Does the pH always decrease as the phosphorus concentration increases? Has anything been observed to increase pH as phosphorus increases?
Are you talking about soil..??..
Usually , as the soil pH goes down with leaching of divalent bases under the influence of rainfall as it happens in case of Alfisols/Ultisols/Oxisols , the phosphorous starts getting precipitated with aluminium and toxic level of iron to form aluminium -and ferrous phosphates, respectively, far less available to plants . On the other as well, as the soil pH increases , the same phosphorous gets precipitated predominantly as calcium phosphates , this is again far less available to plants . If a soil is waterlogged and soil pH is buffered around neutrality , here phosphorous would be in maximum quantum , all three forms viz., Fe-P, Al-P and Ca-P , apart from occluded form of P, find their way collectively to be available to plants.
It happens often in sodic /alkali soils with dominance of exchangeable sodium , the formation of sodium phosphates turns phosphorous on a higher side in available form, but high ESP coupled with EC and pH , collectively debar plants in a position to absorb this form of phosphorous, so its a complex chemistry of phosphorous.
@ Mina, the ideal pH for maximum P availability in the soil to plants is between 7.5-8.5. If the soil pH is below 5.5 then the P availability is limited due to the fixation by aluminum and iron as rightly mentioned by Dr. Anup and if the soil pH is above 8.5 then again P availability is limited due to fixation by calcium. Therefore, if your soil pH is 7.0, then with increase in pH you may expect more concentration of P till pH near about 8.5.
Phosphorus availability is controlled by three primary factors: soil pH, amount of organic matter, and proper placement of fertilizer phosphorus. Acid soils should be limed to bring soil pH up to ideal levels (pH 6-7). ... Soil pH values below 5.5 and between 7.5 and 8.5 limit phosphate availability to plants. As attached figure. Phosphorus does not readily leach out of the root zone; potential for P-loss is mainly associated with erosion and runoff. Soils and sites that are most prone to erosion and runoff, or are in close proximity to streams, lakes and other water bodies need to be closely managed to avoid P loss.
In pure solutions, you can observe the pH increment or reduction using different phosphorous salt source. For example, if you use H2PO4- dihydrogen phosphate, HPO4-2 hydrogen phosphate , or PO4-3 phosphate (orthophosphate)
Based on below equations, when you use H2PO4- salts (e.g., NaH2PO4) you will decrease your solution pH. However, when you use PO4- salts (e.g., Na3PO4) you will increase your solution pH, increasing the salt concentration.
(1) H3PO4 = H+ + H2PO4- pKa = 2.14
(2) H2PO4- = H+ + HPO4-2 pKa = 7.207
(3) HPO4-2 = H+ + PO4-3 pKa = 12.346
When you want to fix the solution pH in different P concentrations, you should use two those kid of salts to form a buffer solution. For example, to keep the pH ~7.2 , you need a solution with 50% H2PO4- and 50% HPO42- (see attached file).
In soil solution you can observe an increasing of pH due the high P fixation on Fe and Al oxides clay (you need high P concentration to see this effect). This occurs because phosphate can substitute -OH clay surface binding during fixation reaction (third reaction in the 2nd figure attached)
Mina,
In response to your question, I ran a Pearson's correlation analysis (r) using soil phosphorus and soil pH. The database consists of 414 data points of three different types of phosphorus species from agricultural soils in the Midwest United States and their respective pH. The results for these typical agricultural soil tests reveal there is no correlation between fixed phosphorus and soil pH (r = 0.543, p
You can simplify your assessment. Phosphates of the apatite-group are stable under neutral to alkaline pore fluid conditions. Lowering the pH and more acidic conditions at around pH 6 causes apatite-group phosphates to get decomposed the phosphate is dissolved and removed from the system according to the hydraulic conditions. The opposite you can observe as you consider Fe (III)-Al (III)phosphate (APS minerals). They are stable under acidic conditions where they result from the decomposition of the more common Ca phosphates of the apatite group mentioned above in contact with clay minerals (sourec of Al, Fe..). These APS minerals can be arranged as to their stability according to their cations , e.g., Sr, Ca, REE etc. and the substitution of sulfate, arsenate and vanadate for phosphate among the anions. In other words the "APS-apatite" system is a perfect yardstick for the acidity of pore and meteoric solutions and commonly denominated as a "geo-acidometer".. There are plenty of pH-soluble equations and diagrams to obtains precise figures for these parameters.
Is your question more fundamental, i.e. about the molar stoichiometry of P and proton binding?
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