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.