Zn

Critical levels of zinc in soil

Critical Limits in plants

Zn content in plant                                            Rating

Below 20 ppm                                 Deficiency

20 - 150 ppm                                  Adequacy level

Above 400 ppm                              Toxic

For rice plant

If Zn content is less than 5 ppm (is the critical limit) - very high response is obtained to the application of zinc

5 - 10 ppm – High response

10 - 15 ppm – Possible response

15 - 20 ppm – No response

Factors affecting availability of zinc in soils

1. Soil pH: Zinc is generally more available in acid soils than in alkali ones. Mostly pH induced deficiencies of zinc occur in pH range of 6 to 8. The critical pH for availability of Zn is 5.5 to 6.5. In the alkaline range particularly above pH 7.85, zinc forms negatively charged ions called zincate ions, thus reducing the availability due to the formation of calcium zincate.

2. Organic matter: The presence of organic matter may promote the availability of zinc by complexing with zinc.

3. CaCO3: Zinc deficiencies are more common in calcareous soils. Zinc was adsorbed on CaCO3 rendering it unavailable.

4. Clay minerals : Zinc adsorption on montmorionite clays from ZnCl2 solutionoccurs as Zn2+, Zn Cl+ and Zn (OH)2 ions.

5. Phosphates : Zn deficiency is very common in soils containing very high amounts of phosphates.

Functions of Zn

·          Essential too formation of growth harmones

·          Helps in reproduction of certain plants.

·          Stabilise rhibosomal fractions

·          Influence the activity of dehydrogenase enzymes

·          Involves in auxin metabolism like tryptophan synthetase.

Zinc

Normal concentration in plant 25 to 150 ppm. Deficiency level is < 20 ppm. Toxic level is > 400 ppm. Zinc content of lithosphere is about 80 ppm. Zn in soil : 10-300 ppm, average 50 ppm. Plant roots absorb as a cation (Zn +2) and as a component of synthetic and natural organic complexes. Zn is involved in many enzymatic activities, but it is not known whether it acts as a functional, structural, or regulatory cofactor.

 Important in the synthesis of tryptophane, a component of some proteins and a compound needed for the production of growth hormones (auxins) such as indoleacetic acid. Reduced growth hormone production in Zn deficient plants causes shortening of internodes and smaller than normal leaves. Zn is involved in chlorophyll synthesis, enzyme activation, and cell membrane integrity.

Sources

Rocks

·               Igneous rocks – 70 ppm

·               Sedimentary rocks – 95 ppm

·               Lime stone – 20 ppm

·               Sand stone – 16 ppm

Zinc bearing minerals

·               Franklinite (ZnFe2O4)

·               Smithsonite (ZnCO3)

·               Willemite (ZnSiO4)

Zn is present in all parts of the plants. In general root contain more zinc than fruits.

         Cereals                    : 27.8 ppm

         Pulses          : 34.8 ppm

         Vegetables  : 28.2 ppm

         Fruits           : 36.6 ppm

Zinc deficienciency commonly found in :

•   Acid sandy soils

•   Neutral / basic / calcareous soils

•   Fine textured soils

•   Soils high in available P

Amelioration of zinc deficiency

·  Application of fertilizer like zinc sulphate, zinc sulphate mono-hydrate, zinc phosphate, chelated zinc.

·  Regular application of farmyard manure, poultry manure or piggery manure.

·  Application of biogas slurry, compost, organic manure.

·  Foliar spray of ZnSo4.

Recommended fertlizer doses

·  Foliar spray of 0.2 to 0.5% ZnSO4.

·  zinc sulphate at  the rate of 5.5 kg/ha.

·  Application of FYM 10t/ha.

·  Poultry manure 5t/ha.

(i) Interaction with other nutrients:

 (a) Phosphorous

§  High P availability can induce Zn deficiency

§  Zn –P reactions in soil leads to formation of  Zn3(PO4)2, causes Zn deficiency.

(ii) Flooding

•  when soils are submerged, conc. of many nutrients increases but not Zn.

•   In acid soils, Zn deficiency may be attributed  to the increase in pH under reducing         conditions.

•   Decreasing pH in submerged caleareous soils would increase Zn solubility.

(iii) Climate

• •  Zn deficiency: Mole pronounced during cool, wet seasons and disappear in warm                        weather.

  •  Increase in soil temperature, increase the solubility of Zn.

  •   Maize, beans are very susceptible for Zn deficiency

  •  Fruit trees : citrus and peach

Zn deficiency symptoms in major crops

Paddy

•   15 – 20 days after transplanting small, scattered light yellow spots appear on the older leaves which later enlarge and turn deep brown known as ‘Khaira’ disease.

Wheat

•   At tillering stage, third leaf from top of the plant develop a band of white / yellow tissue between midrib.

•   Followed by interveinal chlorotic mottling

•   Panicle initiation and maturity delayed.

Maize

•   In 10 – 15 days old plants, middle leaves second/third from the top show light yellow tissue with reddish veins between midrib and edges of the leaf.

•   At 25-35 days old stage, fourth fifth and sixth based leaves show white patches known as ‘white bud’

•   Citrus : little leaf

Remedial measures

Application of Zn by

•  ZnSO4  - most efficient and cheapest

•   ZnCl2

•    ZnO    

Method of application

•   Broad  cast / band placement

•   Time of application : Basal

•   Foliar spray 0.25 – 0.5 % ZnSO4 at 7 – 10 days interval.