When rain falls on a clay soil it enters less readily because of the closeness of the particles, and during long rains or heavy showers some of the water may run off the surface.  If the surface has been recently broken and softened with the plow or cultivator the rain enters more readily.  What does enter is held and is not allowed to run through as in the case of the sand.

Humus soil absorbs the rain as readily as the sand and holds it with a firmer grip than clay.

This fact gives us a hint as to how we may improve the sand and clay.

Organic matter mixed with these soils by applying manures or plowing under green crops will cause the sand to hold the rain better and the clay to absorb it more readily.

CHAPTER V

FORMS OF SOIL WATER

Water which comes to the soil and is absorbed exists in the soil principally in two forms:  Free water and capillary water.

FREE WATER

Free water is that form of water which fills our wells, is found in the bottoms of holes dug in the ground during wet seasons and is often found standing on the surface of the soil after heavy or long continued rains.  It is sometimes called ground water or standing water and flows under the influence of gravity.

Is free water good for the roots of farm plants?  If we remember how the root takes its food and moisture, namely through the delicate root hairs; and also remember the experiment which showed us that roots need air, we can readily see that free water would give the root hairs enough moisture, but it would at the same time drown them by cutting off the air.  Therefore free water is not directly useful to the roots of house plants or farm plants, excepting such as are naturally swamp plants, like rice, which grows part of the time with its roots covered with free water.

[Illustration:  FIG. 26.—­CAPILLARY TUBES.  To show how water rises in small tubes or is drawn into small spaces.]

[Illustration:  FIG. 27.—­CAPILLARY PLATES.  Water is drawn to the highest point where the glass plates are closest together.]

[Illustration:  FIG. 28.  A cone of soil to show capillarity.  Water poured about the base of this cone of soil has been drawn by capillary force half-way to the top.]

[Illustration:  FIG. 29.  To show the relative amounts of film-moisture held by coarse and fine soils.  The colored water in bottle A represents the amount of water required to cover the half pound of pebbles in the tumbler B with a film of moisture.  The colored water in bottle C shows the amount required to cover the soil grains in the half pound of sand in tumbler D.]

CAPILLARY WATER