Growing Green Manure: A Fertilizer from the Garden

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As Coleman's planting of corn undersown with sweet clover shows, gardeners can have their food and soil fertility too.

Eliot Coleman, farm manageof MiltoAcademy’s Mountain School Program in Vermonthas been a leading practitioner of organic growing for 20 yearsThifall bringthpublication of his first book, The New Organic Grower–atreatise of grow­ing techniquethat‘s thbest market­-gardening guide we’ve seen in manyearColeman’s book hamuch useful information that can be adapteto home gardens as well, such as the following excerpt on innovative ways to combinboth soil-building and edible crops.

Not all crops are for sale. Green
manure crops are grown not for cash but to contribute to the care and feeding of the
soil. A
green-manure crop in­corporated into the soil improves fertility, but the eventual benefits are even greater.

Green-manure crops help protect against erosion, retain nutrients that might otherwise
be leached from the soil, suppress the ger­mination and growth of weeds, cycle nutrients from the lower to the upper layers of the soil, and–in the case of legumes–­leave to the following crop a considerable quantity of nitrogen. Other contributions of a green manure are improved soil structure, additional organic matter, enhanced drought tolerance and increased nutrient availability.

The value of green manures has been ap­preciated since the earliest
days of agricul­ture, yet the full potential of green-manure use is still underappreciated and unexploit­ed. Growing green manures has traditional­ly been viewed as an
either-or situation. You grew either a paying crop or a green manure. If the use of green manures means replacing a cash crop, then the lack of interest in them is understandable. However, there are other options.

There are three ways in which green­-manure crops can be managed: as overwinter crops, main crops and undersown crops.

First, they can be sown for overwintering after a market crop has been harvested. For example, a leguminous green manure
could be sown after pea harvest and would occupy the ground until it was tilled
in the follow­ing spring. The benefit from a wintered-over legume that provides ideal
growing condi­tions for next year’s crop is a strong incen­tive for growing it.

In the second case, the green manure oc­cupies ground during the
growing season in place of a market crop. If extra land is avail­able, this is a highly recommended
practice, and when the green-manure crop can be grazed by livestock, it
serves a double pur­pose. If a grower prefers to put all the land into market crops,
however, a choice must be made between losing the future benefits of the
green-manure crop to the potential in­come from a market crop. Since this is a choice
that usually goes against green ma­nures, often at the expense of the soil, I
recommend a third management option, one that allows the grower to have a
leguminous green-manure crop and the cash too: undersowing.

Undersowing, also known as overseeding or companion seeding, is
the practice of growing a green manure along with the mar­ket crop. When done
correctly, undersowing provides the best of both worlds. It is estab­lished
practice in small-grain growing. The clovers or other legumes are sown with or
shortly after the wheat or oats, for example, and grow slowly in the understory
until the grain crop is harvested. In vegetable grow­ing this practice is not common, to the best
of my knowledge, and only recently has be­gun to be seriously considered.

Timing: When to Undersow

The advantage of undersowing is that the green-manure crop is
already established at harvesttime. In our northern New England climate, winter
rye is the only green manure that can be seeded after fall harvest. A leg­ume
cannot be established that late in the sea­son. Since in my experience legumes are the most beneficial green
manures, I try to use them whenever possible. The only way I can do that
without taking land out of cash-crop production is to undersow them.

The practice of undersowing is something like planting desirable
weeds between the crop rows. In a way that is very similar to the relationship
between weed competition and crop growth, the effect of the undersown plant–the
deliberate weed–upon the crop plant depends upon the age of the crop. Weeds can overwhelm young
crops if they both start at the same time. Weed research has shown that crops
will do fine if they have an adequate head start. If most crops are kept weed-free
for the first four to five weeks af­ter establishment, later competition from
low-growing weeds will have little effect on them. If we interpret that correctly, then the best crops for
undersowing would be low-growing, and the best sowing date for green manures
would be four to five weeks after the establishment of the crop plants. My ex­perience
bears that out.

Where timing is important, there is a tend­ency to err on the
safe side. Why not wait six weeks or more before undersowing the green manure, just to be sure? The
problem is that then the balance is tipped too far in the other direction.
Since the undersown “weed” is deliberate, I want to be sure it grows.
If I wait too long before undersowing, the crop plants will be large enough to
overwhelm the green manure. The trick is to undersow when the crop plants are
well enough along not to be adversely affected by the undersowing, but not so
well established as to hinder growth of the undersown green manure.

How does this timing work out in actual practice? Here in
Vermont, where our springs are cool and crops such as corn, beans, squash and
late brassicas are not set out until June 1, I find the Fourth of July to be
just about perfect, year in and year out, as the date for undersowing to those
crops. Obviously, later crops or succession crops will have their own dates. In all cases, the four-
to five-week delay has proven to be a reliable yardstick.

Preparing the Seedbed for Undersowing

Successful undersowing requires a clean, weed-free seedbed.
Sowing the green manure is no different from sowing the crop: When seeds are
planted into a weedy mess, they be­come the seeds for failure. I have often thought that
another side benefit of under­sowing is that it motivates the grower to pay
attention to clean cultivation right from the start, simply because there is
one more rea­son to do so. Like any problem “nipped in the bud,” weeds are
easiest to control early in the season. The clean seedbed prepared for undersowing is a by-product of early weed control. At least three cultivations should be made prior to undersowing, the last one just a day or two beforehand.

The goal of the grower is to provide every opportunity for the undersowing to get well established without weed competition. Un­less the garden has a lot of weed pressure, the canopy of the undersowing
will join with the crop canopy to keep later weeds from germinating. The few that do pop through should be pulled before they go to
seed. Occasional forays down the rows will keep these competitors from becoming a problem.

Seeding Undersown Crops

I have seeded undersown crops both by broadcasting and drilling, and I emphatically recommend drilling. If the undersown crop is broadcast in the standing market crop, there is no way to cover all the seeds to en­sure
their establishment and germination. In a hot, dry spell, the green manure can be a total failure. When I use a cultural
practice, I want to be able to depend on its performance. If the undersown crop is drilled be­tween the crop rows with either a single-­row or multi-row drill, the seeds are planted at the proper depth, in contact with moist soil where they are much more certain to germinate.

The single-row drill is the same garden seeder I use for corn, carrots and peas. It is equipped with an appropriate plate for whatever green-manure seed
I am planting. The multi-row drill consists of five of these single-row seeders bolted together side by side, with common axles and a common push bar. That gives the tool a
total width of 20 inches, which fits nicely between the 30-inch row at which corn, beans and brassicas are

When using the multirow model, all five hoppers can be filled with either the same seed or different seeds. Each hopper can be fitted with its own seed plate. Under some conditions, the grower might want to alter­nate rows of different legumes or legumes and grasses.

Planning a Green Manure Crop

Sowing dates and equipment for under­sown green manures should be
as well thought out as those for the cash crops. Sow­ing dates should be marked
on the calendar. The seeds should be ordered ahead. The equipment should be efficient, in good work­ing order
and simple to

Obviously, green manures are most effec­tive when they are
considered as an impor­tant component of the crop-rotation planning. There is
another parallel between green manures and crop rotation that should be noted.
Variety in green manures is as im­portant as variety in the market crops. Be­cause green-manure plants also
have different faults and virtues that affect the soil and following crops in
different ways, green manures should be rotated to include as many varieties as

Principles of Growing Green Manure

Green-manure varieties and combinations are endless and are not
limited to the ones listed here. The varieties I mention worked for me as I
developed the biological produc­tion techniques for my particular soil and cli­mate.
But while I’ve shared my own specific, regional choices for the area of Vermont
in which I garden (see the sidebar “Coleman’s 8-Year Garden Rotation”),
I want to emphasize principles that are more nearly universal–not only because
different parts of the country require different green manures but because
there are no hard-and­-fast rules.

Time of seeding. Early,
late, intercrop, un­dersown, overwinter, year-round?

EstablishmentThe ideal crop is easy to es­tablish and grows rapidly.

Time oincorporation into thsoilHow ma­ture is the green manure? What is the follow­ing crop, seed or transplant? Legumes turned
under in fall lose 70% of added nitrogen but only 38% when turned under in
spring. With a winter-killed green
manure it may be pos­sible to transplant the spring crop directly without incorporating the
green-manure residues into the soil.

Rotational fit. The green manure should
not share diseases with the crop plants.

Soil microorganisms. Rape,
for example, stimulates the biological activity of the soil. Soybeans improve scab
control in potatoes.

Beneficial insectsSome green manures can serve as nurse crops for useful insects. This is an emerging field of knowledge with much to be learned!


Shade tolerance.

Ability to grow with the crop.

Effects, including competition, on this yearcrop.

Beneficial effects on nexyear‘s crop.

Erosion control.

Winter hardiness(In some situations, a crop that is winter-killed is preferred, to avoid having a vigorous residue in the way of an early spring

Weecontrol(Rapid growth and broad leaves are a plus.)

Remember, although it is possible to present the broad outline of a biological sys­tem inside a book, the
fine-tuning that goes on within that outline is the province of the grower. The best innovations and
improve­ments usually come from
the grower and not from any chart or list, no matter how com­plete it supposedly is. Whatever an expert
does or does not say should not limit your op­tions. The more active a grower becomes
in taking charge of perfecting the system pro­posed here, the more independent, reliable and sustainable his
or her own system will become. 

Spacing Undersown Crops: Examples

The garden is most easily visualized as a series of long strips five feet wide and 100 feet
long. Forty-eight inches of that width is given over to the crops, and 12 inches is used as an access path. For crops such as carrots, onions and lettuce planted in
l2-inch rows, there are four rows side by side with a l2-inch spacing.

12- to 18-inch spacing. The
one-row seeder can be used to drill three rows in the path. A single un­dersown
row can be drilled between each crop row. In this case, I usually sow dwarf
white clover in the paths. White clover or biennial sweet clover can be
used between rows of onions or carrots. The rows spaced at 18 inches are similarly undersown, with three rows in the
path and one row between the crops.

30-inch spacing. For the
30-inch spacing at which corn, beans, brassicas and so on are planted, the
five-row drill is used. One pass is made down the center between each crop row. Depending on the crop,
dwarf white clover, sweet clover, red clover, hairy vetch and soybeans have all
worked well between these crops.

60-inch spacing. In the
crops spaced at 60 inches (tomatoes and melons, for example), two passes are made with the five-row drill. Dwarf white, red,
and sweet clover are all good choices here.

120-inch spacing. At the
widest spacing, 120 inches for pumpkins and winter squash, everything except a
strip about two feet wide on either side ofthe row is drilled (four passes with
the five-row drill). My favorite undersown crop for squash and pumpkins is
biennial sweet clover. 

Coleman’s 8-Year Garden Rotation

Undersown green manures are used extensively within my eight-­year crop rotation. The
following sequence has worked out very well in practice. It is described here both verbally and visually to show how all
the pieces fit together.

Note that six of the eight rotational plots are undersown, a
seventh is sown to legumes after early harvest, and only one–­potatoes–is
seeded to rye after fall harvest. The ground is never bare. The soil is always
growing either a market crop or next year’s fertility. For much of the summer it is grow­ing both!

Year 1. Beanare undersown to winter vetch. It is a dependable preceding green-manure crop for

Year 2Tomatoes are undersown to oats or some other non-winter-hardy grass crop. Certain
grasses have been found to be excellent preceding crops for legumes such as
peas, since they produce an allopathic effect that suppresses grasses and other weeds but not legumes. It is important to choose
a non-winter-hardy
cultivar so there will not be a mass of fresh growth in the spring to impede ear­ly soil preparation and planting of the pea crop.

Year 3Peaare not undersown but are
followed by a mix of clovers as soon as the peas can be cleared. This combination of legumes grows un­til it is turned under the following May, by which time enough
nitrogen has been fixed to ensure a splendid crop of brassicas.

Year 4The cabbage family is undersown to sweet clover, which is one of the best leguminous green manures to turn
under for next year’s corn crop. It grows well under the cabbage family because it is a taprooted
crop that does not seem to interfere with the more shallowly rooted brassicas.

Year 5Sweet corn is undersown to soybeans be­cause research shows a soybean crop almost total­ly inhibits potato-scab organisms in the soil. The soybeans also grow well in the understory of the corn and provide excellent weed

Year 6Potatoes cannot
be undersown easily if the cultivation method used is hilling. I have grown
potatoes without hilling by planting at a depth of six inches and filling the
furrow partially at first, then completely after the potatoes emerge. Vetch can then be planted
as an undersown legume. If the green manure is to be established following the potato harvest, winter rye is probably
the best choice.

Year 7. Squash is undersown to sweet clover in the empty strips between the squash rows. Beets, carrots and other root crops grow very well follow­ing sweet
clover. The onion crop, on the other hand, has always grown best with no
preceding green manure, so the onions are planted in the strips that were
occupied by the squash plants themselves.

Year 8Root
are undersown to dwarf white clover (both in the paths and between the rows, because they will grow in the crop
understory and because they provide good erosion protection for the soil over winter).