I prefer a very coarse seed starting mix. If necessary, I'll often broadcast seeds on a mix like these

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and topdress with a thin cover of peat or Turface fines left over from the screening process. Roots love a highly aerated, damp (not wet) mix.

Al

Thanks for the info, Tapla. This is for a community garden and I'd like to try to make use of what's already available. Budget is limited so I want to avoid purchasing anything else.