promoting the growth of bacteria, and it may be that the percentages of total sulfur which are found in the tissues of legumes are due to the presence of the symbiotic nitrogen-gathering bacteria in the nodules on the roots of these plants. This point has not yet been investigated, however.

Sodium is probably not essential to plant growth, although it is present in small proportions in the ash from practically all plants. In cases of insufficient supply of potassium, sodium can apparently perform at least a part of the rôle of the former element; but this seems not to be a normal relationship or use.

Chlorine is found in small amounts in the sap and in the ash of nearly all plants. However, it does not appear to be essential to the growth of a plant, except possibly in the case of certain species, such as asparagus, buckwheat, and, perhaps, turnips and some other root crops. Whether the benefit which these crops derive from the application of common salt to the soil in which they are growing is due to the direct food value of either the chlorine, or the sodium, or to some indirect effect, is not yet known. The presence of chlorine in the sap of plants is undoubtedly due to the inevitable absorption of soluble chlorides from the soil and apparently has no connection with the nutritional needs of the plant.

Silicon is always considered as a non-essential element, although it occurs in such large proportions in some plants as to indicate that it cannot be wholly useless. It accumulates in the stems of plants, chiefly in the cell-wall, and has sometimes been supposed to aid in giving stiffness to the stems. But large numbers of analyses have failed to show any direct correlation between the stiffness of straw of cereal plants and the percentage of silicon which they contain. Further, plants will grow to full maturity and with erect stems when no silicon is present in the mineral nutrients which are furnished to them. On the other hand, certain experiments appear to indicate that silicon can perform some of the functions of phosphorus, if soluble silicates are supplied to phosphorus-starved plants. But under normal conditions of plant nutrition, it seems to have no such function.

INORGANIC PLANT TOXINS AND STIMULANTS

Much study has been given during recent years to the question of the supposed poisonous, or toxic, effects upon plants of various soil constituents. There seems to be no doubt that certain organic compounds which are injurious to plant life are often present in the soil, either as the normal excretions of plant roots or as products of the decomposition of preceding plant growths. A consideration of these supposedly toxic organic substances would be out of place in this discussion of mineral soil nutrients. But there seems to be no doubt that there may also be mineral substances in the soil which may sometimes exert deleterious influences upon plant growth. In fact, most metallic salts, except those of the few metals which are required for plant nutrition, appear to be toxic to plants. The exact nature of the physiological effects which are produced by these mineral toxins is not clearly understood; indeed, it is probably different in the case of different metals. Further, it is certain that both the stimulating and the toxic effect of metallic compounds upon low forms of plants is quite different from the effects of the same substances upon the more complex tissues of higher plants, a fact which is utilized to advantage in the application of fungicides for the control of parasitic growths on common farm crops.

Among the elements whose physiological effects upon higher plants, such as the cereal crops, etc., when their soluble compounds are present in the soil, have been carefully studied, there are three fairly distinct types of injurious mineral elements. The first of these, represented by copper, zinc, and arsenic, apparently exert their toxic effect regardless of the proportion in which they are present in the nutrient solution which is presented to the plant; although the degree of injury varies with the amount of injurious substance present, of course. The second type, of which boron and manganese are representatives, apparently exerts a definite stimulating effect upon plants when supplied to them in concentrations below certain clearly defined limits; but are toxic in concentrations above these. The third includes many soluble salts of magnesium, sodium, potassium, etc., which while either innocuous or else definite sources of essential plant foods when in lower concentrations, become highly toxic, or corrosive, when present in the soil solution in concentrations above the limits of "toleration" of individual plants for these soluble salts. The tolerance shown by the different species of plants toward these soluble salts (the so-called "alkali" in soils) varies widely; indeed, there seems to be considerable variation in the resistance of different individual plants of the same species to injury from this cause.

With reference to the toxic effect of the third type of substances, i.e., the common soluble salts, it is known that single salts of potassium, magnesium, sodium, or calcium, in certain concentrations, are toxic to plants, while mixtures of the same salts in the same concentrations are not. Thus, solutions of sodium chloride, magnesium sulfate, potassium chloride, and calcium chloride which, when used singly, killed plants whose roots were immersed in them for only a few minutes, formed when mixed together a nutrient solution in which the same plants grew normally. The remarkable remedial effect of calcium salts in overcoming the injurious effects of other soluble salts has already been mentioned. One explanation of these relationships between mineral soil constituents and the living plant is that the life phenomena depend upon a balanced adjustment between the compounds of these different mineral elements with the proteins (producing the so-called "metal proteids") which constitute the active material of the cell protoplasm. According to this theory, any excess or deficiency of any one or more of these elements in the plant juices which surround a given cell will, of course, cause an interchange with the mineral components of the supposed "metal proteids" which upsets the assumed essential balance between them, with disastrous results. A more recent, and much more satisfactory, explanation of the "antagonism" between mineral elements in their toxic effects upon plants, which has both theoretical and experimental confirmation, is that single salts disturb the colloidal condition (see Chapter XV) of the protoplasm of the plant cells in such a way as to destroy its permeability to nutrient substances, while mixtures of salts restore the proper state of colloidal dispersion and permit the normal functioning of the protoplasm.

It is apparent from the above brief discussions that the rôle of the different soil elements as plant food, and their relations to the complex processes which constitute plant growth, afford an interesting and promising field for further study.

References