greatly limits the ability of plants to aggressively find water. A
compacted subsoil or even a thin compressed layer such as plowpan
may function as such a barrier. Though moisture will still rise
slowly by capillarity and recharge soil above plowpan, plants obtain
much more water by rooting into unoccupied, damp soil. Soils close
to rivers or on floodplains may appear loose and infinitely deep but
may hide subsoil streaks of droughty gravel that effectively stops
root growth. Some of these conditions are correctable and some are
not.

Plowpan is very commonly encountered by homesteaders on farm soils
and may be found in suburbia too, but fortunately it is the easiest
obstacle to remedy. Traditionally, American croplands have been
tilled with the moldboard plow. As this implement first cuts and
then flips a 6-or 7-inch-deep slice of soil over, the sole--the part
supporting the plow's weight--presses heavily on the earth about 7
inches below the surface. With each subsequent plowing the plow sole
rides at the same 7-inch depth and an even more compacted layer
develops. Once formed plowpan prevents the crop from rooting into
the subsoil. Since winter rains leach nutrients from the topsoil and
deposit them in the subsoil, plowpan prevents access to these
nutrients and effectively impoverishes the field. So wise farmers
periodically use a subsoil plow to fracture the pan.

Plowpan can seem as firm as a rammed-earth house; once established,
it can last a long, long time. My own garden land is part of what
was once an old wheat farm, one of the first homesteads of the
Oregon Territory. From about 1860 through the 1930s, the field
produced small grains. After wheat became unprofitable, probably
because of changing market conditions and soil exhaustion, the field