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 passed 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 passed 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 glass 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 glass 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, coarse 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 illustrated 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 coarse, 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 coarse 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 coarse 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.

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.ToList

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