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Chapter 4. Micronutrients in Crop Production
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.
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
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
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,
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
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
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
Potential Micronutrient Deﬁciencies or Toxicities Associated with Major Soil Groupsa
Mollisols (Aqu), Inceptisols,
Entisols, etc. (poorly drained)
Mollisols (Aridis) (Udolls)
Mollisols (Rendolls) (shallow)
(Natric) (high alkali)
Aridisols (high salt)
Fe, Mn, Zn
Cu, Mn, Zn
Fe, Mn, Zn
Co, Fe, Zn
Cu, Fe, Mn, Zn
Cu, Fe, Mn, Zn
Modiﬁed from Baligar and Fageria (1999); Clark (1982); Dudal (1976); S. W. Buol, North
Carolina State University, Raleigh; H. Eswaren, USDA-NRCS, Washington, DC.