Phosphorous (P) is one of three vital macro-nutrients (‘NPK‘) needed for photosynthesis and overall plant health (root growth, flowering, fruit-set) and yield (see  also nitrogen and potassium). Calcareous soils whose clay content is high may be low in phosphorous.

Bibliography

Dr Ian d’Agata, Native Wine Grape Terroirs of Italy (University of California Press, 2019) p.337.