Zn Nutrient cycle
Introduction
 The zinc cycle is a biogeochemical cycle that transports zinc
through the lithosphere, hydrosphere, and biosphere
 Zn is an important micronutrient for plant growth.
 Essentiality of Zn was discovered by- A.L. Sommer and C.P.
Lipman.
 In plant Zn content varies from- 27 ppm to 100 ppm
 In Soil Zn content in soils varies from1. Arid/semi-arid climate - 20-89 mg/kg
2. Humid/sub-humid tropics – 22-74 mg/kg
3. Vertisols - 69-76 mg/kg
4. Oxisols (coarse textured)- 24-30 mg/kg
Role of Zinc in Plant system
 Low Molecular weight complexes of Zinc- In plant leaves
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soluble Zinc occurs mainly as anionic compound possibly associated
with amino acid.
Carbohydrate metabolismPhotosynthesis- Zinc is a constituent of Carbonic anhydrase enzyme,
which have role in co2 fixation.
Protein metabolism- Zinc is necessary for the activity of RNA
polymerase enzyme and it protects ribosomal RNA from attack by the
enzyme ribonuclease.
Membrane integrity- The role of Zinc in maintaining the integrity
of cellular membranes involving structural orientation of
macromolecules and maintenance of ion transport systems.
Auxin metabolism- Zinc is required for synthesis of Auxin, zinc is
required for synthesis of tryptophan which is precursor of Auxin.
Zinc distribution
 The total amount of zinc in soils is distributed over 5
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fractions (or pools). These comprise:
i) The water soluble pool/Soil solution form of Zn:
present in the soil solution,
ii) Exchangeable pool: ions bound to soil particles by
electrical charges,
iii) Organically bound pool: ions adsorbed, chelated or
complexed with organic ligands
iv) Pool of zinc sorbed non-exhangeably onto clay
minerals and insoluble metallic oxides,
v) Pool of weathering primary minerals
Factors affecting Zn availability
 Soil pH and Liming
 Hydrous oxides of iron and aluminium
 Carbonates of calcium and magnesium
 Organic matter
 Soil submergence
 Nutrients other than Zinc
 Seasons
Form of Zinc in Soil
1.Mineral form- Zinc exist as Zinc sulphides, Zinc
carbonates, and Zinc silicates. On weathring Zn ion
released.
Sphalarite- ZnS
Smithsonite- ZnCO3
Willemite- ZnSiO4
Franklinite- ZnFe2O4
Mineral form
Sphalarite- ZnS
Smithsonite- ZnCO3
Willemite- ZnSiO4
Franklinite- ZnFe2O4
Cont’d
2. Adorbed form- Zn is adsorbed on the surface of
clays, oxide minerals, carbonates and organic matters.
3.Solution form- In soil solution Zn exists as Zn ion and
Zn(OH)+.
4.Organic complex form-Zn form stable complex with
organic colloids. This form is not readily available to
plants
How does Zn get into the soil
The most important sources of anthropogenic zinc
in soil come from
 Discharges of smelter slags and wastes, mine
tailings,
 Coal and bottom fly ash, and
 The use of commercial products such as
fertilizers and
 Wood preservatives that contain zinc.Zinc does
not volatilize from soil.
Cont’d
Smelter slugs and wastes
Mine tailings
Zn Fertilizer
Coal and bottom fly ash
Wood Preservative
Biological Transformation of Zinc
The distribution of zinc between these forms is governed
by the reaction include
 precipitation and dissolution,
 complexation and decomplexation,
 adsorption and desorption
Precipitation and Dissolution,
 Precipitation
 This reactions occur when two or more compounds react to
form a new, insoluble compound (the precipitate) that "falls
out" of solution.
 Dissolution
 It is the process where a solute in gaseous, liquid, or solid
phase dissolves in a solvent to form a solution.
 At neutral to alkaline pH values precipitation—dissolution
reactions of zinc may take place.
Complexation and Decomplexation
 Complexation
 A complexation reaction can be described as a reaction that
forms a complex compound.
 A complex is defined as a molecular structure that consists
of a metal atom or ion in the center, surrounded by ligands
 Decomplexation
 It refers to the removal of a ligand from a coordination
complex. Decomplexation is of particular interest when the
ligand has been synthesized within the coordination sphere of
the metal, as is often the case in organometallic chemistry.
Adsorption and Desorption
 Adsorption
 It is a process in which zinc present in soil solution is
attached/bound to the surface of soil particles
 Adsorption is a fast process and reversible in nature
 Desorption
 It means that adsorbed Zinc can be released into soil solution via a
process known as desorption and will be available for plant
uptake.
 The concentrations of zinc in equilibrium solutions with soil clay
fractions and whole soil samples at pH values below 7 are
determined exclusively by adsorption—desorption reactions for
various pH's