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 in both mixes. My observation is that adding soil and compost had a beneficial effect. Wouldn’t you agree?
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 have rocketed past the Stone seedlings. Maybe the soil and compost have added nutrients and microbes that the germinating seeds didn’t use but that the baby plants have since appreciated.
Why do I suspect this? Here’s a video of Karl Hammer of Vermont Compost explaining why he adds so much compost to his potting mixes, used by many successful organic vegetable farmers. Also, here is a publication by George Keupper that explains the contributions of both soil and compost, as well as other ingredients you might add to seed-starting and general potting mixes.
Anyway, I’ll definitely add soil and compost to my seed-starting mixes from now on.