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 became an unplowed pasture.  Then in the 1970s it grew daffodil bulbs, occasioning more plowing.  All through the ’80s my soil again rested under grass.  In 1987, when I began using the land, there was still a 2-inch-thick, very hard layer starting about 7 inches down.  Below 9 inches the open earth is soft as butter as far as I’ve ever dug.

On a garden-sized plot, plowpan or compacted subsoil is easily opened with a spading fork or a very sharp common shovel.  After normal rotary tilling, either tool can fairly easily be wiggled 12 inches into the earth and small bites of plowpan loosened.  Once this laborious chore is accomplished the first time, deep tillage will be far easier.  In fact, it becomes so easy that I’ve been looking for a custom-made fork with longer tines.

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.