phosphorus Chapter 5

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  

Phosphorus in plant Nutrition Plant tissue contains 0.2-0.4% 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

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

The phosphorus problem in soil Low levels (200-2000kg/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.

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 ↓

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

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

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

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.

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

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

Soil pH affects the forms of P in soil

Organic forms (% of org P) Types of organic P Inositol C6(PO4)6 (10-50%) Nucleic acids (1-5%) Phopholipids (0.2-2.5%)

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)

Phosphorus compounds in soils In solution

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

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

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

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  

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.

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

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.