Clayey subsoil can supply surprisingly larger amounts of moisture than the granular sandy surface might imply, but only if the earth is opened deeply and becomes more accessible to root growth.  Fortunately, once root development increases at greater depths, the organic matter content and accessibility of this clayey layer can be maintained through intelligent green manuring, postponing for years the need to subsoil again.  Green manuring is discussed in detail shortly.

Other sites may have gooey, very fine clay topsoils, almost inevitably with gooey, very fine clay subsoils as well.  Though incorporation of extraordinarily large quantities of organic matter can turn the top few inches into something that behaves a little like loam, it is quite impractical to work in humus to a depth of 4 or 5 feet.  Root development will still be limited to the surface layer.  Very fine clays don’t make likely dry gardens.

Not all clay soils are “fine clay soils,” totally compacted and airless.  For example, on the gentler slopes of the geologic old Cascades, those 50-million-year-old black basalts that form the Cascades foothills and appear in other places throughout the maritime Northwest, a deep, friable, red clay soil called (in Oregon) Jori often forms.  Jori clays can be 6 to 8 feet deep and are sufficiently porous and well drained to have been used for highly productive orchard crops.  Water-wise gardeners can do wonders with Joris and other similar soils, though clays never grow the best root crops.

Spotting a Likely Site

Observing the condition of wild plants can reveal a good site to garden without much irrigation.  Where Himalaya or Evergreen blackberries grow 2 feet tall and produce small, dull-tasting fruit, there is not much available soil moisture.  Where they grow 6 feet tall and the berries are sweet and good sized, there is deep, open soil.  When the berry vines are 8 or more feet tall and the fruits are especially huge, usually there is both deep, loose soil and a higher than usual amount of fertility.

Other native vegetation can also reveal a lot about soil moisture reserves.  For years I wondered at the short leaders and sad appearance of Douglas fir in the vicinity of Yelm, Washington.  Were they due to extreme soil infertility?  Then I learned that conifer trees respond more to summertime soil moisture than to fertility.  I obtained a soil survey of Thurston County and discovered that much of that area was very sandy with gravelly subsoil.  Eureka!

The Soil Conservation Service (SCS), a U.S.  Government agency, has probably put a soil auger into your very land or a plot close by.  Its tests have been correlated and mapped; the soils underlying the maritime Northwest have been named and categorized by texture, depth, and ability to provide available moisture.  The maps are precise and detailed enough to approximately locate a city or suburban lot.  In 1987, when I was in the market for a new homestead, I first went to my county SCS office, mapped out locations where the soil was suitable, and then went hunting.  Most counties have their own office.