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[88] 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.
[88] 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,[89] 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.
[89] 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.[90]
[90] 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.[91] 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.[92] 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.
[91] 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.
[92] 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.