How Soil Loses Water

Suppose we tilled a plot about April 1 and then measured soil
moisture loss until October. Because plants growing around the edge
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
moisture. But if the cracks are filled with dust by surface
cultivation, even this soil type ceases to lose water.
Soil functions as our bank account, holding available water in
storage. In our climate soil is inevitably charged to capacity by
winter rains, and then all summer growing plants make heavy
withdrawals. But hot sun and wind working directly on soil don't
remove much water; that is caused by hot sun and wind working on
plant leaves, making them transpire moisture drawn from the earth
through their root systems. Plants desiccate soil to the ultimate
depth and lateral extent of their rooting ability, and then some.
The size of vegetable root systems is greater than most gardeners
would think. The amount of moisture potentially available to sustain
vegetable growth is also greater than most gardeners think.
Rain and irrigation are not the only ways to replace soil moisture.
If the soil body is deep, water will gradually come up from below
the root zone by capillarity. Capillarity works by the very same
force of adhesion that makes moisture stick to a soil particle. A
column of water in a vertical tube (like a thin straw) adheres to
the tube's inner surfaces. This adhesion tends to lift the edges of
the column of water. As the tube's diameter becomes smaller the
amount of lift becomes greater. Soil particles form interconnected
pores that allow an inefficient capillary flow, recharging dry soil
above. However, the drier soil becomes, the less effective capillary
flow becomes. _That is why a thoroughly desiccated surface layer
only a few inches thick acts as a powerful mulch._
Industrial farming and modern gardening tend to discount the
replacement of surface moisture by capillarity, considering this
flow an insignificant factor compared with the moisture needs of
crops. But conventional agriculture focuses on maximized yields
through high plant densities. Capillarity is too slow to support
dense crop stands where numerous root systems are competing, but
when a single plant can, without any competition, occupy a large
enough area, moisture replacement by capillarity becomes

How Plants Obtain Water I. CROPS REMAINING ENTIRE SEASON facebooktwittergoogle_plusredditpinterestlinkedinmail