Curing Clayey Soils

In humid climates like ours, sandy soils may seem very open and
friable on the surface but frequently hold some unpleasant subsoil
surprises. Over geologic time spans, mineral grains are slowly
destroyed by weak soil acids and clay is formed from the breakdown
products. Then heavy winter rainfall transports these minuscule clay
particles deeper into the earth, where they concentrate. It is not
unusual to find a sandy topsoil underlaid with a dense, cement-like,
clayey sand subsoil extending down several feet. If very impervious,
a thick, dense deposition like this may be called hardpan.
The spading fork cannot cure this condition as simply as it can
eliminate thin plowpan. Here is one situation where, if I had a
neighbor with a large tractor and subsoil plow, I'd hire him to
fracture my land 3 or 4 feet deep. Painstakingly double or even
triple digging will also loosen this layer. Another possible
strategy for a smaller garden would be to rent a gasoline-powered
posthole auger, spread manure or compost an inch or two thick, and
then bore numerous, almost adjoining holes 4 feet deep all over the
garden.
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.