The graph shows the change in crop yield of wheat when different masses of nitrogen fertiliser are added to fields.
State the mass of nitrogen fertiliser at which nitrogen ceases to be a limiting factor for the growth of the wheat crop.
Calculate the percentage increase in wheat yield when the mass of nitrogen fertiliser applied is increased from 100 kg/ha100\text{ kg/ha}100 kg/ha to 150 kg/ha150\text{ kg/ha}150 kg/ha.
Suggest a biological reason why wheat crops can still grow and produce a yield when no nitrogen fertiliser is added.
