I think there's a yield curve based on the amount of fertilizer added to the crop. I saw on one video that a "normal" fertilizer application on his crop yields 2x the amount vs no fertilizer. The yield change is a non-linear curve based on the amount of fertilizer application.

So farmer will have to estimate what the price will be at the time of harvest and then optimize this curve.

(edit here's the video):

https://www.youtube.com/watch?v=-mIBz5r29nw