Which soils have the greatest capacity for film water?

=Experiment.=--Place in a tumbler or bottle one-half pound of pebbles about the size of a pea or bean; pour a few drops of water on them and shake them; continue adding water and shaking them till every pebble is covered with a film of water; let any surplus water drain off. Then weigh again; the difference in the two weights will be approximately the weight of the film water that the pebbles can carry. Repeat this with sand and compare the two amounts of water. A striking ill.u.s.tration can be made by taking two slender bottles and placing in them amounts of colored water equal to the amounts of film water held by the pebbles and sand respectively. In the accompanying ill.u.s.tration (Fig. 29), _A_ represents the amount of water that was found necessary to cover the pebbles in tumbler _B_ with a film of moisture. _C_ is the amount that was necessary to cover with a film the particles of sand in _D_. The finer soil has the greater area for film moisture. It has been estimated that the particles of a cubic foot of clay loam have a possible aggregate film surface of three-fourths of an acre.

CHAPTER VI

LOSS OF SOIL WATER

LOSS OF SOIL WATER AND MEANS OF CHECKING THE LOSS

We noticed in previous paragraphs that soil might at times have too much water in it for proper ventilation and so check the growth of the roots of the plant. Now is it possible that soil water may be lost or wasted and if so can we check the loss?

In the experiment to find out how well the soils would take in the rainfall (page 40) we noticed that the clay soil took in the water very slowly and that on a field of clay soil part of the rain water would be likely to run off over the surface and be lost. Free water may be lost then, by surface wash.

We noticed methods of checking this loss, namely, pulverizing the soil with the tillage tools and putting organic matter into it to make it absorb the rain more readily.

We noticed that water poured on the sand ran through it very quickly and was apt to be lost by leaching or percolation. This we found could be checked by rolling the soil and by putting organic matter into it to close the pores.

We learned that roots take water from the soil for the use of the plant and send it up to the leaves, which in turn send it out into the air, or transpire it, as this process is called. We learned also that the amount transpired is very great. Now water that is pumped up and transpired by the crops we are growing we consider properly used. But when weeds grow with the crop and pump and transpire water we consider this water as lost or wasted.

Water may be lost then by being pumped up and transpired by weeds. And this is the way weeds do their greatest injury to crops during dry weather. The remedy is easily pointed out. Kill the weeds or do not let them get a start.

There is another way, which we are not apt to notice, by which water may be lost from the soil. When the soil in the pans in a previous experiment (page 26) had been wet and set aside a few days it became very dry. How did the water get out of this soil? That at the surface of the soil evaporated or was changed into vapor and pa.s.sed into the air. Then water from below the surface was pumped up by capillary force to take its place just as the water was pumped up in the tubes of soil. This in turn was evaporated and the process repeated till all of the water in the soil had pa.s.sed into the air. Now this process is going on in the field whenever it is not raining or the ground is not frozen very hard.

Water then may be lost by evaporation.

How can we check this loss?

Suppose we try the experiment of covering the soil with some material that cannot pump water readily.

=Experiment.=--Take four gla.s.s fruit jars, two-quart size, with straight sides. If you cannot get them with straight sides cut off the tops with a hot iron just below the shoulder; tin pails will do if the gla.s.s jars cannot be had. Fill these with moist soil from the field or garden, packing it till it is as hard as the unplowed or unspaded soil. Leave one of them in this condition; from two of them remove an inch or two of soil and replace it in the case of one with clean, dry, coa.r.s.e sand, and in the case of the other with chaff or straw cut into half-inch lengths. Stir the soil in the fourth one to a depth of one inch, leaving it light and crumbly. Now weigh the jars and set them aside. Weigh each day for several days. The four jars ill.u.s.trated in Fig. 30 were prepared in this way and allowed to stand seven days. In that time they lost the following amounts of water:

Amounts of water lost from jars of prepared soil in seven days.

No. 1 packed soil--lost 5.5 oz. equal to about 75 tons per acre.

No. 2 covered with straw--lost 2 oz. equal to about 27 tons per acre.

No. 3 covered with dry sand--lost 0 oz. equal to about tons per acre.

No. 4 covered with crumbled soil--lost 2.5 oz., equal to about 34 tons per acre.

Why did not 2, 3 and 4 lose as much water as No. 1?

The soil in jar No. 1 was packed and water was pumped to the surface by capillary force and was evaporated as fast as it came to the surface.

In No. 2 the water could rise rapidly until it reached the straw, then it was stopped almost entirely. But the straw being coa.r.s.e, the air circulated in it more or less freely and there was a slow loss by evaporation. In jar No. 3 the water could rise only to the sand, which was so coa.r.s.e that the water could not climb on it to the surface, and the air circulated in the sand so slowly that there was not sufficient evaporation to affect scales weighing to one-quarter ounce. No. 4 lost less than No. 1 because, as in the case of the sand, the water could not climb rapidly to the surface on the coa.r.s.e crumbs of soil. The loss that did take place from No. 4 was what the air took from the loosely stirred soil on the surface with a very little from the lower soil. Simply stirring the surface of the sod in No. 4 reduced the loss of water to less than half the loss from the hard soil in No. 1.

This experiment gives us the clew to the method of checking loss of water from the soil by evaporation. It is to keep the water from climbing up to the surface, or check the power of the soil to pump the water to the surface by making it loose on top. This loose soil is called a soil mulch. Everything that we do to the soil that loosens and crumbles the surface tends to check the loss of water by evaporation from the soil below.

[Ill.u.s.tration: FIG. 30.--TO SHOW THE EFFECT OF A SOIL MULCH 1. Packed soil, lost in 7 days 5.5 ozs. water, equal to 75 tons per acre.

2. Packed soil, covered with straw, lost in 7 days 2 ozs. water, equal to 27 tons per acre.

3. Packed soil, covered with sand, lost in 7 days 0 ozs. water, equal to tons per acre.

4. Packed soil, covered with soil mulch, lost in 7 days 2.5 ozs.

water, equal to 34 tons per acre.]

CHAPTER VII

SOIL TEMPERATURE

We learned that roots need heat for their growth and development. Now what is the relation of the different kinds of soil toward heat or what are their relative powers to absorb and hold heat?

=Experiment.=--Some days before this experiment, spread on a dry floor about a half bushel each of sand, clay and decayed leaf mould or black woods soil. Stir them occasionally till they are thoroughly dry. When they are dry place them separately in three boxes or large flower pots and keep dry. In three similar boxes or pots place wet sand, wet clay, and wet humus. Place a thermometer in each of the soils, placing the bulb between one and two inches below the surface (Fig. 31). Then place the soils out of doors where the sun can shine on them and leave them several days. If a rain should come up protect the dry soils.

Observe and make a record of the temperatures of each soil several times a day. Chart the average of several days observations. Fig. 32 shows the averages of several days observations on a certain set of soils.

It will be noticed that the temperature of the soils increased until the early part of the afternoon and after that time they lost heat.

[Ill.u.s.tration: FIG. 31.--SOIL TEMPERATURE EXPERIMENT.

Thermometer in pot of soil.]

HOW SOILS ARE WARMED

=Experiment.=--Hold your hand in bright sunlight or near a warm stove or radiator. Your hand is warmed by heat radiated from the sun or warm stove through the air to your body. In the same manner the rays of the sun heat the surface of the soil.

=Experiment.=--Take the stove poker or any small iron rod and hold one end of it in the fire or hold one end of a piece of wire in a candle or lamp flame. The end of the rod or wire will quickly become very hot and heat will gradually be carried its entire length until it becomes too hot to hold. This carrying of the heat from particle to particle through the length of the rod is called heating by conduction. Now when the warm rays of the sun reach the soil, or a warm wind blows over it, the surface particles are warmed and then pa.s.s the heat on to the next ones below, and these in turn pa.s.s it to others and so on till the soil becomes heated to a considerable depth by conduction.

A clay soil will absorb heat by conduction faster than a sandy soil because the particles of the clay lie so close together that the heat pa.s.ses more readily from one to another than in the case of the coa.r.s.er sand.

If the soil is open and porous, warm air and warm rains can enter readily and carry heat to the lower soil.

You have noticed how a pile of stable manure steams in cold weather.

You doubtless know that manure from the horse stable is often used to furnish heat for hotbeds and for sweet potato beds.

Now the heat which warms the manure and sends the steam out of it, and warms the hotbed and sweet potato bed, is produced by the decaying or rotting of the manure. More or less heat is produced by the decay of all kinds of organic matter. So if the soil is well supplied with organic matter, the decay of this material will add somewhat to the warmth of the soil.

HOW SOILS LOSE HEAT

Wet one of your fingers and hold your hand up in the air. The wet finger will feel colder than the others and will gradually become dry.

This is because some of the heat of your finger is being used to dry up the water or change it into a vapor, or in other words to evaporate it.