The graph shows the change in crop yield of barley 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 barley crop.
Calculate the percentage increase in barley yield when the mass of phosphate 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 barley crops can still grow and produce a yield when no phosphate fertiliser is added.
