1. GES175, Science of Soils
Lecture 10
Phosphorus

2. Phosphorus Soil-Plant Relations
Slide 10.2
Phosphorus Soil-Plant Relations
* Energy and reproduction
* Growth and development
a root growth
a maturity (seed set, flowering,...)
* Maintained by organic matter cycling

3. Water Quality accelerated eutrophication
Slide 10.3
Water Quality
accelerated eutrophication
P (often) promotes algae growth
- may promote anoxia and lead to ‘dead’ zones
P from point and nonpoint sources

4. Phosphorus Fixation * Limited Biological Availability
Slide 10.4
Phosphorus Fixation
* Limited Biological Availability
• P reacts strongly with soil material
- limits bioavailability
- limits transport through soil
- movement occurs via erosion
• Adsorbs and precipitates

5. Adsorption Reactions * Strong adsorption on soil minerals
Slide 10.5
Adsorption Reactions
* Strong adsorption on soil minerals
- adsorption on Fe- and Al-oxides
- adsorption on ‘edge’ sites of silicate clays (dominantly kaolinite)

6. Slide 10.6
Volcanic Ash ëë

7. Inorganic P Compounds (precipitates)
Slide 10.7
Inorganic P Compounds (precipitates)
Acid soils
8 Fe and Al phosphates
FePO4l2H2O, AlPO4l2H2O
Alkaline soils
4 Ca and Mg phosphates

8. Inorganic P Compounds â Ca(H2PO4)2 decreasing solubility Slide 10.8 6
â monocalcium phosphate
â CaHPO4
â dicalcium phosphate
â Ca3(PO4)2
â tricalcium phosphate
â 3Ca3(PO4)2lCa(OH)2
â hydroxyapatite
â 3Ca3(PO4)2lCaCO3
â carbonate apatite 6 pH 8

9. Nomenclature H3PO4 = phosphoric acid H2PO4- = monobasic
Slide 10.9
Nomenclature
H3PO4 = phosphoric acid
H2PO4- = monobasic
HPO4-2 = dibasic
PO4-3 = tribasic

10. Phosphate Ion: Protonation
Slide 10.10
Phosphate Ion: Protonation
acid soils
alkaline
soils

11. Most Available P between pH 6 - 7
Slide 10.11
Most Available P between pH 6 - 7

12. Organic Soil Phosphorus
Slide 10.12
Organic Soil Phosphorus
* % of total soil P is organic
* Mostly inositol phosphates,
C6H6(OH)6
% of organic-P
- some nucleic acid and phospholipids

13. Cycling: A slow release mechanism
Slide 10.13
Cycling: A slow release mechanism
Organic-P
immobilization
mineralization
(available P)
HxPO4x-3
Solid Phase-PO4
(unavailable)

14. Symbiotic Relation: Fungi and Plants
Mycorrhizae root infections, a key to phosphorus uptake
Plant root
Fungal hyphae
mycorrihizae

15. The End

16. Reactions at High pH Values
Slide 10.11
Reactions at High pH Values
* P converts to less soluble
Ca and Mg compounds
Ca(H2PO4)2 + CaCO3 + H2O ® 2 CaHPO4·2H2O + CO2
very less
soluble soluble
6 CaHPO4·2H2O + 3 CaCO3 ® 3 Ca3(PO4)2 + 3 CO2 + 5 H2O
less
soluble
3 Ca3(PO4)2 + CaCO3 ® 3Ca3(PO4)2·CaCO3
very insoluble
- most serious in calcareous soils of arid regions