v0.34:What is the "seas fert" option for?
"Seas Fert" is short for "Seasonal Fertilize", which tells your dwarves to automatically fertilize this particular farm plot at the beginning of each season. Fertilized crops produce better yields (larger stacks of plants), which leads to greater efficiency in most stages of post-processing (ex. plump helmet  -> dwarven wine , using only a single barrel).
Note that seasonal fertilization will be ignored if your dwarves do not have a ready source of potash. It is also advisable to place a stockpile of potash near your fields to prevent your farmers from carrying across the entire map.