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Self-renewing Fertility in Edible Forest Gardens: Part 2
Part 2 of a 2-part series. For Part 1, click here
This part continues the presentation of in-depth information on nitrogen-fixing and dynamic accumulator plants.
Diversify the Leaf Litter to Aid Nutrient Cycling
Research has shown that diverse forms of litter on the forest floor aid nutrient cycling in the litter layer and topsoil. Diverse litter provides for better decomposition and diversity in the decomposer food web. Therefore, using various kinds of mulch and planting plants that provide diverse kinds of litter will improve self-renewing fertility.
Grow Your Own Mulch
You can dedicate an area of your landscape to growing plants that produce prodigious amounts of organic matter and that regrow when cut, so you can use them as mulch sources. You can place this "mulch garden" around or downhill of your compost or manure piles to catch nutrients that leach away from the piles. Or place it in an area where runoff from neighboring lawns or gardens carries nutrients onto your land. The mulch garden will also catch and filter any toxic substances that wash onto your land, but it will provide you with a usable product nonetheless. In addition, you can create patches of mulch plants in various places throughout your garden, such as at the back of planting beds in harder-to-reach zones. Ideal plant candidates for mulch gardens include species that act as nitrogen fixers or dynamic accumulators and woody species that coppice.
Use Nitrogen-Fixing Plants
Nitrogen is one of the most important plant nutrients and often a limiting factor to plant growth. We can import nitrogen from off-site in many forms, such as blood meal, fish fertilizers, and other amendments. In forest gardening, we seek to reduce off-site inputs as much as possible by populating our ecologies with plants that "fix" nitrogen from the atmosphere.
Nitrogen fixation results from a symbiotic relationship between certain types of plants and bacteria. If the right strain of bacteria is in the soil near roots from the right species of plant, a "beneficial infection" of the roots occurs. Nodules form on the roots and provide sites for an exchange-the bacteria fix atmospheric nitrogen into a plant-usable form, and in return, the plants provide the bacteria with carbohydrates.
The amount of energy this exchange requires means that most of these plants can fix nitrogen only in full sun, or something close to it. As a result, the vast majority of nitrogen-fixing plants act as pioneer species in succession and do best in open, disturbed sites. They can colonize low fertility sites because they provide their own fertilizer. Nitrogen fixers typically exhibit many adaptations characteristic of pioneer species, including a tendency toward successfully spreading their seed around; that is, they can be weedy. They are often among the first on the scene after a disturbance. For instance, witness the success of the actinorhizal nitrogen fixer autumn olive (Eleagnus umbelata) as it takes advantage of numerous disturbed sites throughout eastern North America. Some actinorhizal species can stick it out longer in the successional sequence than most legumes and hence can grow in shadier situations.
Numerous studies have shown that nitrogen fixers make nitrogen available to surrounding plants through decomposition of leaf and twig litter. Root dieback can account for up to 50 percent of the nitrogen released. The nitrogen provided thus becomes available slowly, through decomposition, and is much less vulnerable to leaching away than are chemical fertilizers. This is particularly appropriate for tree crops, since too much nitrogen can cause fast, weak growth that is susceptible to late dormancy and winterkill as well as disease and insect problems. Easily available nitrogen fertilizer also tends to hold back succession, supporting competitor-strategist and ruderal plants.
These soil-building functions allow other, less tolerant species to recolonize and move the system toward a rich, diverse, and complex ecosystem. The nitrogen added to the ecosystem in early succession is critical for the long-term productivity of mature successional stages. At a community level, nitrogen fixation tends to decline with successional age due to decreasing sunlight availability.
Researchers recently demonstrated that nitrogen fixing trees and shrubs can fix amounts of nitrogen similar to such standbys as clover and alfalfa (90 to 150 lbs/ac/yr, or 100 to 170 kg/ha/yr). Interplanting with nitrogen fixers can increase the yields of orchard trees and the growth rates of timber trees.
Martin Crawford has done excellent research on the use of nitrogen-fixing trees and shrubs to provide the nitrogen requirements of fruit, nut, and berry crops. He has estimated ratios between the canopy area of mature nitrogen fixers to the canopy area of crop trees that provide the complete nitrogen requirements of the crop trees. He breaks tree crops into three categories based on nitrogen requirements.
In urban and suburban areas, polluted rainfall can deposit more nitrogen than the low-nitrogen demanding crops need, up to half the nitrogen moderate-demand crops need, and 40 percent of what high-demand crops need. Cut your nitrogen fixer ratio as appropriate in these regions until we clean up the air.
Use Plants as Nutrient Pumps: Dynamic Accumulators
Dynamic accumulators are another kind of fertility improving plant. Their roots reach down into and extract nutrients from the subsoil for their own use. The nutrients become available in the topsoil as the plants' foliage and roots decompose and leaching from their tissues occurs. Like nitrogen fixers, many of these plants colonize disturbed sites because their dynamic-accumulator abilities allow them to survive in nutrient-poor environments. Including dynamic accumulators in your garden is a useful strategy for creating self-renewing fertility.
Integrate these plants throughout the forest garden to offer their benefits to adjacent polyculture members. They also work well at the backs of beds, with crop plants in the more easily accessed zones. In England, many forest gardeners grow large patches of comfrey, nettles, and other important accumulators in "pockets of production" or mulch gardens outside their forest gardens. They then cut these plants for mulch or compost them. They also make them into fermented "teas" by submerging a large bunch of cut plants in a barrel of water until the leaves decompose into a slimy liquid, or the water turns a dark color (it also ferments and usually smells pretty bad). Once the tea is ready, you can water your garden with your own "green gold." Even if plants are not dynamic accumulators, you can use them to make such teas; rhizomatous weeds such as quackgrass are good candidates for drowning this way, since their roots can so readily grow even after being pulled. These techniques allow you to grow much of your own fertilizer. Of course, many dynamic accumulators also make fine edibles or perform multiple other functions.
Little research has been done on dynamic accumulators. It is highly likely that many more plant species are accumulators. Some plants that concentrate heavy metals are being used commercially to remove toxins from water and soils. Could we select and breed plants for improved accumulation of subsurface minerals, so they become improved sources of fertilizer for forest gardens? Only time and effort will tell.
This is Part 2 of a 2-Part series that was excerpted with the kind permission of the publisher and authors from:
Jacke, Dave, with Eric Toensmeier. 2005. Edible Forest Gardens, Vol. 2, Ecological Design and Practice for Temperate Climate Permaculture. Chelsea Green Publishing, White River Jct., Vermont. 654 pp.
Eric Toensmeier has studied and practiced permaculture since 1990. He has spent much of his adult life exploring edible and useful plants of the world and their use in perennial agroecosystems. He is the author of Perennial Vegetables and co-author of Edible Forest Gardens with Dave Jacke. Both books have received multiple awards. Eric manages an urban farm project for Nuestras Raices Inc., which provides immigrants and refugees with access to plots and start-up support on a 30-acre farm. He gives courses and presentations in English, Spanish, and Botanical Latin.