riod 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
soil to the surface and therefore check loss of heat by radiation from
the surface.

VALUE OF ORGANIC MATTER

Figure 33 illustrates a simple way to show the value of organic matter
in the soil. The boxes are about twelve inches square and ten inches
deep. They were filled with a clay subsoil taken from the second foot
below the surface of the field. To the second box was added sufficient
commercial fertilizer to supply the plants with all necessary plant
food. To the third box was added some peat or decayed leaves, in
amount about ten per cent. of the clay subsoil. The corn was then
planted and the boxes were all given the same care. The better growth
of the corn in the third box was due to the fact that the organic
matter not only furnished food for the corn but during its decay
prepared mineral plant food that was locked up in the clay, and also
brought about better conditions of air and moisture by improving the
texture of the soil. The plants in the second box had sufficient plant
food, but did not make better growth because poor texture prevented
proper conditions of air and moisture. "And that's another witness"
for organic matter. Decaying organic matter or humus is really the
life of the soil and it is greatly needed in most of the farm soils of
the eastern part of the country. It closes the pores of sand