Updated: Apr 12, 2022
Actually it's not soil, which consists of sand, silt and clay and ideally organic matter.
This seed starting mix has peat moss as the base ingredient, which in fact is pure organic decomposed matter harvested from peat bogs. Peat moss comes in different qualities, not all sterile and varying a great degree in organic matter content and microbial activity. It absorbs a lot of water and it also dries out quite fast. But as a growing medium it is light and fluffy, it practically babies the little seedlings if you keep them watered well.
Recipe and how to improvise (if you have to)
I follow Elliot Coleman's (check out his farm and teachings) https://www.fourseasonfarm.com/ recipe for the soil(less) starter. It has worked well for me for years, even though I have a tendency to follow recipes rather loosely, depending on availability of ingredients and the type of plants I am starting. I suggest you use it as your base and adjust it where you need to.
1. Mix equal parts of blood meal + rock phosphate + greensand.
2. Mix 14 lbs. of #1 per yd3 of soilless mix (peat moss + perlite or vermiculite).
3. Allow the complete mix to sit for a month or more before planting.
The blood meal, rock phosphate, and greensand supply N, P, and K. The month after mixing and before planting presumably allows the fertilizer materials to partially breakdown and release plant-available nutrients. Test this recipe on a small number of plants before you adopt it for all your plants.
I scale it way down to fit my size operation. I assume you also don't need a 2 tons of seed starter:)
So you can measure it by 1 big bag of peat moss = 107 litres takes appr. 1/2 lb of fertilizer mix.
(I do double on the fertilizer as I live way up north and my seedlings often spend a fair bit of time in their starter trays and need to nibble on those nutrients a little longer.)
Also I omit vermiculite. For no particular reason. I just have never used it.
Blood meal is a very valuable source of organic nitrogen. But it is less and less available now and can be replaced with other sources high in nitrogen. I have used GaiaGreen feather meal https://www.gaiagreen.com/product-page/feather-meal just because I have a big bag of it sitting around. The N-P-K is 13-0-0 which is very comparable to blood meal which has an N-P-K of 12-0-0.
Sometimes I add a little compost for good measure.
And, of course, I made my soil starter last week and I need it now. So while it is better to wait for a month to let the mix of minerals and fertilizer blend and become more available for the plants, sometimes it just doesn't work out that way.
Please consider: The harvesting of peat moss is a largely unsustainable practice. Boglands store huge amounts of carbon dioxide which is released into the atmosphere when they are drained and harvested. It's a very big industry, and I don't recommend using peat moss abundantly.
While I appreciate its place in seed starting (and our composting toilet) I don't use it at all in my garden beds or greenhouses. Ecological footprint, as I understand it, is much defined by the volume of what we do and consume.