Phosphorus in Plant Nutrition: Role, Compounds, and Deficiency Signs
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Phosphorus is an essential plant nutrient that plants absorb as the orthophosphate anion, PO4. Unlike nitrogen, phosphorus is not reduced inside the plant, so every phosphorus compound found in plant tissue can be regarded as a derivative of phosphoric acid.
Why is phosphorus essential to plants?
Phosphorus is essential because it forms part of a whole range of vital, life-sustaining compounds in the plant (for more detail, see the chemical composition of plants). Phosphorus improves and accelerates plant development, stimulates flowering, and promotes fruiting. When phosphorus is in short supply, growth slows, leaves take on a bluish-green hue, and blossoms and set fruit drop from fruit trees.
Up to 50% of the phosphorus present in a plant is bound in organic compounds. Phosphorus is a component of nucleic acids, nucleotides, phospholipids, certain enzymes, and adenosine triphosphate and adenosine diphosphate.
Nucleic acids combine with proteins to form nucleoproteins, which are structural parts of the cytoplasm and the nucleus. Phosphorus compounds also play a major role in plant respiration. Up to 50% of a plant's phosphorus remains in mineral form as dibasic and monobasic salts of phosphoric acid, dissolved in the cell sap, where together with other substances they help create the cell's osmotic potential.
Dissolved phosphoric acid salts also act as a buffer, holding the pH of the cell sap at a stable level.
How is phosphorus distributed within the plant?
Phosphorus is distributed unevenly throughout the plant. Seeds contain 5 to 10 times more phosphorus than the vegetative organs. In seeds, phosphorus is stored mainly as phytin, a reserve phosphorus-containing substance.
Under phosphate starvation, phosphorus — like nitrogen — can be reused, a process called reutilization. Complex phosphorus compounds break down and the phosphorus migrates to the younger parts of the plant. Because phosphorus is never reduced inside the plant and stays as the PO4 anion, the phosphorus cycle in nature is a simple one.
How does the phosphorus cycle work in the soil?
When plants and animals die, their organic phosphorus compounds undergo mineralization and are released as phosphoric acid, which immediately reacts with soil bases to form sparingly soluble salts of calcium, magnesium, and iron. Because these salts dissolve poorly, they cannot fully supply the plant with phosphorus.
Dissolving these salts — the mobilization of phosphoric acid — happens through biochemical processes accompanied by acid formation, for example during nitrification. Root exudates and physiologically acidic salts are particularly important for mobilizing phosphoric acid.
In nature, the activity of anaerobic bacteria (for more detail, see anaerobic respiration in plants) can drive processes that reduce phosphorus to phosphorous and hypophosphorous acids, and even to phosphine. These reactions occur only under anaerobic conditions, so they can be suppressed by good soil cultivation that keeps the soil aerated.
What are the signs of phosphorus deficiency?
Phosphorus deficiency shows up as stunted growth of both the above-ground organs and the roots, caused by a slowdown in the rate of cell division. The visible symptoms include:
- Leaves turning a bluish or grayish-green color;
- Lower leaves yellowing and dying off while the upper leaves keep a dark-green color;
- Slowed overall growth and weak development of the root system;
- Blossoms and fruit set dropping from fruit trees.
Sometimes plants suffer from phosphorus shortage because of heavy strain on the soil: frequent plowing or digging destroys the soil bacteria. Those bacteria are exactly what make the phosphorus already abundant in the soil available to plants.
Phosphorus starvation during the early growth period is especially damaging. It can disrupt metabolism so severely that even with good phosphorus nutrition afterward, the plants are unable to return to a normal state. This is why ensuring adequate phosphorus supply from the seedling stage onward matters more than correcting a shortage later.
