Keeping the Subsoil Open with Green Manuring

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
Selected Seeds (see Chapter 5 for their address). Poppy seed used
for cooking will often sprout. Sown densely in October, it forms a
thick carpet of frilly spring greens underlaid with countless
massive taproots that decompose very rapidly if the plants are
tilled in in April before flower stalks begin to appear. Beware if
using poppies as a green manure crop: be sure to till them in early
to avoid trouble with the DEA or other authorities.
For country gardeners, the best rotations include several years of
perennial grass-legume-herb mixtures to maintain the openness of the
subsoil followed by a few years of vegetables and then back (see
Newman Turner's book in more reading). I plan my own garden this
way. In October, after a few inches of rain has softened the earth,
I spread 50 pounds of agricultural lime per 1,000 square feet and
break the thick pasture sod covering next year's garden plot by
shallow rotary tilling. Early the next spring I broadcast a
concoction I call "complete organic fertilizer" (see _Growing
Vegetables West of the Cascades_ or the _Territorial Seed Company
Catalog_), till again after the soil dries down a bit, and then use
a spading fork to open the subsoil before making a seedbed. The
first time around, I had to break the century-old plowpan--forking
compacted earth a foot deep is a lot of work. In subsequent
rotations it is much much easier.
For a couple of years, vegetables will grow vigorously on this new
ground supported only with a complete organic fertilizer. But
vegetable gardening makes humus levels decline rapidly. So every few
years I start a new garden on another plot and replant the old
garden to green manures. I never remove vegetation during the long
rebuilding under green manures, but merely mow it once or twice a
year and allow the organic matter content of the soil to redevelop.
If there ever were a place where chemical fertilizers might be
appropriate around a garden, it would be to affordably enhance the
growth of biomass during green manuring.
Were I a serious city vegetable gardener, I'd consider growing
vegetables in the front yard for a few years and then switching to
the back yard. Having lots of space, as I do now, I keep three or
four garden plots available, one in vegetables and the others
restoring their organic matter content under grass.

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