I think you're mistaking "Damp" for "Muddy." "Damp" stone is worse than worthless; it's there to tell you that there is a water square adjacent to that wall so you don't accidentally dig it out, but when you dig out next to a damp tile that you previously hadn't seen, the game pauses itself and centers on that tile. Very annoying.
You cannot farm on damp tiles unless they are also muddy or soil, nor do plants grow on damp tiles unless they are muddy or soil.
The only way to muddy a tile, making it possible for plants to grow on otherwise-stone tiles, is to flood it. Having water next to, above, or below a tile will not muddy it; the water has to be in the tile itself for the tile to become muddied.
If you want to make a system to repeatedly muddy an area in case something screws up otherwise, I would suggest having the area to be muddied sealable (via doors/drawbridges/etc.), then have a cistern, floored with retracting bridges, directly above the area to be muddied. The cistern should be no more than 2/7 the size of the area to be muddied and no less than 1/7 that size. Design the cistern such that its input can be sealed as well, so you can have a precisely-measured amount of water to be dropped onto the would-be farm.
Like so:
z-level +0:
WWWDWWW
W+++++W
W+++++W
D+++++D
W+++++W
W+++++W
WWW+WWW
z-level +1:
WWWWWWW
WWWWWWW
WWWWWWW
WWBBD++
WWBBWWW
WWWWWWW
WWWWWWW
Where Ws are walls, Ds are doors, and Bs is a 2x2 retracting bridge. At least the door on the upper level should be linked to a lever, while the bridge should be linked to a different lever. Or, replace the upper-level's door with a 1-tile drawbridge and use just 1 lever.