Transcript PHOSPHORUS - Soil and Environmental Biogeochemistry

GES175, Science of Soils
Lecture 10
Phosphorus
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
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
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
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)
Slide 10.6
Volcanic Ash 
Inorganic P Compounds
(precipitates)
Acid soils
8 Fe and Al phosphates
FePO4l2H2O, AlPO4l2H2O
Alkaline soils
4 Ca and Mg phosphates
Slide 10.7
Slide 10.8
Inorganic P Compounds
decreasing solubility
6
pH

Ca(H2PO4)2
monocalcium phosphate


CaHPO4

dicalcium phosphate

Ca3(PO4)2

tricalcium phosphate

3Ca3(PO4)2 Ca(OH)2

hydroxyapatite

3Ca3(PO4)2 CaCO3
carbonate apatite
l
8
l
Slide 10.9
Nomenclature
H3PO4 = phosphoric acid
H2PO4- = monobasic
HPO4-2 = dibasic
PO4-3 = tribasic
Slide 10.10
Phosphate Ion: Protonation
acid soils
alkaline
soils
Slide 10.11
Most Available P between pH 6 - 7
Slide 10.12
Organic Soil Phosphorus
* 20 - 80 % of total soil P is organic
* Mostly inositol phosphates,
C6H6(OH)6
- 10 - 50 % of organic-P
- some nucleic acid and phospholipids
Cycling: A slow release mechanism
mineralization
immobilization
Organic-P
(available P)
HxPO4x-3
Solid Phase-PO4
(unavailable)
Slide 10.13
Symbiotic Relation:
Fungi and Plants
Mycorrhizae root infections, a key to phosphorus uptake
Plant root
Fungal hyphae
mycorrihizae
The End
Reactions at High pH Values
Slide 10.11
* P converts to less soluble
Ca and Mg compounds
Ca(H2PO4)2 + CaCO3 + H2O  2 CaHPO42H2O + CO2
very
soluble
less
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