might extend roots into our test plot and extract moisture, we'll
make our tilled area 50 feet by 50 feet and make all our
measurements in the center. And let's locate this imaginary plot in
full sun on flat, uniform soil. And let's plant absolutely nothing
in this bare earth. And all season let's rigorously hoe out every
weed while it is still very tiny.

Let's also suppose it's been a typical maritime Northwest rainy
winter, so on April 1 the soil is at field capacity, holding all the
moisture it can. From early April until well into September the hot
sun will beat down on this bare plot. Our summer rains generally
come in insignificant installments and do not penetrate deeply; all
of the rain quickly evaporates from the surface few inches without
recharging deeper layers. Most readers would reason that a soil
moisture measurement taken 6 inches down on September 1, should show
very little water left. One foot down seems like it should be just
as dry, and in fact, most gardeners would expect that there would be
very little water found in the soil until we got down quite a few
feet if there were several feet of soil.

But that is not what happens! The hot sun does dry out the surface
inches, but if we dig down 6 inches or so there will be almost as
much water present in September as there was in April. Bare earth
does not lose much water at all. _Once a thin surface layer is
completely desiccated, be it loose or compacted, virtually no
further loss of moisture can occur._

The only soils that continue to dry out when bare are certain kinds
of very heavy clays that form deep cracks. These ever-deepening
openings allow atmospheric air to freely evaporate additional