Clayey soil can provide plants with three times as much available water as sand, six times as much as a very coarse sandy soil.  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.  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.