I came across this video of a grower way up north who does the huge veggie thing...his secret is the compost tea....some of these are ridiculously sized...maybe some ideas for your onion project?
I am setting up a new compost tea approach this year...focusing on a mix of my normal manure and greens compost half and half with a hardwood mulch compost...from what I have been reading, I was getting the good bacterias, just not so much the good fungal side of the spectrum...which the hardwood mulch compost will provide.
With the root systems on an onion being so small and shallow, I wonder if they would benefit greatly from mycorrhizal fungi supplements? It would allow the small root system to intake many more nutrients. We used this technique on some fruit trees and pumpkins with amazing results...it fosters a great symbiotic relationship which helps the roots draw more nutrients in, and also makes more of them that were previously unavailable to the plant reachable. Might be worth a test on a few of them to see. Mycorrhizal fungi supplements are available at most garden centres.
