1. phosphorus
Chapter 5

2. Phosphorus - P
 Phosphorus levels are low in soils- in archeology P enrichment is evidence of human influence
P is environmentally important
Eutrophication /pollution- P from sewage sludge and agricultural lands

3. Phosphorus in plant Nutrition
Plant tissue contains % P in DM
Essential component of ATP (.. energy currency):
Nutrients uptake; transport
Component of bi-omolecules: DNA, RNA, Phospholipids ; bones and teeth
Essential in the processes
Photosynthesis; N fixation,
Reproduction; maturation
crop quality and structural strength

4. Deficiency Symptoms Stunting, thin stems
Dark green foliage (bluish green)
Yellowing and senescence of leaves
Purpling in some plants in leaves and stems
 Delayed maturity, sparse flowering, poor seed quality
Mobile in plants therefore older leaves show deficiency first

5. The phosphorus problem in soil
Low levels ( kg/Ha to 15cm depth
P compounds largely insoluble and unavailable
fixed and rendered unavailable in soils- even that applied as manure and fertilizer
Lack of P is usually the first limiting factor in crop production.

6. Land degradation
low soil P land degradation esp in humid region- highly weathered soils
insoluble Al-P compounds dominant
clearing of forests induces P losses in R/off
P cycling reserves P in soils as long as no disturbances occur to the system
 Soil P levels decline and re-growth of vegetation bcmes sparse:
leguminous BNF ↓, erosion ↑, Om, WHC and fertilty↓, soil degradation ↑, vegtn cover ↓

7. Water quality degradation
Through eutrophication from point sources and non-point sources
Agric operations increase soil P levels in R/O
Fertilizers & animal manures from P-enriched feeds
Particulate P losses in sediments (erosion)
Tillage promotes loss of clay and OM fractions rich in P leaving behind the coarser fractions which are lower in P levels.
from livestock wastes, P in run off and P from fertilized lands.
Point sources- outflow from sewage effluent and industrial plants
Non-point sources- Run off water and sediments from scattered soils though out the affected watershed

8. The Phosphorus cycle
Effective P management in soils only possible with an understanding of the various pools of soil P and the interactions between them

10. Plant and animal residues
P cycle
fertilizer
Plant and animal residues
Soil organic matter
(non labile P
(Labile P)
minerlizn
imoblzn
adsorbed
Plant uptake
Adsorbed P
(labile P)
dissolution
Secondary minerals
Fe/Al PO4
CaHPO4
(non Labile)
Primary minerals
(non labile)
leaching
precipitation
Solution P H2PO4- HPO4-
Desorbed

11. Soil Phosphorus forms usu < 0.01% of P total in soil
Phosphorus is present in soil as :
Soil solution inorganic/Plant available P
usu < 0.01% of P total in soil
Organic Phosphorus –Phosphate esters (inositol phosphates); nucleic acids and phospholipids
inorganic phosphorus compounds
Calcium Phosphate copds (in alkaline soils)
Iron-, manganese and aluminium-P (in acid soils)
All three forms of phosphorus are insoluble and only contribute slowly to soil solution inorganic P.

12. Soil phosphorus pools
· solution P
· active P
· fixed P

13. P in Solution Very low- 0.001-1 mg/L
Forms taken up by plants are strongly pH dependent
H2PO4- { - monovalent - predominates at pH = 4 to 4.5}
HPO42- {- divalent - predominates under alkaline conditions}
Soluble org Phosphorus
Both divalent and monovalent present at pH around neutral

14. Soil pH affects the forms of P in soil

16. Organic forms (% of org P)
Types of organic P
Inositol C6(PO4)6 (10-50%)
Nucleic acids (1-5%)
Phopholipids ( %)

17. Organic P- mineralization & immobilization
Fe2+, Al3+, Ca2+
Org P Microbes H2PO4- Fe2+, Al3+, Ca2+ Phosphates microbes insoluble mineralization
Immobilization: C: P> 300: 1
Mineralization: C:P < 200: 1
Organic P contributes more to plant Phosphorus needs in intensely weathered soils like oxisols ultisols ( rich in Fe and Al oxides that are extremely insoluble)

18. Phosphorus compounds in soils
In solution

19. Inorganic P minerals-soil P copds
 Mainly in 2 groups:
Ca—P compounds: eg Apatite
predominant in alkaline soils, more soluble as pH decreases ,
Apatite minerals are the least soluble
Fe, Al or Mn—P copds: (strengite & variscite)
Iron and aluminium hydroxy-phosphates:
very low solubility in acid soils. Solubility increases with pH in
unstable in alkaline soils , predominant in acid soils.
Ca—P compounds and Al, Fe or Mn—P compounds (calcium phosphate compounds or aluminium—, Iron— or Manganese—phosphate compounds

20. 4 Dustbin (%) Soil pH 100 50 5 6 7 8 fixation by soluble Fe, Al & Mn
fixation by hydrous oxides of Fe & Al
fixation mostly as Calcium phosphates
Rxns with silicate minerals
Dustbin (%)
Soil pH
100 50 4 5 6 7 8
Relatively avail phosphates
Rel. avail. phosphates

21. Factors affecting P fixation in soils
P fixing capacity- dependent on clay type and clay content
2:1 clays<<1:1 clays < carbonate xstals< xstalline Al, Fe, Mn oxides< armophous Al, Fe, Mn oxides< allophone
Soil pH - p fixtion highest at pH extremes i.e. very acid and very alkaline soil conditions
phosphate fixation is lowest at pH = 6 – 7

22. Effect of Organic matter:- addition of decomposable OM reduces P fixation
OM molecules mask fixation sites and prevent P fixation on those sites
Organic matter anions produced during decomposition by plant roots and microbes compete with P for exchange or fixation sites on clay colloids
Chelation of Fe, Mn and Al and their subsequent removal from forming insoluble P copds

23. Practical control of P in soils
Quench soil P fixing capacity by adding P in excess of crop requirements
Localized placement-banded application of P to reduce costs
In no-till systems, P may be broadcast on the surface forming a horizontal band.
Combine ammonium and phosphorus fertilizers:
ammonium and phosphorus fertilizers are mixed in a band
ammonium fertilizers releases acidity when NH4+ oxidizes to NO3-
Uptake of excess NH4+ ions also produces acidity which aids solubilization of P compounds.

24. Cycling of organic matter
microbial break down of OM releases P slowly facilitating uptake by mycchorrhizae and plants before fixation occurs
organic compounds can reduce P fixation capacity of soils
Enhanced by the use of P efficient plants which encourage cycling
Control of soil pH between 6 and 7

25. Enhance mychorrhizal symbiosis
Include good michorhizal host plants
Reduce tillage
Inoculation with appropriate fungi
vi) choose efficient plants- some plants require less p in soil solution
extensive root systems by monocots
N-fixing legumes acidify their rhizosphere by taking little nitrate
Some plants secrete such compounds as piscidic acid (pigeon pea) which complexes Fe thereby increasing availability of Fe—P cpds
Plants in the cruciferae family (cabbage, mustard seed) excrete malic and citric acids and also form extensive root systems.
Choice of plant will aid restorative ecology approaches and initiatives.