Page:Popular Science Monthly Volume 41.djvu/103

Rh discussed in detail, it appears safe to estimate the soil evaporation in the Middle States at approximately twice the amount exhaled by a growing crop of fair luxuriance. Of an annual rainfall of thirty-two inches, or over, fairly distributed, we may then assume, with apparent good reason, that about sixteen inches will be disposed of by evaporation from a fertile, well-drained soil, and about eight inches by exhalation from a growing crop, or an aggregate of about twenty-four inches will be disposed of in the form of vapor from soil and crop, involving an expenditure of energy represented by the heat produced by burning 320 tons of coal per acre, or the equivalent of the work of seventy-three horses, day and night, without intermission, for six months. If to this is added the energy expended in constructive metabolism and in warming the soil, which we will not now estimate in specific terms, the sum would represent the normal demands for energy in growing a crop of one acre.

This enormous expenditure of energy appears to be quite as essential to the well-being of the crop as the supply of food constituents, to which attention has been too exclusively directed, and any conditions that tend to materially increase or diminish it must be looked upon as injurious.

From this standpoint the principle of the conservation of energy furnishes most satisfactory data for discussing the philosophy of farm drainage. On undrained, retentive soils, the rain that falls in excess of the normal requirements of the crop and soil metabolism must be removed by evaporation, and this calls for a very considerable expenditure of energy that on drained land might be made available in useful work, to say nothing of the influence of removing surplus water by evaporation on the physical and biological characteristics of the soil.

For each inch of surplus rainfall removed from the soil by evaporation, the energy expended would be represented by 26,600 pounds of coal per acre. With an annual rainfall of forty inches, which is not unusual in the Middle States, and is considerably exceeded in some localities, there would be sixteen inches of water in excess of the normal demands of an ordinary farm crop, and to remove this by evaporation would require the equivalent of about 213 tons of coal per acre, representing the continuous work of forty-eight horses, day and night, for six months. The removal of this surplus water by drainage would obviate the necessity for this enormous expenditure of energy, besides other incidental advantages which we need not notice here.

In the economy of animals the manifestations of the law of the conservation of energy are quite as striking and significant. The potential energy of their food is the sole source of the energy expended in work, and in their processes of nutrition and growth.