Growing Green Manure: A Fertilizer from the Garden

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 planted.

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 use.

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 possible.

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?

Establishment. The ideal crop is easy to es­tablish and grows rapidly.

Time of incorporation into the soil. How 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 insects. Some green manures can serve as nurse crops for useful insects. This is an emerging field of knowledge with much to be learned!

Cost. 

Shade tolerance. 

Ability to grow with the crop. 

Effects, including competition, on this year's crop. 

Beneficial effects on next year'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 sowing.)

Weed control. (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. 

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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. Beans are undersown to winter vetch. It is a dependable preceding green-manure crop for tomatoes.

Year 2. Tomatoes 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 3. Peas are 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 4. The 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 5. Sweet 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 suppression.

Year 6. Potatoes 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 8. Root crops 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).