Source. Phosphorus is present in the soil in two general forms, organic and inorganic. Plants do not absorb organic phosphorus, either from the solid or solution phase of soil. However, organic compounds are decomposed and phosphorus is made available to plants an inorganic form. Soil solution contains phosphorus in inorganic forms as the phosphate ions H2PO4-and HPO42- When pH is low phosphate ions are present in the form of H2PO4- and when pH is high, phosphate ions are represented in HPO42-. The amount of phosphorus in plant is about 0.5 per cent to 1 per cent.
Regions. Regions of plant in which phosphorus is required are younger tissues. It is withdrawn from older, metabolically less active cells. It promotes healthy root growth and fruit ripening.
Functions. It is present in plasma membrane, nucleic acid, nucleotides, many coenzymes and organic molecules as ATP. Phosphorus plays an indispensable role in energy metabolism i.e., hydrolysis of pyrophosphate and various organic phosphate bonds being used to drive chemical reactions. Thus it is required for all phosphorylation reactions.
Deficiency symptoms
(i) Loss of older leaves.
(ii) Poor growth.
(iii) Dull green color of older leaves.
(iv) Striking increase in the activity of enzyme phosphatase.
(v) Anthocyanin development in the stem and leaf veins and in extreme cases development of necrotic areas.
(vi) Leads to accumulation of carbohydrates in soya bean (Glycine max).
(vii) Lower leaves may turn yellow.