THE BALANCE BETWEEN SUPPLY AND REMOVAL OF MINERAL PLANT NUTRIENTS.
The mechanism of the solution and transport of mineral nutrients
developed in the preceding pages makes it of interest to determine
the relation between the possible or probable supply of mineral
plant nutrients and crop demands over large areas. The inquiry can
be formulated more specifically: Is the movement of mineral plant
nutrients towards the surface soil equal to or in excess of the removal
by drainage waters and garnered crops? Satisfactory data are yet
wanting for anything like exact computations, but approximate figures
are available which appear sufficient for the present purpose.
The rainfall (R) can be considered as disposed in three portions, the
fly-off (_f_), the run-off (_r_), and the cut-off (_c_). Stating this
as an equation,
R = _f_ + _r_ + _c_.
The cut-off can be resolved into the portion (_a_) seeping through the
soil to ultimately join the run-off, and the portion (_b_) returning to
the surface to ultimately join the fly-off. Stated as equations,
R = _f_ + _r_ + _a_ + _b_
= _f_ + _b_ + (_r_ + _a_).
In other words, the rainfall can also be considered as made up of the
fly-off, the capillary water of the soil and the drainage from the
area. According to Murray,[93] Geikie,[94] Newell,[95] and others, the
drainage water for humid areas, or such an area as the United States
as a whole, would be between 20 and 30 per cent. of the rainfall, the
major portion coming from seepage water rather than surface drainage.
Assuming the higher figure, and making the further very probable
assumption that the capillary water in the soil (_b_) is never less
than the fly-off or the water that evaporates during rain (_f_),
it follows from the equations given that the capillary water is at
least 35 per cent. of the rainfall. If we assume the lower value for
the drainage, then the capillary water is at least 40 per cent. of
the rainfall, and if we assume the extreme case—that the fly-off is
practically negligible—the capillary water becomes 80 per cent. of the
rainfall. It appears, therefore, that in all probability the proportion
of the cut-off water which returns to the surface as film water or
capillary water is always greater, and generally much greater, than the
portion which seeps through the soil to join the run-off.
[93] On the total annual rainfall on the land of the globe, and the
relation of rainfall to the annual discharge of rivers, by Sir John
Murray, Scot. Geog. Mag., =3=, 65-77, (1887).
[94] Textbook of Geology, by Sir Archibald Geikie, p. 484, (1903).
[95] _In_ Principles and conditions of the movements of ground water,
by F. H. King, Ann. rept. U. S. Geol. Surv., =19=, II, 59-294,
(1897-98).
From the available data, it appears that the average concentration of
the run-off waters of the United States is about 1.8 parts per million
of potassium (K) and about 0.6 parts per million of phosphoric acid
(PO₄),[96] while the concentration of the capillary groundwater is some
ten or twelve times greater. But even if these concentrations were the
same, it is altogether probable that very much the greater part of the
mineral plant nutrients dissolved by meteoric waters is continually, if
slowly, moving towards the surface of the soil.
The average rainfall of the United States may be taken as approximately
30 inches.[97] If it be assumed that the discharge into the sea is
25 per cent., then the capillary cut-off water is at least 37.5, and
probably nearer 70 per cent. of the rainfall. King’s experimental
work[98] indicates that the higher figure is much nearer the truth.
Computing from the concentrations just cited, with the equations given
above, it is found that approximately 3,500,000 tons of potassium (K)
and 1,200,000 tons of phosphoric acid (PO₄) are carried into the sea
annually from the United States, while from 48,000,000 to 100,000,000
tons of potassium and 18,000,000 to 40,000,000 tons of phosphoric acid
are being carried towards the surface of the soil. If it be assumed
that an average of one ton per acre of dry crop containing one per
cent. potash and 0.6 per cent. phosphoric acid[99] be removed from the
entire area of the United States, then the annual loss from this source
would be 24,000,000 tons of potassium and 14,000,000 tons of phosphoric
acid. Consequently, there is an ample margin between the losses by
cropping and seepage waters, and the supply of capillary waters. It is
true that cases exist where the production of vegetable matter is much
greater than a ton to the acre, productions of five tons or even more
being on record. But such cases occur only where the water supply is
also greater, either through natural rainfall or artificial irrigation;
and it should also be borne in mind that the production of so large a
mass of green crop involves a considerable drawing power on the water
in the soil in addition to the evaporation which would take place at
the surface under ordinary conditions. In other words, the plant would
then be playing no small part in drawing to itself its needed supplies
of water and dissolved mineral nutrients.
[96] Estimated from data in Bull. No. 330, U. S. Geological Survey, The
data of geochemistry, by Frank Wigglesworth Clarke, 1908, p. 53-90.
[97] The latest authoritative statement is that the average annual
rainfall of the United States is 29.4 inches; see: Water Resources,
by W. J. McGee, vol. 1, p. 39-49, and Distribution of rainfall, by
Henry Gannett, vol. 2, p. 10-12, Report of the National conservation
commission, Senate doc. No. 676, 60th Congress, 2d session, 1909.
[98] King: loc. cit., p. 85.
[99] Estimated from Wolff’s tables, How crops grow, by Samuel W.
Johnson, 1890, appendix.
The question may be asked, if the processes outlined above are
generally operative, why accumulations of soluble mineral substances
are not usually found at the surface of the soil. As a matter of fact
such accumulations do occur normally when the evaporation at the
surface is relatively large, that is, under arid conditions. And under
humid conditions it appears to be a general rule that the surface
soil contains more readily soluble or absorbed mineral matter than do
subsoils.[100] No great accumulation occurs at the surface normally
under humid conditions because the rainfall is sufficiently distributed
throughout the year to enable the cut-off water to carry back promptly
into the lower soil levels any excessive amount of soluble material,
there to start anew its slower ascent towards the surface.
[100] See, for instance: Investigations in soil management, by F. H.
King, Madison, Wis., 1904, p. 62 _et seq._ This tendency towards a
higher content of absorbed soluble mineral matter in the surface soil
has been amply confirmed by other experiments. It has been advanced
as an argument against the assumption that the hydrolysis of the soil
minerals is a reversible process. But as pointed out elsewhere in the
text, many of the soil minerals can be made in the wet way at more or
less elevated temperatures and the more rational explanation is simply
that at ordinary temperatures the rate of formation is exceedingly slow.
Calculations such as those here presented are at the best open to many
objections, and it is wise to avoid giving them too much emphasis. So
far as the available data justify any conclusion, however, it appears
that the rise of capillary water is entirely capable of maintaining
a sufficient supply of mineral nutrients for crop requirements; and
furthermore, it is obvious that the problem of the supply of mineral
plant nutrients is dynamic and cannot be successfully attacked by
considerations which are essentially static.