How close do nitrogen fixing plants need to be?

I am trying to get my Spring tree order together and thinking about ordering some Siberian Pea Shrubs to put near a couple of my apple trees. How far away should I plan to put them? I am sure the info is in Gaia's Garden, but I've scanned the book again and can't find it. Would I be better off with a N-fixing groundcover that can be planted directly under the tree? I was hoping for the Siberian Peas to feed to the chickens...

Howdy Renee, welcome to Permies. Great question, I would like to know what the answer to this is also.

From what I've seen and read about... it depends.

Mostly it depends on:
- which nitrogen fixers you plan to use
- what you are trying to accomplish
- how you're planning to manage the overall system

Example:
If you are planning to plant a new siberian pea shrub under an apple tree, specifically to benefit that individual tree.
Then plant it in the same hole, or as close as possible to an existing tree.
Chop & drop the pea shrub when it starts getting too tall for you.
Using that method, the pea shrub should stay a small shrub, and continue to pump nitrogen into the soil.

Rule of thumb from the arborist literature is that tree roots extend 1.5 to 3 times the drip line, depending on soil texture... trees reach farther in sandy soil, perhaps due to the poverty of water and nutrients, but also ease of foraging. Also feeder root density tends to be highest around the drip line. I've heard others offer Matt's suggestion as well, planting a N fixer in the same hole, and just pruning it to allow the target tree to keep dominance for light. There is a goof G.Lawton video where he talks about a shifting dominance of soil builders to crop plants over the maturation period of a food forest (first 20 years), so at the beginning N-fixers are everywhere, annual, forb, shrub AND tree. Over time human intervention suppresses and recycles the soil builders in favor of the crop plants.

So I'd tend to plant both herbaceous and woody N-fixers to start a planting (locally I use cottonwood, willow, clovers, goumi, lupine, alder). I am still getting my head around processing and managing the quantity of wood that this can generate! This years brush piles become next years plantings. If you are water limited and not particularly nutrient limited, you may have to think through how you reduce competition for water (either by water harvesting, grey water, or timing your slashing to reduce summer leaf area in you n-crop).

EDIT - here's my Lawton groupie page with that video

the guys in the NW plant em in the tree hole. they have excellent results.

there is a discussion here somewhere....