Acidic and Basic soils experience phosphate sequestration through binding with either calcium, aluminum, or iron. How do plant that thrive in acidic and basic soils obtain the necessary phosphate? Blueberries are an example that I particularly enjoy.

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Thanks to the University of Hawaii for posting this picture.

  • $\begingroup$ Phycology would seem to be an ideal place to study mechanisms of phosphate absorption at varying pH. $\endgroup$ – Dale Sep 15 '13 at 18:40
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    $\begingroup$ Why is pH6- pH7 the highest range for P availability ?It seems to me like the lowest. $\endgroup$ – biogirl Sep 16 '13 at 15:25
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    $\begingroup$ @biogirl it would be because the phosphorous is more likely to dissolve in water at this pH than to react with iron, aluminum or calcium. This makes it available to the plant since it cannot absorb insoluble compounds. $\endgroup$ – S. Albano Sep 21 '13 at 6:28

Fixation of phosphorous occurs when the chemical equations tip in favour of less soluble compounds. Plants are able to absorb Orthophosphate ions, such as H2PO4- and HPO42-.

Orthophosphate is most available to plants at pH values near neutrality. It is believed that in relatively acidic soils, orthophosphate ions are precipitated or sorbed by species of Al(III) and Fe(III). In alkaline soils, orthophosphate may react with calcium carbonate to form relatively insoluble hydroxyapatite:

3HPO42- + 5CaCo3(s) + 2H2O -> Ca5(PO4)3(OH)(s) +5HCO3- + OH-

Environmental Chemistry, Seventh Edition -Stanley E. Manahan

This does tend to make it more difficult for plants to obtain their phosphorous when it is stuck to something that they cannot absorb, but it is not hopeless. These equations are equilibria, so they can be made to go both directions by adding or removing reagents from each side.

  • The plants will pull from the left side of the equations by removing the orthophosphate ions. More will be released to maintain the equilibrium.
  • In acidic or basic environments, rainwater will serve as a base or acid to push the equations from the right.
  • Decaying organic matter releases orthophosphates that the plant can absorb.

In this way, plants living in acidic or basic environments would still obtain enough phosphorous.

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