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.

[Illustration:  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?