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Phosphorus Behavior in Soils

Phosphorus Behavior in Soils. Plant and Soil Sciences Department Oklahoma State University. G.V. Johnson. Understanding phosphorus behavior in soils. Start with “stripped” down, bare P. “P” stands for Norwegian Goddess Phyllis.

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Phosphorus Behavior in Soils

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  1. Phosphorus Behavior in Soils Plant and Soil Sciences Department Oklahoma State University G.V. Johnson

  2. Understanding phosphorus behavior in soils • Start with “stripped” down, bare P. • “P” stands for Norwegian Goddess Phyllis. • Like elemental phosphorus, we remember the ancient story of Phyllis, and how violent she reacted when she swam to shore and, as she stepped onto the beach, found herself naked! • P

  3. Understanding phosphorus behavior in soils • Phyllis (P) ran to the nearest beach-front store and quickly bought a two-piece bathing suit (O2) and matching beach sandals (O2). • She was much more comfortable. • P + O2 + O2 = PO43-

  4. Understanding phosphorus behavior in soils • But, Phyllis was chilled when it got cloudy and the wind blew, so she went back to the store and bought a “beach –wear” cover (H+)and matching shoes (2 H+). • Now she was comfortable anywhere along the beach. • PO43- + H+ +2 H+ = H3PO4

  5. Understanding phosphorus behavior in soils • Even a Norwegian Goddess can’t spend all her time at the beach. Phyllis needed to move about the land. • She was conspicuous in her beach-wear. It rained a lot. People were always staring at her. • She went shopping again. • She bought a jump-suit (Al3+) and got rid of the beach-wear. • H3PO4 + Al3+ = AlPO4 + 3 H+ • Now she was comfortable anywhere on land along the beach

  6. Understanding phosphorus behavior in soils • In time Phyllis became lonely, she missed her family, especially her twin sister Phylline. • Phylline lived in dry, southern California, but not too far from the beach. • She was quite stylish, but unstable, and wore OSU sweats (Ca2+) and OU running shoes (H+). • CaHPO4

  7. Understanding phosphorus behavior in soils • Phyllis went to visit her sister Phylline. • At first they weren't comfortable together. Their styles clashed. • CaHPO4 + AlPO4

  8. Understanding phosphorus behavior in soils • With time the sisters learned to overcome their differences. • Phyllis finally found a sweat-suit like Philline’s (Ca2+), and a two-for-one sale for matching (OSU) shoes (Ca2+) • The sisters became inseparable and went everywhere together. • CaHPO4 + AlPO4 + 2 Ca2+ Ca3(PO4)2

  9. O ll -O - P - O- l O- Understanding phosphorus behavior in soils • Review • In nature, P always exists in combination with oxygen (O) in the form of phosphates. • PO43- • Three separate sites for reaction with a single charge cation (H+,K+,NH4+).

  10. Understanding phosphorus behavior in soils • PO43- will react with: • whatever cation is in greatest abundance and, • whatever cation is held with the strongest bond. • In nature, there is plenty of water around: H2O === H+ + OH- Concentration of H+ = OH- = 10-7

  11. O ll HO - P - OH l OH Understanding phosphorus behavior in soils • Review • When the charges on phosphate are all satisfied by H+, in the laboratory, the compound phosphoric acid is formed. • H3PO4

  12. Understanding phosphorus behavior in soils • The H+ leave (dissociate from) phosphoric acid in a stepwise manner when the acid is reacted with base, like sodium hydroxide (NaOH). • H3PO4  H++ H2PO4- • H2PO4- H++HPO42- • HPO42- H++PO43- • One or more of the phosphate forms will be present in solution, depending upon the solution pH.

  13. Understanding phosphorus behavior in soils • One or more of the phosphate forms will be present in solution, depending upon the solution pH.

  14. Understanding phosphorus behavior in soils • In the normal pH range of soils the concentration of H+ in the soil solution is too weak to effectively compete for reaction with phosphate anions. • At pH 6, the H+ concentration is 1 x 10-6 mole/liter. • = 0.000001 g/liter. • = 0.001 ppm. • = 0.001 milliequivalent weights. • At pH 5 the H+ concentration is 10 times greater and at pH 7 it is 10 less than at pH 6.

  15. Understanding phosphorus behavior in soils • Since the H+ concentration is so low in the normal pH range of soils, the concentration of H2PO4- and HPO42- might also be expected to be low. • If there is not enough H+ to react with PO43- in the soil, what else can PO43- react with? • Consider the lyotropic series and the relationship of soil pH and percent base saturation.

  16. 7 6 5 4 Soil pH 0 20 40 60 80 100 Percent Base Saturation If there is not enough H+ to react with PO43- in the soil, what else can PO43- react with? • The lyotropic series is the order of adsorption strength of cations adsorbed on soil colloids: • Al3+~ H+ > Ca2+ ~ Mg2+ > K+ ~ NH4+ > Na+ • Soil pH and % base saturation are directly related:

  17. 7 6 5 4 Soil pH 0 20 40 60 80 100 Percent Base Saturation If there is not enough H+ to react with PO43- in the soil, what else can PO43- react with? • The lyotropic series also relates to acidic and basic cations: • Al3+~ H+ > Ca2+ ~ Mg2+ > K+ ~ NH4+ > Na+ • Strongly acid======== Strongly alkaline:

  18. 7 6 5 4 Soil pH 0 20 40 60 80 100 Percent Base Saturation If there is not enough H+ to react with PO43- in the soil, what else can PO43- react with? • Above soil pH 5.5 there is an abundance of: • Ca2+ ~ Mg2+ > K+ ~ NH4+ > Na+ • Below soil pH 5.5 there are increasing amounts of Al3+~ H+.

  19. Below soil pH 5.5 there are increasing amounts ofAl3+~ H+. Al3+ + H3PO4 == AlPO4 DAPAl3+ + (NH4)2HPO4 ==  AlPO4 + 2 NH4+ MAPAl3+ + NH4H2PO4 ==  AlPO4 + NH4+ • AlPO4is very insoluble. • The reaction rate depends on the concentrations of Al3+ and H3PO4 in the soil solution. • 27 lbsAl3+ will react with about 71 lbs P2O5 to form AlPO4.

  20. Below soil pH 5.5 there are increasing amounts ofAl3+~ H+. • 27 lbsAl3+ will react with about 71 lbs P2O5 to form AlPO4. • When P2O5 is banded with seed, the P2O5 concentration is several times greater than the Al3+ concentration in the band zone. • The concentrations of H2PO4- and HPO42- are very low in acid soils because of their reaction with Al3+.

  21. 7 6 5 4 Soil pH 0 20 40 60 80 100 Percent Base Saturation Above soil pH 5.5 there is an abundance of:Ca2+ ~ Mg2+ > K+ ~ NH4+ > Na+ Ca2++ H3PO4  Ca(H2PO4)2 (0-46-0) DAPCa2++ (NH4)2HPO4  CaHPO4 + 2 NH4+ MAPCa2++ NH4H2PO4  Ca(H2PO4)2 + NH4+ • Ca(H2PO4)2 and CaHPO4 are “highly” water soluble.

  22. Above soil pH 5.5 there is an abundance of:Ca2+ ~ Mg2+ > K+ ~ NH4+ > Na+ • Ca(H2PO4)2 and CaHPO4 are highly water soluble. • These phosphates revert to apatite (rock-phosphate).

  23. Understanding phosphorus behavior in soils: Fertilizers • Phosphate fertilizers dissolve slowly, but react quickly in soils to form solid calcium and aluminum phosphates, which, in time become very insoluble. • DAP (solubility = 23g/100mL) 100 H2O + 23 (NH4)2HPO4 (dissolves slowly)=> HPO42-+ 2 NH4+ (solid) (solution) Ca2+ + HPO42-<===(precipitates quickly)CaHPO4 Equilibrium P concentration is ~ 60 ppm Equilibrium Ca2+ concentration is ~ 70 ppm. Most soil Ca2+ concentrations will be > > 400 ppm

  24. Understanding phosphorus behavior in soils: Fertilizers • Phosphate fertilizers dissolve slowly, but reactquickly in soils to form solid calcium and aluminum phosphates, which, in time become very insoluble.

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