I had a suggestion from a horticulture instructor at Meramec Community College on the potting mix they use. It’s call Hummerts Dyna-Mix for Raised Beds. For 2.8 cu ft. the cost is around $20.
They then suggest adding 12-12-12 to the mix when planting and then supplementing with Jack’s 20-20-20 water soluble fertilizer once a week.
I do like Jack’s fertilizer, but find it extremely expensive retail. Hummert’s does sell it for less, but then you need to buy a 25 pound bag for $45.60.
I’ve done some calculations and while they call this Dyna Mix product – Raised Bed Mix – it would be fairly expensive. For a 4′ x 8′ x 12″ bed, you would need 11.4 bags. At $20 a bag, that equals $228. You could buy the same amount of “Garden Mix” soil from St. Louis Composting for around $33.00.