hairy vetch is a great nitro fixer
goumi and autumn olive are as well
i read somewhere that "scotch broom" is the best one.
alfalfa has a chemical i it that spurs growth also.
i just watched about 20 hours of videos on soil science.
it was very enlightening.
bare soil is the worst thing you can do, and grass next to fruit trees
also isnt a good idea, so, i use a cover crop on some
sweet potato is good. it can be planted several feet or yards away
and the vines will travel and cover the ground.
the leaves are also edible.
i use "chop and drop" technique.
when a nitro fixer gets large, i basically cut it back (coppice)
and drop the leaves on the ground under the fruit trees.
if you cut down and kill a nitro fixer, leave the roots in the soil
when they decay is where you get the best nutrients.
compost or compost tea is the best way to condition the soil
at least, that was my impression from the videos.
healthy soil organisms are the key to good growth, water retention, drainage, disease etc...
worm tea is as good, if not better.
i had a lot of extra compost worms from my bin
and put a few under my fruit trees - underneath the heavy mulch and compost layer.
i had explosive growth from my small loquat and papaya already.