The graph shows the change in crop yield of maize when different masses of phosphate fertiliser are added to fields.
State the mass of phosphate fertiliser at which phosphate ceases to be a limiting factor for the growth of the maize crop.
Calculate the percentage increase in maize yield when the mass of phosphate fertiliser applied is increased from 80 kg/ha80\text{ kg/ha}80 kg/ha to 120 kg/ha120\text{ kg/ha}120 kg/ha.
Suggest a biological reason why maize crops can still grow and produce a yield when no phosphate fertiliser is added.
