Which soils have greatest power to hold the water which enters them?

=Experiment.=--Use the same or similar apparatus as for the last experiment. After placing the cloth caps over the ends of the tubes label and carefully weigh each one, keeping a record of each; then fill them with the dry soils and weigh again. Now place the tubes in the rack and pour water in the upper ends until the entire soil is wet; cover the tops and allow the surplus water to drain out; when the dripping stops, weigh the tubes again, and by subtraction find the amount of water held by the soil in each tube; compute the percentage.

It will be found that the organic matter will hold a much larger percentage of water than the other soils; and the clay more than the sand. The tube of organic soil will actually hold a larger amount of water than the other tubes. (See also Fig. 25.)

In the experiment on page 40 we noticed that the sand took in the water poured on its surface and let it run through very quickly. This is a fault of sandy soils.

What can we do for our sandy soils to help them to hold better the moisture which falls on them and tends to leach through them? For immediate effect we can close the pores somewhat by compacting the soil with the roller. For more lasting effects, we can fill them with organic matter.

Which soils will hold longest the water which they have absorbed? Or which soils will keep moist longest in dry weather?

[Ill.u.s.tration: FIG. 23.

To show how bottles may be used in place of lamp chimneys shown in Figs 22 and 24.]

[Ill.u.s.tration: FIG. 24.--CAPILLARITY OF SOILS To show the relative powers of soils to take water from below.]

[Ill.u.s.tration: FIG. 25.--WATER-ABSORBING AND WATER-HOLDING POWERS OF SOILS.]

=Experiment.=--Fill a pan or bucket with moist sand, another with moist clay, and a third with moist organic matter; set them in the sun to dry and notice which dries last. The organic matter will be found to hold moisture much longer than the other soils. The power of the other soils to hold moisture through dry weather can be improved by mixing organic matter with them.

We find then that the power of soils to absorb and hold moisture depends on the amount of sand, clay, or humus which they contain, and the compactness of the particles. We see also how useful organic matter is in improving sandy and clayey soils.

THE EFFECT OF WORKING SOILS WHEN WET

By this time the soils we left in the pans (see page 26), sand, clay, humus and garden soil, must be dry. If so, examine them. We find that the clay which was stirred when wet has dried into an almost bricklike ma.s.s, while that which was not stirred is not so hard, though it has a thick, hard crust. The sand is not much affected by stirring when wet.

The organic matter which was stirred when wet has perhaps stiffened a little, but very easily crumbles; the unstirred part was not much affected by the wetting and drying.

The garden soil after drying is not as stiff as the clay nor as loose as the sand and humus. This is because it is very likely a mixture of all three, the sand and the humus checking the baking. This teaches us that it is not a good plan to work soils when they are wet if they are stiff and sticky; and that our stiff clay soils can be kept from drying hard or baking by the use of organic matter. "And that's a witness" for organic matter.

The relation of the soil to moisture is very important, for moisture is one of the greatest factors if not the greatest in the growth of the crop.

The power to absorb or soak up moisture from any source is greatest in those soils whose particles are smaller and fit closer together.

It is for this reason that strong loams and clay soils absorb and hold three times as much water as sandy soils do, while peaty or humus soils absorb a still larger proportion.

The reason why crops burn up so quickly on sandy soils during dry seasons is because of their weak power to hold water.

The clay and humus soils carry crops through dry weather better because of their power to hold moisture and to absorb or soak up moisture from below. It is for this reason also that clay and peaty soils more often need draining than sandy soils.

When rain falls on a sandy soil it enters readily, but it is apt to pa.s.s rapidly down and be, to a great extent, lost in the subsoil, for the sand has not sufficient power to hold much of it.

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 princ.i.p.ally 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.

[Ill.u.s.tration: FIG. 26.--CAPILLARY TUBES.

To show how water rises in small tubes or is drawn into small s.p.a.ces.]

[Ill.u.s.tration: FIG. 27.--CAPILLARY PLATES.

Water is drawn to the highest point where the gla.s.s plates are closest together.]

[Ill.u.s.tration: 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.]

[Ill.u.s.tration: FIG. 29.

To show the relative amounts of film-moisture held by coa.r.s.e 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

If you will take a number of gla.s.s tubes of different sizes, the largest not more than one-fourth of an inch in diameter, and hold them with one end of each in water or some colored liquid, you will notice that the water rises in the tubes (Fig. 26), and that it rises highest in the smallest tube. The force which causes the water to rise in these tubes is called the capillary force, from the old Latin word _capillum_ (a hair), because it is most marked in hair-like tubes, the smaller the tube the higher the water will rise. The water which rises in the tubes is called capillary water.

Another method of ill.u.s.trating capillary water is to tie or hold together two flat pieces of gla.s.s, keeping two of the edges close together and separating the opposite two about one-eighth of an inch with a sliver of wood. Then set them in a plate of water or colored liquid and notice how the water rises between the pieces of gla.s.s, rising higher the smaller the s.p.a.ce (Fig. 27). It is the capillary force which causes water to rise in a piece of cloth or paper dipped in water.

Take a plate and pour onto it a cone-shaped pile of dry sand or fine soil; then pour water around the base of the pile and note how the water is drawn up into the soil by capillary force (Fig. 28).

Capillary water is the other important form of water in the soil. This is moisture which is drawn by capillary force or soaks into the s.p.a.ces between the soil particles and covers each particle with a thin film of moisture.

FILM WATER

Take a marble or a pebble, dip it into water and notice the thin layer or film of water that clings to it. This is a form of capillary water and is sometimes called film water or film moisture. Take a handful of soil that is moist but not wet, notice that it does not wet the hand, and yet there is moisture all through it; each particle is covered with a very thin film of water.

Now this film water is just the form of water that can supply the very slender root hairs without drowning them, that is, without keeping the air from them. And the plant grower should see to it that the roots of his plants are well supplied with film water and are not drowned by the presence of free water. Capillary water may sometimes completely fill the s.p.a.ces between the soil particles; when this occurs the roots are drowned just as in the case of free water as we saw when cuttings were placed in the puddled clay (see Fig. 18). Free water is indirectly of use to the plant because it serves as a supply for capillary and film moisture.

Now I think we can answer the question which was asked when we were studying the habit of growth of roots but was left unanswered at the time (see page 14). The question was this: Of what value is it to the farmer to know that roots enter the soil to a depth of three to six feet? We know that roots will not grow without air. We also know that if the soil is full of free water there is no air in it, and, therefore, roots of most plants will not grow in it. It is, therefore, of interest to the farmer to see that free water does not come within at least three or four feet of the surface of the soil so that the roots of his crops may have plenty of well ventilated soil in which to develop. If there is a tendency for free water to fill the soil a large part of the time, the farmer can get rid of it by draining the land. We get here a lesson for the grower of house plants also. It is that we must be careful that the soil in the pots or boxes in which our plants are growing is always supplied with film water and not wet and soggy with free water. Water should not be left standing long in the saucer under the pot of a growing plant. It is best to water the pot from the top and let the surplus water drain into the saucer and then empty it out.