Below is a walkthrough for the Use a Fertilizer Bin task in Rune Factory 5.
Client: Priscilla
Objective: Place some grass in the fertilizer bin
Reward(s): Withered Grass x3
After accepting this task from the Task Board, Priscilla will show up and gift you a Fertilizer Bin along with some weeds. Interact with the bin to pick it up then carry it to your farm and set it down wherever you like. You’ll then need to place the weeds in your hand and interact with the bin to add in the weeds. Once this is done, return to the Task Board to complete the task.