Phosphorus Behavior in Soils

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Phosphorus Behavior in Soils

• Plant and Soil Sciences Department
• Oklahoma State University

G.V. Johnson
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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

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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-

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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

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Understanding phosphorus behavior in soils

• Even a Norwegian Goddess cant 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

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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
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Understanding phosphorus behavior in soils

• Phyllis went to visit her sister Phylline.

• At first they weren't comfortable together.
  Their styles clashed.

CaHPO4 AlPO4
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Understanding phosphorus behavior in soils

• With time the sisters learned to overcome their
  differences.
• Phyllis finally found a sweat-suit like
  Phillines (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
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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).

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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

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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

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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.

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Understanding phosphorus behavior in soils

• One or more of the phosphate forms will be
  present in solution, depending upon the solution
  pH.

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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.

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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.

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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 gt Ca2 Mg2 gt K NH4 gt Na
• Soil pH and base saturation are directly
  related

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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 gt Ca2 Mg2 gt K NH4 gt Na
• Strongly acid?? Strongly alkaline

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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 gt K NH4 gt Na
• Below soil pH 5.5 there are increasing amounts of
  Al3 H.

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Below soil pH 5.5 there are increasing amounts of
Al3 H.

• Al3 H3PO4 ?? AlPO4
• DAP Al3 (NH4)2HPO4 ? ? AlPO4 2 NH4
• MAP Al3 NH4H2PO4 ? ? AlPO4 NH4
• AlPO4 is very insoluble.
• The reaction rate depends on the concentrations
  of Al3 and H3PO4 in the soil solution.
• 27 lbs Al3 will react with about 71 lbs P2O5 to
  form AlPO4.

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Below soil pH 5.5 there are increasing amounts of
Al3 H.

• 27 lbs Al3 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.

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Above soil pH 5.5 there is an abundance ofCa2
Mg2 gt K NH4 gt Na

• Ca2 H3PO4 ?? Ca(H2PO4)2 (0-46-0)
• DAP Ca2 (NH4)2HPO4 ?? CaHPO4 2 NH4
• MAP Ca2 NH4H2PO4 ?? Ca(H2PO4)2 NH4
• Ca(H2PO4)2 and CaHPO4 are highly water soluble.

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Above soil pH 5.5 there is an abundance ofCa2
Mg2 gt K NH4 gt Na

• Ca(H2PO4)2 and CaHPO4 are highly water soluble.
• These phosphates revert to apatite
  (rock-phosphate).

Mineral Reversion time Chemical formula Ca/P Solubility g/100 mL
Monocalcium phosphate Ca(H2PO4)2 . H2O 0.5 1.8
Dicalcium phosphate 3 weeks CaHPO4 1.0 0.03
Octacalcium phosphate 2 to 5 mo. Ca4H(PO4)3 . 2.5 H2O 1.33
Tricalcium phosphate 8 to 10 mo. Ca3(PO4)2 1.5 0.002 (4 ppm)
Hydroxyapatite 1 to 2 yr. Ca5(PO4)3OH 1.66
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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) gt
  HPO42- 2 NH4
• (solid) (solution)
• Ca2 HPO42- lt (precipitates quickly)?CaHPO4
  
• Equilibrium P concentration is 60 ppm
• Equilibrium Ca2 concentration is 70 ppm.
• Most soil Ca2 concentrations will be gt gt 400 ppm

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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.