Hi, I don't grow veg, but I would think that mushroom compost would be too rich for onions, and you would just get loads of top growth, at the expense of the bulb, I think it would be much more use spread on your flower beds, Derek.

Hi, I don't grow veg, but I would think that mushroom compost would be too rich for onions, and you would just get loads of top growth, at the expense of the bulb

Sorry about the double post, Derek.

Most mushroom compost is actually spent so has very little goodness and best used as a soil conditioner or a mulch. Has added gypsum (chalk) to make it sweet, rather neutral ph. Would be best to use sieved top soil mixed with perhaps blended stable manure or your own compost. Do make sure that you dig over the sub soil as onions feed quite deeply. I do assume that you mean onions from sets rather than spring onions.