Water, entering the soil, moves downward under the influence of gravity as gravitational water, until under the attractive influence of the soil it has been converted into capillary water and adheres to the soil particles as a film.  If the soil were dry, and the film therefore thin, the rain water would move downward only a short distance as gravitational water; if the soil were wet, and the film therefore thick, the water would move down to a greater distance before being exhausted.  If, as is often the case in humid districts, the soil is saturated, that is, the film is as thick as the particles can hold, the water would pass right through the soil and connect with the standing water below.  This, of course, is seldom the case in dry-farm districts.  In any soil, excepting one already saturated, the addition of water will produce a thickening of the soil-water film to the full descent of the water.  This immediately destroys the conditions of equilibrium formerly existing, for the moisture is not now uniformly distributed.  Consequently a process of redistribution begins which continues until the nearest approach to equilibrium is restored.  In this process water will pass in every direction from the wet portion of the soil to the drier; it does not necessarily mean that water will actually pass from the wet portion to the drier portion; usually, at the driest point a little water is drawn from the adjoining point, which in turn draws from the next, and that from the next, until the redistribution is complete.  The process is very much like stuffing wool into a sack which already is loosely filled.  The new wool does not reach the bottom of the sack, yet there is more wool in the bottom than there was before.

If a plant-root is actively feeding some distance under the soil surface, the reverse process occurs.  At the feeding point the root continually abstracts water from the soil grains and thus makes the film thinner in that locality.  This causes a movement of moisture similar to the one above described, from the wetter portions of the soil to the portion being dried out by the action of the plant-root.  Soil many feet or even rods distant may assist in supplying such an active root with moisture.  When the thousands of tiny roots sent out by each plant are recalled. it may well be understood what a confusion of pulls and counter-pulls upon the soil-moisture exists in any cultivated soil.  In fact, the soil-water film may be viewed as being in a state of trembling activity, tending to place itself in full equilibrium with the surrounding contending forces which, themselves, constantly change.  Were it not that the water film held closely around the soil particles is possessed of extreme mobility, it would not be possible to meet the demands of the plants upon the water at comparatively great distances.  Even as it is, it frequently happens that when crops are planted too thickly on dry-farms, the soil-moisture cannot move quickly enough to the absorbing roots to maintain plant growth, and crop failure results.  Incidentally, this points to planting that shall be proportional to the moisture contained by the soil.  See Chapter XI.