Answer
Changing the amount of fertilizer used would be a short-run adjustment.
Work Step by Step
Although the changing of the amount of fertilizer for a farmer would most likely imply a significant change in cost of production, regardless of the production size of the farm, fertilizer is still a variable cost since it can be varied in the short run (presuming that there is no contract that binds the farmer to buy a fixed quantity of fertilizer in the long run). Thus, since it can be varied easily, it is a short run adjustment.
