This green manure was drill-seeded so that the entire field benefits from soil protection, increased organic matter and a bonus of nitrogen. (SHNS photo courtesy Maureen Gilmer)
These beautiful purple flowers are from a strain of hairy vetch under analysis by the USDA for improved green manure varieties. (SHNS photo courtesy Maureen Gilmer)
Long before the advent of synthetic plant foods, farmers had only two ways to make their fields more fertile. One method was to spread manure from livestock, which proved a labor-intensive method that dates back to the dawn of agriculture. The other option, known as “green manure,” doesn’t use real manure at all, but provides even better results.
Early on, farmers learned that their crops grew better where clover was present. Science later explained this phenomenon as nitrogen fixation, which is prevalent among all members of the pea family, particularly a group known as legumes. With these crops, nitrogen is not obtained from the soil like with other plants. Instead, these species draw atmospheric nitrogen into their leaves and send it down into the roots where it moves out into the surrounding soil.
Somebody got the bright idea of sowing clover all over a crop field in the fall so it could build up nitrogen over the winter months. By spring, these plants were rich with nitrogen throughout their stems and roots. When the time came to start the new garden, the cover crop was tilled into the ground so it decomposed, thereby infusing the soil with fresh organic matter and a bonus dose of nitrogen. This practice caught on and became known as “green manure.”
Today, sowing green-manure cover crops in the fall is a big part of organic gardening for the same reason it was practiced before commercial fertilizers. It works better for larger gardens where a tiller is used. The power of a tiller or rotovator is needed to chop the plants up as it turns the ground.
Green manure is an excellent way to improve soil on a larger site. Consecutive years of green-manuring have helped turn very poor soils into rich ground. It’s a super problem-solver where gardens are being created in heavy clay because, for example, the deep rooting of green-manure plants helps open up dense subsoils.
Those with newly built homes on infertile earth, on cut and fill sites, and on former forest ground, will find the ground lacking in nutrients. To make it suitable for vegetable crops and landscaping in the future, plant a cover crop this fall.
A great resource for learning all the benefits of green manure is GreenCoverSeed.com. This Nebraska-based website is focused on organic-market gardeners. It details some of the most common legumes, such as hairy vetch and crimson clover. Each plant has an extensive fact sheet.
Above all, the company offers seed for the amazing “Nitro radish” (Raphanus sativus), which produces such a deep fat root that it’s ideal for opening up superheavy clays. This is an alternative to what farmers call “deep tillage” for the enhanced drainage done with tractors and specialty implements.
Planting Nitro radish directly into the remnants of this year’s crops achieves similar results without disturbing the soil in a process called “bio-drilling.” The main root can reach 20 inches long, and its smaller taproot goes down 6 feet. Residues of this plant are well-known to release many nutrients, adding as much as 100 pounds of nitrogen per acre in the spring.
As your garden fizzles in these shorter days of fall, consider sowing an experimental crop on your garden ground. It’s the lazy gardener’s path to fertility. Green manures don’t let your ground lie fallow all winter, but enrich it.
Maureen Gilmer is an author, horticulturist and landscape designer. Learn more at www.MoPlants.com. Contact her at mogilmer@yahoo.com or P.O. Box 891, Morongo Valley, CA 92256.
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