The Movement Of Film Water

In Chapter II it was stated that by far the most important kind of water in the soil is that which surrounds the soil grains like a film; because it is this, not free water, which the roots of plants use. This water is held to the surface of the soil grains by tension or adhesion, as a film of water adheres to a pebble dipped into the brook. There is also more or less water in the spaces between the grains. These films of water are not all of the same thickness. Some grains have more water on them than others; therefore parts of the soil are dryer than others. The dryness of some parts of the soil may be due to the fact that they have received less water from rainfall. It may also be caused by the roots of thirsty plants. The movement of film water takes place in this way: The minute root hairs are always absorbing water, together with the plant food that is dissolved in it; not free water, but the film water clinging to the grains of soil. The soil grains which thus pay tribute to the plants become dry. But they touch grains that are not in direct contact with the plant pump; part of the film moisture clinging to these is passed along to the dry grains, so that both become equally moist. Now the grains a little further off have more moisture than these which have given a part of theirs to the dry grains in the grasp of the root hairs.

These, likewise, give of their abundance to the soil grains less favoured. So it comes about that there is always a steady current of film water passing to every root hair of every thirsty, growing plant; not flowing through the soil, but creeping from particle to particle, and space to space. In exactly the same way there is always a current of film water passing upward on every summer day to replace the water that the uppermost soil grains have lost by evaporation. The amount of water lost from common farm soils by evaporation may be as much as five inches a month during the summer. There must be inequalities in the dryness of the soil that are due to other causes, as difference in texture or composition; but for the most part we may think of this great volume of film water, equal to a layer of water over fifteen inches deep in the first five feet of some soils, as settling strongly in two currents — toward the surface, to replace the loss of water by evaporation, and toward the roots of plants. These invisible currents are not affected by the law of gravitation; they travel up, down, or sidewise in the endeavour to make the soil equally moist throughout its bulk. But this result is never brought to pass; it is prevented by the frequent downward passage of water, constant evaporation from the surface and continued absorption by roots.

We are not concerned about checking the current of film moisture toward the roots, except to increase it. Usually the larger the loss of film water in this way, the greater the gain to the farmer. But we are greatly concerned about the current of film water that is passing upward to the surface of the soil and is then lost in the air as water vapour. We cannot afford to lose this water; and we cannot afford to lose, even temporarily, the plant food that is dissolved in it. When the water evaporates, this is left upon the surface of the soil where it is useless to plants, until washed down into the root feeding area. We would rather have the water evaporate, not from the soil, but through the leaves of crops, after it has given to the plants the food that it contains. The sun is the mightiest of pumps. The amount of water that is evaporated from the soil in one summer day is astonishing even to those who have observed how quickly the soil becomes dry in midsummer after a heavy rain. King found that each square foot of an ordinary farm soil lost 1.3 pounds of water daily by evaporation from the surface. Capillary Action. — The movement of film water in the soil is frequently called "capillary action." The soil being made of millions of tiny grains, there are likewise millions of tiny spaces between the grains, as in a pile of wheat; so it follows that there is a more or less continuous passage from one space to another, making many small and very crooked tubes — hence the term "capillary," hairlike.

Film water passes up, down and sidewise through these tubes, but mostly upward, for there is where the soil is most likely to become dry. For the purpose of illustration, then, we may conceive that every farm soil is permeated with very fine hair-like tubes which reach deep into the subsoil; that it is, we will say, something like a bundle of wheat straw. The lower ends of the tubes rest upon the water table — which may be two, six or thirty feet below the surface, according to the depth at which free water is found. The upper ends of the tubes open upon the surface. Water is drawn up through these tubes, from the water table to the surface, by a kind of suction called "capillary action." Capillary action is something like the process by which oil is drawn up through a wick; the flame that burns the oil is like the sun that evaporates the water; as oil creeps up through the strands of the wick, so soil water creeps up through tiny pores of the soil. Whenever the sun is hot, or a drying wind hugs the ground, water is drawn up through these tubes. In reality the tubes are as crooked and irregular as the holes in a piece of cheese, yet the principle and the results are the same. How to Prevent the Loss of Film Water. — How can this great loss of water — sometimes amounting to over one and one-half inches of rain in a single week — be checked ? Obviously there is but one way to do it — by stopping the mouths of the tubes. One need not travel far to find illustrations of how this may be done. Turn over a board or stone lying on the ground; the soil beneath is more moist than the adjacent soil; the pores of the earth have been closed, and the current of water passing upward has been stopped. That is why fishermen hunt for earthworms beneath stones, when the weather is very dry. A layer of small flat rocks scattered over the surface of the ground would prevent a large part of the film water from escaping, were it practicable. The woodpile offers another illustration, for the soil is always moist beneath the layer of chips, showing that evaporation has been checked. But a layer of straw does just as well and is easier to apply. Any material that is spread upon the soil to stop up the mouths of the water tubes and shade the surface from the sun, thus preventing the loss of soil water, is called a mulch. The most effective and practicable mulches are coarse hay, straw, and farm manures; not only because they are easy to apply, but also because they benefit the soil in other ways, chiefly through the humus that they add.

Occasionally other materials are used to mulch the soil, as leaves, straw waste, coal ashes, sea-weed. Mulching to save soil water is rarely practised in growing common farm crops. Small fruits, especially the strawberry, currant and gooseberry and also, to a slight extent, the tree fruits, are frequently mulched with these materials. The Soil Mulch. — The most practicable mulch in general farming is made of loose, dry soil. This is obtained by stirring the surface of the soil with the implements of tillage, as the plow, harrow and cultivator. Stirring the soil makes it much looser. The pores are broken. Water can creep from one soil grain to another only when the grains are close together — when the soil is compact. Stirring the soil spreads the grains so far apart that water cannot pass from one grain to another, or but very slowly. So it comes no further than the mouths of the tubes, which are now not on the surface but eight inches, six inches or three inches below the surface, according to the depth to which the soil has been loosened.