As they were darker in color and the clay was more compact than the
sand, they must have absorbed more heat, but they also held more
water, and, therefore, lost more heat by evaporation.

[Illustration: FIG. 32.
Charts showing average temperature of a set of dry and wet soils
during a period of five days. _H_, humus; _C_, clay; _S_, sand.]

[Illustration: FIG. 33.
To show the value of organic matter. 1 contains clay subsoil; 2, clay
subsoil and fertilizer; 3, clay subsoil and organic matter. All
planted at the same time.]

Of the dry soils, then, the humus averaged warmest, because, on
account of its dark color, it absorbed heat more readily than the
others. The dry clay was warmer than the sand on account of its color
and compact texture. Of the wet soils the sand was the warmest,
because, on account of its holding less moisture, less heat was
required to raise its temperature and there was less cooling by
evaporation, while the other soils, although they absorbed more heat
than the sand, lost more on account of greater evaporation, due to
their holding more moisture. Why are sandy soils called warm soils and
clay soils said to be cold?

How may we check losses of heat from the soil?

If we make a mulch on the surface of the soil evaporation will be
checked and therefore loss of heat by evaporation will be checked
also. The mulch will also check the conduction of heat from the lower