
I thought that any of the three above ideas would help.
As I mentioned, picking rocks really effects soil quality in real life, and sifting the soil is part of gardening. Some unrealism is expected of course, since we alread have easier than real life stump removal. I think a tiny bit of lp, and a heavy stamina drain should help with grinding, but this helps noobs who want to integrate with people have something better to do than wander around camp repairing things and grinding clay digs so they can be allowed to touch someone's q50 wheat feild finally.
Throwing rocks really should be credited to Flame. I recognize for game balance that combat skills are made more difficult to get, but even I, as a low skilled university student, can kill someone with a cinder block dropped of my balcony if I found myself so inclined (say, to fight off a mob of zombies). The throw rock skill is good for the moment when, as a noob farmer, the bloody boar has spawned between you and where you're going, and also lets gangs of weaker characters toss rocks to defend someone as they try to flee. I think, to preserve game balance, that a fixed amount of damage might be a good idea? Plus, Flame's rock tossing comic was pretty darn cute.
As for compost/manure heaps, I'm not sure how they'd help the crop since I'm not a game balance expert, but I think that making sacks of fertilizer, like making bricks, could be something that non-metal holding characters could do to improve trade with their neighbours depending on how fast and how much crap one would have to add to the dump bin to get enough to do a modest sized feild.