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III. Soil Factors Affecting Availability
FAGERIA et al.
Inﬂuence of Soil pH on Micronutrient Concentrations in Soil and Plant Uptakea
Inﬂuence on concentration/uptake
Increasing soil pH favors adsorption of B. This element generally becomes less available
to plants. Availability and uptake of B decrease dramatically at pH > 6.0.
Chloride is bound tightly by most soils in mildly acid to neutral pH soils and becomes
negligible to pH 7.0. Appreciable amounts can be adsorbed with increasing soil acidity,
particularly by Oxisols and Ultisols, which are dominated by kaolinitic clay. Increasing
soil pH generally increases Cl uptake by plants.
Solubility of Cu2+ is very soil pH dependent and decreases 100-fold for each unit
increase in pH. Plant uptake also decreases.
Ferric (Fe3+) and ferrous (Fe2+) activities in soil solution decrease 1000-fold and
100-fold, respectively, for each unit increase in soil pH. In most oxidized soils, uptake
of Fe by crop plants decreases with increasing soil pH.
The principal ionic Mn species in soil solution is Mn2+, and concentrations decrease
100-fold for each unit increase in soil pH. In extremely acid soils, Mn2+ solubility can
be sufﬁciently high to induce toxicity problems in sensitive crop species.
Above soil pH 4.2, MoO42− is dominant. Concentration of this species increases with
increasing soil pH and plant uptake also increases. Water-soluble Mo increases sixfold
as pH increases from 4.7 to 7.5. Replacement of adsorbed Mo by OH− is responsible
for increases in water-soluble Mo as soil pH increases.
Zinc solubility is highly soil pH dependent and decreases 100-fold for each unit increase
in pH, and uptake by plants decreases as a consequence.
Ni2+ is relatively stable over wide ranges of soil pH and redox conditions. However,
availability is usually higher in acidic than in alkaline soils. At pH 7 and higher,
retention and precipitation increase. Increasing the pH of serpentine soils through
liming from 4 to 7 reduced Ni in plant tissue.
Solubility and availability of Co decrease with extreme soil pH. Presence of CaCO3, and
high Fe, Mn, SOM, and moisture.
Adriano (1986), Fageria, Baligar, and Jones (1997), and Tisdale et al. (1985).
as inﬂuenced by soil pH and consequent acquisition by plants. Table V has been
provided to show acquisition of Cu, Fe, Mn, and Zn by rice grown at various soil
Boron is the only micronutrient to exist in solution as a nonionized molecule
over soil pH ranges suitable for the growth of most plants. Increasing soil pH decreases B availability by increasing B adsorption onto clay and Al and Fe hydroxyl
surfaces, especially at high soil pH (Keren and Bingham, 1985). The highest availability of B was at pH 5.5–7.5, and the availability decreased below or above this
pH range. In other studies, B adsorption increased from pH 3 to 8 on kaolinite,
MICRONUTRIENTS IN CROP PRODUCTION
Inﬂuence of Soil pH on Acquisition of Cu, Fe, Mn, and Zn by Upland Rice
Grown in an Oxisol of Brazila
Cu (μg plant−1)
Fe (μg plant−1)
Mn (μg plant−1)
Zn (μg plant−1)
P < 0.05.
P < 0.01.
montmorillonite, and two arid zone soils with peak adsorption at pH 8–10 and decreases from pH 10 to 12 (Goldberg, Forster, Lesch et al., 1996). Reduced B availability occurs from liming (called “B ﬁxation”)(Fleming, 1980) as CaCO3 acts as an
adsorption surface. As such, B deﬁciency may occur in plants grown in limed acid
Chloride is bound only lightly by most soil-exchange sites in acid to neutral
soils and becomes negligible to pH 7.0. Chloride is easily leached from soil.
Considerable soil Cu is speciﬁcally adsorbed as pH increases. For example,
increasing the pH from 4 to 7 increased Cu adsorption (Cavallaro and McBride,
1984), and Cu was adsorbed on inorganic soil components and occluded by soil
hydroxide and oxides (Martens and Westermann, 1991). Increases in soil pH above
6.0 induces hydrolysis of hydrated Cu which can lead to stronger Cu adsorption
to clay minerals and OM. Readily soluble sources of Cu (exchangeable or sorbed)
were highly toxic to citrus, and Cu concentrations decreased considerably with
soil pH increases above 6.5 (Alva et al., 2000). Over-liming acid soils may also
lead to Cu deﬁciency. SOM is a primary constituent for Cu adsorption and readily
complexes Cu. As the pH increases, the sizes of organic colloids of high molecular
weight diminish, thus increasing the surfaces where Cu can be adsorbed (Geering
and Hodgson, 1969).
The solubility of Fe decreases by ∼1000-fold for each unit increase of soil pH in
the range of 4 to 9 compared to ∼100-fold decreases in the activity of Mn, Cu, and
Zn (Lindsay, 1979). Iron exists in Fe0 (metallic), Fe2+ (ferrous), and Fe3+ (ferric)
forms. Under acidic conditions, Fe0 readily oxidizes to Fe2+, and Fe2+ oxidizes to
Fe3+ as the pH increases above 5. Ferric Fe (Fe3+) is reduced to Fe2+ and is readily
FAGERIA et al.
available to plants in acidic soils, but precipitates in alkaline soils. Iron oxides
are dominant in governing Fe solubility in soils. Minimum Fe solubility occurs
between pH 7.5 and 8.5, which is the pH range of many calcareous soils (Lindsay,
1991). The increases in soil pH or Eh shift Fe from exchangeable organic forms
to water-soluble and Fe oxide forms. The solubility of Fe in well-aerated soils
is controlled by dissolution and precipitation of Fe3+ (Moraghan and Mascagni,
1991). Decreasing rhizosphere pH with added N (NH4–N) and/or K (KCl and/or
K2SO4) was effective for increasing Fe uptake by plants (Barak and Chen, 1984).
Applying FeSO4 with acid-forming fertilizer also increased Fe availability to plants
(Moraghan and Mascagni, 1991).
Soil pH affects solubility, adsorption, desorption, oxidation of Mn, and reduction of Mn oxides in soil. As the pH decreases, Mn is mobilized from various fractions and increases Mn soil solution concentrations and availability. Exchangeable
Mn (plant available form) was high at low soil pH (<5.2), while organic and Fe
oxide fractions of Mn (low availability form) were high at high pH (Sims, 1986). In
sandy soil, increasing pH also increased organic fractions of Mn (Shuman, 1991).
Increasing soil pH with Mg applications on peanut decreased Mn toxicity and
leaf and stem Mn concentrations (Davis, 1996). The reduction of Mn4+ to Mn2+
is greatest at low soil pH, and acid soil conditions (<5) lead to Mn toxicities for
many sensitive plant species (Mortvedt, 2000). In addition, high-molecular-weight
organic colloids diminish as soil pH increases to increase surfaces where Mn as
well as Cu and Fe can be adsorbed (Geering and Hodgson, 1969). Soil solution
Mn increased 1.6-fold for each unit decrease in pH in a well-drained Mollisol
acidiﬁed with high N fertilizer, indicating that soil acidity and aeration are important for Mn availability (Fageria and Gheyi, 1999). Manganese, Cu, and Fe are
generally more available under conditions of restricted drainage or in ﬂooded soils
Molybdenum is the only micronutrient whose availability normally increases
with increases in soil pH. The active form of Mo is normally MoO42−, which
tends to polymerize when in solution. This condition is enhanced by acidiﬁcation
which could partially explain the low availability of Mo in some acid soils (KabataPendias and Pendias, 1984). The solubility of CaMoO4 and H2MoO4 (molybdic
acid) increases with increases in soil pH. Molybdenum sorption on Fe oxides
increased with decreases in soil pH in the range of 7.8 to 4.5 (Hodgson, 1963).
Adsorption of Mo on Al and Fe oxides was maximum at pH <5, and decreased
as the pH increased >5 with little or no adsorption at pH 8 (Goldberg, Forster,
and Godfrey, 1996). Soil pH had pronounced effects on Mo adsorption between
3 and 10.5 with virtually no adsorption at pH 8 (Goldberg and Foster, 1998).
Adsorption of Mo on hydrous Fe and Al oxides decreased as soil pH increased,
and the addition of lime to soil normally increased Mo solubility and a cquisition
by plants (Williams and Thornton, 1972). In addition, maximum Mo adsorption
on Al and Fe oxides was at pH 4–5, but adsorption was maximum at pH 3.5 with
MICRONUTRIENTS IN CROP PRODUCTION
humic acid and decreased as soil pH increased (Biback and Borggaard, 1994).
Different mechanisms were apparent for Mo adsorption with humic acid compared
to Al/Fe oxides, which involved complex formation between carboxyl and phenolic
groups. Harmful effects occasionally arise for legumes grown in acid soils, as Mo
deﬁciency may be more dominant than Al toxicity (Bohn et al., 1979). In some
cases, both lime and Mo applications may be needed to provide adequate Mo to
plants (Lindsay, 1991).
Soil pH is more important than any other single property for controlling Zn
mobility in soils (Anderson and Christensen, 1988). Increasing soil pH generally
decreased Zn availability to plants (Saeed and Fox, 1977), and such decreases
were usually due to higher adsorption of Zn. As soil pH increases above pH 5.5,
Zn is adsorbed on hydrous oxides of Al, Fe, and Mn (Moraghan and Mascagni,
1991). However, the extent to which Zn is retained on Fe and Al hydrous oxides
is inﬂuenced by the nature of clay minerals, surface conditions, and pH (Harter,
1991). In some cases, a soil pH higher than 7 may increase soil solution Zn due
to solubilization of OM and also forms Zn(OH)+ and increased complexation of
Zn with a lower positive charge (Barber, 1995). Gradual decreases in Zn activity
as soil pH increases have been attributed to increased cation-exchange capacity
(Stahl and James, 1991). Thirtyfold decreases in Zn concentration in acid soil
have been reported for each unit increase in soil pH between 5 and 7 (McBride
and Blasiak, 1979). Zinc was preferentially adsorbed over Cu on exchange sites
indicating that chemisorption of hydrolyzed Zn occurs. Zinc adsorption is a major
factor contributing to low concentrations of solution Zn in Zn-deﬁcient soils. Soil
pH affected Zn adsorption either by changing the number of sites available for
adsorption or by changing the concentration of Zn species that is preferentially
absorbed by plants (Barrow, 1986). Over-liming of soil may induce Zn deﬁciency
and decrease Zn availability, especially at a high soil pH. Zinc absorption by wheat
decreased as H+ concentrations increased, presumably because of the direct effects
of H+ toxicity and the indirect effects of competition between Zn2+ and H+ for
uptake sites on root surfaces (Chairidchai and Ritchie, 1993). The effect of pH may
also be modiﬁed by organic ligands, and these ligands may decrease Zn uptake
by plants as soil pH increases. Zinc deﬁciency may be expected in slightly acid
and particularly in alkaline soils where inorganic Zn in equilibrium with soil Zn
decreases between 10−8 and 10−10 M (Lindsay, 1991).
Chemisorption of Ni on oxides, noncyrstalline alumino silicates, and layer silicate clays is favored at soil pH > 6, but exchangeable and soluble Ni 2+ is favored
under lower pH conditions (McBride, 1994). The mobility of Ni is moderate in acid
soils and becomes low in neutral and alkaline soils. Cobalt solubility decreases
with increases in soil pH because of increased chemisorption on oxides and silicate clay, complexation by OM, and possible precipitation of Co(OH)2 (McBride,
1994). Cobalt is somewhat mobile in acid soils, but reduces as soil pH approaches
FAGERIA et al.
B. ORGANIC MATTER
Soil OM may be grouped into water-insoluble (humic acids or humin) and
water-soluble (fulvic acids and small molecular weight microbial products) compounds. Humic acids contain many anionic oxygen groups (phenolic hydroxyl and
carboxyl, aliphatic carboxyl, alcoholic hydroxyl), which may interact with metal
cations (Tate, 1987). Predominant reactions between humic acids and metals are
ionic-bonding or complexation reactions. The increases in humiﬁcation of OM increased these reactive groups and enhanced the potential for reaction with metallic
cations (Stevenson, 1986). Metal complexation with humic substances normally
forms strong metal complexes, while ionic bonding with low-molecular- weight
organic acids (acetic, citric, malic) form relatively weak bonds. Both types of bonding normally result in the enhancement of metal mobility and/or plant availability
(Tate, 1987), but some complexes are not readily available to plants (Harmsen
and Vlek, 1985). Chemical reactions involved with SOM and metals have been
reviewed (Stevenson, 1982).
Native soil B was signiﬁcantly and positively correlated with organic C
(Elrashidi and O’Connor, 1982). Soil OM adsorbs B by ligand exchange and
such adsorption is vital to B availability (Goldberg, 1997). Organic matter is the
main source of B in acid soil, as relatively little B is adsorbed on mineral fractions
at low pH. Born adsorption by soil-composed OM increases with increased SOM
content and with increased soil pH (Yermiyahu et al., 1995). Even though reactions
of B with OM are not well understood, B may be involved in reactions with hydroxyl groups on organic complexes (Ofﬁah and Axley, 1993). Boron complexes
with dihydroxyl compounds in OM, and these compounds retain considerable
amounts of B (Marzadori et al., 1991). Soil OM also appears to be responsible
for occluding important adsorption sites and reduces possible hysteretic reactions
(confers reversibility characteristics) with adsorption sites (Marzadori et al., 1991).
Because B is so closely associated with OM, it is usually more available in surface
compared to subsurface soils because of higher amounts of OM in surface soil
(Tisdale et al., 1985).
Chloride bioavailability does not complex with and is not related to OM content
in soil (Mortvedt, 2000).
Copper is tightly bound to compounds in SOM, even more so than the other
micronutrients, and is generally unavailable to plants (Mathur and Levesque, 1983).
Much of the Cu in soil solution is also associated with OM (Kline and Rust, 1966).
Low Cu levels in soil and Cu complexation into insoluble forms when soils have
high OM lead to Cu deﬁciency in some plants (Moraghan and Mascagni, 1991).
The major portions of total Cu were organically bound in an acid sandy soil, but
precipitated when soil pH was high (Alva et al., 2000). The solubility of Cu in soil
is usually decreased by complexation with clay–humus particles and/or formation
of insoluble humic complexes (Stevenson and Fitch, 1981). In Cu-deﬁcient soils,
MICRONUTRIENTS IN CROP PRODUCTION
humic and fulvic acids probably form highly stable complexes with Cu to reduce
its availability. Complexation of Cu with OM occurs mainly at solution pH values
above 6.5 (Barber, 1995), and increased Cu complex formation usually occurs
with increased pH, decreased ionic strength, and increased OM/Cu ratios (Sanders
and Bloomﬁeld, 1980). Inorganic Cu commonly complexes with hydroxyls and
carbonates when soil solution pH is >7.0 (McBride, 1981). The breakdown of
crop residues by soil microbes may release signiﬁcant amounts of Cu, but natural
complexing substances produced during OM decomposition could complex Cu
into unavailable forms (Moraghan and Mascagni, 1991).
Iron forms stable complexes with organic compounds that occur in both soil
and solid phases (Barber, 1995). Organic acids such as citric, malic, oxalic, and
phenolic that form soluble Fe complexes are released when OM decomposes. These
Fe complexes enhance the mobility and bioavailability of Fe (Lindsay, 1991). Even
though Fe complexes with OM, Fe bioavailability is affected more by soil pH than
by OM content. Fulvic and humic ligands form the most stable complexes with
Fe compared to the other transition metals, and the effectiveness of these complexes
increases with increasing pH because of the enhanced dispersion and ionization
of surface ligands (Stevenson, 1991). The formation of soluble Fe complexes by
naturally occurring chelating ligands may also increase Fe solubility in soil. The
addition of OM to soil leads to reducing conditions, and Fe is changed from
less soluble to exchangeable and organic forms under these conditions (Shuman,
1991).The biological degradation of OM also releases electrons or other reducing
agents to lower soil redox potentials and signiﬁcantly increases the solubility of
Fe (Lindsay, 1991). Increases in oxalate-extractable Fe (and Al) occurred after
decomposition of OM, and Fe and Al oxide adsorption sites became coated or
occluded with OM and were active only after removal of OM (Marzadori et al.,
1991). In addition, Fe availability improved with the addition of OM in drained
and water-logged soils (Tisdale et al., 1985).
Soil OM content has been related to increased, decreased, and no effects on Mn
availability to crop plants (Reisenauer, 1988). Within soil fractions, exchangeable
and organically bound forms of Mn are important to plant availability. The higher
accumulation of Mn in surface soil horizons has been reported to indicate that
Mn may be closely associated with OM (McDaniel and Buol, 1991). Positive
correlations between OM and Mn indicate that Mn has a strong afﬁnity for OM, and
higher Mn concentrations in surface soil compared to lower layers are likely due
to higher OM in surface horizons (Zhang et al., 1997b). Sites of Mn retention have
also been associated not only with OM but also with CaCO3 in pH 8 calcareous soils
(Karimian and Gholamalizadeh Ahangar, 1998). Mn2+ especially forms complexes
with fulvic and humic acids and humins, and with organic ligands such as organic,
amino, and sugar acids, hydroxamates, phenolics, siderophores, and other organic
compounds produced by various organisms in soil solution (Marschner, 1995;
Stevenson, 1986; Tate, 1987). Hydrated Mn2+ forms complexes with carboxyl
FAGERIA et al.
groups of OM, which helps explain observations that Mn binds weakly to OM
compared to Fe, Cu, and Zn (Bloom, 1981). Manganese availability in soils high
in OM may also decrease because of the formation of unavailable Mn complexes.
Unavailable Mn complexes form in peaty or muck soils.
Soil OM appears to have smaller effects on the availability of Mo than does
soil pH. Molybdenum availability under acid soil conditions is primarily affected
through the adsorption of MoO42− onto inorganic soil components. However, evidence exists that Mo is ﬁxed by OM (Moraghan and Mascagni, 1991). In southeastern United States soils, adsorbed Mo increased with increases in SOM and
Fe–oxide contents (Karimian and Cox, 1978). Organic matter may also potentially increase the mobilization of Mo under conditions of impeded drainage.
Soil OM appears to affect the availability of Zn by (i) increasing the solubility
of Zn through the formation of complexes with organic, amino, or fulvic acids;
(ii) forming insoluble Zn–organic complexes that decrease the solubility of Zn;
(iii) roots releasing exudates and ligands that may complex Zn in the rhizosphere;
and (iv) microbes immobilizing and mineralizing decreased or increased soilavailable Zn (Lindsay, 1972). Increased levels of OM increase exchangeable and
organic fractions of Zn and decrease oxide fractions of Zn in soil because of
reducing conditions to enhance Zn bioavailability. A widespread Zn deﬁciency in
lowland rice in Asia was related to high soil pH, low available soil Zn, and OM
content (Yoshida et al., 1973). The decomposition of OM releases OH−, HCO3−,
and organic ligands that tend to immobilize Zn in the root rhizosphere (Yoon et al.,
1975). In practice, ﬁne-textured soils and soils with horizons containing high levels
of OM had higher Zn sorption capacities than sandy-textured, low OM soils (Stahl
and James, 1991). Adsorption of organic anions may also increase negative charges
on particle surfaces to enhance Zn adsorption. On the other hand, organic ligands
in solution may decrease Zn adsorption by competing with surface sites for Zn.
Zinc adsorption onto clays and hydrous oxides may be increased or decreased with
organic ligands (Chairidchai and Ritchie, 1990).
High SOM levels in Ni-rich soils can solubilize Ni2+ as organic complexes at
high soil pH (McBride, 1994). At high soil pH, Co complexes with SOM, and Co
bioavailability increases when it is complexed with SOM (McBride, 1994).
C. TEMPERATURE, MOISTURE, AND LIGHT
Temperature and moisture are important factors affecting the availability of micronutrients in soils (Cooper, 1973; Fageria, Baligar, and Jones, 1997). The availability of most micronutrients tends to decrease at low temperatures and moisture
contents because of reduced root activity and low rates of dissolution and diffusion
of nutrients. In soils with low moisture, colloidal particles may become immobilized as a result of micronutrient adsorption on surfaces of soil particles (Harmsen
and Vlek, 1985). Light affects mostly metabolic processes of plants.
MICRONUTRIENTS IN CROP PRODUCTION
Boron and Cl uptake are inﬂuenced more than any other mineral for plants
grown under hot and dry conditions. For example, increased temperatures in nutrient solutions enhanced B concentrations in shoots of plants by increasing B
uptake with increased transpiration (Moraghan and Mascagni, 1991; Vlamis and
Williams, 1970). On the other hand, turnip was B deﬁcient when grown in soil
with <0.3 mg B kg−1 of hot water-extractable B, but became B deﬁcient when
grown in the ﬁeld with 0.5–0.6 mg kg−1 hot water-extractable B during a dry
summer (Batey, 1971). The availability of B decreases under drought conditions
most likely because of the reduced mobility of B by mass ﬂow to roots (Barber,
1995). Boron can move relatively long distances by mass ﬂow and diffusion to
roots. Soil drying reduces B diffusion by reducing the mobility of soil solution and
increasing the diffusion path length (Scott et al., 1975). Boron deﬁciency in crops
commonly occurs during drought periods because of restricted water ﬂow to roots,
and B deﬁciency may also restrict root growth to reduce acquisition of water and
intensify drought stress effects (Bouma, 1969). The lack of soil moisture reduces
the rate of transpiration, thereby reducing B transport to shoots (Lovatt, 1985).
Wetting and drying cycles and increasing soil temperature (e.g., 25 to 45◦ C) also
increased B ﬁxation by montmorillonitic and kaolinitic clays (Biggar and Fireman,
1960). Low temperature in spring and fall seasons of temperate regions reduced
availability of B to forage legumes, while increased temperature enhanced B concentrations for sugarcane (Gupta, 1993). Another aspect of drought-induced B
deﬁciency involves moisture stress that may restrict the mineralization and availability of organically bound soil B (Evans and Sparks, 1983; Flannery, 1985). High
light intensity may also induce B deﬁciency and reduce B toxicity (Moraghan and
Temperature can affect mobilization/immobilization reactions to decrease/
increase solubility of organically bound soil Cu and its acquisition by plants
(Moraghan and Mascagni, 1991; Stevenson and Fitch, 1981). For example, increasing temperature from 8 to 20◦ C increased Cu uptake by carrot grown in
acid organic soil (MacMillan and Hamilton, 1971). Soil moisture had no consistent effect on Cu levels available to alsike clover (Kubota et al., 1963), but Cu
availability to annual ryegrass increased when roots had access to subsoil water
(Nambiar, 1977). Flooding soil also decreased Cu availability to rice (Beckwith
et al., 1975). Low Cu acquisition by plants was attributed to low soil moisture
conditions in the zone of Cu application (Mortvedt, 2000).
Iron deﬁciency, which occurs predominantly in calcareous and alkaline soils, is
commonly enhanced by low soil temperature and high water (wet) and/or poorly
aerated conditions (Marschner, 1995). Low soil temperatures reduce root growth
and metabolic activity and increase HCO3− levels in the soil solution to increase
the severity of Fe deﬁciency with the increased solubility of CO2 in soil solutions
(Inskeep and Bloom, 1986). On the other hand, high soil temperature may decrease
Fe acquisition by increasing the microbial decomposition of organic materials to
stimulate microbial activity and CO2 production to increase the severity of Fe
FAGERIA et al.
deﬁciency (Inskeep and Bloom, 1986; Moraghan and Mascagni, 1991). High aerial
or soil temperatures may also stimulate relative growth rates to enhance the induction of Fe deﬁciency (Inskeep and Bloom, 1986). High soil temperature may also
increase P uptake to enhance P-induced Fe deﬁciency (Moraghan and Mascagni,
1991). Since absorption of Fe by plant roots is largely restricted to actively growing
root tips, restricted root growth in dry surface layers (zone with highest amount
of available Fe) may partially explain the appearance of Fe deﬁciency for some
plants grown under hot, dry conditions (Moraghan and Mascagni, 1991). Soil temperature generally has less effect on Fe deﬁciency in Strategy II (root release of
phytosiderophores) than in Strategy I (root release of organic acids and increased
root reducing power) plants (Răomheld and Marschner, 1986). Increasing light intensity enhanced the release of phytosiderophores by cereal roots, which could
increase uptake of both Fe and Zn (Cakmak et al., 1998).
Drying and ﬂooding of acid sulfate soils increase the risk of Fe toxicity
(Sahrawat, 1979), since water-logging enhances the accumulation of high amounts
of soluble Fe2+, especially in acid soils. Good soil drainage increases oxidation,
and Fe toxicity in rice is commonly reduced (Gunawardena et al., 1982). The ratio
of Fe2+/(Fe2+ + Mn2+ + Ca2+ + Mg2+) in the soil solution, rather than the activity
of Fe2+ alone, controlled Fe acquisition by ﬂooded rice grown in the acid sulfate
soils of Thailand (Moore and Patrick, 1989). High levels of Mn, Ca, and Mg also
reduced the likelihood of Fe toxicity for plants grown in acid soils.
Low soil temperature may induce Mn deﬁciency. For example, Mn deﬁciency
of ﬁeld-grown soybean was more severe at low temperature despite having high
Mn concentrations in shoot tissue (Ghazali and Cox, 1981). Critical Mn concentrations in leaves are often lower at low than at high soil temperatures (Rufty
et al., 1979). High soil temperatures may increase the solubility of soil Mn and
enhance the Mn availability, and air drying often combined with high temperature
increased extractable and exchangeable Mn, which sometimes has led to Mn toxicity (Moraghan and Mascagni, 1991). Soil temperature increases of 10 to 25◦ C
approximately tripled Mn accumulation in shoots of barley grown in organic soil
(Reid and Racz, 1985), while soybean grown at 16, not 24◦ C, developed severe
Mn toxicity symptoms when grown in calcareous soil (Moraghan et al., 1986).
Plant tolerance to Mn toxicity increased in tobacco and soybean with increased
temperatures despite higher Mn absorption, which was attributed to faster plant
growth to provide larger leaf vacuoles to sequester potentially toxic Mn (Heenan
and Carter, 1976; Rufty et al., 1979).
Excess moisture favors Mn-reducing conditions and water-logging, even for
relatively short periods of time, and enhanced Mn accumulation could possibly
induce Mn toxicity (Moraghan and Mascagni, 1991; Siman et al., 1974). Excess
soil moisture can restrict diffusion of O2 within soils and favor Mn reduction. At
lower soil redox potentials, high levels of Fe2+ may also be formed which could lead
to Mn–Fe antagonisms (Vlamis and Williams, 1962, 1964). Manganese deﬁciency
MICRONUTRIENTS IN CROP PRODUCTION
has rarely been observed in rice grown under ﬂooded conditions, and Mn toxicity
was aggravated in alfalfa grown under hot dry conditions in Australia (Siman
et al., 1974). Manganese deﬁciency on various crops in Sweden disappeared after
a heavy rainfall following a dry period (Stahlberg and Sombatpanit, 1974).
High and low light intensities may intensify Mn deﬁciency and toxicity symptoms on plants (Hewitt, 1966). High intensity stimulated Mn absorption and accentuated the severity of Mn toxicity (El-Jaoual and Cox, 1998; Horiguchi, 1998).
The Mn-induced chlorosis symptoms on leaves with high light intensity were attributed to the oxidation of chlorophyll (El-Jaoual and Cox, 1998). On the other
hand, decreased light intensity appeared to lower Mn concentrations in leaves
through reduced water transport and increased the leaf area to dilute internal Mn
(Campbell and Nable, 1988).
Molybdenum is associated with N2 ﬁxation, and low temperatures will suppress this process and lower Mo requirements (Anderson, 1956). The temperature had little effect on plant incidence or severity of Mo deﬁciency (Moraghan
and Mascagni, 1991). The adsorption of Mo increased when the temperature increased from 10 to 40◦ C (Goldberg and Forster, 1998). The acquisition of Mo
decreased in plants grown under dry conditions (Gupta and Sutcliffe, 1968). Submerged acid soils had increased soluble Mo fractions because of decreased MoO42−
adsorption (Ponnamperuma, 1972).
Temperatures lower than optimum normally decrease Zn acquisition by crop
plants. Zinc deﬁciency symptoms were relatively severe at low soil temperature,
but Zn concentrations increased when the temperature was increased (Martin et al.,
1965). Added P also induced Zn deﬁciency at low soil temperature. Cool and wet
conditions induced Zn deﬁciency, which were related to reduced mineralization of
Zn and reduced root growth (Moraghan and Mascagni, 1991). Spring-seeded crops
like maize, edible bean, and potato grown in western United States soils exhibited
early season Zn deﬁciency symptoms, which did not appear in newer growth
later in the season (Viets, 1967). The detrimental effects of low root temperatures
on Zn accumulation by maize grown in nutrient solution were partially due to
decreased translocation from roots to shoots (Edwards and Kamprath, 1974). Since
Zn moves to roots mainly by diffusion, Zn deﬁciency is common also in crops
grown under dry conditions (Warncke and Barber, 1972). Mycorrhizae associated
with roots enhances uptake of Zn (Clark and Zeto, 2000), and low soil temperatures
may severely reduce root colonization with mycorrhizae and induce Zn deﬁciency
(Hayman, 1974). Zinc deﬁciency was less severe for plants grown under low
light and cool conditions compared to optimum growth conditions (Moraghan and
Zinc deﬁciency frequently associated with ﬂooded soil may be the result of Zn
reactions with free sulﬁde (Sajwan and Lindsay, 1988). Under ﬂooded conditions,
Zn may precipitate as ZnS or possibly form organic–Zn complexes which can
lead to reduced availability. Zinc may also react with sesquioxides under ﬂooded
FAGERIA et al.
conditions to enhance Zn deﬁciency (Sajwan and Lindsay, 1986). Added OM also
suppressed Zn acquisition due to redox processes and buildup of Fe2+ (Giordano
et al., 1974). Flooding–drying and alternating wetting–drying decreased Zn adsorption, whereas preﬂooding increased Zn adsorption, and the addition of OM
increased Zn adsorption under these water treatments (Mandal and Hazra, 1997).
Under reducing conditions Ni2+ is incorporated into sulﬁdes that restrict its
mobility to very low levels, and strong oxidizing soil conditions favor adsorption
of Co (McBride, 1994).
IV. FACTORS ASSOCIATED WITH SUPPLY
Sufﬁcient concentrations and/or available forms of micronutrients must be at
or near root surfaces to meet plant acquisition needs. Nutrient supplies to plants
are governed by such factors as concentrations inside plants and in soil solution,
supply and chemistry at root surfaces or in the rhizosphere, and interactions of one
nutrient with another. At any given time, concentrations of nutrients in the solution
immediately adjacent to roots appear to be one of the best measures for assessing
absorption potential, although plant and rhizosphere factors may inﬂuence the rates
of absorption (Fageria, Baligar, and Wright, 1997). Information about mechanisms
and processes associated with mineral nutrient uptake and translocation are not
discussed here, since many review articles are available on the subject (Barber,
1995; Clarkson and Hanson, 1980; Fageria, Baligar, and Jones, 1997; Glass, 1989;
Kochian, 1991; Marschner, 1995; Moore, 1972; Tifﬁn, 1972). This article will
focus on supply and general acquisition processes.
A. DEFICIENCIES AND TOXICITIES
For plants to obtain micronutrients for proper physiological and biochemical
functioning (Table VI), these mineral nutrients need to be at appropriate concentrations. Micronutrient deﬁciencies and toxicities are widespread and have
been documented in various soils throughout the world. The deﬁciency of essential micronutrients induces abnormal pigmentation, size, and shape of plant tissues, reduces leaf photosynthetic rates, and leads to various detrimental conditions
(Masoni et al., 1996). Speciﬁc deﬁciency symptoms appear on all plant parts, but
discoloration of leaves is most commonly observed. Deﬁciency symptoms of low
mobile nutrients (Fe, B, Mn, Zn, and Mo) appear initially and primarily on upper
leaves or leaf tips, while symptoms of mobile nutrients (N, P, K, and Mg) appear
primarily on lower leaves. Deﬁciency and toxicity symptoms may be confused