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It's Role in Plants

Phosphorus (P) is the second major fertilizer element. Its role in plant growth is related to the part played by Nitrogen (N). When N is lacking in the solution, the plant's absorption of P ions also decreases. Excess P ions remain `stored- up' in the plant tissue because enough N is not available to utilize P and complete its assigned job. Likewise, N will accumulate and stay inactive in the plant tissue when P is deficient. In other words, N and P go hand-in-hand to keep the various plant's life process working actively.

Phosphorus Uptake by Plants:
Plants take up P as an orthophosphate ion (H2PO4-) an anion with a single negative charge. Orthophosphate is classified as a very weak acid.

Phosphorus Movement in Plants:
P moves readily within the plant as phosphates. In cases when P becomes deficient during the plant's growing period, P that is stored in older leaves translocates to the younger developing leaves and even to developing fruits. P moves very rapidly in the plant.

Phosphorus Role in Plants:
Phosphates ions contribute greatly to various growth mechanism in the plant: The plant's photosynthesis and respiration (oxidation) processes depend upon phosphate ions. Photosynthesis is the process that manufactures sugar. Respiration is the "burning" of sugar for plant's energy. As with Nitrogen, P is also necessary for division and enlargement of plant cells.
P influences root development, and formations of blossoms and fruits (seeds). P controls the movement of fatty compounds which in turn influences the structure of the plant's fibers. It is invalued in the production of nucleoproteins which are vital to the hereditary characteristics of the plant.

Phosphorus Deficiency in Plants:
P deficiency can be detected in plants by the following symptoms:
1 Due to poorly developed roots, plants tend to be smaller (stunted). Since P is involved in cell division and enlargement
and the release of energy, P deficiency will result in poor root growth. 
2 Leaves on plants turn purple. This is the result of accumulated sugar that turns leaf pigmentation to red or purple.
3 Fruits do not mature on time, usually late maturity. Seeds do not form properly and have low germination potential. 
4. P deficiency first appears in the older leaves; since, P deficiency will cause what P is in the plant to relocate to the younger leaves.

Availability of Phosphorus:
Between pH of 6.5-7.5, H2PO4 ion predominates. This is the orthophosphate ion that the plants take up. Therefore, P availability is greatest between these two pH ranges. As the solution becomes alkaline (higher pH), first the HP04 ion, and then the P04 ion predominate. (see Fig. 1 above)

Root development, blossoms and fruits formations depend upon P. Research has shown that the interplay between N and P significantly affects the growth and production (yield) by the plant.