Iron is essential to chlorophyll-formation. It is not a constituent of the chlorophyll molecule, as is magnesium; but in the absence of iron from the culture solution, a plant fails to produce chlorophyll and a green plant which is deprived of a supply of iron rapidly becomes etiolated. The way in which iron is related to chlorophyll-formation is not known.
Iron is taken from the soil by plants in the smallest proportions of any of the essential elements. Only soluble ferric compounds seem to serve as a suitable source of supply of the element; ferrous compounds being usually highly toxic to plants.
Sulfur is an essential element of plant food. The amounts required by plants were supposed, until recently, to be relatively small. This was due to the fact that earlier studies took account only of the sulfur which, on analysis, appeared as sulfates in the ash. Improved methods of analysis, which insure that the sulfur which is present in the plant tissue in organic combinations is oxidized under such conditions that it is not lost by volatilization during the combustion of the material, have shown that the total sulfur which is present in many plants approaches the quantity of phosphorus which is present in the same tissue. Furthermore, recent field and pot experiments have shown that at least a considerable part of the beneficial effects of many fertilizers, which has previously been attributed to the calcium, potassium, or phosphorus which they contain, is actually due to the sulfur present as sulfates in the fertilizers used.
Sulfur occurs in the organic compounds of plants, associated with phosphorus. It seems probable that its physiological uses are similar to those of the latter element; but there is as yet no experimental evidence to establish its exact rôle in the economy of plant growth. It appears to be needed in largest proportion by plants which contain high percentages of nitrogen in their foliage, such as the legumes. There is some evidence that sulfur has a particular rôle in 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.