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Cover Crops and Green Manure

I ask a lot from my soil. The very same patch of soil may grow collards, lettuce, beans and turnips – all within one year. To keep the soil healthy and fertile, I have a simple regimen: homemade compost, the Bountea Compost Tea system and cover crops. Cover crops, or green manures, are a crucial element of sustainable organic cultivation -- not only for soil renovation, protection and nourishment but also to control weeds, disease and pests.

I always keep a few bags of cover crop seeds on hand: tiny grains of clover and alfalfa -- black triangular buckwheat -- the familiar cereals, barley and rye. Whenever I create a new garden bed, or a patch of soil looks depleted and tired, I reach for particular crops. If I plan to grow the same vegetable in the same place (not a recommended practice), a quick seeding of green manure between the two plantings creates a kind of speedy crop rotation -- adding nutrients and reducing diseases.

My longtime favorites are barley and white clover planted together very early in the spring. After I turn the crop over in late April, the soil will sparkle with new life and texture. In the hot summer months, a fast growing crop of buckwheat is useful to supplement soil phosphorus. As fall approaches, my fallow garden beds are sown with alfalfa and rye to clothe the soil during winter. There is a cover crop for every season and use.

Growing Cover Crops
Crops come in three main types: leafy fast growing grasses -- oats, rye, barley and various types of wheat; nitrogen-fixing legumes such as alfalfa, clover, vetches, peas and beans; and finally a few specialist crops such as buckwheat and mustard. The type you decide to sow depends on the season of planting, your particular soil conditions, and what you want to achieve.

The table below offers a short sample of the most easily available cover crops with their particular benefits and the conditions they prefer. Most are fairly hardy and easy to cultivate even in poor soils. When planting, I add a little compost and treat the soil and seedlings with Bountea. I sow seeds fairly thickly, water well and protect with a floating row cover in the early spring or shade in the heat of summer.

Cereals grow faster than legumes, so if you use a mix such as oats and clover or wheat and peas, keep the grasses cut at around 6” high so the legumes have light and space to develop. Do not let the crops grow too big before you turn them in. Rye, with its deep roots, can be very hard to dig if you let it get out of hand.

One important caution: never let cover crop go to seed. Many years ago, I prepared a new flowerbed with a planting of hairy vetch -- and let it get away from me. Now each spring I find straggling tendrils with pretty purple bells interwoven with my perennials. That is not so bad at my altitude but vetch and other crops can become irrepressible weeds in more temperate zones. Keep on top of your crops!

When you are ready to turn the crop into the ground, cut it all down and mash up the leaves with a strimmer or similar. Use a shallow fork to turn over the soil, covering up any green shoots and leaving the roots on the surface. Lightly break up the soil and spread with compost or organic mulch. After 10 days to 3 weeks, the crop will have decomposed and the bed is ready to replant.

Weed and Pest Control
If a portion of garden will not be used for a few months, a clothing of greenery is a perfect way to protect the soil; it also helps control or even eradicate weeds. Fast growing cereals or buckwheat crowd out other plants. Rye that is cut and left to lie on the ground, releases a weed-suppressing chemical. Once a crop is turned into the soil, any weed seeds will quickly germinate. The bare soil should be watered and then lightly hoed as the weeds grow. Replanting another crop chokes out any further growth and after a couple of similar cycles, the soil will be free of annoying seeds.

A low growing crop such as clover can be used as a living mulch. Inter-planted with fruits and vegetables such as asparagus, tomatoes or grape vines, these legumes reduce weeding, retain moisture and provide nitrogen. An added benefit is that clover or similar plants attract pollinating insects to your fruit flowers.

Recent research on the use of mustard as a cover crop shows that it not only provide plenty of organic matter, it also helps control root diseases and soil borne pests. It seems that the powerful organic chemicals in mustard essentially fumigate the soil when it is incorporated. If you are tormented by root-gnawing nematodes, a mustard crop may be just what you need. Mustard is particularly useful if you are preparing ground for potatoes.

Cover Crops and Soil Life
Cover crops have a powerful effect on soil life. The roots of the legume family attract and nurture Rhizobium bacteria that form nodules on the plant roots. These bacteria fix and hold nitrogen from the atmosphere. When the cover crop is incorporated into the soil, that nitrogen becomes available for the next crop in line.

Crops of the grass family quickly create a network of fine roots that penetrate deep into the subsoil drawing up essential minerals. These roots also attract and feed the bacterial soil life with minute drops of sap. Mycorrhizae link into the root fibers and send out masses of hyphae, tiny filaments that transport nutrients and create the sticky carbon glomalin that gives soil its texture.

When the cover crop is incorporated, sugars and nitrogen in the leaves boost bacterial activity to a high level. Soil fungi begin breaking down the fibrous cellulose into life enhancing humus. Worms and other members of the soil food web clean up the larger debris. The soil becomes a hive of activity.

This is not the best time to plant seeds for your next crop. Allow a little time for all the hustle and bustle to settle down. After a few weeks, the organic matter and humus stabilizes and masses of nutrients become available to your new plants. It is as if you just treated the soil with a healthy dose of premium compost – without the heavy lifting and carrying.

If you are not yet using cover crops, you are missing out on one of the most important methods of sustaining soil fertility and supporting plant health. Now is always a good time to start planning which cover crop to plant next. In this way you can continue to expand and enhance your existing program of organic soil management.

Cover Crop

Conditions

When to Sow

Benefits

Alfalfa

Prefers well-drained alkali soils

Spring or late summer

Fixes nitrogen - good for fallow ground - some varieties are perennial 

Barley

Prefers alkali soils – cold tolerant

Early spring and late summer

Weed-reducing -- fast-growing - lots of bulk – shallow roots

Buckwheat

Tolerates heat but not frost – adaptable to most soil

Late spring and summer

Accumulates phosphorus,
Fast growing

Field Peas

Intolerant of wet soils - withstands frost but not heat

Early spring or late fall

Fixes nitrogen - lots of bulk - easy to kill - decomposes rapidly - grows well with cereals
Fast growing

Hairy Vetch

Very tolerant - hard to eradicate if goes to seed

Late summer or fall

Fixes nitrogen - loosens compacted soil –

Mustard

Very adaptable

Early spring

Reduces root nematodes and fungal disease

Oats

Prefers clay loam and cool moist weather

Very early spring and fall

Weed-reducing – easy tilling – lots of bulk

Red Clover

Tolerates acidic poorly drained soil – intolerant of heat

Spring or late summer

Fixes nitrogen - attracts pollinators

White Clover

Tolerates drought, shade and heat

Any time

Fixes nitrogen – good living mulch – very hardy

Winter Rye

Prefers well-drained soil - adaptable

Fall

Protects soil in winter – deep roots - reduces weeds

 

Resources
Agricultural information: http://attra.ncat.org/attra-pub/covercrop.html
Mustard information: http://grant-adams.wsu.edu/agriculture/covercrops/pubs/eb1952e.pdf
Seeds:             Pinetree Garden Seeds – http://www.superseeds.com
Fedco Seeds – http://www.fedcoseeds.com
Main Street Seed and Supply - http://www.mainstreetseedandsupply.com/vsmustard.htm
Peaceful Valley -- http://www.groworganic.com/item_SCN755_Mustard_Mix_Lb.html