Plant Green Manure to Boost Soil and Add Nitrogen

Credit: Holt Studios/Bob Gibbons, Michael Mayer, Nigel Cattlin

Here’s a quick, inexpensive way to build and aerate your soil and boost nitrogen in your garden

Fall is the perfect time to take advantage of Mother Nature and improve our garden soil. As we reap the harvest of the vegetable garden, we need to consider how to give back nutrients to the soil for the following spring. After adding compost, manure and store-bought soil amendments, the next step is to sow a cover crop, sometimes called green manure. While a cover crop improves the soil in many ways, a primary benefit is the addition of nitrogen, allowing the plants to make good growth in the absence of nitrogen fertilizer and in cold soil. Many cover crops are from the legume family, which have roots that are colonized by rhizobia bacteria that extract atmospheric nitrogen from the air and “fix” it into the usable nitrogen required for their own growth. When the bacteria are done with this nitrogen, it becomes available to the host plant. Typically legumes with these nitrogen-fixing bacteria have swellings or nodules on the roots that are soft. Leaving these roots in the soil will also leave a population of the bacteria for the next crop of legumes. Some soil may lack these beneficial rhizobia bacteria and may benefit from the addition of “inoculants,” commercially prepared rhizobia bacteria, available for purchase from cover-crop seed suppliers. Each legume species requires a specific species of rhizobia. Cover crops also build up organic matter when tilled or dug in at the end of the growing season, or make great compost or mulch if pulled from the soil or cut and harvested.

Finally, a cover crop offers soil protection from winter weather; stabilizing it and preventing wind and rain erosion, nutrient leaching and compaction. With all these advantages, it is easy to see why growing a cover crop is much preferable to leaving the soil bare. Cover crops may be sown on vacant soil, seeded between rows of winter veggies, or transplanted as seedlings into the empty areas of perennial or shrub borders. The crop you choose to grow depends on how early and harsh your winter is, what nutrients you are most in need of and, most importantly, whether you want to till in your green manure or simply cut it down to use as compost or mulch. All green manures will add valuable nutrients to the compost; so if you choose not to dig it in, simply cut and toss, leaving the roots in the garden bed. Sow cover crops as soon as the soil is bare. Although they are primarily planted in the spring or fall, you can germinate cover crops at any frost-free time of the year. For fall choose a winter-hardy grower, such as vetch or peas. It is miraculous how these cool-season growers can trap the energy of our winter sun and turn it into nutrients for our gardens. I’ve had great success simply broadcasting my seeds at the recommended rate and gently raking them into the top inch of soil. I follow up by watering in with liquid fish fertilizer; within two weeks I see lush growth.

As with any crop, be sure to rotate and alternate your cover crops. Yes, even cover crops can become victims of pests or diseases. Rotation will also ensure that the soil benefits from many different sources. One season it may get an extra boost of nitrogen, the next season it may get improved texture or extra calcium. Just be sure to note the main benefit of the cover crop you are growing and alternate it with others that have different qualities. For garden soil, variety in cover crops is the spice of life! Your cover crop will begin to fix nitrogen and nutrients in the soil immediately, and its roots will aerate and break up the soil. You’ll know it’s time to dig the plants in or cut them off as soon as they begin to show the first flower or tassel. Never let cover crops produce seed; the plant begins to rob the soil of nutrients at this stage. The exception to this rule is clover; the flowers are attractive and beneficial to bees. After you dig green manure into the soil, allow the soil to rest for three weeks before planting. If you choose the no-dig method and cut your crop, simply throw the tops into the compost and allow the soil to rest for a week before planting. Whichever crop or method you choose, you can expect better soil structure, fewer weeds and improved nutrients – all the results of wonderful planning and the art of green manure.

SHEENA’S BEST COVER CROPS

Red Clover

(Trifolium pratense) – hardy to zone 5 Winter-hardy, short-lived and a superb nitrogen fixer – this is considered one of the best green manures. Sow it in early spring through September. Attractive with its traditional cloverleaf and honey-scented flowers, it reaches a mature height of only 30 cm (12 in.), with a taproot three times as long. A food source for bees and shelter for beneficial ground beetles. This clover is a useful cover crop in summer or winter. If grown in an apple orchard, farmers report the trees will produce tastier fruit. Do not grow red clover with camellias or gooseberries because it harbours a mite that can cause fruit drop in the gooseberries and premature budding in the camellias.

Fava Bean

(Vicia faba) – hardy to zone 7 or 8 This is an excellent nitrogen fixer with an extraordinarily long taproot useful for breaking up clay or compacted soils. When tilled in, the leaves decompose rapidly; however, the fibrous stem will loosen heavy soils. It may also be cut and composted, leaving the nitrogen-fixing roots in the soil. Plant in early fall or early spring, as it thrives in cooler temperatures. For extra winter hardiness, seek out the Banner variety. As with all legumes, be sure to rotate crops within the garden. Pull the plants or till them in before they form pods.

Fall Rye

(Secale cereale) – hardy to zone 3 This cover crop suppresses weeds and prevents erosion and soil compaction. It grows well from fall through spring, when it should be tilled in. It is quite fibrous and should be tilled in at least three weeks before planting. It is an excellent soil amender and supplier of nutrients, particularly phosphorus.

Alfalfa

(Medicago sativa) – hardy to zone 5 Alfalfa fixes nitrogen and is an excellent weed suppressor. Its roots can go down 1.2 m (4 ft.) to reach nutrients deep in the earth and break up the subsoil. It can be grown year-round; it will die back in severely cold winters but should resprout in spring, when it can grow until it is time to till it in. It should be double-tilled to prevent resprouting. Alfalfa does not like water-logged or acidic soil.

Austrian Winter Pea

(Pisum arvense) – hardy to zone 6 This very hardy pea is an excellent nitrogen fixer. Sow in fall or early spring. The wiry stems can be tilled in, as they compost quickly. The crop provides a home for many beneficial insects, such as ladybugs and predatory mites. Till, or cut and compost, when flowers appear. This one can be companion-planted with fall rye.

Hairy Vetch

(Vicia villosa) – hardy to zone 4 This is an excellent weed suppressor and supplier of nitrogen. Winter-hardy and tolerant of poor soil conditions, this can grow where no other cover crop can survive. Sow in fall or spring and till or dig in at the first sight of blooms.

HEAT UP YOUR COMPOST FOR WINTER

When the temperature outside cools down, so does the heat of our compost piles. It seems the microorganisms and beneficial bacteria like to take a winter sleep – I call it “compost hibernation.” At the same time, fall and early winter are when most compost piles are at their full capacity, with clippings, fallen leaves and leftovers from the vegetable garden. Here are a few things you can do to turn the heat up in your winter compost pile. 1. Come fall, before you start loading the pile up, remove up to three-quarters of any completely composted material and add it to the garden. The one-quarter you leave behind is to ensure there are lots of beneficial bacteria, worms and microorganisms left to start the new pile. 2. When adding kitchen and garden waste, give them an extra chop. Starting with smaller pieces will give a faster finished product. 3. Be sure to alternate dry waste, such as newspaper or shredded fallen leaves, with wet waste. 4. Every winter month add Winter Compost Booster (page 34) to the pile. This is a good time to note the dampness of your pile; if it seems too dry, water it with a liquid fish or kelp fertilizer at the recommended rate. This not only adds nitrogen to heat the pile up, it also contributes valuable trace elements and minerals to your finished compost. 5. Increase your population of red wriggler worms; they are ferocious eaters and will break down a pile quickly. 6. If you live in an area where your compost bin freezes, consider burying half of it in the ground or surrounding the bin with bales of straw. Although it doesn’t hurt the compost pile to freeze, the warmer it is, the faster it will break down. 7. Place a glass window sloping south on your bin to trap solar energy and increase the temperature. 8. None of us want to slog through wet or snowy weather to get to our compost piles, so keep a portable composter on the back porch or in the garage to ensure that all kitchen scraps are recycled and not thrown in the trash. 9. The final strategy of winter composting is simple: be sure to blend your compost every two weeks. A good mixing up is essential for faster breakdown. Use these helpful tips for your winter composting and come spring, you’ll have a fresh blend of compost to get your garden off to a good start.