| "Tim Wescott" wrote in
| message
| ...
| We have this big ol' pile of horse manure out
| back, which we can't spread out in the pasture
| because it is full of rocks that are just about
| exactly the right size for causing problems with
| horses feet (according to SWMBO, at least). The
| sizes range from sand to 3-4 inches.

I was just thinking about that deck grating used in industrial
environments. It's a bunch of metal strips laid on end and held together by
various means, so that ought to sift out anything larger than about 3/4" or
so. It's used for a billion things, and great for stables and barns so that
you have something solid to walk on while muck falls away between the slats
and can be washed away later.
Stand it up on a mild incline somehow and connect it by whatever means
works for you to whatever piece of machinery you have that vibrates the
most. The rocks will continue out to the low side of the grate while the
manure falls through.
Another thing that came to me was to connect it to a front loader so
that all you have to do is to drive the grate into the pile, pick up a load,
back away over the piles of clean manure previously deposited, which will
give you a nice shaking motion, then dump the rocks left behind wherever you
see fit. This way you get to move the rocks and screen them out at the same
time.

And you should smack whoever had the bright idea of mixing rocky soil
and manure together.