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Helen, Unfortunately there is no replicable answer to this because for the large trays, usually reserved for seedlings I am growing lots of or are at a more advanced stage of growing on, I just use all sorts of recycled mix --- both bagged seedling mix as well as heavier, true soil (compost, potting soil etc). The Hellebores were rescued from moist packing as they were just starting to germinate and just spread out in the large tray willy nilly.