Good organic matter or compost.

agree, a couple inches of compost will do the trick, and water it in.

regular watering will keep it moist. it is the same in greenhouses with soil in the borders.

Thank you in 2 of the raised beds I have put a couple of bags of rotted horse manure which I have had for a couple of years

Oh and given the whole thing a good soaking

I suspect that if the soil is as light as you describe it you'll need ore than two bags of rotted horse manure to make much of a difference.