Hi Stef –
The soil in raised beds is usually superior to that in row gardens in part because it never gets stepped on and therefore does not get compacted. Beyond that, filling and maintaining beds usually becomes an opportunity to get high-quality soil and to fine-tune the mix of fertilizer and amendments.
If youโre not sure about the quality of your starter soil, or if you just want to add to what you are already using, mix in organic compost — no sludge or biosolids — and plenty of it. Compost will make your soil heavier and help retain moisture, which is a good thing since raised beds dry out faster than the surrounding earth. You can also add dry fertilizers such as blood meal for nitrogen, bone meal for phosphorus, and greensand for potassium. These slow-release nutrients will break down over time and feed your gardens all season long.
Since your beds will settle with time, you may need to add more soil/ compost later, but do not pile it too high and gently grade the surface into a slight mound, with the dirt sloping away from the center of the planter. The excellent soil in most raised beds makes high yields possible, but even the best soil will need rejuvenating over time.
Hope it helps!