Have a lot of old woodchips and want to make potting soil for tree seedlings

I have a couple large piles of old woodchips and one of soil from under the woodchip piles.  
I want to turn this into quality potting soil for seedling and grafted trees.
I was thinking of adding alfalfa pellets and SEA-90, along with some top soil.  Doing this will create several years worth of potting soil.
I keep turning these piles to control weeds. (nice to have a small tractor)

Any suggestions?  Should I mix all now and cover with a tarp so all the nutrients don't wash away, or should I just do the mixing as needed?

I'd take a walk through your nearby woods, collecting at least one of every mushroom type you see, then bring them home, crumble them up and mix them into the pile. My wood piles that get good and myceliated break down into a compost I want to use much faster than sort of conventional composting. (I guess I can't speak how the substantial difference in our climate might change best practices...)

Sifting it and mixing it with alfalfa was what I thought of immediately.
With the huge amount you have, and because it can already grow weeds, I wonder you could sift, fill a bed with that sifted material and plant it with inoculated legumes, thus gowing your nitrogen into your potting soil.

William Bronson wrote: Sifting it and mixing it with alfalfa was what I thought of immediately.
With the huge amount you have, and because it can already grow weeds, I wonder you could sift, fill a bed with that sifted material and plant it with inoculated legumes, thus gowing your nitrogen into your potting soil.

I would like to keep it in piles to conserve space.  I should be able to flatten them out but they would still be 3 foot tall.