Phosphorous, or phosphate, (P) is one of the macronutrients, needed by plants in relatively large amounts. It is important in good plant growth and many of the plant’s internal metabolic processes.
Phosphorous, phosphate, deficiency is relatively uncommon, but is caused when there are insufficient suitable phosphorous-containing materials in the growing medium (soil or compost). It can also be caused when plants are growing in dry soil, which prevents the roots taking up the phosphates present from the soil.
Phosphorous, phosphate, deficiency can occur on heavy clay soils, which lock in the phosphates making them unavailable to plants, in soils that are extremely acidic or extremely alkaline, and in locations with very high rainfall.
To check if your soil is short of phosphorous, you can use a simple soil-testing kit.