Green Manure to Boost Soil and Add Nitrogen-fixing
(Sheena Adams GardenWise online, September 2011)
Fall is the time to give back nutrients to the soil. After adding compost, manure and store-bought soil amendments, sow a cover crop (sometimes called green manure).
Cover crops add nitrogen, which allows 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. Leaving these roots in the soil will also leave a population of the bacteria for the next crop of legumes.
Cover crops 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. They offer soil protection from winter weather; stabilizing it and preventing wind and rain erosion, nutrient leaching and compaction.
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 what nutrients you are most in need of and, whether you want to till in your green manure or simply cut it down to use as compost or mulch.
Sow cover crops as soon as the soil is bare. Distribute seeds at the recommended rate and gently rake them into the top inch of soil and water. You can also water with liquid fish fertilizer. Your crop will begin to fix nitrogen and nutrients in the soil immediately, and its roots will aerate and break up the soil. 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 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.
Red Clover
(Trifolium pratense) Winter-hardy, short-lived and a superb nitrogen fixer – considered one of the best green manures. Sow in early spring through September. Food source for bees and shelter for beneficial ground beetles. This clover is a useful cover crop in summer or winter.
Fava Bean
(Vicia faba) Excellent nitrogen fixer with extraordinarily long taproot useful for breaking up clay or compacted soils. When tilled in, leaves decompose rapidly; and the fibrous stem loosens heavy soils. May also be cut and composted, leaving the nitrogen-fixing roots in the soil. Plant in early fall or early spring. Pull or till plants pods.
Fall Rye
(Secale cereale) Suppresses weeds and prevents erosion and soil compaction. Grows from fall through spring, when it should be tilled in. Quite fibrous and should be tilled in at least three weeks before planting. Excellent soil amender and supplier of nutrients, particularly phosphorus.
Alfalfa
(Medicago sativa) Fixes nitrogen, suppresses weeds. Roots can go down 1.2 m (4 ft.) to reach nutrients deep in the earth and break up the subsoil. Can be grown year-round. Double-till to prevent resprouting. Does not like acid soil.
Austrian Winter Pea
(Pisum arvense) Excellent nitrogen fixer. Sow in fall or early spring. Wiry stems can be tilled in, as they compost quickly. Provides a home for many beneficial insects, such as ladybugs and predatory mites. Till, or cut and compost, when flowers appear. Can be companion-planted with fall rye.
Hairy Vetch
(Vicia villosa) Excellent weed suppressor and supplier of nitrogen. Winter-hardy and tolerant of poor soil conditions. 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.