It
might seem logical to conclude that a clayey garden would be the
most drought resistant. But there's more to it. For some crops, deep
sandy loams can provide just about as much usable moisture as clays.
Sandy soils usually allow more extensive root development, so a
plant with a naturally aggressive and deep root system may be able
to occupy a much larger volume of sandy loam, ultimately coming up
with more moisture than it could obtain from a heavy, airless clay.
And sandy loams often have a clayey, moisture-rich subsoil.
_Because of this interplay of factors, how much available water your
own unique garden soil is actually capable of providing and how much
you will have to supplement it with irrigation can only be
discovered by trial._
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.
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