Chapter X.

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, Geikie, Newell, 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.

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).

Textbook of Geology, by Sir Archibald Geikie, p. 484, (1903).

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), 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. 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 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 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.

Estimated from data in Bull. No. 330, U. S. Geological Survey, The data of geochemistry, by Frank Wigglesworth Clarke, 1908, p. 53-90.

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.

King: loc. cit., p. 85.

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. 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.

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.