One year when someone delivered mountains of horse manure to the community garden, I piled it as high as I could in a couple of my garden beds -- probably 12 inches -- in late fall. I added a few inches of mulch on top. By spring, the beds had shrunk by half. To plant each tomato seedling, I pulled aside the partly decomposed horse manure to make a hole, positioned the plant in the middle of the hole, and filled the hole with a bucket of homemade compost. I already had garden soil underneath all that manure. (I've never worked with sandy soil, however.) My plants did well. I was still finding lumps a year later, so the horse manure was not fully decomposed.

I'd be a little more cautious about using "straight" horse manure with leafy vegetables or anything that grows too close to the surface, though adding a good layer of mulch on top mitigates any risk.

Here's a reference that might help.
http://ag.arizona.edu/pubs/garden/mg/...