If you grow a lot of transplants, you’ll rapidly realize that it takes a lot of potting soil! While baby seedlings which have just sprouted do best in a special seedling mix, older transplants need something with a bit of food in it for them, and don’t need a sterile mix. So, you can mix your own.
My recipe is adapted from one in Steve Solomon’s excellent book Growing Vegetables West of the Cascades. My measures are a bucket which holds about 3 gallons, and an old measuring cup retired from the kitchen. Measurements don’t need to be exact, though. The compost and peat or coir should be damp, not soaking wet and not dry. See below for a video version!
3 buckets homemade compost, screened
2 buckets coir or peat, without too many big lumps
2 buckets horticultural vermiculite or perlite
3 cups complete dry organic fertilizer
1 cup lime if you have acid soil
Layer it all together in a big wheelbarrow or other container and mix with a shovel or fork. It’s not rocket science: just keep mixing until everything is well distributed and there are no clumps.
Store in a ventilated container: I use an old plastic garbage can with ventilation holes drilled in it. Be aware that without ventilation any non-sterile soil mix can go “sour” and grow micro-organisms which your seedlings will not be happy with. Been there, done that.
This works very well for hanging baskets and container gardens as well as growing small plants. If the plants are in the mix all season though, they will run out of nutrients: feed with liquid organic feed like compost or manure tea, fish fertilizer, or similar.
Here’s a video showing exactly how I do it.