ys in hot weather and then lift it up on a hot day you will
see that the soil underneath is quite moist; you may also find several
slugs or other animals that have gone there for the sake of the
coolness and the moisture. Plants and trees also keep off the sun's
heat and so make the soil cold and moist. Grass land is in summer and
autumn, and even in early winter, cooler near the surface than bare
land. At Harpenden we found:--

Soil temperature
Date Grass land Bare land

1910
Sept. 24th 1/2 inch 13 17.5
3 inches 12.5 12.5
6 inches 12.5 12.25

Oct. 5th 1/2 inch 17.5 17
3 inches 15 16.7
6 inches 14.5 15.5

Even if the ground is not covered a certain amount of protection is
still possible. Trees are often planted round ponds to prevent
evaporation of the water. The wind helps to dry the soil very much,
and a hedge {92} that shields from the wind not only protects the crop
but also keeps the soil moist: a road with high hedges at each side
remains wet for a long time after more exposed parts have dried. The
effect on the temperature can be well seen on a day when a N.E. wind is
blowing. Fix up on a piece of the experimental ground a little hedge
made of small pea-stakes or brushwood, and take the soil temperature at
one inch depth, both on the windward and on the leeward side. Two
results were:--

Temperature at 1 inch depth--sheltered side 15.5
" " " " windward side 14

We have already seen that on the hot day, June 20th, the top half-inch
of soil was hotter than the air: the mercury in the thermometer rose
directly it was put into the soil. There is nothing very unusual about
this; if you touch a piece of iron lying on the soil you find it hotter
than the air. Lower down the soil had the same temperature as the air,
and still lower it was cooler[1]. The sun's heat travels so slowly
into the soil in summer that months pass before it gets far down, but
then, as it takes so long to get in, it also takes a long time to get
out, and it takes still longer to get either in or out if there is a
mulch or if grass is growing.

During the early winter you may notice that the first fall of snow soon
melts on the arable land but r