I've got some new raised beds, 16inch deep on top of gravel so not over soil, and am thinking of filling them with pure horse manure compost, a little top soil and small amount of sharp sand. Would using just manure compost make the bed too high in nitrogen? Or is it ok to fill up the bed with mostly manure compost.
Some plants such as squash will do pretty well on nothing but manure; others will not do well at all. One vegetable garden rotation method suggests using four separate beds - each year in a 4 year rotation you load up one bed high with manure and grow nothing but squash in it. In year 2 another bed gets the same treatment and you grow squash there. Same for years 3 and 4 until in year 5 the squash treatment goes back to the first bed.
In year 2 the first bed, which by now has sunk a good deal, is used for potatoes. By now the nutrient levels are declining, so in year 3 it can rotate to green crops and in year 4 it is used for root crops.
So how to ease yourself into such a routine from scratch? In year 1 you could load up all beds with manure, plant squash in all four, trade excess squash for other needs. In year 2 you have one squash bed and 3 potatoes. In year 3 you have one greens, two potatoes and one squash and so on. At the end of year 4 you should be good to go. Consider adding regular garden soil from time to time to all beds to ensure that the range of nutrients available is as wide as the plants expect.