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IV. Factors Associated with Supply and Acquisition
MICRONUTRIENTS IN CROP PRODUCTION
Functions of Micronutrients in Plantsa
Activates certain dehydrogenese enzymes. Involved in carbohydrate metabolism.
Synthesis of cell wall components. Essential for cell division and development.
Essential for photosynthesis and as an activator of enzymes. Involved in splitting water.
Functions in osmoregulation of plants growing on saline soils.
Constituent of a number of important oxidase enzymes including cytochrome oxidase,
ascorbic acid oxidase, and lactase. Important in photosynthesis and protein and
Important in chlorophyll formation and an essential component of several peroxidase,
catalase, and cytochrome oxidase enzymes. Found in key metabolic functions such as
N2 ﬁxation, photosynthesis, and electron transfer.
Activates decarboxylase, dehydrogenese, and oxidase enzymes. Involved in
photosynthesis, N metabolism, and assimilation.
An essential component of nitrate reductase and N2-ﬁxation enzymes and required for
normal assimilation of N.
Essential component of several dehydrogenase, proteinase, and peptidase enzymes.
Promotes growth hormones, starch formation, and seed maturation.
Component of urease enzyme. Participates in redox reactions. Improves hydrogenase
acitivity, urea hydrolysis. Stimulates germination and growth.
Nodule development, rhizobium infection, N2 ﬁxation, component of coenzyme
cobalamin (vitamin B12).
From Brady and Weil (1996), Fageria, Baligar, and Jones (1997), Marschner (1995), and Stevenson
with drought, disease, insect, and other damage, so correct diagnosis may be difﬁcult without experience. Critical concentration ranges of micronutrients in soil for
important ﬁeld crops (Table VII) and some description of deﬁciency and toxicity
symptoms associated with many crop plants (Table VIII and Table IX) have been
Boron deﬁciency is common for plants grown in arid, semiarid, and heavy
rainfall areas in calcareous, sandy, light textured, acid, and low OM soils (Bradford,
1966; Gupta, 1993). Soils supplied with high amounts of municipal compost,
sludge, and biosolids tend to accumulate high amounts of B which may result in B
toxicity. Boron toxicities are commonly associated with crops receiving irrigation
water containing high B. Differences between B sufﬁciency and toxicity are narrow
Chlorine deﬁciencies under ﬁeld conditions have been reported for oil palm,
sugarcane, hard red spring wheat, and potato (Martens and Westermann, 1991).
Soybean grown in Atlantic coastal plain soils with added KCl developed Cl
FAGERIA et al.
Critical Micronutrient Concentrations (mg kg−1) in Soil for Some Field Cropsa
Alfalfa, sugar beet,
cotton, maize, peanut
Wheat, barley, oat
Maize and small grains
Barley and oat
Maize, soybean and
Sorghum and soybean
Bean (common), maize,
rice, sorghum, ﬂax
0.01 M Ca(NO3)2
0.05 M K2SO4
0.05 M EDTA
0.05 M HCl
0.1 M HCl
0.05 M HCl
From Cox (1987), Martens and Lindsay (1990), Sims (2000), and Sims and Johnson
toxicity (Parker et al., 1983). Crops that are grown in salt-affected soils and receive
irrigation (sprinkler) often have enhanced symptoms of Cl toxicity.
Copper deﬁciency is often observed on plants grown in soils inherently low
in Cu (coarse-textured and calcareous soils) and in soils high in OM, where Cu
is readily complexed (Alloway and Tills, 1984). Higher than normal Cu supplies
usually inhibit root growth more than shoot growth (Lexmond and Vorm, 1981).
The use of Cu-containing fungicides and antihelminthic compounds (insecticides)
in agriculture has resulted in Cu toxicity in some plants, but naturally occurring
Cu toxicity is relatively uncommon (Welch et al., 1991).
MICRONUTRIENTS IN CROP PRODUCTION
Micronutrient Deﬁciency Symptoms in Crop Plantsa
Death of growing points of shoot and root. Failure of ﬂower buds to develop.
Blackening and death of tissues, especially inner tissue of brassica plants.
Reduced leaf size. Yellowing, bronzing and necrosis on leaves. Roots reduced in growth
and without hairs.
Yellowing of young leaves. Rolling and dieback of leaf tips. Leaves are small. Tillering
is retarded. Growth is stunted.
Interveinal yellowing of younger leaves with distinct green veins. Entire leaves become
dark yellow or white with severe deﬁciency, and leaf borders turn brown and die.
Interveinal tissue becomes light green with veins and surrounding tissue remaining
green on dicots (Christmas tree design) and long interveinal leaf streaks on cereals.
Develop necrosis in advanced stages.
Mottled pale appearance in young leaves. Bleaching and withering of leaves and
sometimes tip death. Legumes suffering Mo deﬁciency have pale green to yellowish
leaves. Growth stunted. Seed production is poor.
Deep yellowing of whorl leaves (cereals). Dwarﬁng (rosette) and yellowing of growing
points of leaves and roots (dicots). Rusting in strip on older leaves with yellowing in
mature leaves. Leaf size reduced. Main vein of leaf or vascular bundle tissue
becomes silver-white, and marked stripes appear in middle of leaf.
Chlorosis of newest leaves. Ultimately leads to necrosis of meristems. Reduced
germination and seedling vigor (low seed viability).
Diffuse yellowing in leaves. Young shoots and older leaves have severe localized
From Baligar et al. (1998), Bennett (1993), Bould et al. (1983), Brown et al. (1987), Clark and
Baligar (2000), and Fageria, Baligar, and Jones (1997).
Iron deﬁciency is a worldwide problem and occurs in numerous crops (Korcak,
1987; Marschner, 1995; Vose, 1982). Iron deﬁciency occurs not because of Fe
scarcity in soil but because of various soil and plant factors that affect Fe availability
to inhibit its absorption or impair its metabolic use (Marschner, 1995; Welch
et al., 1991). In the majority of soils, the total concentration of soluble Fe in the
rhizosphere is nearly always far below the level required for adequate plant growth
(Marschner, 1995). Induced Fe-deﬁcient chlorosis is widespread and is a major
concern for plants growing on calcareous or alkaline soils due to their high pH and
low Fe (Korcak, 1987). Bicarbonate, nitrate, and environmental factors inﬂuence
the occurrence of Fe-deﬁcient chlorosis in plants, which occurs in young leaves
due to inhibited chloroplast chlorophyll syntheses as a consequence of the low
Fe nutrition status of plants (Lucena, 2000). Plant species that commonly become
Fe deﬁcient are apple, peach, citrus, grape, peanut, soybean, sorghum, and upland
FAGERIA et al.
General Description of Mineral Toxicity Symptoms on Plantsa
High B may induce some interveinal necrosis, and severe cases turn leaf margins straw
color (dead) with distinct boundaries between dead and green tissue. Roots appear
High Cl results in burning leaf tips or margins, reduced leaf size, sometimes yellowing,
resembles K deﬁciency, and root tips die.
High Cu may induce Fe deﬁciency (chlorosis). Light colored leaves with red steaks along
margins. Plants become stunted with reduced branching, and roots are often short or
barbed (like wire). Laterals may be dense and compact.
Excess Fe is a common problem for plants grown in ﬂooded acidic soil. May induce P, K,
and Zn deﬁciencies. Bronze or blackish-straw colored leaves extending from margins to
midrib. Roots may be dark red and slimy.
Excess Mn may cause leaves to be dark green with extensive reddish-purple specks before
turning bronze yellow, especially interveinal tissue. Uneven distribution of chlorophyll.
Margins and leaf tips turn brown and die. Sometimes Fe deﬁciency appears, and main
roots become stunted with increased number and density of laterals.
Excess Mo induces symptoms similar to P deﬁciency (red bands along leaf margins), and
roots often have no abnormal symptoms.
Excess Zn may enhance Fe deﬁciency. Leaves become light colored with uniform necrotic
lesions in interveinal tissue, sometimes damping off near tips. Roots may be dense or
compact and may resemble bared wire.
High Ni results in white interveinal banding alternating with green semichlorotic areas
with irregular oblique streaking, dark green veins, longitudinal white stripes, and brown
patches. Yellowing of leaves may resemble Fe or Mn deﬁciency. Distortion of young
leaﬂets (peg-like or hook type).
Pale green leaves with pale longitudinal stripes.
From Baligar et al. (1998); Bould et al. (1983); Clark and Baligar (2000), and Fageria, Baligar,
and Jones (1997).
rice. Iron toxicity (bronzing) can be a serious disorder for the production of crops
in water-logged soils. For wetland rice, Fe toxicity is the second most severe
yield-limiting mineral disorder after P deﬁciency. Audebert and Sahrawat (2000)
reported that the application of P, K, and Zn with N to an iron-toxic lowland soil in
the Ivory Coast reduced Fe toxicity symptoms and increased lowland rice yields.
Manganese toxicity is probably more of a problem than Mn deﬁciency throughout the world. Manganese deﬁciency occurs on plants grown in organic, alkaline,
calcareous, poorly drained, slightly acid soils, and coarse-textured sandy soils
(Martens and Westermann, 1991). Over-liming of acid soils may induce Mn deﬁciency. Manganese toxicity is a major factor for reduced production of crops
grown in acid soils, as is Al toxicity. Plant ability to tolerate Mn toxicity is affected by plant genotype, concentration of Si in soils, temperature, light intensity,
MICRONUTRIENTS IN CROP PRODUCTION
and physiological age of leaves (Horst, 1988). The conditions that lead to the
buildup of high levels of Mn in soil solution are high levels of total Mn, soil pH
below 5.5, high soluble Mn relative to Ca, reduction of Mn under low oxygen
caused by poor drainage, soil compaction, and excess water from irrigation or
rainfall (Reisenauer, 1988).
Molybdenum deﬁciency is widespread in legumes, maize, and cauliﬂower grown
in acid mineral soils containing high amounts of iron oxides and hydroxides.
Copper/Mo ratios <2 will normally reduce Mo deﬁciency in plants (Miltmore
and Mason, 1971). The appearance of Mo toxicity is rare, but high levels of Mo
in forages may induce Cu deﬁciency in animals. Molybdenum concentrations
>5 to 10 mg kg−1 dry wt in forage tissue have induced toxicity in ruminants
(“molybdenosis or teart”) (Marschner, 1995). Such disorders of Cu occur in forage
grown in poorly drained and high organic soils.
Zinc deﬁciency in plants is widespread throughout the world (Bould et al., 1983;
Viets, 1966). Increasing pH due to liming reduces plant available Zn. High clay
and P supply and low soil temperatures are also known to promote Zn deﬁciency
(Marschner, 1995). Lowland rice grown in limed or calcareous soils often exhibit
Zn deﬁciency (Ponnamperuma, 1972). Chaney (1993) indicated that after “natural”
phytotoxicity from Al or Mn in strongly acid soils, Zn phytotoxicity is the next
most extensive micronutrient phytotoxicity compared to Cu, Ni, Co, Cd, or other
trace element toxicities. As soil pH decreases, Zn solubility and uptake increase,
and the potential for Zn phytotoxicity increases. At comparable soil pH and total
Zn contents, Zn phytotoxicity is more severe on plants grown in light-textured than
in heavy-textured soils. This is mainly because of the differences in the speciﬁc
Zn adsorption capacities of soil. Continued applications of Zn to alkaline sandy
soils low in OM and clay tend to develop Zn toxicity in plants, even though
the occurrence of Zn toxicity is relatively rare under ﬁeld conditions (Rattan and
Shukla, 1984). Liming was effective in overcoming Zn toxicity on peanut (Keisling
et al., 1977). Even though no clear evidence exists for Ni deﬁciency in plants, Ni
toxicity is of concern for plants grown in soil receiving sewage sludge and industrial
by-products. Nickel as well as Co toxicity may also be found on plants grown in
soils formed from serpentinite or other ultrabasic rocks (McBride, 1994). Cobalt
deﬁciency may occur on plants grown in highly leached sandy soils derived from
acid igneous rocks and in calcareous or peaty soils (Martens and Westermann,
1991) and in coarse-textured, acid-leaching alkaline or calcareous soils and humic
rich soils (McBride, 1994).
B. SUPPLY AND UPTAKE
Micronutrient uptake by roots depends on nutrient concentrations at root surfaces, root absorption capacity, and plant demand. Micronutrient acquisition
includes dynamic processes in which mineral nutrients must be continuously
FAGERIA et al.
Estimated Proportions of Micronutrients Potentially
Supplied by Mass Flow, Diffusion, and Root Interception
to Maize Roots Grown in a Fertile Alﬁsola
of total uptake
From Barber (1966).
replenished in soil solution from the soil solid phase and transported to roots
as uptake proceeds. Mineral nutrient transport to roots, absorption by roots, and
translocation from roots to shoots occur simultaneously, which means that rate
changes of one process will ultimately inﬂuence other processes involved in uptake (Fageria, Baligar, and Jones, 1997). In soil systems, mineral nutrients move
to plant roots by mass ﬂow, diffusion, and root interception (Barber, 1995).
Mass ﬂow is the passive transport of minerals to roots as water moves through
soil and occurs when solutes are transported to roots with convective ﬂow of water
(soil solution) from soil. The amount of minerals supplied to roots depends on
the rates of water ﬂow to roots and the average mineral content of the water.
The amounts of mineral nutrients reaching roots by this process depend on the
concentrations of nutrients in soil solution and the rates of water transport to and
into roots. Diffusion and mass ﬂow could meet plant micronutrient requirements
for B, Cu, and Zn, provided sufﬁcient nutrient concentrations are in soil solution.
Table X provides estimates of nutrients supplied to maize roots by mass ﬂow,
diffusion, and root interception in a fertile Alﬁsol.
Diffusion is deﬁned as the movement of nutrients from regions of high concentration to regions of low concentration. When the nutrient supply to root surfaces
is not sufﬁcient to satisfy plant demands by mass ﬂow and root interception, concentration gradients develop and nutrients move by diffusion (Barber, 1966, 1995).
Considerable quantities of B, Mn, and Fe move by diffusion.
Root interception is another process by which roots obtain minerals. As roots
grow in soil, they push soil particles aside and root surfaces come in direct contact
with mineral nutrients. Mineral interception by roots depends on soil volume
occupied by roots, root morphology, and concentrations of minerals in the soil
volume occupied by roots. On average, soil volume occupied by roots of crop
MICRONUTRIENTS IN CROP PRODUCTION
plants is about 0.7 to 0.9% (Fageria, Baligar, and Wright, 1997). Root interception
can provide signiﬁcant amounts of plant requirements for B, Zn, and Mn.
The interaction of soil and plant factors inﬂuences the processes of mineral ﬂux
in soil. The major soil factors that inﬂuence mineral ﬂux are concentrations of
mineral ions on exchange sites and in solution, soil buffer capacity, diffusion coefﬁcient, type of clay, soil structure, nature of OM, water content, and temperature.
Soil capacity to adsorb mineral nutrients is important in mineral transport to roots.
If soil ion-exchange capacity is low, ions are usually freely mobile in solution. In
addition, diffusion coefﬁcients of Cu, Mn, and Zn decrease ∼10-fold for various
clays in the order of kaolinite > illite > montmorillonite > vermiculite (Lindsay,
1979). The major plant factors that contribute to mineral ﬂuxes are root and root
hair density and length, plant demand for mineral nutrients and water, and plant
modiﬁcation of the rhizosphere (Fageria and Baligar, 1993, 1997a,b).
The amount of minerals in soil, concentration in soil solution, and transport
to roots are key factors inﬂuencing mineral uptake by roots. Since B, Mn, and
Fe move to plant roots primarily by diffusion, soil properties that affect diffusion
govern micronutrient availability to plant roots. Mineral nutrient supply, whether
at adequate or toxic levels, can strongly inﬂuence root growth, morphology, and
distribution of root systems in soil (Baligar et al., 1998; Barber, 1995; Marschner
1995). As most micronutrients may be supplied by diffusion, the size of roots has
profound effects on plant ability to acquire required mineral concentrations. Toxic
levels of Al, Mn, and H in acid soils and the presence of H2CO3, Na2CO3, B, Na,
Mo, SO4–S, and Cl in alkaline or high-salt soils can directly reduce root growth
and inhibit ability of roots to explore large soil volumes for minerals and water.
Soil weathering, anthropogenic activities, addition of agricultural amendments
(fertilizers, organic manures, lime, slags, sewage sludge), and pesticides have contributed to increased levels of essential micronutrients and nonessential trace elements in soil (Baligar et al., 1998). The mobility and bioavailability of these
minerals in soil are inﬂuenced by pH, temperature, redox potential, cation exchange, anion ligand formation, and composition and quantity in soil solution
(Alloway, 1995a,b; Baligar et al., 1998). At any given pH, the relative mobility of
some micronutrients in acid soil decreases in the order of B > Ni > Zn > Mn > Cu.
Mineral nutrient deﬁciencies and excesses affect growth (dry mass, root : shoot
ratio) and morphology (length, thickness, surface area, density) of roots and root
hairs (Baligar et al., 1998). Nutrient deﬁciencies usually lead to ﬁner roots and
trace element toxicities stimulate initiation and growth of second- and third-order
lateral roots, while tap roots and ﬁrst-order laterals (seminal/basal) become suppressed (Hagemeyer and Breckle, 1996). Additional information about toxicity
and constraints of micronutrients and trace elements on root growth is available (Baligar et al., 1998). Changes in root growth and morphology affect plant
ability to absorb minerals from soil to meet plant demands. Mineral uptake involves selectivity [where certain minerals are absorbed preferentially over others
FAGERIA et al.
(discrimination or exclusion)], accumulation (where minerals accumulate at higher
concentrations in cell sap than in external soil solution), and genotype (where distinct differences exist among plant species and within species) (Marschner, 1995).
A detailed discussion and reviews of plant and soil factors that affect micronutrient
uptake, transport, and utilization in plants are available (Barber, 1995; Chen and
Hadar, 1991; Graham et al., 1988; Gupta, 1993; Marschner, 1995; Mengel and
Kirkby, 1982; Mortvedt et al., 1991; Robson, 1993; Sumner, 2000; Welch, 1995).
Micronutrient cations in soil solution also commonly form organic complexes of
varying stability, size, and charge (Tifﬁn, 1972).
Kochian (1991) stated that to understand the overall mechanisms of micronutrient cation uptake in plants there is a need to consider the form of metal chelates
in the root rhizosphere at the root–cell plasma membrane, forms of micronutrient
cations transported into plant cells, and the nature of the metal chelate complexes,
both within cells and involved in long-distance transport. A detailed discussion of
the processes associated with mineral uptake and transport is provided in several
review articles (Epstein, 1972; Kochian, 1991; Marschner, 1995; Moore, 1972;
Mengel and Kirkby, 1982; Tifﬁn, 1972).
Boron is absorbed by roots as undissociated boric acid [B(OH)3 or H3BO3],
and it is not clear whether uptake is active or passive (Marschner, 1995; Mengel
and Kirkby, 1982). Nevertheless, B uptake by rice appeared to be passive under
normal B supplies and active under low B supplies (Yu and Bell, 1998) and was the
result of passive assimilation of undissociated boric acid (Hu and Brown, 1997). At
high B supplies, passive uptake and active excretion of B were also noted (Yu and
Bell, 1998). Boron as well as Cl distribution in plant tissue appear to be primarily
governed by transpiration, since B and Cl in soil are highly mobile and move with
water. Boron is supplied to roots primarily by mass ﬂow. The factors affecting
B uptake include soil type, B content, soil pH, amount of water soil receives,
and plant species (Welch et al., 1991). Soil pH affects B absorption kinetics of
roots, adsorption on soil particles, and maintenance of B concentrations in soil
solution (Barber, 1995). The absorption of B by monocotyledonous plants was less
than that by dicotyledonous plants and was passive (Shelp, 1993). Long-distance
transport of B from roots to shoots occurs in the xylem and is related to the rates
of transpiration (Brown and Shelp, 1997).
Copper uptake is an active process (Dokiya et al., 1964) and is inﬂuenced by plant
species, growth stage, plant part, various soil properties, and added amendments.
Copper is relatively immobile in soil, so that large portions of Cu are derived
from root interception in soils low in labile Cu (Oliver and Barber, 1966). The
exploitation of soil by roots (root volume, density) inﬂuenced the Cu absorbed by
roots (Barber, 1995). Soil pH did not affect Cu uptake extensively because the soil
maintained sufﬁcient levels of Cu, even when free Cu2+ had been reduced with
increased soil pH (Barber 1995). Mycorrhizal associations with roots improved
Cu uptake by 53 to 62% in white clover (Li et al., 1991).
MICRONUTRIENTS IN CROP PRODUCTION
In soil solution, Fe3+ dominates and forms organic complexes with degraded
OM (fulvic acid) or siderophores (Fe-complexing compounds released by soil microbes and/or plant roots) (Powell et al., 1982). In well-aerated soils, complexed
Fe3+ is the major form of Fe. Higher plants use nonspeciﬁc and speciﬁc processes to increase the solubility and uptake of Fe from the rhizosphere. Uptake
of cations over anions is one of the most important nonspeciﬁc processes that
results in pH decreases in the rhizosphere to increase Fe availability and uptake
(Răomheld and Marschner, 1986). The factors that interfere with ionic balances
in plants and contribute to Fe uptake are N source, K supply, plant P status, and
genotypic differences (Zaharieva and Răomheld, 1991). Strategy I processes used
by dicotyledons and nongrass monocotyledons (nongraminaceous species) in responding to Fe deﬁciency are to excrete protons (acidiﬁcation of rhizosphere) and
increase reductase activity at the root–soil interphase. The iron deﬁciency in dicotyledonous plants is reduced by lowering the rhizosphere pH from the root H+
excretion (proton excretion), root exudation of organic acids (mainly phenolics),
enhanced root reduction of Fe3+ to Fe2+, and activated root-reducing capacity at
cell plasma membranes. Increased medium acidiﬁcation and Fe3+ reduction are
brought about by plasmalemma-linked H+: ATPase and NADH:Fe3+ reductase activities (Dell’Orto et al., 2000). Organic anions such as citrate and oxalate exudated
from the roots contribute to the Fe mobilization in soil, and such a response appears
to be the factors under P deﬁciency for species such as rape or lupin. (Hinsinger,
1998; Jones et al., 1996). In Strategy I plants, reduction activity at the root–soil
interface appears to play a dominant role in Fe aquisiton (Bertrand and Hinsinger,
2000; Brown, 1978; Chaney et al., 1972). In Strategy I, plant response to Fe deﬁciency is the increased capacity of the roots to reduce ferric chelates (Bienfait,
1988), which is affected by HCO3−, Fe, and other metals (Alc´antara et al., 2000).
Many monocotyledonous plants, especially those of Poaceae (grasses), transport Fe3+-phytosiderophores (root-derived chelates) across root cells (Strategy II
plants), which is an important mechanism by which Fe is acquired by these plants.
Strategy II processes are used by graminaceous species, which excrete several
types of phytosiderophores as adaptive mechanisms to Fe deﬁciency (Kanazawa
et al., 1993; Takagi et al., 1984). Phytosiderophores are low-molecular-weight
polydentate (nonproteinogenic amino acids) ligands which bind Fe3+ to facilitate transport (Kochian, 1991; Marschner, 1995; Răomheld, 1991; Răomheld, and
Marschner, 1986). Overall, the high pH, redox state, pH buffer (HCO3−, active
lime, OM), nitrate, and Fe mineral types affect Fe uptake by plants (Lindsay,
1994; Lucena, 2000; Marschner, 1995; Răomheld and Marschner, 1986). The rate
of phytosiderophore release in cereals under Fe deﬁciency greatly differs between
species, and these differences are positively correlated with the resistance of cereals to Fe deciency (Marschner et al., 1986; Răomheld and Marschner, 1990).
Besides Fe, phytosiderophores also mobilize Zn, Mn, and Cu (Cakmak, Oztă
et al., 1996; Hopkins et al., 1998; Răomheld, 1991).
FAGERIA et al.
Manganese uptake is metabolically mediated, and uptake increases from pH 4
to 6 (Maas et al., 1969). Above pH 6, oxidation of Mn2+ to Mn4+ occurs, and
Mn2+ uptake is reduced. Soil pH and redox potentials control the Mn supply to
roots by mass ﬂow and diffusion. Deﬁciency of Mn usually occurs when soil pH
is >6.2, but Mn2+ may be sufﬁcient in some soils, even though the pH is ≥7.5
(Barber, 1995). The prevailing source of Mn at root surfaces is Mn2+. Manganese
forms complexes with organic compounds (trihydroxamic acid, sideramines) of
microbial and plant origin, which increases the Mn mobility in soil (Clarkson,
1988). The three major sources of Mn in soils that are primarily responsible for
the Mn supply to roots are exchangeable Mn, organically complexed Mn, and
Mn oxides (Marschner, 1988). The proportion of these Mn forms vary with soil
type, soil pH, and OM. As the soil pH decreases, the proportion of exchangeable
Mn increases dramatically, while the proportions of Mn oxides and Mn bound to
Mn and Fe oxides decrease. In soils low in available Fe, root reductase activity is
stimulated because of acidiﬁcation of the rhizosphere and may lead to higher Mn
mobility and uptake. Greater ranges in foliage Mn were noted for different species
of plants growing in the same soil compared to Cu, Fe, or Zn (Gladstones and
Loneragan, 1970). These differences were attributed to species ability to acidify
soil in the rhizosphere rather than to the Mn requirement.
Molybdenum is absorbed as an anion (MoO42−) and is energy dependent; S can
interfere, and P enhances Mo uptake (Barber, 1995; Mengel and Kirkby, 1982).
Mass ﬂow and diffusion supply Mo to roots in soil (Table X).
Zinc is absorbed primarily as a divalent cation (Zn2+) and may be absorbed at
high soil pH as a monovalent cation (ZnOH+). It is not clear whether Zn uptake
is active or passive, even though Mengel and Kirkby (1982) indicated that Zn was
actively absorbed. Zinc is not reduced or oxidized as are Mn, Fe, and Cu. The low
availability of Zn in high pH calcareous soils is due to the adsorption of Zn on clay
or CaCO3 (Trehan and Sekhon, 1977). In addition, high concentrations of HCO3−
inhibit Zn uptake and translocation (Dogar and van Hai, 1980). Zinc uptake is
ă urk et al., 1996; Hopkins et al.,
also enhanced by phytosiderophores (Cakmak, Oztă
C. OXIDATION AND REDUCTION
Oxidationreduction reactions occur when electrons are transferred from a
donor to an acceptor. The donor loses electrons to increase in oxidation number,
and the acceptor gains electrons to decrease in oxidation number. Redox reactions
with various forms of Mn (Mn2+ and Mn4+), Fe (Fe2+ and Fe3+), and Cu (Cu+
and Cu2+) are common in soils (Lindsay, 1979), but Fe and Mn redox reactions
are considerably more important than Cu because of their higher concentrations in
soil. The primary source of electrons for biological redox reactions in soil is OM,
MICRONUTRIENTS IN CROP PRODUCTION
but aeration, pH, and root and microbial activities also inﬂuence these reactions.
Redox reactions in soil can also be inﬂuenced by organic metabolites produced by
roots and microorganisms.
Certain forms of micronutrients are more available to plants than others, and
concentrations of each mineral form depend on soil conditions affecting redox.
The most water-soluble and available forms to plants are Mn2+, Fe2+, and Cu2+,
and these may be altered greatly depending on redox conditions. In general, a high
pH favors oxidation and a low pH favors reduction of these minerals. The availability of Fe and Mn increases, and sometimes they become toxic to plants grown
under highly reducing conditions (ﬂooding). Redox of Mn is thermodynamically
favored at relatively higher redox potentials compared to Fe at given pH values.
For example, the critical redox potential at which Fe2+ appeared was 100 mV
and Mn2+ appeared at 200 mV in a Crowley silt loam soil at pH 6.5 (Patrick and
Jugsujinda, 1992). As a result, demonstrated spatial relationships between Mn and
Fe precipitation in horizontal sand columns relative to increased redox potentials
were observed (Collins and Buol, 1970). Iron precipitated at relatively lower redox
potentials compared to Mn, which did not precipitate until reaching more oxidized
portions in columns. Liming soil to pH > 5.6 increased oxidation processes and
reduced or prevented Mn toxicity (Kamprath and Foy, 1985). Increased reduction
of Mn oxides occurred with increased soil temperature (Ross and Bartlett, 1981;
Sparrow and Uren, 1987). Hence, warm soils may induce Mn toxicity more readily
than cooler soils.
Flooding (reducing conditions) had no inﬂuence on B concentrations in soils,
and B did not undergo redox reactions (Ponnamperuma, 1972). Increasing soil Eh
values (oxidation) redistributed Cu from exchangeable and organic fractions to Fe
oxide fractions, thereby reducing Cu availability to plants (Shuman, 1991). Under
ﬂooded conditions, Cu was adsorbed onto surfaces of reduced Mn and Fe oxides
(Iu et al., 1981).
Reducing conditions in soil mobilized Fe oxide fractions, which became associated with exchangeable, organic, and Mn oxide fractions to make Fe more
available to plants (Shuman, 1991). Increases in Eh or soil pH shifted Fe from
exchangeable and organic forms to water-soluble and Fe oxide fractions. Under
alternate wetting and drying conditions, adding OM led to reducing conditions and
enhanced Fe availability (Shuman, 1988). As redox potentials and/or soil pH increase, the plant availability of Fe decreases due to the insolubility of Fe3+ oxides.
The critical redox potential for Fe3+ was −100 mV at pH 8, +100 mV at pH 7, and
+300 mV at pH 6 (Gotoh and Patrick, 1974). Water-logging resulted in a decreased
redox potential, and a low pH led to increased water-soluble and exchangeable Fe.
Excess water in calcareous soil increased the buildup of HCO3−, which reduced
soluble Fe3+ and induced Fe deﬁciency (Moraghan and Mascagni, 1991).
Soil pH and redox potential are responsible for Mn transformation from insoluble to water-soluble and extractable forms. Under reducing conditions, Mn