Soils 205 Lecture 15

Phosphorus

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A. Soil-Plant Relations

1. Energy and reproduction- found in biomolecules such as ADP, ATP, DNA, and RNA

2. Growth and development- essential macronutrient

{ root growth

{ maturity (seed set, flowering,...)

3. Deficiency - subtle

B. "P fixation"

1. Convert soluble, plant-available P

to

insoluble, unavailable form

2. P reacts strongly with soil constituents

à limits bioavailability

à limits transport through soil

à P tends to build up in heavily fertilized soils

C. Environmental Quality

1. Land Degradation - highly weathered soils

n P maintained by organic cycling

n remove vegetation

- high fixation

- depleted P

- poor growth and erosion

- high P fertilizer requirement

2. Water Quality

n accelerated or cultural eutrophication

n P limits algae growth in many aquatic systems

n P from points and nonpoint sources

- encourages weeds and algae growth

- decaying organic matter = low O₂

D. pH and phosphate ions

H₃PO₄ ¬¾¾® H₂PO₄^- ¬¾¾® HPO₄^-2 ¬¾¾® PO₄^-3

very	é	very
acid	é	alkaline
	pH = 7
	|¬¾¾ at soil pH values ¾¾® |

E. Organic P in Soils

1. 20 - 80 % of total soil P is organic

2. mostly inositol phosphates

- phosphate ester of inositol, C₆H₆(OH)₆

- 10 - 50 % of organic-P

- quite reactive with soil constituents = stable in soil

- some nucleic acid and phospholipids

3. mineralization

- release of P to solution

- especially important in high-fixing soils

4. P is not fixed by organic matter

F. Inorganic P Compounds in soils

1. Acid soils

l Fe and Al phosphates

Fe(OH)₂H₂PO₄ or FePO₄· 2H₂O

and Al(OH)₂H₂PO₄ or AlPO₄·2H₂O

2. Alkaline soils

l Ca and Mg phosphates

decreasing solubility

¯ 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

G. P Solubility in Acid soils

1. Precipitation by soluble Fe and Al

(a) Al(OH)₃ + H⁺ ¬¾® Al(OH)₂⁺ + H₂O

insoluble

Al(OH)₂⁺ + H₂PO₄^- ¬¾® Al(OH)₂H₂PO₄

insoluble

(b) Al and Fe dissolve in strongly acid soils
and reacts with soluble P

2. Reaction with hydrous oxides and silicate clays

(a) Anion sorption to + charge sites

- acid soil + Fe and Al oxides = + charge

Al/Fe(OH)_x + HCl ¬¾® Al/Fe(OH)_x H + -Cl^-

+ H₂PO₄^- ¬¾® A/Fel(OH)_x H + -H₂PO₄^- + Cl^-

(b) Reaction with surface OH groups

Al/Fe(OH)_x -OH + H₂PO₄^- + H⁺

¾® Al/Fe(OH)_x -H₂PO₄ + H₂O

enhanced by acidity

- with time (aging) = very insoluble Fe and Al compounds

(c) large in soils with hydrous oxides

u tropical

u volcanic ash-influenced

(d) 1:1 silicate clays (kaolinite)

u [Al] + H₂PO₄^- ® Al(OH)₂H₂PO₄
in silicate clay

uoccurs over wide pH range

H. P Solubility at High pH Values

1. 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

2. most serious in calcareous soils of arid regions

I. Least fixation in neutral (pH 6 - 7) soils

- liming acid soil = lowers P fixation

- more rapid in acid soils

- O.M. enhances solubility and bioavailability

J. P Sorption

1. Procedure

1. add known amount of P to soil suspension and shake

2. analyze P remaining in solution (I)

3. calculate P sorbed (Q) = added P - solution P

4. plot Q vs I or solution P vs sorbed P

2. Intensity (I) factor

ü soil solution P level

ü about 0.2 mg/L for maximum yield

 

3. Quantity (Q) factor - labile pool

ü source of P for I factor

ü replenishment of P taken up by plants

ü freshly precipitated or sorbed P

4. High P-fixing or sorbing soils

ü difficult plant growth

ü good water treatment