Did a little experiment with mixes for starting seeds. The “Stone” medium on the left is an E.B. Stone “Seed Starter” Mix that I purchased. The bag says it contains primarily peat moss and perlite.
The “Alder” medium on the right is a mixture that I made up using two parts E.B. Stone “Seed Starter Mix” (so, peat moss and perlite), two parts compost (from the Miramar Greenery in San Diego), and one part dirt from my yard (sandy loam).
My aim was to see what effect including soil and compost would have. I sowed Corvair spinach and Gabriella onions in both mixes. Here is what the onions looked like after a few weeks:
Not much of a difference, right? Maybe the Alder mix seedlings are doing a little better.
The spinach seedlings showed more of a difference. See photo at top and here:
My general conclusion is that adding soil and compost had, if any effect, a beneficial one.
Why? Not sure. The spinach seeds in the Stone mix actually emerged a day earlier than the seeds in the Alder mix, but since then the seedlings in the Alder mix rocketed past the Stone seedlings. Maybe the soil and compost added nutrients and microbes that the germinating seeds didn’t use but that the baby plants since appreciated. Just a suspicion.
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Thanks. This is helpful. I’m looking at doing soil blocking and am considering a fancy seed starting mix from the East Coast but it’s $$$ (even without shipping). My go-to has been E.B. Stone but it doesn’t have compost. I think the reason to avoid biologically active organics is to reduce bacteria/fungal pathogens but it looks like compost actually helps! Thanks for sharing your results!