Soooo, I'm happy with the soil mix I've come up with, but now I'm sweating how much it costs. Coco coir, pool filter sand, perlite, earthworm castings.....it's pricy stuff by the bag when you have a full-blown grafting addiction. I have all these grafted seedlings in tree pots, I'm doing the math and realizing I'm going to need something north of 50 gallons of mix when I upsize their pots in a few months. I'm going to keep using my soil mix for favorites, but I want to also give trees to my family, friends & neighbors when they are about 3 gallon pot-sized big (I'm giving them free trees, but not my pricey mix). There's a Big Earth near me that sells bulk soils
https://bigearthsupply.com/product-category/soil/bulk-soils/. Do I just buy a yard of compost soil, yard of sand, and a yard of top soil to mix together? How do the nurseries do it and keep costs at a minimum?