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

Understanding phosphorus behavior in soils Start with “stripped” down, bare P. Start with “stripped” down, bare P. “P” stands for Norwegian Goddess Phyllis. “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! 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! Start with “stripped” down, bare P. Start with “stripped” down, bare P. “P” stands for Norwegian Goddess Phyllis. “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! 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 P

Understanding phosphorus behavior in soils Phyllis (P) ran to the nearest beach- front store and quickly bought a two- piece bathing suit (O 2 ) and matching beach sandals (O 2 ). Phyllis (P) ran to the nearest beach- front store and quickly bought a two- piece bathing suit (O 2 ) and matching beach sandals (O 2 ). P + O 2 + O 2 = PO 4 3- P + O 2 + O 2 = PO 4 3- She was much more comfortable. She was much more comfortable.

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 + ). 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 + ). PO H + +2 H + = H 3 PO 4 PO H + +2 H + = H 3 PO 4 Now she was comfortable anywhere along the beach. Now she was comfortable anywhere along the beach.

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. 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. Even a Norwegian Goddess can’t spend all her time at the beach. Phyllis needed to move about the land. 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. H 3 PO 4 + Al 3+ = AlPO H + She bought a jump-suit (Al 3+ ) and got rid of the beach-wear. She bought a jump-suit (Al 3+ ) and got rid of the beach-wear. She went shopping again. She went shopping again. Now she was comfortable anywhere on land along the beach Now she was comfortable anywhere on land along the beach

Understanding phosphorus behavior in soils CaHPO 4 In time Phyllis became lonely, she missed her family, especially her twin sister Phylline. 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. Phylline lived in dry, southern California, but not too far from the beach. She was quite stylish, but unstable, and wore OSU sweats (Ca 2+ ) and OU running shoes (H + ). She was quite stylish, but unstable, and wore OSU sweats (Ca 2+ ) and OU running shoes (H + ). In time Phyllis became lonely, she missed her family, especially her twin sister Phylline. 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. Phylline lived in dry, southern California, but not too far from the beach. She was quite stylish, but unstable, and wore OSU sweats (Ca 2+ ) and OU running shoes (H + ). She was quite stylish, but unstable, and wore OSU sweats (Ca 2+ ) and OU running shoes (H + ).

Understanding phosphorus behavior in soils CaHPO 4 + AlPO 4 Phyllis went to visit her sister Phylline. Phyllis went to visit her sister Phylline. At first they weren't comfortable together. Their styles clashed. At first they weren't comfortable together. Their styles clashed.

Understanding phosphorus behavior in soils With time the sisters learned to overcome their differences. With time the sisters learned to overcome their differences. Phyllis finally found a sweat-suit like Philline’s (Ca 2+ ), and a two-for-one sale for matching (OSU) shoes (Ca 2+ ) Phyllis finally found a sweat-suit like Philline’s (Ca 2+ ), and a two-for-one sale for matching (OSU) shoes (Ca 2+ ) With time the sisters learned to overcome their differences. With time the sisters learned to overcome their differences. Phyllis finally found a sweat-suit like Philline’s (Ca 2+ ), and a two-for-one sale for matching (OSU) shoes (Ca 2+ ) Phyllis finally found a sweat-suit like Philline’s (Ca 2+ ), and a two-for-one sale for matching (OSU) shoes (Ca 2+ ) The sisters became inseparable and went everywhere together. The sisters became inseparable and went everywhere together. CaHPO 4 + AlPO Ca 2+  Ca 3 (PO 4 ) 2

Understanding phosphorus behavior in soils In nature, P always exists in combination with oxygen (O) in the form of phosphates. In nature, P always exists in combination with oxygen (O) in the form of phosphates. PO 4 3- PO 4 3- In nature, P always exists in combination with oxygen (O) in the form of phosphates. In nature, P always exists in combination with oxygen (O) in the form of phosphates. PO 4 3- PO 4 3- Review Review - O - P - O - OllOll l l lO-O- l lO-O- l l lO-O- l lO-O- Three separate sites for reaction with a single charge cation (H +, K +, NH 4 + ). Three separate sites for reaction with a single charge cation (H +, K +, NH 4 + ).

Understanding phosphorus behavior in soils PO 4 3- will react with: PO 4 3- will react with: whatever cation is in greatest abundance and, whatever cation is in greatest abundance and, whatever cation is held with the strongest bond. whatever cation is held with the strongest bond. In nature, there is plenty of water around: In nature, there is plenty of water around: H 2 O  ===  H + + OH - Concentration of H + = OH - = PO 4 3- will react with: PO 4 3- will react with: whatever cation is in greatest abundance and, whatever cation is in greatest abundance and, whatever cation is held with the strongest bond. whatever cation is held with the strongest bond. In nature, there is plenty of water around: In nature, there is plenty of water around: H 2 O  ===  H + + OH - Concentration of H + = OH - = 10 -7

Understanding phosphorus behavior in soils When the charges on phosphate are all satisfied by H +, in the laboratory, the compound phosphoric acid is formed. When the charges on phosphate are all satisfied by H +, in the laboratory, the compound phosphoric acid is formed. H 3 PO 4 H 3 PO 4 When the charges on phosphate are all satisfied by H +, in the laboratory, the compound phosphoric acid is formed. When the charges on phosphate are all satisfied by H +, in the laboratory, the compound phosphoric acid is formed. H 3 PO 4 H 3 PO 4 Review Review HO - P - OH OllOll l l lOHOH l lOHOH l l lOHOH l lOHOH

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). The H + leave (dissociate from) phosphoric acid in a stepwise manner when the acid is reacted with base, like sodium hydroxide (NaOH). H 3 PO 4  H + + H 2 PO 4 - H 3 PO 4  H + + H 2 PO 4 - H 2 PO 4 -  H + + HPO H 2 PO 4 -  H + + HPO HPO  H + + PO HPO  H + + PO One or more of the phosphate forms will be present in solution, depending upon the solution pH. One or more of the phosphate forms will be present in solution, depending upon the solution pH. The H + leave (dissociate from) phosphoric acid in a stepwise manner when the acid is reacted with base, like sodium hydroxide (NaOH). The H + leave (dissociate from) phosphoric acid in a stepwise manner when the acid is reacted with base, like sodium hydroxide (NaOH). H 3 PO 4  H + + H 2 PO 4 - H 3 PO 4  H + + H 2 PO 4 - H 2 PO 4 -  H + + HPO H 2 PO 4 -  H + + HPO HPO  H + + PO HPO  H + + PO One or more of the phosphate forms will be present in solution, depending upon the solution pH. One or more of the phosphate forms will be present in solution, depending upon the solution pH.

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

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. 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 mole/liter. At pH 6, the H + concentration is 1 x mole/liter. = g/liter. = g/liter. = ppm. = ppm. = milliequivalent weights. = milliequivalent weights. At pH 5 the H + concentration is 10 times greater and at pH 7 it is 10 less than at pH 6. At pH 5 the H + concentration is 10 times greater and at pH 7 it is 10 less than at pH 6. 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. 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 mole/liter. At pH 6, the H + concentration is 1 x mole/liter. = g/liter. = g/liter. = ppm. = ppm. = milliequivalent weights. = milliequivalent weights. At pH 5 the H + concentration is 10 times greater and at pH 7 it is 10 less than at pH 6. At pH 5 the H + concentration is 10 times greater and at pH 7 it is 10 less than at pH 6.

Understanding phosphorus behavior in soils Since the H + concentration is so low in the normal pH range of soils, the concentration of H 2 PO 4 - and HPO 4 2- might also be expected to be low. Since the H + concentration is so low in the normal pH range of soils, the concentration of H 2 PO 4 - and HPO 4 2- might also be expected to be low. If there is not enough H + to react with PO 4 3- in the soil, what else can PO 4 3- react with? If there is not enough H + to react with PO 4 3- in the soil, what else can PO 4 3- react with? Consider the lyotropic series and the relationship of soil pH and percent base saturation. Consider the lyotropic series and the relationship of soil pH and percent base saturation. Since the H + concentration is so low in the normal pH range of soils, the concentration of H 2 PO 4 - and HPO 4 2- might also be expected to be low. Since the H + concentration is so low in the normal pH range of soils, the concentration of H 2 PO 4 - and HPO 4 2- might also be expected to be low. If there is not enough H + to react with PO 4 3- in the soil, what else can PO 4 3- react with? If there is not enough H + to react with PO 4 3- in the soil, what else can PO 4 3- react with? Consider the lyotropic series and the relationship of soil pH and percent base saturation. Consider the lyotropic series and the relationship of soil pH and percent base saturation.

If there is not enough H + to react with PO 4 3- in the soil, what else can PO 4 3- react with? The lyotropic series is the order of adsorption strength of cations adsorbed on soil colloids: The lyotropic series is the order of adsorption strength of cations adsorbed on soil colloids: Al 3+ ~ H + > Ca 2+ ~ Mg 2+ > K + ~ NH 4 + > Na + Al 3+ ~ H + > Ca 2+ ~ Mg 2+ > K + ~ NH 4 + > Na + Soil pH and % base saturation are directly related: Soil pH and % base saturation are directly related: The lyotropic series is the order of adsorption strength of cations adsorbed on soil colloids: The lyotropic series is the order of adsorption strength of cations adsorbed on soil colloids: Al 3+ ~ H + > Ca 2+ ~ Mg 2+ > K + ~ NH 4 + > Na + Al 3+ ~ H + > Ca 2+ ~ Mg 2+ > K + ~ NH 4 + > Na + Soil pH and % base saturation are directly related: Soil pH and % base saturation are directly related: Percent Base Saturation Soil pH

The lyotropic series also relates to acidic and basic cations: The lyotropic series also relates to acidic and basic cations: Al 3+ ~ H + > Ca 2+ ~ Mg 2+ > K + ~ NH 4 + > Na + Al 3+ ~ H + > Ca 2+ ~ Mg 2+ > K + ~ NH 4 + > Na + Strongly acid  ========  Strongly alkaline: Strongly acid  ========  Strongly alkaline: The lyotropic series also relates to acidic and basic cations: The lyotropic series also relates to acidic and basic cations: Al 3+ ~ H + > Ca 2+ ~ Mg 2+ > K + ~ NH 4 + > Na + Al 3+ ~ H + > Ca 2+ ~ Mg 2+ > K + ~ NH 4 + > Na + Strongly acid  ========  Strongly alkaline: Strongly acid  ========  Strongly alkaline: Percent Base Saturation Soil pH If there is not enough H + to react with PO 4 3- in the soil, what else can PO 4 3- react with?

Above soil pH 5.5 there is an abundance of: Above soil pH 5.5 there is an abundance of: Ca 2+ ~ Mg 2+ > K + ~ NH 4 + > Na + Ca 2+ ~ Mg 2+ > K + ~ NH 4 + > Na + Below soil pH 5.5 there are increasing amounts of Al 3+ ~ H +. Below soil pH 5.5 there are increasing amounts of Al 3+ ~ H +. Above soil pH 5.5 there is an abundance of: Above soil pH 5.5 there is an abundance of: Ca 2+ ~ Mg 2+ > K + ~ NH 4 + > Na + Ca 2+ ~ Mg 2+ > K + ~ NH 4 + > Na + Below soil pH 5.5 there are increasing amounts of Al 3+ ~ H +. Below soil pH 5.5 there are increasing amounts of Al 3+ ~ H Percent Base Saturation Soil pH If there is not enough H + to react with PO 4 3- in the soil, what else can PO 4 3- react with?

Al 3+ + H 3 PO 4  = =  AlPO 4 DAP Al 3+ + (NH 4 ) 2 HPO 4  = =  AlPO NH 4 + MAP Al 3+ + NH 4 H 2 PO 4  = =  AlPO 4 + NH 4 + MAP Al 3+ + NH 4 H 2 PO 4  = =  AlPO 4 + NH 4 + AlPO 4 is very insoluble. AlPO 4 is very insoluble. The reaction rate depends on the concentrations of Al 3+ and H 3 PO 4 in the soil solution. The reaction rate depends on the concentrations of Al 3+ and H 3 PO 4 in the soil solution. 27 lbs Al 3+ will react with about 71 lbs P 2 O 5 to form AlPO lbs Al 3+ will react with about 71 lbs P 2 O 5 to form AlPO 4. Al 3+ + H 3 PO 4  = =  AlPO 4 DAP Al 3+ + (NH 4 ) 2 HPO 4  = =  AlPO NH 4 + MAP Al 3+ + NH 4 H 2 PO 4  = =  AlPO 4 + NH 4 + MAP Al 3+ + NH 4 H 2 PO 4  = =  AlPO 4 + NH 4 + AlPO 4 is very insoluble. AlPO 4 is very insoluble. The reaction rate depends on the concentrations of Al 3+ and H 3 PO 4 in the soil solution. The reaction rate depends on the concentrations of Al 3+ and H 3 PO 4 in the soil solution. 27 lbs Al 3+ will react with about 71 lbs P 2 O 5 to form AlPO lbs Al 3+ will react with about 71 lbs P 2 O 5 to form AlPO 4. Below soil pH 5.5 there are increasing amounts of Al 3+ ~ H +.

27 lbs Al 3+ will react with about 71 lbs P 2 O 5 to form AlPO lbs Al 3+ will react with about 71 lbs P 2 O 5 to form AlPO 4. When P 2 O 5 is banded with seed, the P 2 O 5 concentration is several times greater than the Al 3+ concentration in the band zone. When P 2 O 5 is banded with seed, the P 2 O 5 concentration is several times greater than the Al 3+ concentration in the band zone. The concentrations of H 2 PO 4 - and HPO 4 2- are very low in acid soils because of their reaction with Al 3+. The concentrations of H 2 PO 4 - and HPO 4 2- are very low in acid soils because of their reaction with Al lbs Al 3+ will react with about 71 lbs P 2 O 5 to form AlPO lbs Al 3+ will react with about 71 lbs P 2 O 5 to form AlPO 4. When P 2 O 5 is banded with seed, the P 2 O 5 concentration is several times greater than the Al 3+ concentration in the band zone. When P 2 O 5 is banded with seed, the P 2 O 5 concentration is several times greater than the Al 3+ concentration in the band zone. The concentrations of H 2 PO 4 - and HPO 4 2- are very low in acid soils because of their reaction with Al 3+. The concentrations of H 2 PO 4 - and HPO 4 2- are very low in acid soils because of their reaction with Al 3+. Below soil pH 5.5 there are increasing amounts of Al 3+ ~ H +.

Ca 2+ + H 3 PO 4  Ca(H 2 PO 4 ) 2 (0-46-0) DAP Ca 2+ + (NH 4 ) 2 HPO 4  CaHPO NH 4 + MAP Ca 2+ + NH 4 H 2 PO 4  Ca(H 2 PO 4 ) 2 + NH 4 + MAP Ca 2+ + NH 4 H 2 PO 4  Ca(H 2 PO 4 ) 2 + NH 4 + Ca(H 2 PO 4 ) 2 and CaHPO 4 are “highly” water soluble. Ca(H 2 PO 4 ) 2 and CaHPO 4 are “highly” water soluble. Ca 2+ + H 3 PO 4  Ca(H 2 PO 4 ) 2 (0-46-0) DAP Ca 2+ + (NH 4 ) 2 HPO 4  CaHPO NH 4 + MAP Ca 2+ + NH 4 H 2 PO 4  Ca(H 2 PO 4 ) 2 + NH 4 + MAP Ca 2+ + NH 4 H 2 PO 4  Ca(H 2 PO 4 ) 2 + NH 4 + Ca(H 2 PO 4 ) 2 and CaHPO 4 are “highly” water soluble. Ca(H 2 PO 4 ) 2 and CaHPO 4 are “highly” water soluble Percent Base Saturation Soil pH Above soil pH 5.5 there is an abundance of: Ca 2+ ~ Mg 2+ > K + ~ NH 4 + > Na +

Ca(H 2 PO 4 ) 2 and CaHPO 4 are highly water soluble. Ca(H 2 PO 4 ) 2 and CaHPO 4 are highly water soluble. These phosphates revert to apatite (rock-phosphate). These phosphates revert to apatite (rock-phosphate). Ca(H 2 PO 4 ) 2 and CaHPO 4 are highly water soluble. Ca(H 2 PO 4 ) 2 and CaHPO 4 are highly water soluble. These phosphates revert to apatite (rock-phosphate). These phosphates revert to apatite (rock-phosphate). Above soil pH 5.5 there is an abundance of: Ca 2+ ~ Mg 2+ > K + ~ NH 4 + > Na + Mineral Reversion time Chemical formula Ca/P Solubility g/100 mL Monocalcium phosphate Ca(H 2 PO 4 ) 2. H 2 O Dicalcium phosphate 3 + weeks CaHPO Octacalcium phosphate 2 to 5 mo. Ca 4 H(PO 4 ) H 2 O 1.33 Tricalcium phosphate 8 to 10 mo. Ca 3 (PO 4 ) (4 ppm) Hydroxyapatite 1 to 2 yr. Ca 5 (PO 4 ) 3 OH1.66

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. 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) DAP (solubility = 23g/100mL) 100 H 2 O + 23 (NH 4 ) 2 HPO 4  (dissolves slowly) = > HPO NH 4 + (solid)(solution) Ca 2+ + HPO 4 2- < == = (precipitates quickly)  CaHPO 4 Equilibrium P concentration is ~ 60 ppm Equilibrium Ca 2+ concentration is ~ 70 ppm. Most soil Ca 2+ concentrations will be > > 400 ppm Phosphate fertilizers dissolve slowly, but react quickly in soils to form solid calcium and aluminum phosphates, which, in time become very insoluble. 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) DAP (solubility = 23g/100mL) 100 H 2 O + 23 (NH 4 ) 2 HPO 4  (dissolves slowly) = > HPO NH 4 + (solid)(solution) Ca 2+ + HPO 4 2- < == = (precipitates quickly)  CaHPO 4 Equilibrium P concentration is ~ 60 ppm Equilibrium Ca 2+ concentration is ~ 70 ppm. Most soil Ca 2+ concentrations will be > > 400 ppm

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. Phosphate fertilizers dissolve slowly, but react quickly in soils to form solid calcium and aluminum phosphates, which, in time become very insoluble.