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Polyculture Pattern Language: Nitrogen Fixation Patterns

When Dave Jacke and I were writing Edible Forest Gardens, we discussed polyculture design extensively. We felt that we were beginning to see the vague outlines of an international pattern language of polycultures – a set of basic templates that might be used regardless of climate or the particular species in question.

Give the huge diversity of crop species and possible combinations, there may be an almost infinite number of polycultures grown in the world today. In my garden I grow many, and I’ve been fortunate to be able to travel and see many more. Reading and research have introduced me to a great number including commercial systems from around the world.

That pattern language is beginning to seem clearer to me. For example, though agroforestry systems including food forests are practiced around the world and in many climates, I only see a few basic patterns representing the majority of the use of nitrogen fixers. Here they are:

The nitrogen-fixing canopy

In this pattern, nitrogen-fixing trees are the tallest component. They may be dense as in shade coffee, or more widely dispersed as in Martin Crawford’s alder-dominated forest garden in England. Commonly used in production of shade crops from cacao to coffee, black pepper to kava around the world. Pros: Frees up all lower levels for food production. Cons: Shade cast by the nitrogen-fixers may limit productivity below; pruning of tall overstory trees. Crawford manages this by pruning his alders to minimize shade.

Bananas and coffee under nitrogen-fixing Trema overstory. Veracruz, Mexico.
Bananas and coffee under nitrogen-fixing Trema overstory. Veracruz, Mexico.

Nitrogen-fixing understory

Here the nitrogen fixation is provided by a shade-tolerant nitrogen-fixing ground cover. For example, many tropical orchards feature perennial peanut (Arachis pintoi), also known as forage peanut or pintoi peanut. Pros: allows all the rest of the system to be edible plants. Cons: no opportunity to grow crops in the lowest level.

Bananas with understory of nitrogen-fixing, shade-loving perennial peanut. ECHO, Florida, USA.
Bananas with understory of nitrogen-fixing, shade-loving perennial peanut. ECHO, Florida, USA.

Early-succession nitrogen-fixers

In these systems, an initial cover crop or other arrangement provides a strong pulse of nitrogen in the initial years, to be slowly replaced with food crops. Geoff Lawton has made this strategy rightfully famous. Pros: can also suppress weeds and reduce site establishment costs and labor. Cons: reduces food yields in early years by ceding space to fertility plants.

Food forest establishment with pigeon pea and sunn hemp. Earth Learning, Florida USA.
Food forest establishment with pigeon pea and sunn hemp. Earth Learning, Florida USA.

Irregularly intercropped nitrogen-fixers

In this system nitrogen fixing trees and shrubs are grown throughout the planting, either to grow as standards or as chop-and-drop coppice or heavily pruned plantings.

Food forest with apple, pear, stone fruits, lovage, and comfrey, intercropped with Siberian pea shrub as coppiced nitrogen-fixer. Central Rocky Mountain Permaculture Institute, Colorado, USA.
Food forest with apple, pear, stone fruits, lovage, and comfrey, intercropped with Siberian pea shrub as coppiced nitrogen-fixer. Central Rocky Mountain Permaculture Institute, Colorado, USA.

Alternating rows of nitrogen-fixers

These systems feature rows of nitrogen-fixing trees alternating with rows of crops. Alley cropping and contour hedgerows are examples, as are nitrogen fixing tree rows planted on keyline. This is also done with perennial crops, such as the case of banana rows alternating with coppiced legume rows. Pros: ease of management, including mechanized harvest of the crops. Cons: Nitrogen fixers are occupying prime sunny space that could be used for crops.

A contour line of alders intercropped with annuals (here a tuber crop of taro). Las Cañadas, Veracruz, Mexico.
A contour line of alders intercropped with annuals (here a tuber crop of taro). Las Cañadas, Veracruz, Mexico.

The nitrogen-fixing living trellis

In this design, productive vines are grown on nitrogen-fixing trees or shrubs. This system is commonly used in commercial plantings of passion fruit, black pepper, and yams. Pros: no need to construct a trellis. Cons: need to wait until trellis trees are established before planting vines; need for ongoing pruning of trellis trees.

Silk-tree mimosa serving as trellis for arctic kiwifruit. Paradise Lot, Massachusetts, USA.
Silk-tree mimosa serving as trellis for arctic kiwifruit. Paradise Lot, Massachusetts, USA.

Nitrogen fixers in pasture mix

In silvopastoral systems that incorporate livestock with crop trees, legumes are often sown as part of the pasture mix in the understory. Pros: seeding is an inexpensive establishment strategy. Cons: Lower density of legumes in grass-legume pastures can reduce overall nitrogen input on a per-acre or per-hectare basis.

 Sheep grazing on grass-clover pasture in understory of agroforestry planting including honey locust, black walnut, persimmon, mulberry, pear and more. Heifer Overlook Farm, Massachusetts, USA.

Sheep grazing on grass-clover pasture in understory of agroforestry planting including honey locust, black walnut, persimmon, mulberry, pear and more. Heifer Overlook Farm, Massachusetts, USA.

What other patterns of nitrogen fixation have you observed, or planted?

I feel strongly that an understanding of these “meta-patterns” can make design easier for all of us. I’m also posting an article here on permaculture news this week to share a crowdsourced effort to develop this pattern language.

To read more of Eric’s articles head over to his author page here.