Answer
We can see a sketch of the graph of the crop yield as a function of the amount of fertilizer used.
Work Step by Step
Initially, a little bit of fertilizer does not have much effect. After a certain minimal amount of fertilizer is reached, the yield starts to increase steeply as more fertilizer is added. After a certain point, additional fertilizer does not increase the yield very much.
After reaching the maximum, adding more fertilizer could actually be harmful and prevent crops from growing at their optimal rate, so adding too much fertilizer can lead to a decreasing yield. If too much fertilizer is used, the yield could actually fall below the level when no fertilizer is used.
