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V. Improving Supply and Acquisition

V. Improving Supply and Acquisition

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FAGERIA et al.



Fageria, 1997; Fischer, 1998). Acid soils present special micronutrient nutritional

problems for plants because of the high availability of Mn and Fe and the reduced availability of Zn and Mo (Baligar and Fageria, 1997; Fageria, Baligar,

and Edwards, 1990; Fageria, Baligar, and Wright, 1990; Sumner et al., 1991).

In addition, factors enhancing acidification not only lead to micronutrient toxicities/deficiencies but also to soil degradation (Baligar and Ahlrichs, 1998; Baligar

et al., 1998; Dudal, 1976; Sumner et al., 1991). Micronutrients commonly occurring in toxic concentrations in salt-affected soils-include Mo and B (Gupta and

Abrol, 1990).

In recent years, the addition of toxic trace elements like Cd, Cr, Ni, Pb, Cu,

Zn, As, Co, and Mn (some of which are considered micronutrients) to agricultural

soils has increased from enhanced anthropogenic activity (burning fossil fuels,

application of sewage, industrial, mine, municipal products), use of amendments

(fertilizers, manures, lime), application of pesticides, and deposition of atmospheric particles (Adriano, 1986; Alloway, 1995a,b; Kabata-Pendias and Pendias,

1992). Excessive levels of trace elements pose phytotoxicities to plants and may

reduce growth and acquisition of micronutrients (Baligar et al., 1998; KabataPendias and Pendias, 1992; Marschner, 1995). Temperature, pH, redox potentials,

anion ligand formation, and composition and quantity of solution greatly influence the mobility and bioavailability of micronutrients and other trace elements

in soil (Alloway, 1995b). The bioavailability of most trace elements is high at low

soil pH.

Adverse soil physical properties affect longitudinal and radial root growth, root

distribution, morphological (stunting, thickening, reduction of lateral roots) and

anatomical changes (Bennie, 1996; Russell, 1977; Taylor et al., 1972). High mechanical impedance leads to the loss of root caps and the reduction of root thickening, primarily due to short and wide cells of the same cortex volume (Camp and

Lund, 1964) and thick cortex cells (Baligar et al., 1975). Mechanical impedance

may also cause changes in the structure of the endodermis and pericycle cells

(Baligar et al., 1975; Bennie, 1996). Such changes in root size and internal and

external morphology will influence root ability to explore large soil volumes

for micronutrients. Excessive or deficient micronutrients also affect morphology

(length, thickness, surface areas, density) and growth (dry mass, root : shoot ratio)

of roots and root hairs (Baligar et al., 1998; Bennett, 1993; Hagemeyer and Breckle,

1996; Fageria, Baligar, and Jones, 1997; Fageria, Baligar, and Wright, 1997; Foy,

1992; Kafkafi and Bernstein, 1996; Marschner, 1995). Maize root : shoot ratios

increased when Zn was decreased and decreased when Mn and Cu were decreased

(Clark, 1970).

Organic matter helps maintain good soil aggregation, increases water holding

capacity and exchangeable ions, leaching of nutrients, and Mn and Fe toxicities

(Baligar and Fageria, 1997; Fageria, 1992; von Uexkull, 1986). The addition of

crop residues, green manures, composts, animal manures, growing cover crops,



MICRONUTRIENTS IN CROP PRODUCTION



229



using reduced tillage, and avoiding elimination (burning) of crop residues can

significantly improve SOM levels and eventually lead to improved plant growth

and acquisition of micronutrients.

Liming has also been effective in correcting soil chemical constraints (Adams,

1984) and has improved the availability of Mo and decreased the availability of

Mn, Fe, B, Zn, and Cu, and reduced Mn toxicity (von Uexkull, 1986). Liming

also improves root growth to increase plant ability to absorb micronutrients. In

addition, liming improves soil capacity to supply needed micronutrients to plants

(Baligar and Fageria, 1997; Fageria, 1992; Fageria et al., 1995).Since lime has

low mobility in soil, surface-applied lime has little or no effect on improving

problems in subsurface soil. However, the tendency for downward movement

of Ca from surface-applied gypsum (CaSO4) is high (Farina and Channon,

1988; Farina et al., 2000; Ritchey et al., 1980, 2000) and has long-term positive

effects on plant growth (Farina et al., 2000; Toma et al., 1999). The downward

movement of Ca in soil improved the rooting depth and increased the levels of

micronutrients for maize grown in Cerrado acid soils of Brazil (Sousa et al.,

1992). The reduction of subsoil acidity problems usually leads to deeper rooting

and improves micronutrient uptake by plants.



B. SOIL AND FOLIAR FERTILIZATION

The sources of micronutrients may be inorganic, synthetic chelates, and/or natural organic complexes. The potential exists for creating toxic levels of micronutrient in soil by misapplication, since only small amounts are leached from soil

(except B) or small quantities are absorbed by plants (Martens and Westermann,

1991). Micronutrient toxicities are undesirable as they lower yields and product

quality, and excessive levels may enter the food chain. The remediation of soils

with high levels of micronutrients is relatively difficult. The factors influencing

availability and plant acquisition of micronutrients have been discussed in earlier

sections.

Both organic and inorganic micronutrient sources are used to correct deficiencies

in soil. Soil application includes band or broadcast applications before planting or

foliar sprays during vegetative growth. Micronutrients are usually blended with or

coated onto granular N, P, and K fertilizers or mixed with fluid fertilizers (Mortvedt,

1991, 2000). To prevent chemical alteration of micronutrients, blending should

occur relatively soon before application (Mortvedt, 1991).

Foliar applications are used to supply micronutrients more rapidly for correction

of severe deficiencies commonly induced during the early stages of growth, and

are temporary solutions to the problem. Several problems associated with foliar

applications include low penetration rates in thick leaves, run-off from hydrophobic surfaces or being washed off by rain, rapid drying of spray solution, limited



230



FAGERIA et al.



translocation from uptake site to other plant parts, limited amounts of nutrients

that can be supplied and often do not meet plant demands, and leaf damage/burn

(Marschner, 1995). Reducing the pH of spray solutions may reduce leaf damage.

The addition of Si-based surfactants appears to reduce leaf damage and increase

spray effectiveness (Horesh and Leavy, 1981). The disadvantages of foliar application are maximum yields which may not be possible if spraying is delayed

until deficiency symptoms appear and residual effects from foliar sprays are little,

thus multiple sprays may be required for season-long correction (Mortvedt, 2000).

However, foliar fertilization has many advantages which include: rates applied

are considerably lower than soil applications; uniform applications are possible;

crop response to applied micronutrient is almost immediate so that deficiency

can be corrected relatively rapidly; problems often associated with inactivation of

soil-applied micronutrients may be overcome (Mortvedt, 2000). Plant (leaf age,

species, nutritional status and requirements), climatic (light, temperature, humidity), and chemical (form, carrier, adjuvant) factors affect foliar spray effectiveness

(Kannan, 1990). Greater absorption by leaves is favored under low light, optimum temperature, and high humidity conditions. Young leaves are metabolically

more active than older leaves and are more effective with absorption. Hygroscopic

compounds keep micronutrients in solution longer, thereby helping plants absorb

these elements more effectively than nonhygroscopic compounds. To increase the

effectiveness of foliar uptake, wetting agents are usually added to sprays. These

chemicals are neutral nonionic compounds which reduce surface tension and increase wetting of leaf surfaces to enable larger amounts of solution to be absorbed

(Kannan, 1990).

1. Correcting Deficiencies

The measures for correcting micronutrients are summarized in Table XI. This

information includes concentrations of nutrients for soil and foliar spray applications. The concentrations listed are approximate and may vary depending on

original soil level, crop species/cultivar, crop yield desired, and climatic conditions. Issues related to soil and foliar fertilization of micronutrients and correcting

their deficiencies in soil and plants have been discussed (Martens and Westermann,

1991; Mortvedt, 1991, 2000). Crop recovery of micronutrients is relatively low

(5 to 10%) compared to that of macronutrients (10 to 50%) because of poor distribution from low rates applied, fertilizer reactions with soil to form unavailable

products, and low mobility in soil (Mortvedt, 1994). The principal sources of

micronutrient fertilizers used have been listed in Table XII.

Boron is usually applied at 0.25 to 3 kg ha−1, and higher rates are required for

broadcast than for band application or foliar sprays (Mortvedt and Woodruff, 1993).

Legumes and certain root crops require 2 to 4 kg B ha−1, while lower rates are

usually necessary for maximum yields of other crops (Martens and Westermann,



MICRONUTRIENTS IN CROP PRODUCTION



231



Table XI

Methods of Correcting Micronutrient Deficienciesa

Corrective measure

Element

B

Cl

Cu



Fe



Soil applicationb

0.25–7 kg borax ha−1 (soil

application preferred)

20–50 kg KCl ha−1

1–20 kg CuSO4 ha−1

(every 5–10 years)



Zn



30–100 kg FeSO4 or FeEDDHA

ha−1 (need annual treatment of

0.5–10 kg ha−1)

5–50 kg Mn source ha−1 (soil

application not recommended)

0.01–1 kg Mo source ha−1 (0.3 Na or

NH4 molybdate ha−1) or lime to

pH 6.5

0.5–35 kg ZnSO4 or ZnEDTA ha−1



Ni

Co



Usually not needed

1–6 kg Co source ha−1 (broadcast)



Mn

Mo



Foliar applicationc

0.1–0.25% B solution or

1–10 kg B ha−1

Unknown

0.1–0.2% solution CuSO4·5H2O or

0.1–4.0 kg Cu ha−1 as CuCl2·2H2O,

CuSO4·5H2O, or CuO

2% FeSO4·7H2O or 0.02–0.05%

FeEDTA solution (several sprays

needed)

0.1% MnSO4·H2O solution or 0.3–6 kg

Mn ha−1

0.07–0.1% Na or NH4 molybdate

(100 g Mo ha−1)

0.1–0.5% ZnSO4·7H2O solution

(0.17–1.5 kg ha−1)

May be applied as spray

500 mg Co L−1 solution or 500 mg

Co kg−1 seed treatment



a

From Bould et al. (1983), Fageria, Baligar, and Jones (1997), and Martens and Westermann

(1991).

b

Lower values for soil applications are applicable for band application and higher values are for

broadcast applications.

c

400 liters of solution is sufficient to spray 1 ha of field crop.



1991). Using the concept of Ca/B ratios, the application of foliar (0.3%) or soil

(10 kg ha−1) B ensured adequate B (Moraghan and Mascagni, 1991). Borax or

other soluble borates are usually applied to soil before planting. Boron fertilizer

should not be placed in contact with seeds or at levels that may be toxic to crops.

Boron availability commonly decreases during drought and when acid soils are

limed (Martens and Westermann, 1991).

Even though Cl has been recognized as essential to plants, comparatively little

attention has been given to Cl as a fertilizer because soil levels from inputs and

rain are considered adequate to meet crop requirements. Chlorine may become

limiting for high yields in intensive production practices. Positive yield responses

were noted for application of 400 kg Cl ha−1 for maize (Heckman, 1995). Winter

wheat yields were also increased with Cl applications at seven of nine experimental

sites (Engel et al., 1994). Only a few land areas are deficient in Cl, and crops grown



232



FAGERIA et al.

Table XII

Principal Sources of Micronutrient Fertilizers to Correct Deficienciesa



Element

B



Cl



Cu



Fe



Mn



Mo



Zn



Source



Formula



Boric acid

Borax

Na borate (anhydrous)

Na pentaborate

Na tetraborate

Boron frits

K chloride

Zn chloride

Ca chloride

Mn chloride



H3BO3 [B(OH)3]

Na2B4O7·10H2O

Na2B4O7

Na2B10O16·10H2O

Na2B4O7·5H2O

Fritted glass

KCl

ZnCl2

CaCl2

MnCl2



Cu sulfate (monohydrate)

Cu sulfate (pentahydrate)

Cu chloride

Cuprous oxide

Cupric oxide

Cu chelate

Cu chelate

Ferrous sulfate (monohydrate)

Ferrous sulfate (heptahydrate)

Ferrous ammonium sulfate

Ferric sulfate

Fe chelate

Fe chelate

Fe chelate

Fe chelate

Fe frits

Mn sulfate (anhydrous)

Mn sulfate (tetrahydrate)

Mn chloride

Mn carbonate

Mn oxide

Mn chelate

Mn frits

Na molybdate

Ammonium molybdate

Mo trioxide

Molybdic acid

Mo frits

Zn sulfate (monohydrate)

Zn sulfate (heptahydrate)

Zn chloride

Zn oxide

Basic Zn sulfate



CuSO4·H2O

CuSO4·5H2O

CuCl2

Cu2O

CuO

Na2CuEDTA

NaCuHEDTA

FeSO4·H2O

FeSO4·7H2O

(NH4)2SO4·FeSO4·6H2O

Fe2(SO4)3·4H2O

NaFeEDTA

NaFEHEDTA

NaFeEDDHA

NaFEDTPA

Fritted glass

MnSO4

MnSO4·4H2O

MnCl2

MnCO3

MnO

Na2MnEDTA

Fritted glass

Na2MoO24·2H2O

(NH4)6Mo7O24·4H2O

MoO3

H2MoO4·H2O

Fritted glass

ZnSO4·H2O

ZnSO4·7H2O

ZnCl2

ZnO

ZnSO4·4Zn(OH)2



Element (%)



Solubilitya



17

11

20

18

14

1.5–2.5

48

52

64

44

35

25

47

89

75

13

9



Soluble

Soluble

Soluble

Soluble

Soluble

Sl. solubleb

Soluble

Soluble

Soluble

Soluble

Soluble

Soluble

Soluble

Insoluble

Insoluble

Soluble

Soluble



33

19

14

23

5–14

5–9

6

10

2–6



Soluble

Soluble

Soluble

Soluble

Soluble

Soluble

Soluble

Soluble

Sl. soluble



23–28

26–28

17

31

41–68

5–12

2–10



Soluble

Soluble

Soluble

Insoluble

Insoluble

Soluble

Sl. soluble



39

54

66

53

0.1–0.4



Soluble

Soluble

Sl. soluble

Soluble

Sl. soluble



36

23

48–50

50–80

55



Soluble

Soluble

Soluble

Insoluble

Sl. soluble

continues



MICRONUTRIENTS IN CROP PRODUCTION



233



Table XII—continued

Element



Ni



Co



a

b



Source

Zn chelate

Zn chelate

Zn frits

Ni chloride

Ni nitrate

Ni oxide

Co sulfate

Co nitrate



Formula

Na2ZnEDTA

NaZnEDTA

Fritted glass

NiCl2·6H2O

Ni(NO3)2·6H2O

NiO

CoSO4·7H2O

Co(NO3)2·6H2O



Element%



Solubilitya



14

9

4–9

25

20

79

21

20



Soluble

Soluble

Sl. soluble

Soluble

Soluble

Insoluble

Soluble

Soluble



From Mortvedt (1991, 2000), and Martens and Westermann (1991).

Slightly soluble.



on salt-affected soils often exhibit symptoms of Cl toxicity. Seed germination may

be inhibited with high concentrations of Cl, so Cl fertilizers need to be applied in

advance of planting (Bould et al., 1983).

Copper deficiency can generally be corrected by applying 3.3 to 14.5 kg Cu ha−1

as broadcast CuSO4 (Martens and Westermann, 1991). The rates of banded CuSO4

required to correct Cu deficiency have been as low as 1.1 kg ha−1 for vegetables

and as high as 6.6 kg Cu ha−1 for alfalfa, oat, and wheat. Copper deficiency can

be corrected by banding or broadcasting Cu to soil or as foliar sprays. Lower rates

of Cu application are required to correct Cu deficiency with banded compared to

broadcast CuSO4. Foliar sprays are emergency measures, as Cu deficiency is most

frequently corrected by soil applications (Murphy and Walsh, 1972) which are

more effective than foliar sprays (Solberg et al., 1993). Soil application of CuSO4

is usually more effective than CuO, and Cu might need frequent applications

when problems persist (Karamanos et al., 1986). The differences in the rates of

Cu required to correct Cu deficiency vary with soil properties, crop requirement,

and concentrations of extractable soil Cu. In semiarid regions, drying of top soil

reduces Cu availability.

Iron deficiency is corrected mainly by foliar sprays because soil applications

are generally ineffective unless very high rates are applied. Typical Fe compounds

used for foliar application to crops are FeSO4, Fe(NO3)2, and FeDTPA, and a

200 kg ha−1 FeSO4 rate was required to obtain maximum yields of annual crops

(Mortvedt, 1991). More than one foliar spray and often three to four are needed during vegetative growth periods to obtain optimum production of crops like sorghum,

soybean, and rice. Tree injection with ferric ammonium citrate (8% Fe) and seed

treatment with FeEDDHA have had limited success in correcting Fe deficiency.

Inorganic Fe sources applied to soils are rapidly converted to unavailable forms

(oxidation of Fe2+ to Fe3+) in well-aerated soils, especially as soil pH increases.

In Oxisols from central Brazil, Fe deficiency on upland rice was frequently reported where soil had been limed to pH ∼ 6 for the production of common bean



234



FAGERIA et al.



and soybean in rotations (Fageria et al., 1994). Synthetic Fe chelates are generally

the most effective Fe sources for soil and foliar applications, but their cost may

be prohibitive. A common source of Fe applied to annual crops is FeSO4, but Fe

chelates may be cost-effective if crops are of high value (fruits and berries). Fritted

materials are sometimes used in acid soils to maintain Fe for plants (Martens and

Westermann, 1991).

A common source of Mn applied to soils and as foliar sprays is MnSO4. Soybean

and rice commonly develop Mn deficiency during their growth on many soils.

Optimum soybean yields were obtained with MnSO4 broadcast (14 kg ha−1) and

band (3 kg ha−1) applied, and Mn deficiency was corrected by broadcasting MnSO4

(11 kg ha−1) or banding at half that rate or by timely foliar applications (1–2 kg

ha−1) (Hatfield and Hickey, 1981). In other studies, 10 to 40 kg MnSO4 ha−1 was

required to achieve maximum soybean yields (Anderson and Mortvedt, 1982).

Manganese deficiency on soybeans grown in a Brazilian Cerrado Oxisol at pH 6.7

was corrected with applications of 15 mg MnSO4 kg−1 soil (Novais et al., 1989).

Manganese deficiency on rice grown in a drained Histosol at pH ∼ 7 was alleviated

with soil applications of ∼15 kg MnSO4 (Snyder et al., 1990). Seed applications

of Mn also prevented Mn deficiency and provided near-maximum grain yields,

and banded MnSO4 with seed has been equally as effective as sprayed Mn. Soil

applications of Mn with acid-forming macronutrient fertilizers in neutral to high pH

soils generally increase Mn effectiveness, and Mn deficiencies on plants grown in

acid soils may be avoided by not over-liming. Both MnSO4 and MnO were effective

as sources of Mn at rates of 20 kg Mn kg−1 for correcting Mn deficiency on soybeans

grown in an Oxisol at pH 6.9 (Abreu et al., 1996). Chelated Mn (MnEDTA),

MnSO4, and mangasol were equally effective for alleviating Mn deficiency on

lupine (Brennan, 1996). Foliar applications of MnSO4 are effective for small grain

cereals grown in calcareous and alkaline soils, which tend to dry during the growing

season (Reuter et al., 1973). Soybean receiving 1.12 kg MnSO4 foliar sprays during

early growth stages (V6) and again during late growth stages (R1) had higher

yields than plants receiving single early sprays (Gettier et al., 1985). Multiple

applications of foliar MnSO4 are usually superior to single applications on soybean

(Cox, 1968).

Molybdenum deficiency can be corrected by soil and foliar applications and

by seed treatments. Since the availability of Mo increases as soil pH increases,

liming acid soils to pH 6.5–7.0 will frequently prevent or correct Mo deficiencies

(Martens and Westermann, 1991). The application of 0.01 to 0.5 kg Mo ha−1

will generally correct Mo deficiency. Sodium and/or ammonium molybdates are

suitable sources for soil applications. Foliar applications of Mo have usually been

more effective than soil applications for crops grown under dry conditions (Martens

and Westermann, 1991). Foliar applications of 40 g Mo ha−1 increased bean growth

and shoot N concentrations (Viera et al., 1998). High rates of seed-treated Mo



MICRONUTRIENTS IN CROP PRODUCTION



235



could be toxic to rhizobia or my induce seedling injury (Sedberry et al., 1973).

Even though excess Mo applications could lead to Cu deficiencies in animals

(“molybdenosis”), this hazard is low since most Mo becomes relatively insoluble

in well-drained soils (Martens and Westermann, 1991).

Zinc deficiency can be corrected by either foliar or soil applications of ZnSO4

or ZnEDTA (Martens and Westermann, 1991). Foliar Zn is usually applied in

emergencies to salvage crops when Zn deficiencies appear, and one foliar application is usually not adequate for correcting moderate to severe Zn deficiency.

Maximum grain yields were obtained with foliar applications of ∼1 mg Zn kg−1

during the third and fourth weeks after plant emergence for maize grown in an

Oxisol in central Brazil (Galr˜ao, 1994, 1996) and with 6 mg Zn kg−1 soil for

upland rice grown in a greenhouse (Barbosa Filho et al., 1990). Applications of

Zn either by broadcast or band usually are more effective than foliar applications

(Murphy and Walsh, 1972). Zinc deficiency is common on land where subsoils

have been exposed after land leveling, and these normally receive applications of

farmyard manure to alleviate deficiencies and improve soil conditions (Martens

and Westermann, 1991).

Nickel is ubiquitous in soils, and most P fertilizers contain sufficient Ni for plant

productivity, so Ni is not usually applied to soils. However, foliar applications have

corrected Ni deficiency (Chamel and Newmann, 1987). Cobalt deficiency is usually

controlled by soil broadcast applications (0.4 to 6 kg Co ha−1), foliar applications

(500 mg Co L−1), and seed treatments (500 mg Co kg−1) (Raj, 1987; Reddy and

Raj, 1975). Both sulfate and nitrate salts of Co have been used as fertilizers.

2. Residual Effects

Knowledge concerning residual effects of applied micronutrient fertilizers is

important to make sound and economic recommendations for succeeding crops.

Micronutrient fertilizers have longer residual effects in high silt and clay than in

sandy soils. Slightly soluble materials also have longer residual effects than highly

soluble materials. Crop yields also determine residual micronutrient effects in soil.

Information about long-term micronutrient effects is limited. Since crop recovery

of micronutrients is relatively low, long-term residual effects might be expected.

Broadcast applications of 2 kg B ha−1 as Borate-65 to a loam soil provided sufficient B for alfalfa and red clover for 2 years (Gupta, 1993). Recommendations for

correcting Cu deficiency indicated a relatively high residual availability of applied

Cu. For example, residual Cu was effective for 5 to 8 years after application for

several crops (Martens and Westermann, 1991). Soil applications of Fe sources

usually have no or only limited residual effects, since Fe2+ is rapidly converted

to Fe3+ in aerated soils. Band applications of Fe at relatively high rates may be

effective for more than 1 year provided tillage operations do not mix fertilizer



236



FAGERIA et al.



with surrounding soil (Martens and Westermann, 1991). Manganese applied at 20

to 40 kg ha−1 to a sandy loam soil produced maximum soybean yields, but this

Mn was insufficient to alleviate deficiency the next year (Gettier et al., 1984).

However, optimum soybean yields occurred 2 years after broadcasting 30 kg Mn

ha−1 on a clay loam soil (Mascagni and Cox, 1985). Residual effects have usually

been higher for MnSO4 than for MnO (Abreu et al., 1996). The results regarding

residual effects of Mo fertilization showed that effectiveness decreased ∼50% per

year in some soils (Barrow et al., 1985). Broadcast applications of 34 kg ZnSO4

ha−1 were adequate to correct Zn deficiency on maize for 4 to 5 years, but banded

Zn had to be applied at 6.6 kg ha−1 for ∼5 years to assure adequate residual

Zn (Frye et al., 1978). Economical and long-term residual effects were also

obtained for soil applications of Zn on wheat (Yilma et al., 1997).



C. PLANT IMPROVEMENT

The steady increases in yields of major crops during the last half-century have

been achieved through genetic improvement and improved management practices.

The selection of improved genotypes adapted to wide ranges of climatic differences

has contributed greatly to the overall gain in crop productivity during this time. In

spite of these advances, mean yields of major crops are normally two- to fourfold

below recorded maximum potentials (Baligar and Fageria, 1997). Newly developed

genotypes of rice, maize, wheat, and soybean have been more efficient in the

absorption and utilization of micronutrients compared to older cultivars (Clark and

Duncan, 1991; Fageria, 1992). (See Table XV for scientific names of plant species.)

The accumulation of micronutrients varies among plant species and cultivars/

genotypes within species (Marschner, 1995; Welch, 1986). Such differences among

plant species/cultivars have been attributed to genetics, physiological/biochemical

mechanisms, responses to climate variables, tolerance to pest and diseases, and

responses to agronomic management practices. Genetic variations in plant acquisition of micronutrients have been reviewed (Brown et al., 1972; Duncan,

1994, Duncan and Carrow, 1999; Gerloff and Gabelman, 1983; Graham, 1984;

Marschner, 1995). The development of genotypes/cultivars effective in the acquisition and use of micronutrients and with the desired agronomic characteristics is

vital for improving yields and achieving genotypic adaptation to diversified environmental conditions and increased resistance to pests (Baligar and Fageria, 1997;

Duncan, 1994; Graham, 1984). Plant and external factors affecting micronutrient

use by plants and mechanisms and processes influencing genotypic differences in

micronutrient efficiency have been summarized (Table XIII and Table XIV).

Plant species differ considerably for B requirements and tolerance to deficient

and toxic levels of B in soil (Fixen, 1993; Gupta, 1979; Rerkasem and Loneragan,



MICRONUTRIENTS IN CROP PRODUCTION



237



Table XIII

Plant and External Factors Affecting Micronutrient Use by Plantsa

Plant factors



External factors



Genetic control

Species/cultivar/genotype



Agronomic management practices

Liming

Crop rotation

Incorporate crop residue, cover crops

Soil

Aeration/reducing conditions

pH

Organic matter levels and forms

Temperature

Moisture Status

Texture/structure

Compaction

Fertilizers

Source

Timing, depth, method of placement, and

application

Use slow release form

Elements

Toxicities in acid (Al, Mn, pH) and

saline (B, Cl) soils

Deficiencies in acid (Cu, Zn, Mo) and alkaline

(Zn, Fe, Mn, Cu) soils

Others

Arbuscular mycorrhizae, beneficial

soil microbes

Control weeds, diseases, and insects



Physiological

Root length, density of main, laterals, and

root hairs

Higher shoot yield, harvest index, internal

demand

Higher physiological efficiency

Higher nutrient uptake and utilization

Excretion of H+, OH−, and HCO3−

Biochemical

Enzymes: rhodotorulic acid (Fe), ferroxamine

b (Fe), ascorbic acid oxidase (Cu), carbonic

acid anhydrase (Zn)

Metallothionein (trace elements)

Proline, aspharagine pinitol (salinity)

Abscisic acid, proline (drought).

Root exudates (citric, malic, transaconitic acids)

Phytosiderophores

Others

Tolerance to stress (drought, acidity, alkalinity)

Tolerance/resistance to diseases/pests

Arial temperature, light quality, humidity



a



Baligar and Bennett (1986a,b), Baligar and Fageria (1997), Duncan (1994), and Fageria (1992).



1994). Plants with high requirements for B are alfalfa, apple, red beet, turnip,

cabbage, and cauliflower (NRC, 1980). Genotypic differences for tolerance to

high B have been observed in wheat, barley, annual medic, and field peas (Nable

and Paull, 1991; Paull et al., 1992). Such differences sometimes are related to

restricted B uptake and transport. For example, the susceptibility to B deficiency

in tomato was due to the lack of plant ability to transport B from roots to shoots

(Brown et al., 1972). The genetic variability for B uptake and leaf concentration

was noted for maize (Gorsline et al., 1968).

Sensitivity to high Cl concentrations varies widely among plant species and cultivars (Eaton, 1966), but Cl toxicity is more extensive worldwide than Cl deficiency,



238



FAGERIA et al.

Table XIV

Soil and Plant Mechanisms and Processes and Other Factors Influencing Genotypic

Differences in Micronutrient Efficiency in Plants Grown under Mineral Stressesa



Nutrient acquisition

Diffusion and mass flow in soil: buffer capacity, ionic concentration and properties, tortuosity,

moisture, bulk density, temperature

Root morphological factors: number, length, extension, density, root hair density

Physiological: root/shoot ratio, root microorganisms (rhizobia, azotobacter, mycorrhizae),

nutrient status, water uptake, nutrient influx and effux, nutrient transport rates, affinity

for uptake (Km), threshold concentration (Cmin)

Biochemical: enzyme secretion (phosphatases), chelating compounds, phytosiderophores,

proton exudate, organic acid exudates (citric, malic, trans-aconitic, malic)

Nutrient movement in root

Transfer across endodermal cells and transport in roots

Compartmentalization/binding within roots

Rate of nutrient release to xylem

Nutrient accumulation and remobilization in shoots

Demand at cellular level and storage in vacuoles

Retransport from older to younger leaves and from vegetative to reproductive tissues

Rate of chelation in xylem transport

Nutrient utilization and growth

Nutrient metabolism at reduced tissue concentrations

Lower element concentrations in supporting structures, particularly stems

Elemental substitution (Fe for Mn, Mo for P, Co for Ni)

Biochemical: peroxidase for Fe efficiency, ascorbic acid oxidase for Cu, carbonic anhydrase

for Zn, metallothionein for metal toxicities

Other factors

Soil factors

Soil solution: ionic equilibria, solubility, precipitation, competing ions, organic ions, pH,

phytotoxic ions

Physiochemical properties: organic matter, pH, aeration, structure, texture, compaction, moisture

Environmental effects

Intensity and quality of light (solar radiation)

Temperature

Moisture (rainfall, humidity, drought)

Plant diseases, insects, and allelopathy

a



From Baligar and Fageria (1997), Baligar et al. (1990), Duncan and Baligar (1990), Fageria

(1992), and Gerloff (1987).



particularly in arid and semiarid regions. Plant tolerance to Cl has reported strawberry and pea to be very sensitive; lettuce, onion, maize, apple to be moderately

sensitive; potato, cabbage, cauliflower, wheat, and ryegrass to be slightly tolerant;

and red beet, spinach, rape and barley to be highly tolerant (Marschner, 1995).

The genotypic differences in tolerance to Cu and other heavy metals are well

known in certain species and ecotypes of natural vegetation (Woolhouse and



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