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Chapter 4. Micronutrients in Crop Production

Chapter 4. Micronutrients in Crop Production

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186



FAGERIA et al.

activity, SOM, cation-exchange capacity, and clay contents are important in determining the availability of micronutrients in soils. Plant factors such as root and root

hair morphology (length, density, surface area), root-induced changes (secretion of

H+, OH−, HCO3−), root exudation of organic acids (citric, malic, tartaric, oxalic,

phenolic), sugars, and nonproteinogenic amino acids (phytosiderophores), secretion of enzymes (phosphatases), plant demand, plant species/cultivars, and microbial

associations (enhanced CO2 production, rhizobia, mycorrhizae, rhizobacteria) have

profound influences on plant ability to absorb and utilize micronutrients from soil.

The objectives of this article are to report advances in research on the micronutrient

availability and requirements for crops, in correcting deficiencies and toxicities in

soils and plants, and in increasing the ability of plants to acquire needed amounts

C 2002 Elsevier Science (USA).

of micronutrient elements.



I. INTRODUCTION

Essential nutrients may be defined as those without which plants cannot complete their life cycle, irreplaceable by other elements, and directly involved in plant

metabolism. Based on the quantity required, nutrients are divided into macro- and

micronutrients. Macronutrients are required in large quantities by plants compared

to micronutrients. Micronutrients have also been called minor or trace elements,

indicating that their concentrations in plant tissues are minor or in trace amounts

relative to the macronutrients (Mortvedt, 2000 ). The essential micronutrients for

field crops are B, Cu, Fe, Mn, Mo, and Zn. The accumulation of these micronutrients by plants generally follows the order of Mn > Fe > Zn > B > Cu > Mo. This

order may change among plant species and growth conditions (e.g., flooded rice).

Other mineral nutrients at low concentrations considered essential to the growth

of some plants are Ni and Co. Convincing evidence exists to indicate that Ni is

essential for certain plants (Brown et al., 1987; Eskew et al., 1983). Even though Co

stimulates growth of certain plants, it is not considered essential according to the

Arnon and Stout (1939) definition of essentiality. Cobalt is essential for the fixation

of N2 by bacteria, but is not required by higher plants (Ahmed and Evans, 1960;

Marschner, 1995; Needham, 1983). Rhizobia and other N2-fixing microorganisms

have absolute Co requirements whether growing inside or outside root nodules

regardless of N source (N2 fixation or mineral N) (Marschner, 1995). Even so, Co

is essential for animal nutrition as a component of vitamin B12 (Needham, 1983).

Chlorine and Si have often been referred to as micronutrients, even though

their concentrations in plant tissue are often equivalent to those of macronutrients.

Chlorine will be considered in this article, but since recent reviews have appeared

about Si (Epstein, 1994, 1999; Savant et al.,1997, 1999), this element will not be



MICRONUTRIENTS IN CROP PRODUCTION



187



considered. Possibly, other essential micronutrients will be discovered in the future

because of the recent advances in solution culture techniques and the availability

of highly sensitive analytical instruments. Based on physicochemical properties,

the essential plant micronutrients are metals except for B and Cl. Even though

micronutrients are required in small quantities by field crops, their influence is as

important as the macronutrients in crop production.

Except for B and Cl, the micronutrients are commonly constituents of prosthetic

groups that catalyze redox processes by electron transfer such as with the primary

transition elements Fe and Mn and to some extent Cu and Mo. Micronutrients

normally form enzyme–substrate complexes (Fe and Zn) and/or enhance enzyme

reactions by influencing molecular configurations between enzymes and substrates

(Zn) (Răomheld and Marschner, 1991).

Micronutrient deciencies in crop plants are widespread because of (i) increased

micronutrient demands from intensive cropping practices and adaptation of high

yielding cultivars which may have higher micronutrient demand, (ii) enhanced

production of crops on marginal soils that contain low levels of essential nutrients,

(iii) increased use of high analysis fertilizers with low amounts of micronutrient

contamination, (iv) decreased use of animal manures, composts, and crop residues;

(v) use of soils that are inherently low in micronutrient reserves, and (vi) involvement of natural and anthropogenic factors that limit adequate plant availability and

create element imbalances.

Plant acquisition of micronutrients is affected by numerous soil, plant, microbial, and environmental factors. Parent material, minerals containing micronutrients, and soil formation processes influence micronutrient availability to plants.

Solid-phase materials are important in determining solubility relationships of nutrients in soils (Lindsay, 1991). Available micronutrients in soil are derived from

weathering of underlying parent materials, natural processes (e.g., gases from

volcanic eruption, rain/snow, marine aerosols, continental dust, forest fires), and

anthropogenic processes (industrial and automobile discharges, addition of fertilizers, lime, pesticides, manures, sewage sludges). Soil micronutrients exist in

solid phases like primary minerals, secondary precipitates, and adsorbed on clay

surfaces (Lindsay, 1991; Shuman, 1991). Soil adsorption reactions are important

in determining the bioavailability of B, Cu, Mo, and Zn. Micronutrients in solid

phases are not immediately available to plants. Only about 10% of micronutrients

in soil are soluble and/or in exchangeable forms for plant acquisition (Lake et al.,

1984). Fluctuating temperatures, moisture, and anthropogenic factors change micronutrient concentrations, forms, and distribution among various phases in soil.

Soil pH, redox potential, and soil organic matter (SOM) profoundly affect the

bioavailability of micronutrients (Stevenson, 1986; Tate, 1987). For most soils,

soil SOM contains the largest pool of labile micronutrients in soil and influences

micronutrient cycling, distribution of naturally occurring organic ligands, speciation and form (organic or inorganic) of elements in soil solution, and nature



188



FAGERIA et al.



and stability of micronutrient complexes with humic and fulvic acids, especially

with microbe conversion of SOM (Stevenson, 1991). The importance of SOM

for influencing micronutrient retention follows the sequence of Cu > Zn > Mn

(McGrath et al., 1988). Most metallic micronutrients in soil are complexed by

both inorganic and organic ligands. Organic ligands act as carriers to plant roots

(Lindsay, 1979), and Cu, Zn, and Mn form stable complexes, especially with carboxyl and phenolic groups, to make these minerals less available to plants (Stevenson, 1991). Organic substances like humic and fulvic acids formed in SOM degradation and transformation are also important in micronutrient cycling (Stevenson,

1986).

Plant factors such as root and root hair morphology (length, density, surface

area), root-induced changes (secretion of H+, OH−, HCO3−), root exudation of

organic acids (citric, malic, tartaric, oxalic, phenolic), sugars, and nonproteinogenic amino acids (phytosiderophores), secretion of enzymes (phosphatases), plant

demand, plant species/cultivars, and microbial associations (enhanced CO2 production, rhizobia, mycorrhizae, rhizobacteria) have profound influences on plant

ability to absorb and utilize micronutrients from soil (Barber, 1995; Baligar and

Fageria, 1997; Marschner, 1995).

Macro- and micronutrients have long been recognized as being associated with

changes in plant susceptibility or tolerance and resistance to diseases and pests

(Engelhard, 1990; Graham and Webb, 1991). Even though research information

on the mineral nutrition of plants has advanced significantly in recent years, most of

the advances have been associated with macronutrients. Reasons for this may have

been that micronutrients are required in such small amounts, and their deficiencies

have not been systematically verified under field conditions. The objectives of

this article are to report advances in research on the micronutrient availability and

requirements for crops, in correcting deficiencies and toxicities in soils and plants,

and in increasing the ability of plants to acquire needed amounts of micronutrient

elements.



II. STATUS IN WORLD SOILS

The amounts and distribution of micronutrients in soils are influenced by parent

materials, levels and form of SOM, pH, Eh (oxidizing conditions), mineralogy,

particle size distribution, soil horizon, soil age, soil formation processes, drainage,

vegetation, and microbial, anthropogenic, and natural processes (Baligar et al.,

1998; Stevenson, 1986; Tate, 1987). Micronutrient concentrations are generally

higher in surface soil horizons (Ap) and decrease with soil depth. In spite of the

relatively high total concentrations of micronutrients reported in soils on a global

basis, micronutrient deficiencies have been frequently reported on many crops



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MICRONUTRIENTS IN CROP PRODUCTION



grown in various parts of the world (Cakmak, Sari et al., 1996; Fageria, 2000a;

Galr˜ao, 1999; Graham et al., 1992; Grewal and Graham, 1999; Mandal and Mandal,

1990; Martens and Lindsay, 1990). It has been estimated that 3.95 billion ha of

the world’s ice-free land area is subject to mineral stresses for plants, with 14%

of this area being subject to potential micronutrient stresses (Gettier et al., 1985).

The reasons for micronutrient deficiencies are that these elements have not usually been applied regularly to soils through fertilization. Furthermore, increased

crop yields, loss of micronutrients through leaching, liming of soils, decreased

use of manures compared to chemical fertilizers, and increased purity of chemical

fertilizers without micronutrient additions have contributed to accelerated exhaustion of available micronutrients in soils. Hidden micronutrient deficiencies may

be more widespread than has generally been suspected. Potential micronutrient

deficiencies/toxicities associated with major soil groups (Table I), common soil



Table I

Potential Micronutrient Deficiencies or Toxicities Associated with Major Soil Groupsa

Element problem

Soil order

Andosols (Andepts)

Ultisols

Ultilsols/Alfisols

Spodosols (Podsols)

Oxisols

Histosols

Entisols (Psamments)

Entisols (Fluvents)

Mollisols (Aqu), Inceptisols,

Entisols, etc. (poorly drained)

Mollisols (Borolls)

Mollisols (Ustolls)

Mollisols (Aridis) (Udolls)

Mollisols (Rendolls) (shallow)

Vertisols

Aridisols

Alfisols/arid Entisols

Alfisols/Utisols (Albic)

(poorly drained)

Alfisols/Aridisols/Mollisols

(Natric) (high alkali)

Aridisols (high salt)



Soil group



Deficiency



Andosol

Acrisol

Nitosol

Podsol

Ferralsol

Histosol

Arensol

Fluvisol



B, Mo

Most micronutrients



Gleysol

Chernozem

Kastanozem

Phaeozem

Rendzina

Vertisol

Xerosol

Yermosol



Mn

Fe, Mn, Zn

Cu, Mn, Zn

Fe, Mn, Zn

Fe

Fe, Zn

Co, Fe, Zn



Planosol



Most micronutrients



Solenetz

Solonchak



Cu, Fe, Mn, Zn



Most micronutrients

Mo

Cu

Cu, Fe, Mn, Zn



Toxicity



Fe, Mn

Mn

Fe, Mn



Fe, Mn

Fe, Mo



Mo



B, Cl



a

Modified from Baligar and Fageria (1999); Clark (1982); Dudal (1976); S. W. Buol, North

Carolina State University, Raleigh; H. Eswaren, USDA-NRCS, Washington, DC.



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