The [[maximum slope]] on which you can sow seeds is 8, where 8 = (highest north-south tile border's slope) + (highest east-west tile border's slope). For example, the slope can be level EW and 8-steep NS. Or 4-steep EW and 4-steep NS, etc. Attempting to sow seeds on a greater slope results in a message "The ground is not flat enough for crops to grow. You need to flatten it first". Farm tiles function the same with a slope 8, flat or anything in between.
Note: Above is not entirely correct, game is using more complicated [[diagonal slope]] formula to calculate steepness for both diagonal directions of the tile. If that steepness is greater than 8, tile cannot be sown on. --[[User:Aldur|Aldur]] ([[User talk:Aldur|talk]]) 07:10, 28 September 2012 (CDT)
The current minimum harvest yield is 2.