Maine Organic Farmers and Gardeners Association
Comfrey. Jean English photo
A fertility patch might include a row of comfrey (shown here) alternating with three rows of alfalfa. English photo

By Caleb Goossen, Ph.D.

Green manure cover crop demonstrations have been a staple of the Common Ground Country Fair since the ‘80s. That’s when the Fair was on rented ground, and MOFGA’s first organic crop specialist, Eric Sideman, began setting up cover crop displays in apple crates full of soil, for portability.

The crop rotation and soil building provided by green manure cover crops will always be a very important component of organic growing, which is why Eric and I will discuss green manure cover crops and key differences among species from 2 to 3 p.m. each day of the Fair. Come for a talk or just view the plots between the south orchard and the agricultural demonstrations as you walk by.

A few home gardeners, homesteaders and small-acreage farmers tell me that they sometimes struggle with giving up productive growing space to grow green manures, or they’re afraid to use the taller species because they might be difficult to kill or incorporate into the soil. Have no fear though; even tall species can be weed-whacked – or better yet, learn to mow them down with a scythe at one of the Common Ground Country Fair scything workshops.

An alternative soil building tactic that also benefits from scything abilities and can help feed the soil without taking crop beds out of production is called a “fertility patch.” I learned of this concept in Patrick Whitefield’s book “How to Make a Forest Garden,” although the broader concept no doubt has been suggested at other times and under other names. This approach will not be part of the green manure cover crop demonstration plots at the Fair, so I’ll describe it here. A relatively simple design suggested in Whitefield’s book is one row of comfrey alternating with three rows of alfalfa. Once the above-ground growth is established, you can mow it several times a season and mulch crops with the nutrient-rich, quickly decomposing material. (Just be sure you don’t plant comfrey in a place that you anticipate plowing or tilling in the future, as those practices will spread comfrey roots.)

Both comfrey and alfalfa are relatively low-maintenance, large-biomass producers with deep taproots that pull nutrients from the subsoil into their aboveground tissues. Alfalfa, a legume, can also “fix” atmospheric nitrogen (N) and incorporate it into its tissues, bringing otherwise inaccessible N to crop soil. Alfalfa and comfrey accumulate other important nutrients as well, and they add fresh organic matter to soils.

Having a legume such as alfalfa in a fertility patch is also important because every time you cut the top growth and carry it to your cropping space, a proportionate amount of roots slough off as leaves are no longer photosynthesizing to support them. Those N-rich legume roots slowly decay and release N, making it available to feed the comfrey.

A fertility patch can also be a helpful intermediary step when you don’t want to directly incorporate a nutrient source such as manure that may harbor pathogens or weed seed into crop ground.

A fertility patch can bring nutrients to crop ground beyond what a cover crop can, as you are in a sense “robbing Peter to pay Paul” by moving nutrients from one area to another. However, the approach doesn’t deliver all of the advantages of a green manure cover crop grown in place because it doesn’t include the impacts of green manure roots in soil (except in the fertility patch, of course). The root biomass of cover crops can be up to 30 percent that of the aboveground biomass, which can add a lot of organic matter to the soil profile. This is particularly important for low- or no-till growing. When roots – especially larger diameter roots – decompose in place, they can help reduce soil compaction and form preferential pathways where water, air and the roots of the next crop can infiltrate. Growing roots also constantly “leak” carbohydrates into the soil. These carbohydrates are a readily available energy source for soil microbes that help build soil structure, which also improves water and air infiltration and such physical characteristics as tilth.

Please join us to discuss how to fit green manure cover crops into your growing system and how these processes help build your soil.

About the author: Caleb is MOFGA’s organic crop and conservation specialist. You can reach him at [email protected].