What is the "seas fert" option for?
The seas fert (short for "Seasonal Fertilize") option in the farm plot building menu automates the fertilization process on that plot. Every season change, dwarves will attempt to automatically fertilize the farm plot with any available potash. If the amount of potash necessary to fertilize a crop of that size is not available, it will not be fertilized. It is also advisable to place a stockpile of potash near your fields to prevent your farmers from carrying across the entire map.
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).