This document discusses micronutrients and factors affecting their availability in soils and plants. It defines micronutrients as nutrients required in small quantities that are essential for plant growth. Criteria for nutrient essentiality and classifications of micronutrients are provided. Key functions and deficiency symptoms of various micronutrients like iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel are summarized. Factors affecting availability of micronutrient cations and anions in soil are outlined. The role of chelates and various chelating agents in enhancing micronutrient availability is briefly explained.

1. MICRONUTRIENTS – CRITICAL LIMITS IN SOILS AND
PLANTS, FACTORS AFFECTING THEIR AVAILABILITY
AND CORRECTION OF THEIR DEFICIENCIES IN
PLANTS; ROLE OF CHELATES IN NUTRIENT
AVAILABILITY

2. Criteria of Essentiality of Nutrients
• This concept was propounded by Arnon and Stout (1939) For an element be regarded as an essential nutrient, it must
satisfy the following criteria;
• A deficiency of an essential nutrient element makes it impossible for the plant to complete the vegetative or
reproductive stage of its life cycle.
• The deficiency of an element is very specific to the element in question and deficiency can be corrected /prevented
only by supplying that particular element.
• The element must directly be involved in the nutrition and metabolism of the plant and have a direct influence on
plant apart from its possible effects in correcting some micro-biological or chemical conditions of the soil or other
culture medium.
Rattan and Goswami, 2012

3. Essential Nutrients
Macronutrients
Basic elements-
C,H,O
Primary(N,P,K)
Secondary(Ca,Mg,S)
Micronutrients
Cations(Fe,
Zn,Mo,Cu)
Anions(Ni,B,Mn,Cl)

4. MICRONUTRIENTS
• Nutrients that are required in relatively smaller quantities but are as essential as
macronutrients are termed micronutrients. These include Iron, manganese, Zinc, Copper,
Boron, Molybdenum, Chlorine and Nickel.
• Micronutrients are Subdivided into micronutrient cations ( Fe, Mn, Zn, Cu, Ni) and
micronutrient anions ( B, Mo and Cl)
Rattan and Goswami, 2012

5. Functions and Deficiency Symptoms of
Nutrients:
• Iron:
Iron is taken up as ferrous ions (Fe2+) by plants. Its concentration in the
range of 100- 500 mg/kg in mature leaf tissues is regarded sufficient for optimum
crop production. Iron is a transition metal, exhibits two oxidation states – Fe(II) and
Fe(III).
Functions of Iron:
• It is a constituent of two groups of proteins, viz. (a) Heme proteins containing Fe
porphyrin complex as a prosthethic group : Cytochrome oxidase, catalase,
peroxidase, leghaemoglobin
• It activates number of enzymes

6. • It plays an essential role in the nucleic acid metabolism
• It is necessary for synthesis and maintenance of chlorophyll in plants.
• Deficiency Symptoms:
• Plants having less than 50 PPm of Fe are usually classified as iron-
deficient.
• Deficiency of iron results in interveinal chlorosis appearing first on the
younger leaves with leaf margins and veins remaining green.
• Under condition of severe deficiency growth cessation occurs with the
whole plant turning necrotic.

7. Manganese
• Manganese is absorbed by the plants as manganous ions (Mn2+). Healthy
Mn sufficient mature plants contain 20 to 300 ppm of Mn.
• Functions of Manganese:
• Mn plays an important role in photosynthesis and detoxification of
superoxide free radicals.
• Mn is an integral component of the water- splitting enzyme associated with
photosystem II. Because of this role Mn deficiency is associated with
adverse effects on photosynthesis and 02 evolution.

8. Deficiency Symptoms:
• Manganese deficient plants contain less than 25 ppm Mn. Deficiency symptoms of
Mn are more severe on middle leaves than on the younger ones because Mn is
preferentially translocated to the younger tissues.
• Zinc:
• Plants absorb Zn as Zinc ions (Zn2+). Zinc sufficient plants contain 27- 150 ppm Zn in
mature tissues.
• Functions of Zinc:
• Zinc is a constituent of three enzymes i.e: Carbonic anhydrase, superoxide dismutase
and alcoholic dehydrogenase

9. • Zinc is involved in the synthesis of indole acetic acid, metabolism of
gibberellic acid and synthesis of RNA.
• Zinc influences translocation and transport of P in plants. Under Zn
deficiency, excessive translocation of P occurs, resulting in P-toxicity.
• Deficiency Symptoms:
• Plants containing less than 15 ppm Zn are regarded deficient in Zinc.
Common deficiency symptoms of Zn are interveinal chlorosis, first appear
on the young leaves , reduction in size of young leaves bronzing and purple,
violet reddish brown coloration of the foliage.eg. Khaira disease of rice
reported from India, Akagare type II in Japan

10. Copper

11. Copper containing enzymes and their
biological role
Copper containing enzyme Biochemical role
Plastocyanin Located in chloroplasts, it is a component of electron system of
photosystem II and influences photosynthesis
Superoxide dismutase Located in cytoplasm, mitochondria and chloroplasts, it involves
in detoxification of superoxide radicals generated during
photosynthesis.
Diamine oxidase Located in apoplasts of epidermis and xylem tissues, functions as
H2o2 delivery system for peroxidase activity in lignification
Polyphenol oxidase Involved in lignin biosynthesis
Ascorbate oxidase Occuring in cell walls and cytoplasm. It catalyses the oxidation of
ascorbic acid to dehydroascorbic acid.

12. • Copper is important in imparting disease resistance to the plants
• It enhances the fertility of male flowers.
• Deficiency Symptoms:
• Plants having less than 5 ppm Cu are regarded as Cu deficient.
• Male flower sterility, delayed flowering and senescence are the most
important effects of Cu deficiency.
• Necrosis, Die back, chlorosis of younger shoots are the characteristic Cu
deficiency symptoms.

13. Molybdenum
• Molybdenum is the only heavy transition metal taken up by the plants
as molybdate ions (MoO4
2-). A healthy Mo sufficient plant contains 0.1
to 2 ppm of Mo ppm of Mo.
• Functions:
• Molybdenum is a component of nitrate reductase, nitrogenase,
Xanthine oxidase/ dehydrogenase and sulphite oxidase. Because of
these enzymes, Mo has the following functions:
• Biological Nitrogen fixation is catalyzed by the Mo containing
enzyme, nitrogenase.
• Molybdenum affects the formation and viability of pollens and
development of anthers.

14. Deficiency Symptoms:
• The critical concentration of Mo deficiency in plants is
usually less than 0.1 ppm. Mo deficiency resembles the
Nitrogen deficiency. Mo deficiency in cauliflower is termed
as whip tail
• Boron
• Boron is absorbed by the plants mainly as boric acid (H3B03),
it can be absorbed in some of its anionic forms, viz.
dihydrogen borate(H2BO3
-), monohydrogen borate(HBO3
2-)
and Borate(BO3
3-). Normal B sufficient plants have B content
ranging from 10 to 200 ppm.

15. Functions:
• It is responsible for the cell wall formation and stabilization,
lignification and xylem differentiation
• It imparts the drought tolerant to the crops
• It plays an important role in pollen germination.
• Deficiency Symptoms:
• Plants having B concentration of the order of 5 to 30 ppm are
suspected to be B deficient. Critical deficiency range from 5 to 10
ppm.
• In such condition internodes become shorter and give a bushy
appearance.

16. Typical names given to B deficiency in different
crops are:
• Heart rot of sugarbeet
• Browning or hollow stem of cauliflower
• Top sickness of tobacco
• Internal cork of apple
• Nickel
• It facilitates transport of nutrients to the seeds or grains
• In free living Rhizobia, adequate Ni supply ensures optimum
hydrogenase activity .

17. Deficiency Symptoms:
• Critical level of Ni deficiency in barley shoots is 0.1 ppm, as
concentrations below this are accompanied by reduction in dry matter
weight, decrease in amino acid content and accumulation of nitrates.
Characteristic deficiency symptoms of nickel have not yet been
defined adequately.
• Chlorine :
• Chlorine is ubiquitous in nature. It is absorbed as chloride ions (Cl-) by
the plants. Normal plants have Cl content ranging from 100 to 500
ppm. It has often been neglected because it is present in abundance
and its deficiencies have not been reported from any where in India.

18. Functions:
• It plays an important role in osmoregulation.
• Chlorine in abundance suppresses the plant diseases. Viz. grey leaf
spot in coconut palms, common root rot in wheat, stalk rot in corn,
stem rot and sheath blight in rice, hollow heart and brown centre in
potatoes.
• Chlorine supply improves the nutritional quality of vegetables.
• Deficiency symptoms:
• Plants having less than 100 ppm Cl are usually designated as deficient.
Deficiency symptoms of chlorine includes wilting of leaves and
chlorosis.

19. Micronutrient Sources
S.N
0
Micronutrient Source Micronutrient
Percentage(%)
1 Zinc Sulphate heptahydrate(mostly used Zn
fertilizer in India)
Zn, 21
2 Zinc Sulphate monohydrate Zn, 33
3 Manganese Sulphate Mn, 30.5
4 Ferrous Sulphate Fe, 19
5 Copper Sulphate Cu, 24
6 Sodium Borate(Borax) B, 10.5
7 Ammonium molybdate Mo, 52
Sathyanarayana et al.2019

20. Optimum Range of Micronutrient elements in
Plants
S.NO Micronutrients(µg/g) Sufficient or Optimum
range
1 Zn 20-100
2 Fe 50-250
3 Mn 20-300
4 Cu 5-20
5 B 10-100
6 Mo 0.1-0.5
7 Cl 2000-20000
Sathyanarayna et al.2019

21. Critical limits of Micronutrients in Soil (Cate
and Nelson)
S.NO Element Critical Limit Range in Soil (ppm)
1 Zn 0.6
2 Mn 2.0
3 Cu 0.2
4 B 0.5
5 Fe 4.5
6 Mo 0.1
7 S 10
Sathyanarayana et al.2019

22. Factors affecting the availability of
micronutrient Cations and Anions
• Iron, Manganese, Copper, Zinc and Nickel are called micronutrient
Cations as these carry positive charges. Boron, Molybdenum and
Chlorine occur as anions and carry negative charges. The effects of
soil environment on micronutrient cations are different from those of
micronutrient anions. So the effect of soil factors on the availability of
micronutrient cations and anions will be discussed separately.
• Effect of Cations-
• Certain soil factors tend to exert a similar effect on the availability of
all the micronutrient cations. These factors are:

23. 1.Soil pH
• Soil pH is the most important factor which regulates the solubility and
availability of micronutrient cation in soils. A very low pH indicates
very high acidic soils reaction in which all the micronutrient occurs in
the soil solution.
• Under such conditions certain micronutrients became toxic to plants.
On the other hand if pH of the soil is higher than 7.0, the micronutrient
cations get precipitated as insoluble hydroxides. All the hydroxides of
micronutrient cations are sparingly soluble, but vary in degree of their
solubility and availability.

24. • One of the major objectives of liming is to keep the soil pH
around 6.0 so as to reduce the concentration of the
micronutrient cation in soil solution below the toxic range.
Over liming is harmful as it reduces the concentration of
micronutrient cations in soil to such a level that a sensitive
plant may suffer from the deficiency of micronutrient
cations.

25. 2.Oxidation state of micronutrient cations
• Lower oxidation states of Fe, Mn and Cu(reduced states) are enmost
soluble than higher oxidation states at the normal pH range of soils.
The reduced states are encouraged by poor oxygen supply under
submerged conditions.
• The presence of easily decomposable fresh organic matter in soil
encourages anaerobic conditions at high moisture levels due to
vigorous microbial activity which get energy directly from the
oxidation of inorganic ions. It is observed the high pH of soil favours
oxidation, whereas low pH conditions are favourable for the reduction
of micronutrient cations in soil. In well drained aerated calcareous
soils, micronutrient cations exist in the oxidised state and their
availability becomes very low and therefore

26. • Plants suffer from micronutrient deficiency, although the total
content of all the micronutrients may be very high. Under such
conditions the hydroxides and carbonates of high valent forms
are too insoluble to meet the plant needs. Zinc is a divalent
cation, but at high pH it forms Zincate ion, Zn(OH)4
2-

27. 3. Inorganic Reaction:
The availability of iron and zinc may be reduced in the presence of excess phosphates.
From a practical standpoint, phosphate fertilizers should be used in only those
quantities that are required for good plant growth.
4. Organic Combinations:
Each of the micronutrient cations may be held in organic combination. These
complexes may protect the micronutrients from certain harmful reactions, such as the
precipitation of iron by phosphate. On soils high in organic matter, complex formation
by copper is thought to be responsible for the deficiency of this element.

28. Effects on Anions
Anions i.e., chlorine, molybdenum and boron are relatively little common in comparison to cations.
Chlorine, molybdenum and boron are quite different chemically, so very similarity would be expected in
their reaction in soils.
1. Chlorine:
The chloride ions are not tightly adsorbed by negatively charged clays and as a result are subject to
movement. In semi-arid and arid regions, a somewhat higher concentration might be expected, the amount
reaching the point of salt toxicity in some of the poorly drained saline soils.
2. Boron:
The availability and utilization of boron is determined to a considerable extent by pH. Boron is most
soluble under acid conditions. It apparently occurs in acid soils in part as boric acid which is readily
available to plants. Boron is held in organic combinations from which it may be released for crop use.

29. 3. Molybdenum:
• Soil conditions affect the availability of molybdenum much
the same as they do that of phosphorus. For example,
molybdenum in quite unavailable in strongly acid soils. The
liming of acid soils will usually increase the availability of
molybdenum.

30. Chelates and Chelating Agents
• A chelate describes a kind of organic chemical complex in which the
metal part of the molecule is held so tightly that it cannot be 'stolen' by
contact with other substances, which could convert it to an insoluble
form.
• Chelating agents are organic molecules that can trap or encapsulate
certain metal ions like Fe, Zn and Mn and then release these metal ions
slowly so that they become available for plants to take them up.
• A chelate refers to a ring system that results when a metal ion
combines with two or more electron donor groups of a single
molecule. Actually unidentate water molecules, which are coordinated
with a metal ion, are replaced by the most stable bi-, trior poly dentate
groups of the chelating agent. This results in the ring formation.
Sekhon,2003

31. • Some chelating agents used for the production of synthetic micronutrient chelates
are :
EDTA
EHPG
HEEDTA
DTPA
NTA
EDDHMA
EHPG
Glucoheptonic acid

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