Winter project

Everything in moderation. Manure is geat, but 3" every year put on and you're going to get an excess of some nutrients.

He wants a way to remove the rocks and stones. And leave his nive soil where it is.
I think the 'RockHound" skid steer attachment is what a landscaper would use.
I had heard they only get rocks about 2" deep at most.  So moldboard plow (or potato plows, big shoe cultivators), Rockhound, plow, Rockhound.... 3 or 4 times?  And you need a place to dump the rocks. See www.rockhound.com. ; A local rental place would have let me have a tracked skid steer with the attachment for $450 dollars per day. I ferget if they delivered and picked up. But they rent trailers for $85 per day. Perhaps find a local contractor and make a deal.

As for building your own soil screen device, sounds like a lot of work. But an idea I had was to build a large peat roller type device (3-4ft in diameter, 5 ft wide). With loader tractor dig up and dump rocky dirt in roller basket , close door, drive the roller tractor around garden till only rocks in the basket,  drive to dump area, open basket, door, drive fwd until rocks dump out, close door. Repeat. The door would be the trick.  I was thinking a split door that opened in. Looking axially, common door hinge at 12' o'clock, sealing door jambs at 10 and 2 o'clock.  When open the doors drop in towards the center axle it can be latched in that position, and also in the closed position.
Sounds like a 3 person 2 tractor operation. And rocks get concentrated in some other area, to be cleaned up later. Also basket will be running over previously dumped  rocks at some point and that may kill it eventually. It was only half-baked, needs more refinement perhaps, or forget it. But no PTO shafts, chains sprockets, gearboxes and stuff like that. All Flintstones technology.

If I were making a small scale screening outfit, I would find a vibrating plate tamper, with a wore out motor, and put a 'lectric motor on it.  Then mount to a table, and frame, using springs for the screen base to table base connection. Often thought of doing this, as I keep lookin at that damn wore out tamper, sitting outside one of the barns...

That's what I was thinking Dave, but up where he lives, the best crop they get is rocks....they just keep pokin out of the ground!
As for horse manure, it is one of the best things you can put into a garden.  A LOT better than cow because horses don't eat weeds.
The bad part of it is if it's hot.  That'll burn up nutrients and plants.  Just leave it sit for a while and let it decompose.  It works GREAT.  My Mom had the BEST garden!!!  Sure wish she'd of given me a LITTLE bit of that green thumb though.  I'm so bad, I can't hardly grow hair....

friend of mine commandeered sum kids, said he'd pay them a nickle for every rock they picked up outta his fields, he has piles at the fields edges that will fill straight trucks. said it was lots cheaper than paying for repairs or buying a rock picker! plus he's been selling them rocks to people that want them to make fireplace faces.

Not sure if I would use the subsoiler. Might just bring more rocks to the surface. The potato digger would make quick work of separating the bigger ones. A basket or box mounted on the back to catch them. Or let them drop on the ground and pick them up with a fork or a square shovel with the center cut out and 1/8" rods welded in to act as a screen. 2 or 3 passes with the potato digger setting it a little deeper with each pass will be just like running a tiller through it.
Getting the little rocks out will require a lot more complex procedure.
Maybe consider a shaker style potato digger, but instead of rods use screen. Would be a simpler machine to build.