When roots decay, fresh organic matter and large, long-lasting
passageways can be left deep in the soil, allowing easier air
movement and facilitating entry of other roots. But no cover crop
that I am aware of will effectively penetrate firm plowpan or other
resistant physical obstacles. Such a barrier forces all plants to
root almost exclusively in the topsoil. However, once the subsoil
has been mechanically fractured the first time, and if recompaction
is avoided by shunning heavy tractors and other machinery, green
manure crops can maintain the openness of the subsoil.

To accomplish this, correct green manure species selection is
essential. Lawn grasses tend to be shallow rooting, while most
regionally adapted pasture grasses can reach down about 3 feet at
best. However, orchard grass (called coltsfoot in English farming
books) will grow down 4 or more feet while leaving a massive amount
of decaying organic matter in the subsoil after the sod is tilled
in. Sweet clover, a biennial legume that sprouts one spring then
winters over to bloom the next summer, may go down 8 feet. Red
clover, a perennial species, may thickly invade the top 5 feet.
Other useful subsoil busters include densely sown Umbelliferae such
as carrots, parsley, and parsnip. The chicory family also makes very
large and penetrating taproots.

Though seed for wild chicory is hard to obtain, cheap varieties of
endive (a semicivilized relative) are easily available. And several
pounds of your own excellent parsley or parsnip seed can be easily
produced by letting about 10 row feet of overwintering roots form
seed. Orchard grass and red clover can be had quite inexpensively at
many farm supply stores. Sweet clover is not currently grown by our
region's farmers and so can only be found by mail from Johnny's