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? =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.