Cambridge Secondary (7-9). Stage 9. Science. Learner's Book - Unit 1: Photosynthesis and the carbon cycle - Question 91

A farmer carried out an experiment to find out how adding different quantities of nitrate-containing fertiliser affected the quantity of grain that she harvested from her wheat crop. The graph shows the results.

The farmer decided that there was no need to add more than about 60 kg of fertiliser per hectare. Explain how the results of the experiment support her decision. (Remember that fertiliser is expensive.)