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
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