One picture is better than thousand words so here is what I had in mind:

The map only had 1 layer of soil, so I had do make a cave in and drop it down.
Now the tiles behave very oddly:
grass - cannot build farm
dry grass - cannot build farm
silt - farm is ok
Before dropping, all tiles were available for farms.
edit: building and removing construction doesnt help
neither does creating and mining obsidian
ALL tiles have a silt wall directly bellow them.
Farming is weird.
Supposing I really have to have a farm like this, does anyone have a suggestion how to build that stupid farm downthere?
-----------------------
Solution - Irrigate. Muddy floors tricks the game into allowing the farm to be build. This thread was pretty pointless, but what about DF isnt...