Chapter IX.. The Relation of Plant Growth to Concentration.

THE RELATION OF PLANT GROWTH TO CONCENTRATION.

That the concentration of the mineral constituents in the soil solution under normal conditions is competent for plant support, is shown by numerous experiments. Birner and Lucanus in an experiment that has long since become classic, found that they could raise wheat to maturity in a well-water, the concentration of which was approximately 18 parts per million with respect to potassium, and 2 parts per million with respect to phosphoric acid, while the corresponding concentrations of the soil solution are normally about 25-30 parts per million of potassium and 6-8 parts per million of phosphoric acid. Nevertheless Birner and Lucanus report that the wheat grown in the well-water throve even better than that grown at the same time in a rich garden mold. Since then many investigators in numerous trials have obtained similar results. Recently wheat, corn, and some of the common grasses have been grown to a satisfactory maturity in tap water with a concentration of about 7 parts per million of potassium and 0.5 parts per million of phosphoric acid. And repeatedly wheat plants, grasses, cowpeas, vetches, potatoes and other plants have grown in a satisfactory way in solutions made by shaking up a soil in distilled water and separating from the solid particles by means of filters of unglazed porcelain.

Wasserculturversuche mit Hafer, von Dr. Birner und Dr. Lucanus, Landw. Vers.-Sta., =8=, 128-177, (1866).

There can be no doubt, therefore, that the soil solution is normally of a concentration amply sufficient to support ordinary crop plants, and is maintained at a sufficient concentration, so far as mineral plant nutrients are concerned. Undoubtedly, however, variations in the concentration of the soil solution can, and often do, take place, and the results of laboratory experiment indicate that they probably produce effects on plants.

It has been shown in water-culture experiments with wheat, that if a given ratio of mineral nutrients be maintained, relatively small effect is produced on the growing plants by varying the concentration over a wide range, in one case from 75 parts per million to 750 parts per million, and this effect seems to be largely independent of the nature of the particular mixture of solutes. But varying the relative proportions of the mineral constituents has been shown by numerous experiments to produce very marked changes in the growth of plants. Not only does a control of the concentration and proportion of the mineral constituents of a solution produce a more rapid, or a slower growth, a greater or lesser total growth, but it produces differences in the character of growth; as for instance, causing the tops to grow relatively faster than the roots, or vice versa. However, many effects of this type can be produced, and sometimes more readily, by soluble organic substances, or mechanical agencies. The mechanism of these effects is by no means clear, in many cases. That other causes obtain than a sufficient supply of mineral nutrients will be shown in the following chapters. Experiments with wheat seedlings in water cultures, where the weights of the green tops were taken as the measure of growth, showed that the most-favorable ratio was one of phosphoric acid (PO₄) to three or four of potassium (K), about the ratio which has been found to exist normally in the soil solution of humid areas of the United States, namely, 6-8 parts per million of phosphoric acid to 25-30 parts per million of potassium.

Effect of the concentration of the nutrient solution upon wheat cultures, by J. F. Breazeale, Science, n. s., =22=, 146-149, (1905).

All growing plants require for their growth and development various organic compounds containing carbon, hydrogen, oxygen and nitrogen. The higher crop plants with which agricultural investigations appear to be more immediately concerned, seem to have inherent power to produce these needed substances within themselves. But it is becoming more and more evident that the large problem of soil fertility, or the relation of the soil to crop production, frequently if not generally involves the growth and development of lower organisms including ferments and bacteria. These may or may not in particular cases, favor the growth of the desired higher plants. Many of these lower organisms require certain organic compounds or thrive better if these are brought to them in the soil solution, and indeed evidence is not lacking that such may sometimes be the case even with the higher plants. Certainly their growth can be much affected by the presence of different organic substances in the nutrient solution. Enough work has been done in this field of investigation to show that the concentration of the soil solution or artificial nutrient solution with respect to the organic compounds must generally be low; too high a concentration always inhibits growth or even produces death; and there is probably an optimum concentration, or one at which the plant will grow best; but this optimum concentration varies with the specific nature of the plant, the presence of other dissolved substances, mineral or organic, and possibly with other factors. While a notable amount of work has thus been done in a field of inquiry obviously of practical as well as theoretical interest, almost no definite information has as yet been obtained as to the concentration of organic substances in the soil solution, or its effect upon plants under field conditions, excepting in the case of the nitrates, the products of bacterial activities. The concentration with respect to nitrates is known to vary greatly from a few parts to several thousand parts per million, and this sometimes within a few days or even hours. The great changes in concentration with respect to nitrates, the rapidity of the changes, and the correspondingly large effects on growing plants make this a subject requiring special treatment by itself. This at present seems more easily appreciated from a consideration of the bacteria involved, and will not be discussed more fully here.

See: The fixation of atmospheric nitrogen by bacteria, by J. G. Lipman, Bull. No. =81=, Bureau of Chemistry, U. S. Dept. of Agriculture, 1904; A review of investigations in soil bacteriology, by Edward B. Voorhees and Jacob G. Lipman, Bull. No. =194=, Office of Experiment Stations, U. S. Dept. of Agriculture, 1907; The physiology of plants, by W. Pfeffer, translated by A. J. Ewart, vol. I, p. 388 et seq., 1900; The effect of partial sterilization of soil on the production of plant food, by Edward John Russell and Henry Brougham Hutchinson, Jour. Agric. Sci., =3=, 111-144, (1909).

Of the ash constituents of plants, there must be in the soil solution, potassium, magnesium, phosphorus, sulphur and iron for any plant growth, and for the higher crop plants, calcium must also be present. Of these, iron is usually present in barely appreciable concentration and more than this is not desirable, or is even harmful for common crop plants. Under the normal conditions for soils in humid areas, sulphur also is usually present in scarcely more than appreciable quantities and there is no positive evidence to show that higher concentrations are especially desirable, though this may be the case for certain crops, such for instance as the onion. Phosphorus is usually present to the extent of 5 or 6 parts per million of phosphoric acid (P₂O₅), while it has repeatedly been shown that such crops as wheat can thrive and make a good growth with a concentration a tenth of this. It appears to be clear therefore that as far as food supply is concerned there is normally an ample supply of phosphorus in the soil solution; but it does not follow that increasing the concentration of the solution if only temporarily would not result in favorable effects upon growing plants.

A consideration of the bases, however, introduces serious difficulties, which will probably require much further research by the plant physiologist as well as the soil chemist. It is impossible as yet to determine the concentrations at which different plants will not grow. It is even impossible to determine the concentrations at which they will thrive best. It seems certain that different crop plants require different amounts of these minerals, but whether or not they require different concentrations of the constituents in the nutrient solution for their several best growths is yet not clearly shown. It now seems probable that to some extent at least these basic mineral nutrients can replace one another for the plant’s metabolism. It has been shown in the case of certain lower plant organisms that potassium can be more or less successfully replaced by rubidium and caesium, and in the case of some higher plants, possibly calcium, magnesium and potassium can partially replace one another. In spite of the fact that sodium as well as potassium is a necessary constituent for the metabolism of higher animals which feed upon plants, it is generally held that sodium can not replace potassium in the processes of plant growth, although Wheeler and his colleagues have advanced evidence to show that a partial replacement is possible. It seems evident, however, that no generalizations can hold concerning the effect of the concentration of any one base on plant growth which do not include recognition of possible modifications due to the presence of other bases; and the formulation of such generalizations must needs wait upon a more thorough knowledge of the parts played by the several mineral nutrients in the metabolism of different classes of plants.

For a more detailed discussion of this subject, and the functions of the several ash constituents in plant nutrition, see: The physiology of plants, by W. Pfeffer, translated by A. J. Ewart, vol. I, p. 410, et seq., 1900.

The effect of the addition of sodium to deficient amounts of potassium, upon the growth of plants in both water and sand culture, by B. L. Hartwell, H. J. Wheeler and F. R. Pember, Report Rhode Island Agricultural Experiment Station, 1906-7, p. 299-357.

As to forms or chemical combinations in which the inorganic constituents of the soil solution are best adapted to plant growth, but little can yet be said other than that the different combinations do have an importance. Some empirical information is available, such as for instance, that potassium sulphate or carbonate is a better fertilizer for some crops than is potassium chloride. It is known that the mineral nutrients in the plant are partly in inorganic combinations but largely in organic combinations. But the causal relationships are yet to be worked out. And finally, although some meagre experimental data have been obtained as to the effect of certain inorganic constituents on the absorption of others, by particular plants, the mechanism of absorption itself, including the selective powers of the plant, is yet wanting an adequate explanation.