I haven't tried soil additives but have always used the same compost for all my plants but only certain plants are affected, onions are the worst that get affected, and carrots and parsnips, cabbage and broccoli as well. My other plants are never affected by them. Somebody once mentioned that coffee granules are a good way off keeping root maggots out and others have mentioned tobacco can be used as a pesticide, I've tired both them but nothing works. I'd be interested in more soil additives that I could try.
