Phosphorus is a vital component of adenosine triphosphate (ATP), the ‘energy unit’ of plants. ATP forms during photosynthesis, has P in its structure, and processes from the beginning of seedling growth through to the formation of grain and maturity.
The general health and vigor of all plants requires P. Some specific growth factors associated with P include stimulated root development, increased stalk and stem strength, improved flower formation and seed production, more uniform and earlier crop maturity, increased nitrogen (N)-fixing capacity of legumes, improvements in crop quality, and increased resistance to plant diseases.
Phosphorus deficiency is more difficult to diagnose than a deficiency of N or potassium (K). Crops usually display no obvious symptoms of P deficiency other than a general stunting of the plant during early growth, and by the time a visual deficiency is recognized, it may be too late to correct in annual crops.
Some crops, such as corn, tend to show an abnormal discoloration when P is deficient. The plants are usually dark bluish-green in color, with leaves and stem becoming purplish. The genetic make-up of the plant influences the degree of purple, and some hybrids show much greater discoloration than others. The purplish color results from the accumulation of sugars, which favors the synthesis of anthocyanin (a purplish pigment) that occurs in the leaves of the plant.
Phosphorus is highly mobile in plants and, when deficient, may translocation from old plant tissue to young, actively growing areas. Consequently, early vegetative responses to P are often observed. As a plant matures, P translocation into the fruiting areas of the plant, where the formation of seeds and fruit requires high energy. Phosphorus deficiencies late in the growing season affect both seed development and normal crop maturity. The percentage of the total amount of each nutrient taken up is higher for P late in the growing season than for either N or K.
Source: Soil Fertility Manual (2006) by the International Plant Nutrition Institute (IPNI) and the Foundation for Agronomic Research (FAR).