I try all kinds of mixes all the time hoping I hit on a good one! What mix do you like? Most of the time there are things you over look! One mix I like is the layer of compost under a few inches of a huge pine tree. Those huge trees set for 20 years and collect fantastic compost! One thing I noticed is that cats love to use the ground under pine trees for their bathroom! Maybe that is why my friend had such a nice garden after he cut down a 40 year old pine tree and had his garden mixed with the rich dark brown compost that went down over a foot under the tree when he cut it down. I probably got 8 wheel barrels full of the compost to add to his garden several years ago!
There are so many things to add to the mix its almost like cooking in a way! Even a little Epsom salts can help the plants! Mild fertilizers is the key I think! To many people think the more fertilizer the better but that causes problems with the water intake of the plants! I would say sand is a good additive to any mix since it helps drainage and the roots seem to hold to it! Also when you replant a seedling it comes out easier without destroying the roots! Worms are great add to a mix especially if you have some shredded newspapers at the bottom of you mix! I always put it in a few trays with some new mix to see what happens!
I think one of the main things is to try to get a mix that does not dry out to fast! Sometimes I add a little clay to the mix. I really like the cow manure compost and mushroom compost that I add to the mix! peat moss seems to dry to me! Last year I added some sea weed liquid mix with only a few drops in the water I watered with. It worked pretty well! The seaweed mix is expensive but goes a long way! How do you like bone meal? Now blood meal is another product but very small amounts at the very bottom seem to work as the seedlings roots reach the bottom!