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III. Soil Factors Affecting Availability

III. Soil Factors Affecting Availability

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FAGERIA et al.

Table IV

Influence of Soil pH on Micronutrient Concentrations in Soil and Plant Uptakea



Element

B

Cl



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.



Cu



Solubility of Cu2+ is very soil pH dependent and decreases 100-fold for each unit

increase in pH. Plant uptake also decreases.



Fe



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.



Mn



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.



Mo



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.



Zn

Ni



Co



a



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

pH values.

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,



197



MICRONUTRIENTS IN CROP PRODUCTION

Table V

Influence of Soil pH on Acquisition of Cu, Fe, Mn, and Zn by Upland Rice

Grown in an Oxisol of Brazila

Soil pH



Cu (μg plant−1)



Fe (μg plant−1)



Mn (μg plant−1)



Zn (μg plant−1)



4.6

5.7

6.2

6.4

6.6

6.8



75

105

78

64

61

51



4540

1860

1980

1630

1660

1570



11,160

5,010

4,310

3,610

2,760

2,360



1090

300

242

262

163

142



r2



0.89b



0.97c



0.99c



0.98c



a



Fageria (2000c).

P < 0.05.

c

P < 0.01.

b



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

soils.

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



198



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

(Ponnamperuma, 1972).

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



199



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

neutrality.



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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



201



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



202



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



203



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

Mascagni, 1991).

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



204



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



205



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

Mascagni, 1991).

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



206



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

AND ACQUISITION

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



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III. Soil Factors Affecting Availability

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