The gravitational soil-water cannot long remain in that condition; for, necessarily, the pull of gravity moves it downward through the soil pores and if conditions are favorable, it finally reaches the standing water-table, whence it is carried to the great rivers, and finally to the ocean.  In humid soils, under a large precipitation, gravitational water moves down to the standing water-table after every rain.  In dry-farm soils the gravitational water seldom reaches the standing water-table; for, as it moves downward, it wets the soil grains and remains in the capillary condition as a thin film around the soil grains.

To the dry-farmer, the full water capacity is of importance only as it pertains to the upper foot of soil.  If, by proper plowing and cultivation, the upper soil be loose and porous, the precipitation is allowed to soak quickly into the soil, away from the action of the wind and sun.  From this temporary reservoir, the water, in obedience to the pull of gravity, will move slowly downward to the greater soil depths, where it will be stored permanently until needed by plants.  It is for this reason that dry-farmers find it profitable to plow in the fall, as soon as possible after harvesting.  In fact, Campbell advocates that the harvester be followed immediately by the disk, later to be followed by the plow The essential thing is to keep the topsoil open and receptive to a rain.

Capillary soil-water

The so-called capillary soil-water is of greatest importance to the dry-farmer.  This is the water that clings as a film around a marble that has been dipped into water.  There is a natural attraction between water and nearly all known substances, as is witnessed by the fact that nearly all things may be moistened.  The water is held around the marble because the attraction between the marble and the water is greater than the pull of gravity upon the water.  The greater the attraction, the thicker the film; the smaller the attraction, the thinner the film will be.  The water that rises in a capillary glass tube when placed in water does so by virtue of the attraction between water and glass.  Frequently, the force that makes capillary water possible is called surface tension.

Whenever there is a sufficient amount of water available, a thin film of water is found around every soil grain; and where the soil grains touch, or where they are very near together, water is held pretty much as in capillary tubes.  Not only are the soil particles enveloped by such a film, but the plant roots foraging in the soil are likewise covered; that is, the whole system of soil grains and roots is covered, under favorable conditions, with a thin film of capillary water.  It is the water in this form upon which plants draw during their periods of growth.  The hygroscopic water and the gravitational water are of comparatively little value in plant growth.

Field capacity of soils for capillary water