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Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Nutrient availability and leaching / Micronutrient availability: pH, redox and plant mobilisation of Fe and Zn

Micronutrient availability: pH, redox and plant mobilisation of Fe and Zn

Iron, zinc, manganese and copper become far less soluble as pH rises (iron about 1000-fold per pH unit) but more soluble under waterlogging. Plants counter low availability with rhizosphere acidification, reduction and chelators, and about half of the world's cereal soils are low in zinc.

Boundary: where this process starts

Starts with: Fe3+ solubility from oxides falls about 1000-fold per pH unit, and Zn2+, Cu2+ and Mn2+ about 100-fold per unit

Boundary: where it ends

Ends after: Waterlogging reduces Fe3+ and Mn4+ so they dissolve, which can cause Fe or Mn toxicity in acid paddy soil; About half of cereal-growing soils worldwide have low plant-available Zn

PARTICIPANTSRESOURCESIron oxidesPlant rootsSoil organicmatterSoil solutionPlantsIronZincHydrogen ionsCalciteManganeseImpact -3 of 5. Click for details and evidence.Impact +2 of 5. Click for details and evidence.Impact -4 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Deficiency triggers mobilisaImpact +3 of 5. Click for details and evidence.↻ balances · moderate · 0.4 Rhizosphere acidImpact +3 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.Impact +2 of 5. Click for details and evidence.Impact -2 of 5. Click for details and evidence.Impact -2 of 5. Click for details and evidence.Overliming recreates the higImpact +3 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Fe3+ solubility from oxides falls about 1000-fold per pH unit, and Zn2+, Cu2+ and Mn2+ about 100-fold per unit: uses IronFe3+ solubility from oxides falls about 1000-fold per pH unit, and Zn2+, Cu2+ and Mn2+ about 100-fold per unit: uses ZincFe3+ solubility from oxides falls about 1000-fold per pH unit, and Zn2+, Cu2+ and Mn2+ about 100-fold per unit: uses Hydrogen ionsIn calcareous soil at pH 7.5 to 8.5 inorganic Fe3+ falls far below the roughly 10^-8 M plants need, so Fe chlorosis appears: uses IronIn calcareous soil at pH 7.5 to 8.5 inorganic Fe3+ falls far below the roughly 10^-8 M plants need, so Fe chlorosis appears: uses CalciteDicots acidify the rhizosphere, reduce Fe3+ and take up Fe2+, while grasses release phytosiderophores that chelate Fe3+: uses up Ironuses upDicots acidify the rhizosphere, reduce Fe3+ and take up Fe2+, while grasses release phytosiderophores that chelate Fe3+: makes Hydrogen ionsmakesWaterlogging reduces Fe3+ and Mn4+ so they dissolve, which can cause Fe or Mn toxicity in acid paddy soil: uses IronWaterlogging reduces Fe3+ and Mn4+ so they dissolve, which can cause Fe or Mn toxicity in acid paddy soil: uses ManganeseAbout half of cereal-growing soils worldwide have low plant-available Zn: uses ZincOrganic matter binds Cu, Zn and Fe, holding them in forms that are slowly released; Cu is bound most strongly: uses IronOrganic matter binds Cu, Zn and Fe, holding them in forms that are slowly released; Cu is bound most strongly: uses ZincOver-liming pushes pH above 7 and induces Mn, Zn and Fe deficiency: uses ZincOver-liming pushes pH above 7 and induces Mn, Zn and Fe deficiency: uses CalciteOver-liming pushes pH above 7 and induces Mn, Zn and Fe deficiency: uses Manganesesolubility falls with pHIron: solubility falls with pHprecipitates as pH risesZinc: precipitates as pH risesconcentration controlsmetal solubilityHydrogen ions: concentration controls metal solubilityfalls far below plantneedsIron: falls far below plant needsbuffers pH near 8Calcite: buffers pH near 8reduced at root surfaceIron: reduced at root surfaceacidify the rhizosphereHydrogen ions: acidify the rhizospheredissolves underwaterloggingIron: dissolves under waterloggingdissolves, causingtoxicityManganese: dissolves, causing toxicityscarce across cerealsoilsZinc: scarce across cereal soilsbound and slowlyreleasedIron: bound and slowly releasedheld in slowly releasedformsZinc: held in slowly released formsdeficient at high pHZinc: deficient at high pHraises pH above 7Calcite: raises pH above 7induces deficiency athigh pHManganese: induces deficiency at high pHStart: Micronutrientavailability: pH, redoxand plant mobilisati…STARTStart: Micronutrient availability: pH, redox and plant mobilisation of Fe and ZnEnd: Micronutrientavailability: pH, redoxand plant mobilisati…ENDEnd: Micronutrient availability: pH, redox and plant mobilisation of Fe and ZnFe3+ solubility fromoxides falls about1000-fold per pH uni…📄1Fe3+ solubility from oxides falls about 1000-fold per pH unit, and Zn2+, Cu2+ and Mn2+ about 100-fold per unitIn calcareous soil atpH 7.5 to 8.5 inorganicFe3+ falls far bel…📄1In calcareous soil at pH 7.5 to 8.5 inorganic Fe3+ falls far below the roughly 10^-8 M plants need, so Fe chlorosis appearsDicots acidify therhizosphere, reduceFe3+ and take up Fe2…📄1Dicots acidify the rhizosphere, reduce Fe3+ and take up Fe2+, while grasses release phytosiderophores that chelate Fe3+Waterlogging reducesFe3+ and Mn4+ so theydissolve, which c…📄1Waterlogging reduces Fe3+ and Mn4+ so they dissolve, which can cause Fe or Mn toxicity in acid paddy soilAbout half ofcereal-growing soilsworldwide have l…📄1About half of cereal-growing soils worldwide have low plant-available ZnOrganic matter bindsCu, Zn and Fe, holdingthem in forms that a…📄1Organic matter binds Cu, Zn and Fe, holding them in forms that are slowly released; Cu is bound most stronglyOver-liming pushes pHabove 7 and induces Mn,Zn and Fe deficiency📄1Over-liming pushes pH above 7 and induces Mn, Zn and Fe deficiency
Line thickness = size of impact (1 to 5)% on a line = chance a fork takes that branch📄 = pieces of evidence on a stepNegative effectLoops backHypothesisResource used up (arrow into the step)Resource used, not used upResource made or released (arrow out of the step)
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Every step, with its evidence

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Fe3+ solubility from oxides falls about 1000-fold per pH unit, and Zn2+, Cu2+ and Mn2+ about 100-fold per unit
Iron oxidesSoil solutionIronZincHydrogen ions
Evidence (1) · added by Fern
  • Lindsay (1979), Chemical Equilibria in Soils, Wiley: Oxide and hydroxide solubility of Fe3+ varies with the third power of H+ and divalent metals with the second power. (confidence 0.8)
In calcareous soil at pH 7.5 to 8.5 inorganic Fe3+ falls far below the roughly 10^-8 M plants need, so Fe chlorosis appears
CalciteIron oxidesPlantsIronCalcite
Evidence (1) · added by Fern
  • Lindsay & Schwab (1982), The chemistry of iron in soils and its availability to plants, Journal of Plant Nutrition 5:821-840: Inorganic Fe solubility at high pH is many orders of magnitude below plant needs, so chelation and reduction are required. (confidence 0.65)
Dicots acidify the rhizosphere, reduce Fe3+ and take up Fe2+, while grasses release phytosiderophores that chelate Fe3+
Plant rootsRhizosphere microbesIronHydrogen ionsRoot exudates
Evidence (1) · added by Fern
  • Marschner & Romheld (1994), Strategies of plants for acquisition of iron, Plant and Soil 165:261-274: Strategy I and Strategy II plants mobilise Fe by reduction and acidification or by phytosiderophore release. (confidence 0.85)
Waterlogging reduces Fe3+ and Mn4+ so they dissolve, which can cause Fe or Mn toxicity in acid paddy soil
Iron-reducing bacteriaIron oxidesIronManganeseWater
Evidence (1) · added by Fern
  • Ponnamperuma (1972), The chemistry of submerged soils, Advances in Agronomy 24:29-96: Submergence raises soluble Fe2+ and Mn2+, sometimes to toxic concentrations in acid soils. (confidence 0.8)
About half of cereal-growing soils worldwide have low plant-available Zn
PlantsSoilZinc
Evidence (1) · added by Fern
  • Alloway (2009), Soil factors associated with zinc deficiency in crops and humans, Environmental Geochemistry and Health 31:537-548: Zinc deficiency affects roughly half of cereal soils globally, linked to high pH, calcareous and low organic matter soils. (confidence 0.65)
Organic matter binds Cu, Zn and Fe, holding them in forms that are slowly released; Cu is bound most strongly
Soil organic matterSoil organic matterZincIron
Evidence (1) · added by Fern
  • McBride (1994), Environmental Chemistry of Soils, Oxford University Press: Cu2+ forms the strongest complexes with soil organic matter among the divalent micronutrient cations (Irving-Williams order). (confidence 0.7)
Over-liming pushes pH above 7 and induces Mn, Zn and Fe deficiency
LimePlantsCalciteManganeseZinc
Evidence (1) · added by Fern
  • Brady & Weil, The Nature and Properties of Soils, 15th ed. (2016), ch. 15 on micronutrients: Excess lime lowers micronutrient solubility and can induce deficiency. (confidence 0.6)