Phosphorus (P)

  • Structural component of macromolecules such as the nucleic acids DNA and RNA
  • Important role in energy transfer and storage
  • Associated with root growth and root health, increase tolerance to avocado root-rot
  • Fruit quality and disease resistance
  • Stimulates growth and early maturity

Function of Nutrient in Plants

Phosphorus, classified as a macronutrient, is a component of certain enzymes and proteins; adenosine diphosphate (ADP) and adenosine triphosphate (ATP), ribonucleic acids (RNA), deoxyribonucleic acids (DNA), and phytin. ADP and ATP are essential in energy storage and transfer reactions. They provide the energy required by all biological processes. RNA and DNA are the two nucleic acids components of the genetic information. Seeds are high in phosphorus which plays an important part in their development. Phosphorus plays an important role in root health and the ability of plants to better tolerate soil born diseases such as avocado root rot.  Roots flourish when adequate phosphorus exists.

Absorption by Plants

Phosphorus is absorbed by the roots mostly in the ionic form of either  H2PO4-  or HPO42- (orthophosphate). The ionic form that is predominantly absorbed depends on soil pH.  H2PO4- is more readily absorbed in low pH soils whereas HPO42- is preferentially absorbed in high pH soils.  Although it is readily mobilized in plants, availability in the soil solution is usually low since much of the phosphorus is tied up in poorly soluble compounds. The best pH range for phosphorus uptake is pH 6.5 to 7.5.  There is a marked increase in phosphorus uptake in the presence of ammonium nitrogen in comparison to nitrate nitrogen.

Deficiency Symptoms

Slow growing, weak and stunted plants that may be dark green with older leaves showing a purple pigmentation. Since P is fairly mobile within the plant, deficiency symptoms initially occur in older tissue.

Deficiency Effects

Will seriously harm various processes associated with energy storage and transfer. It will effect root growth and bud development.  Poor seed development and poor fruit quality and size may result.  The deficiency symptoms can mask other nutrient deficiencies such as nitrogen and potassium.

Symptoms of Excess

Excess phosphorus is manifested by micronutrient deficiency, with iron (Fe) and zinc (Zn) first affected. Excess P can induce potassium (K) deficiency.

Interactions:

Phosphorous (P) Aluminum (Al): The P concentration is decreased as Al is increased (avocado is sensitive to elevated Al).  Plant Al concentration is decreased as soil P increases.  The manifestation of Al toxicity is thought to be due to its negative effect on the P availability in the plant.

Phosphorous Calcium (Ca): High levels of Ca in the soil may affect P levels in the plant.  High soil Ca can cause the desorption of appreciable quantities of P, especially in clay soils with high pH.  In light soils with low clay content the negative influence of high Ca is even greater.

Phosphorous Micronutrients: Deficiencies of copper (Cu), iron (Fe), and zinc (Zn) are associated with high levels of available P in the soil, particularly in soils with high pH.  Increased availability of these micronutrients in the soil lowers P concentration in the plant. Symptoms of Fe and Zn deficiency are associated with high ratios of P to Fe or Zn.

LINKS
http://www.back-to-basics.net/efu/pdfs/phosphorus.pdf 
http://www.oznet.ksu.edu/library/crpsl2/c665.pdf 
http://www1.uwex.edu/ces/pubs/pdf/A2520.PDF 

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