chemical analysis of soils and fertilizers, though not without considerable importance, is now being relegated to the background in comparison with the physical and the bacteriological conditions. The so-called perfect fertilizers, as sold on the market to the farmer, have been looked upon with some distrust, mainly because they have not produced the results in the production of crops which might be expected from the chemical standpoint. The farmer becomes skeptical and again and again sends samples to the chemist to see if the materials have the proper chemical constituents. The chemist finds it all right, and it is all right from his point of view, but to the farmer the results are still unsatisfactory. It is as much to the futile results of the chemical analyses of soils and fertilizers as applied to the growing of crops as to any other single thing that farmers have been so hostile to 'scientific' farming and to scientific agricultural institutions.
The physical conditions of the soil in regard to fertility are as important perhaps to the farmer as all the others put together, because they are more directly under his control. Yet this has never been fully understood by the farmer, nor has it been developed with agricultural students as it should have been. Air and moisture are the two most important substances in the soil. The conservation of these and the bringing of them to the roots of the plants is, therefore, one of the chief problems. It is merely the management of these two substances, for they are always present, that the farmer should concern himself with. Timely and suitable cultivation, then, for the development of crops is of the highest importance. Indeed, manuring of soil resolves itself largely into a question of supplying moisture and placing soil in such a condition that the air and moisture are in the best situation for use by the plant. In fact, when a farmer plows under a crop of rye he does not add any material to the soil excepting what the plant obtained from the air, and this is always available from the air. The other substances contained in the rye are simply restored to the soil. There is no addition. The rye is, therefore, not a manure in the proper sense, but a means by which the farmer improves the physical conditions.
It is not difficult to see, therefore, that the artificial manures in powder form can not contribute to any very great extent to fertility, if fertility is so largely a physical question. There are conditions of soil, such as size, shape and arrangement of particles, which have to do directly with the air and the water content. The arrangement of the particles can be controlled largely by the farmer.
There is one other force called physical affinity, which is of the highest importance because it is largely through the interaction of this force among the various substances in the soil that plants are capable of extracting solids in solution from it. This physical affinity is exercised among the various soil constituents, each one exerting an influence over the others. Now, if some substance (not in the soil) be added to it, the whole equilibrium may be disturbed by the affinity this may have for the substances already there. A chemical analysis can not determine this. The soil yields up to chemical analysis all that it contains, no matter what the relationship may be among the constituents. To the plant, however, which depends upon a form of physical affinity for its soil food, all the soluble substances will probably not be given up. Therefore, it is a question of physics, as well as one of chemistry, which will determine soil fertility.
When the farmer summer-fallows a field he does not add anything directly by the way of a fertilizer to the field, yet it is much more fertile the year after the summer-fallowing has been done. One benefit of this operation is the clearing the field of weeds, but, aside from this, the soil is much more productive and, it may be said, more fertile. What is done to the soil in the process of cultivation is to break up the lumps and allow the air to permeate freely, to retain the moisture by making the surface portions fine and porous, thus acting as a 'Campbell blanket,' to mix so thoroughly that the physical affinity of the soil substances becomes changed, the soil being renewed to a very large extent. In this way a farmer can control the fertility without adding any fertilizer. Similar to this is the case already mentioned where the farmer plows under a field of rye. He increases the fertility without adding any fertilizer.
Land should never be allowed to bake or become hard on the surface. If so, air is kept from the roots of the plants. Baking of the soil can be largely controlled by the farmer with certain crops. It occurs, more or less, after rain, depending upon the character of the soil. This crust should be broken up, and kept broken up, by cultivation. In hoeing weeds a man is fertilizing the soil. "Weeds, therefore, may be of considerable indirect benefit.
The fable of the farmer dying when on the point of revealing great hidden treasure to his sons illustrates the point. When about to die he told his sons that immense treasure lay buried in the ground on the farm, but death came just before he was able to tell the exact spot. His sons then dug through every inch of soil, over and over again, with the greatest diligence, but did not find any such treasure as they expected to find. The result, however, of such thorough digging was great abundance in the crops. They really did unearth the treasure, but in a far different way from that which they expected.
The bacterial content plays also a very important role in the process. Comparatively little is known in regard to the interrelationship existing among these organisms, and of their relations with plants. The processes of nitrifying and denitrifying are in themselves important, but these are probably only an extremely small part of the bacteriological question. And chemical analysis of soils does not throw much light upon it. It is a more complicated problem than a mere chemical one.
Root-tubercles and their production, although a bacterial proposition, require, because of their importance, special mention. The organism which produces the tubercle is capable of extracting nitrogen from the air and rendering it available for the plant upon which the nodule is produced. And these nodules are capable of being produced on many plants of the 'bean' family, such as clover, alfalfa, peas, vetches and the like. The cultivation of such crops, therefore, is productive of additional fertility in nitrogen. The soil, of course, should contain the bacteria peculiar to the plant upon which they work. The soil may contain them already; if not, it should be inoculated. Each farmer, 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.