Overstory #244 - Self-renewing fertility in edible forest gardens
The forest-gardening approach to fertility emphasizes strategies employed in the design phase that should reduce the need for work and expensive inputs later on down the line. Many basic characteristics of forest gardens support self-renewing fertility by their very nature: perennial plant roots provide consistent root-zone resources to the soil food web; lack of tilling allows undisturbed development of the soil organism community; consistent mulch provides stable food resources for the decomposers and a stable soil environment for everyone who lives down there; and so on.
While all of the following strategies are important for creating self-renewing fertility, this section provides in-depth information on nitrogen-fixing and dynamic accumulator plants.
Use Site Analysis and Assessment and Niche Analysis
To create self-renewing fertility, you must begin with a careful assessment of your soils. Make sure to assess nutrient levels in both your topsoil and your parent materials so you can determine long term fertility prospects and needs. Also, evaluate the soil profile so you can identify possible problems of compaction, high water tables, and other factors that may influence the plants' belowground resources. Desired species niche analysis tells you which soil conditions your plants prefer; The site and niche analyses will guide you in developing a garden design and a site preparation plan that will in turn determine how to use the strategies listed below.
Maximize Root Penetration of the Soil Profile
Making sure that plants can get their roots deep into the soil profile gives them the greatest access to the most soil resources. It allows plants to absorb nutrients· leaching into the deep soil and those released by weathering of the parent materials. This aids nutrient recycling and conservation and ultimately enriches and deepens the topsoil as the plants drop leaves on the surface and their roots die throughout the soil profile.
Create Healthy Soil Food Webs
With proper care, subterranean micro herds and fungal allies offer tremendous nutritional benefits to our plants. To receive these benefits we must treat them well by minimizing compaction, adding organic matter, and minimizing tillage and other forms of soil disturbance. Perennial plants help sustain a year-round root-zone soil food web capable of storing more nutrients than can the decomposer food web by itself Mycorrhizas are especially important and beneficial for plants and for nutrient storage and transport.
Fertilize the Way Nature Does: From the Top Down
By mimicking natural fertilizing processes we enable the ecosystem to develop nutrient processing pathways that will run properly even when we aren't around to do the work. This means using nutrients in the form of organic matter and rock powders whenever possible, and applying them appropriately. Though it may be slower, feeding the soil from the surface with organic materials creates the biological infrastructure the ecosystem needs to function properly. Placing organic matter or compost deep in the soil profile, however (say, in the bottom of a planting hole), can make matters worse: less oxygen is available deeper down, so the organic matter often decomposes anaerobically. This in turn can kill roots and beneficial soil organisms. Mulching and fertilizing the soil surface allows leaching, plant roots, and decomposers to act naturally, reduces disturbance to the soil profile, and works in harmony with natural soil-building processes.
Apply Mulch, Compost, and Amendments Appropriately
Mulch is one of the most critical components of forest gardens. A mimic of the litter layer found on forest floors, mulch has numerous benefits. It is not without its drawbacks-notably the cozy habitat it provides slugs and voles-and one must apply it to an appropriate depth.
Compost provides organic matter and nutrients and also inoculates the soil with beneficial organisms. However, compost is usually expensive. It is best mixed into the soil during site preparation: or used as a top dressing after planting, if the soil needs much help. Never fill tree-planting holes with compost, as it can lead to "pot-bound" roots and anaerobic soil conditions. Always mimic natural soil profile structures by spreading compost on the soil surface, or at most mixing it into the topsoil, then covering it with mulch.
Some perennial gardeners pull back their mulch, add 1 to 2 inches (2.5 to 5 cm) of compost to their gardens, and then remulch-every year. This certainly creates a healthy garden, but it is not self-maintaining, self-renewing fertility by a long shot. If the soil is in rough shape, yearly additions of compost might make sense until the plants get well established and serious nutrient cycling and conservation get going in the ecosystem. Otherwise, an early boost, followed by mulch, should be enough. It can take five to seven years, or more, for forest gardens to firmly establish their own nutrient cycles. However, with good design using fertility-building plants, the garden will get there eventually, making its own compost every year with no effort on your part.
For greater amounts of nutrients, and to make up for serious nutrient deficiencies, you can use a wide variety of organic soil amendments. Again, we should most often use these as an early system establishment boost until the garden cycles get going. In areas with some combination of high rainfall, warm winters, nutrient-poor parent materials, and degraded top soils, you may need to add amendments every few years to account for leaching losses, even after the nutrient cycling and conservation system gets going.
Create Lush Vegetation and Abundant Organic Matter
Plants and organic matter play key roles in the anatomy of self-renewing fertility, co creating a dynamic, nutrient-conserving system. More vegetation means more water transpired from the soil, which reduces nutrient leaching. It also means more nutrients built into biomass, which then becomes organic matter. More organic matter means more cation exchange sites and, thus, more room in the soil to store nutrients.
Emphasize Aggrading, Midsuccession Habitats
The aggradation phase of succession is when ecosystems exhibit the greatest control over nutrient flows. The aggradation phase is also when the ecosystem most rapidly converts the greatest amounts of nutrients and sunlight into biomass and builds other forms of natural capital. Emphasizing the aggradation phase will therefore maximize nutrient conservation, storage, and recycling. This means creating midsuccession habitats ranging from old field mosaics to sun loving pioneer-tree woodlands. Since most of our best crop trees and shrubs are competitor-strategist plants adapted to such environments, we are in luck. We just need to build the complete ecosystem with diverse components to make it work.
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, such as willows, alders, hazels, and so on.
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 article 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.
Book orders: Chelsea Green Publishing, 85 North Main Street, Suite 120, White River Jct., Vermont 05001, USA; Orders: 800.639.4099; Offices: 802.295.6300; Fax: 802.295.6444; or order from Edible Forest Gardens.
About the Authors
Primary author Dave Jacke has been a student of ecology and design since the 1970s, and has run his own ecological design firm-Dynamics Ecological Design-since 1984. Dave is an engaging and passionate teacher of ecological design and permaculture, and a meticulous designer. He has consulted on, designed, built, and planted landscapes, homes, farms, and communities in the many parts of the United States, as well as overseas, but mainly in the Northeast. A cofounder of Land Trust at Gap Mountain in Jaffrey, NH, he homesteaded there for a number of years. He holds a B.A. in Environmental Studies from Simon's Rock College (1980) and a M.A. in Landscape Design from the Conway School of Landscape Design (1984). You may reach Dave by email at: firstname.lastname@example.org
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.
Related Editions of The Overstory
- The Overstory #241: Forests for food and nutritional security
- The Overstory #216--Introduction to temperate edible forest gardens
- The Overstory #140--Nitrogen-Fixing Plants (Temperate)
- The Overstory #135--Medicinal and Aromatic Plants in Agroforestry
- The Overstory #128--Wild Foods
- The Overstory #119--Five Fertility Principles
- The Overstory #117--Between Wildcrafting and Monocultures
- The Overstory #106--The Hidden Bounty of the Urban Forest
- The Overstory #71--Nontimber Forest Products (temperate)
- The Overstory #47--Coppice-with-Standards