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IV. Factors Associated with Supply and Acquisition

IV. Factors Associated with Supply and Acquisition

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MICRONUTRIENTS IN CROP PRODUCTION



207



Table VI

Functions of Micronutrients in Plantsa

Element

B

Cl

Cu



Fe



Mn

Mo

Zn

Ni

Co



Function

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

carbohydrate metabolism.

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).



a

From Brady and Weil (1996), Fageria, Baligar, and Jones (1997), Marschner (1995), and Stevenson

(1986).



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

provided.

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

(Marschner, 1995).

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



208



FAGERIA et al.

Table VII

Critical Micronutrient Concentrations (mg kg−1) in Soil for Some Field Cropsa

Critical concentration

Element

B



Crop



Cl



Alfalfa, sugar beet,

cotton, maize, peanut

Wheat, barley, oat



Cu



Maize and small grains

Barley and oat

Rice

Maize, soybean and

wheat



Fe

Mn



Mo



Zn



Sorghum and soybean

Sorghum

Soybean

Small grains

Maize

Soybean

Forage legumes,

Soybean,

Cauliflower

Bean (common), maize,

rice, sorghum, flax

Maize

Maize

Rice



Extracting solution



Range



Mean



Hot water



0.1–2



0.8



Water

0.01 M Ca(NO3)2

0.05 M K2SO4

CaO

NH4HCO3–DTPA

Mehlich-1

0.05 M EDTA

0.05 M HCl

Mehlich-1

NH4HCO3–DTPA

Mehlich-3

NH4HCO3–DTPA

DTPA–TEA

Mehlich-1

NH4HCO3–DTPA

Mehlich-3

Mehlich-3

NH4–oxalate



NH4HCO3–DTPA

Mehlich-1

0.1 M HCl

DTPA–TEA

0.05 M HCl



>22



0.12–2.5

0.1–10



2.5–5

4–8

1–2



0.8

3

1.1

0.1

0.26

0.53

0.37

4.8

4.5

7

1.4

3

3.9



0.1–0.3



0.25–2

0.5–3

2–10



0.8

1.1

5

0.86

1



a



From Cox (1987), Martens and Lindsay (1990), Sims (2000), and Sims and Johnson

(1991).



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



209



Table VIII

Micronutrient Deficiency Symptoms in Crop Plantsa

Element

B

Cl

Cu

Fe

Mn



Mo



Zn



Ni

Co



Symptoms

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

marginal scorching.



a

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



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FAGERIA et al.

Table IX

General Description of Mineral Toxicity Symptoms on Plantsa



Element

B



Cl

Cu



Fe



Symptoms

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

relatively normal.

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.



Mn



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.



Mo



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).



Zn



Ni



Co



Pale green leaves with pale longitudinal stripes.



a

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



211



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



212



FAGERIA et al.

Table X

Estimated Proportions of Micronutrients Potentially

Supplied by Mass Flow, Diffusion, and Root Interception

to Maize Roots Grown in a Fertile Alfisola

Estimated percentage

of total uptake

Micronutrient



Mass flow



Diffusion



Root interception



B

Cu

Fe

Mn

Zn



1000

219

66

22

230



29

0

21

35

0



29

6

13

43

43



a



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



213



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



214



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



215



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).

ă urk,

Besides Fe, phytosiderophores also mobilize Zn, Mn, and Cu (Cakmak, Oztă

et al., 1996; Hopkins et al., 1998; Răomheld, 1991).



216



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

1998).



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



217



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



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IV. Factors Associated with Supply and Acquisition

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