Here's the recipe I've used for years:
4 parts coco coir
3 parts perlite, 1/8" size
1 part worm castings or finely sieved
compost (I make both at home, and use whichever one is ready)
tablespoon - handful of soil from the vegetable beds (just a little bit relative to the overall volume of the mix you're making)
The coir and perlite make a great texture that both holds water and drains, and is fine enough for small seeds to germinate easily.
The worm castings add a little nutrition and microbial life
The garden soil inoculates the seed mix with the microbiome the seedlings will encounter when they're transplanted, helping them develop their symbiotic relationships early on
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