Citrus Decline: Soil Fertility and Plant Nutrition

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  • This article was downloaded by: [Nipissing University]On: 17 October 2014, At: 02:40Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,UK

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    Citrus Decline: Soil Fertilityand Plant NutritionA. K. Srivastava a & Shyam Singh aa National Research Centre for Citrus , Nagpur,Maharashtra, IndiaPublished online: 29 Jan 2009.

    To cite this article: A. K. Srivastava & Shyam Singh (2009) Citrus Decline: SoilFertility and Plant Nutrition, Journal of Plant Nutrition, 32:2, 197-245, DOI:10.1080/01904160802592706

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  • Journal of Plant Nutrition, 32: 197245, 2009

    Copyright Taylor & Francis Group, LLC

    ISSN: 0190-4167 print / 1532-4087 online

    DOI: 10.1080/01904160802592706

    Citrus Decline: Soil Fertility and Plant Nutrition

    A. K. Srivastava and Shyam Singh

    National Research Centre for Citrus, Nagpur, Maharashtra, India

    ABSTRACT

    Soil physical properties related constraints (clay gradient in soil profile,drainage/irrigation/waterlogging) and soil fertility constraints induced by soil pH, salin-ity (specific ion-and cumulative osmotic pressure effect), calcareousness (pedogenic ornon-pedogenic CaCO3), besides increasing menace of nutrient mining, are the impor-tant pedological factors contributing to citrus decline. But, the orchards established onlater two soil orders confronted with subsurface constraints in form of argillic (clay richhorizon with acidic or alkaline pH and varying intensity/forms of calcareousness) andspodic horizonation (organic hardpan with very acidic pH), in addition to multiple soilfertility constraints. Soil condition-based rootstock alternatives, site specific nutrientmanagement coupled with variable rate application, and integrated soil managementsystems representing different modules of INM, are the viable means of combating anuntimely decline in citrus orchards productivity.

    Keywords: plant nutrition, nutrient models, horticulture crops

    INTRODUCTION

    Citrus is considered to be one of the most remunerative fruit crops that have alasting niche in international trade and world finance. World citrus productionis dominated by the northern hemisphere, followed by the southern hemisphere,and Mediterranean region contributing 45%, 35%, and 20%, respectively (FAO,2005). There are three basic requirements for successful cultivation of citrus,namely, climate relatively free from frost hazards, good quality of irrigationwater, and reasonably deep and uniform fertile soil with good internal drainage(Nemec, 1986). Citrus decline as popularly known in citrus belts of Indian

    Received 13 August 2007; accepted 21 January 2008.Address correspondence to A. K. Srivastava, National Research Centre for Citrus,

    Amravati Road, Nagpur 440 010, Maharashtra, India. E-mail: aksrivas 2007@yahoo.co.in

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  • 198 A. K. Srivastava and S. Singh

    subcontinent has been the subject of considerable research world over. Cit-rus decline is also known by various other names such as blight in Florida,declinio in Brazil, declinamiento in Argentina, and marchitamiento repentinoin Uruguay, depending upon causal factor. It is referred to as frenching, de-cay, chlorosis, neglectosis, and amachamients in Mexico. The causal factors ofcitrus decline vary in magnitude with the nature of factors (Figure 1).

    Analysis on contribution of climate, soil, and management factors on yieldof Satsuma mandarin demonstrated that yield is more affected by physiographicenvironment than either climate or even fertilizer application (Egashira et al.,1990). A definite correlation exists between elevation and particle size com-position of soils. The average concentration of clay fraction increased from 31to 38% as the elevation increased from 850 to 1500 m above mean sea level(Kong-Tau, 1986). Likewise, many of the soil fertility parameters are reportedto be influenced by variation in altitude (Singh and Dutta, 1983; Avasthe andAvasthe, 1995). These observations assume a greater significance consideringthe fact that citrus culture for commercial purposes is being practiced up to analtitude as high as 2400 m above mean sea level (Ding et al., 1990). But, someof the ornamental species of citrus are grown up to 2800 m in the areas closeto equator (Camacho and Saul, 1981).

    The citrus soils differ from other cultivated soils in many respects. Thecultivated soils remain fallow for 36 months every year, which results indepletion of soil organic matter as very little carbon (C) is added during thefallow phase, while biological oxidation of existing C continues at the samerate as in citrus soils (Sharma and Singh, 2001). Hence, the state of nutrition,size, and yield of citrus are closely related to the amount of soil explored by theroot system (Avilan et al., 1987). Many investigations enumerated the declineof citrus trees to unfavorable surface and subsurface soil conditions (Wutscher,1989; Srivastava and Singh, 2004a).

    Taxonomically, highest quantum of citrus production from citrus is har-nessed from the soils represented by soil orders Entisols, Alfisols, Ultisols, and

    Citrus Decline

    Abiotic Factors Biotic Factors

    Soil-relatedconstraints

    Irrigation Cultural Physiolgical Entomological Pathological* Waterlogging* Water stress

    * Pruning/shading* Excessive bearing* Rootstock-scion incompatibility* Union creasing

    * Hormonal imbalance* Alteration inonand offyear cycle

    * Blackfly* Psylla* Bark eating caterpillar* Leaf miner* Aphids* Mealy bug* Mites

    * Fungal diseases* Bacterial diseases* Viral

    * Salinity* Calcareousness diseases* Nutrient toxicity/ deficiency* Clay gradient* Compaction/ hardpan in subsurface

    Figure 1. Classification of causal factors to citrus decline.

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  • Citrus Decline: Soil Fertility and Plant Nutrition 199

    Oxisols (Srivastava and Singh, 2003). Comparative studies made on the soilconditions in a young sweet orange orchard with patches of poor growth and inadjacent areas with normal growth revealed that soil conditions in the affectedareas had the presence of higher total soluble salts, higher concentration ofsoluble and exchangeable sodium (Na), and lower soluble calcium (Ca) andmagnesium (Mg) (Milad et al., 1975; Malewar et al., 1983).

    DIAGNOSIS OF CITRUS DECLINE

    Morphological Symptoms

    The problem of citrus decline is often confused with die-back and most of thetime, citrus decline is considered similar to citrus die-back, considering thefact that no sharp distinction exists between them even today. However, for allpractical or operational purposes, the term die-back denotes a lethal conditionleading to rapid death of the plant from top to downward due to one or morepathogenic causal factors, whereas the decline refers to a gradual reduction inorchards productivity which may occur due to nonpathogenic factor (Ghosh,1985).

    The visual symptoms are variable, often nonspecific, and unreliable todetermine the cause- and -effect relationship. Among the factors responsiblefor citrus decline, malnutrition is frequently ascribed to chlorotic condition oftrees. This condition usually develops after initial few years of excellent onyears. Various symptoms comprise of: light green interveinal areas with themidrib, causing yellowish condition of leaf at advanced stages. In this way,growth of the plant is partially retarded, and the plant bears short twigs withnarrow leaves. The shoots have a tendency to die-back and even cause the wholeof the tree to dry completely in the subsequent years. Kiely (1957) describedthe features of morphological symptoms of citrus decline in Sri Lanka as asymptom of a special chlorosis in form of appearance of silvery grey spotson upper surface of leaf in the areas having sandy soils, which resembledvery much the symptoms of marble chlorosis induced by manganese (Mn)-deficiency. These symptoms also resemble to those of zinc (Zn)and iron(Fe)deficiencies in Kinnow mandarin orchards of northwest India (Randhawaet al., 1967). Of late, multi-micronutrient deficiency in Marathwada region ofMaharashtra, India was established to be the causal factor for sweet orangedecline (Srivastava and Singh, 2004a).

    The declinio in Brazil showed the similar symptoms and characteristics asblight in Florida (Syvertsen et al., 1980). The leaves of affected trees indicateZn-deficiency like symptoms, wilt in the canopy, followed by leaf fall, twigdie-back, abnormal flowering, and general canopy decline Visual symptoms arealso adapted for a rational canopy rating: zinc deficiency symptoms in leaves,wilting of part or all canopy, leaf drop, twig die-back, abnormal lowering,

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  • 200 A. K. Srivastava and S. Singh

    general canopy decline, internal growth of new shoots, or reduced size of fruit(Rossetti et al., 1990). Canopy rating by these visual symptoms is recommendedon the scale of 0.03.0 (0.0 healthy, 0.01.0 initial declinio stage, 1.02.0intermediate stage, and > 3.0 advanced stage).

    Diagnostic Criteria

    Important criteria to distinguish decline-affected trees from healthy trees are:reduced water uptake in the trunk by water injection, presence of amorphousplugs in xylem vessels, rate of canopy decline by visual tree vigor, high zincaccumulation in the trunk wood and phloem, and low or no water flow throughsecondary roots of affected trees (Lee et al., 1984; Brlansky et al., 1986; Rossettiet al., 1990). These criteria are the same as those applied to distinguish blightedtrees in Florida. Soil fertility does not appear to be a direct causal factorin blight incidence, though a considerable redistribution of nutrient elementstakes place within the trees as a secondary transformation. This diagnostic test,along with trunk water uptake, is widely used to identify citrus blight in thecountries like South Africa, Uruguay, and Argentina (Wutscher et al., 1982a;1982b). Young et al. (1980) showed that trunk wood Zn accumulation occurredin outer wood, whereas reduced water movement took place in inner wood.Wutscher and Hardesty (1981) proposed cation-anion ratio and water solublenutrients in soil as promising approaches to explain the citrus blight syndrome.According to many studies, an elevation in wood Zn concentration took placeprior to visual symptoms in 58% of the trees developing blight, and in restof the 42% of these trees accumulation occurred 3 years before the develop-ment of such visual symptoms (Wutscher et al., 1977; Wutscher and Hardesty,1981).

    SOIL PHYSICAL PROPERTIES RELATED CONSTRAINTS

    Most of the soil-plant-water relations are governed by physical properties ofsurface and sub-surface soil. Influence of soil physical properties on tree growthof mid-season cultivars (20-year-old sour lemon and 25-year-old Valencia onsweet lemon rootstock indicated a much better tree growth under grass on thesoil type having clay content 10% at 3070 cm than on the soil type under cleancultivation (Nel, 1980).

    Particle Size

    Particle size distribution in governs soil-water-plant relation. Review of texturalvariation in Ap horizon of citrus growing soils across the commercial citrus belts

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  • Citrus Decline: Soil Fertility and Plant Nutrition 201

    showed that the soils predominantly high in sand with comparatively heaviersubsurface are best suited for top citrus production. These soils preferably be-long to soil orders viz., Entisols, Inceptisols, Alfisols, and Spodosols (Florida,USA) with a small proportion of Ultisols (India, Brazil, SriLanka, China, andJapan), Aridisols (Argid in California and Arizona of the USA, Mexico, SouthAustralia, and South-west Africa), and Vertisols (central India, Greece, Mexico,and Venezuela) having large variation in pH and CaCO3 (Table 1). Compar-ison of physical characteristic of soils belonging to Floridana (Entisols) andHolopaw (Alfisols) families under healthy versus blight affected Hamlin treesshowed a comparatively higher proportion of fine textured particles in the rootzone of latter soil type associated with lower hydraulic conductivity and avail-able water (Shih et al., 1986). Decline of sweet orange trees (marchitamientorepentino) on trifoliate orange (Poncirus trifoliata) occurred only in patchesof heavy clay soil in the area of Concordia, Enter Rios province of Argentina(Swartz et al., 1980), since the amount of clay in soil correlated negatively withfeeder roots in all the soil layers (Koudounas, 1994).

    In central India, in the absence of low temperature (cumulative 810 hoursof 510C for 1214 days during December-January for spring flush) sufficientto induce flowering, soil water deficit stress is usually adopted by withholdingirrigation (duration of which varies as per soil depth and texture) which resultsinto concerted flowering following the resumption of irrigation. The successto induction of flowering is, therefore, largely dependent upon the physico-chemical properties of both surface as well as sub-surface soil (Table 2), whichin turn influenced the orchard efficiency (Srivastava and Singh, 2004b).

    Clay content > 600.0 g kg1 at soil depth below 30 cm was observedundesirable from the point of view of regular flowering, fruit set, and fruityield in Nagpur mandarin (Dass et al., 1998). The clay content during peaksoil moisture deficit stress was observed in the range of 350450 g kg1 at030 cm soil depth, sufficient for the surface soil to deplete its moisture duringthe peak stress period (215 mm m1 soil moisture recorded on an average). Inthe next layer immediately below it and up to 150 cm depth, due to high claycontent (600.0820.0 g kg1), virtually no depletion of moisture took place(378.0 mm m1 soil moisture on an average). Such a differential distributionof clay induced the spontaneous flow of moisture from wetter soil zone (lowersoil depths below 30 cm) to a comparatively drier zone (upper soil depth of30 cm) across the sharp moisture gradient within the soil profile that helpedtrees continue its un-interrupted vegetative growth, instead of trees undergoinga physiological rest, a pre-condition for induction of flowering to take place.This set-up of soil-water-plant relationship failed to induce any floral stimulusin the trees, unless growth retardant like paclobutrazol (915 g tree1 soilapplication at the time of imposition of stress) is used to stop vegetative growthduring the stress period, forcing the trees undergo a physiological stress (Singhet al., 1999). Resultantly, the lower intensity of flowering in soils having highsubsurface clay, in turn adversely influenced the fruit yield. Hernandez et al.

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  • Tabl

    e1

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    (%)

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    Sr.N

    o.C

    itrus

    belts

    Sand

    Silt

    Cla

    y(H

    2O

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    Sm

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    (%)

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    1.A

    ntal

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    urke

    y24

    4432

    7.5

    11

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    d,X

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    lf,C

    alci

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    Ass

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    Egy

    pt61

    1029

    7.6

    1.2

    3.2

    Torr

    ent,

    Torr

    ifluv

    ent,

    Torr

    ipsa

    mm

    ent

    3.C

    ape,

    Sout

    hA

    fric

    a91

    36

    7.6

    0.16

    2.0

    Cam

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    t.4.

    Cen

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    1744

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    0.12

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  • 12.

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    obin

    son,

    1977

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  • 204 A. K. Srivastava and S. Singh

    Table 2Distribution of CaCO3 and clay in regular (Rf) and irregular flowering(Irf)orchards

    OrchardCaCO3(g kg1) Clay (g kg1)

    Type 1 2 3 1 2 3

    Rf. 4.5 8.6 12.3 44.1 42.3 46.6Irf. 4.2 4.4 6.5 51.2 58.1 68.4Sig. (p = 0.05) 2.8 2.9 3.2 5.8 10.2 9.6

    1, 2 and 3 represents depth 015, 1530 and 3060 cm.Source: Dass et al., 1998.

    (1987) comparing the mandarin yield on ferrrallitic red earths (clay 310420 gkg1) against ferrsiallitic yellow earths (clay 510680 g kg1) soil types undersubtropical climate of Western Georgia, demonstrated a higher fruit yield (21.4tons ha1) in former than latter soil types (18.3 tons ha1) due to difference innutrient supply to mandarin trees.

    Soil Compaction and Drainage

    Soil compaction and drainage complementary to each other, but a strong lim-iting factor to root growth and tree performance, is well established. Poordrainage due to clay rich sub-surface or caco3-induration (hardpan or clay pan)restricting the root development is claimed to be one of edaphic causal fac-tors to lower fruit yield of citrus at number of locations like northwest India(Kanwar and Randhawa, 1960), San Joaquin Valley, USA (Chapman, 1961),Aegean region, Turkey (Kovanci et al., 1978), Yaracuy, Venezuela (Sanchezet al., 1998), Concordia and Enter Rios provinces, Argentina (Swartz et al.,1980), and Nelspruit, South Africa (Abercrombie and du Plessis, 1996). Anessential point concerning hardpan, in contrast to clay pan, is that once hardpanis broken-up, it stays broken-up. Clay pan on the other hand, when broken,softens and runs together with first liberal application of water. Reduction inroot- and tree growth of valencia orange due to hard sub-surface induced poordrainage (Merlo et al., 1990) is reported to be determined by soil penetrationresistance exceeding 500 kpa (Nel and Bennie, 1984) to as high as 2000 kpa(Abercrombie and du Plessis, 1996). Factors contributing variation in fruit yieldfrom other angles revealed that 10 cm water table could be infiltered in 11 daysin low-yielding orchards (48 kg per tree), while on the same soil type, sameamount of water could be infiltered within 56 hours in high-yielding orchards(162 kg per tree) according to Nunez-Moreno and Valdez-Gascon (1994).

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  • Citrus Decline: Soil Fertility and Plant Nutrition 205

    Indeed, soils are classified into different potential classes for citrus depend-ing on rooting depth (De la Rosa and Carlisle, 1978). Restricted root depth isone of the major factors limiting yield. Plowing to a soil depth of 80 cm on acompacted soil helped restore a favorable rooting volume for regeneration ofroots (Hoffman and Abercrombie, 1999; Abercrombie and du Plessis, 1995).Besides yield, quality of fruit is also improved substantially following theloosening of compacted soil layer (Okada, 1994). However, such soil manage-ment practices often resulted in pruning of roots, might later affecting the treeperformance. The ability of roots to regenerate following such a treatment isattributed to factors such as soil fertility, rootstock, cultivar, and root thicknessin addition to timing and intensity of treatment (Van Zyl and Van Huyssteen,1987).

    Analysis on causes of low fruit yield (2567 kg 0.67 ha1) of man-darin cultivar Wenzhoumigan on Beach land in Yueqing county of Zhejiang,China showed that heavy clay texture of soil coupled with non-capillary pores(0.150.33%) and highly compacted layer (169 kg cm1) at 1040 cm depth arethe contributory factors. The efficacy of soil improvement measures assessedfrom yield increments decreased with treatment involving combined use offerrous sulfate, alum, sulfur, and gypsum or green manuring and phosphorus(P)- fertilization (Lou and Yin, 1986).

    SOIL FERTILITY RELATED CONSTRAINTS

    The soil-related constraints are highly varied in nature, when compared thecitrus orchards established on plain land to that of hilly land, latter remain-ing loaded with problems in both time and scale. Chief soil constraints are:reduced soil depth due to slicing of fertile layer exposing the laterite/plinthitelayer, exposure to water stress due to curtailed available water capacity, lowsoil air temperature, reduced workability, low cation exchange capacity, de-ficiency of nitrogen (N), phosphorus (P), potassium (K), toxicity of alu-minum (Al) and magnesium (Mn) in addition to non-competitive Fe-toxicityinduced Zn-deficiency (Srivastava and Singh, 2006). Multiple nutrient defi-ciency linked decline in citrus orchard productivity is reported world over(Table 3).

    Soil pH

    It one of the properties that dictates the nutrient availability. Establishment ofHamlin orange trees in Florida flatwood soils showed no relationship betweensoil pH and tree canopy volume or orange yield in the soil pH range of 4.68.0.Below pH 4.6, tree size and yield reduced substantially due to toxic effect ofAl3+ and H+ ions (Obreza, 1973). Analysis of soil samples from old citrus

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  • Tabl

    e3

    Glo

    bald

    istr

    ibut

    ion

    ofnu

    trie

    ntde

    ficie

    ncie

    sin

    citr

    usor

    char

    ds

    S.N

    o.C

    itrus

    regi

    ons

    Nut

    rien

    tdefi

    cien

    cies

    Ref

    eren

    ces

    1.A

    rgen

    tina

    (Tuc

    uman

    )N

    ,Cu,

    Fe,M

    g,Z

    nA

    soan

    dD

    antu

    r,19

    702.

    Aus

    tral

    ia(N

    ewSo

    uth

    Wal

    es,

    Riv

    erla

    nd,S

    unra

    yasi

    a)N

    ,P,C

    u,M

    n,Z

    n,B

    Hal

    se,1

    963;

    Dun

    can,

    1969

    3.B

    razi

    l(Sa

    oPa

    ulo,

    Para

    na)

    Ca,

    Mg,

    P,K

    ,Zn,

    B,

    Cae

    tano

    etal

    .,19

    84;F

    idal

    skia

    ndA

    uler

    ,199

    74.

    Chi

    le(A

    zapa

    ,Elq

    ui,L

    imar

    i,C

    acha

    poal

    )N

    ,Zn,

    Mn,

    P,S

    Ver

    egar

    aet

    al.,

    1973

    5.C

    hina

    (Fuj

    ian,

    Sich

    uan)

    Ca,

    P,Fe

    ,Mn,

    Zn,

    Mo

    Lie

    tal.,

    1998

    ;Yin

    etal

    .,19

    986.

    Cos

    taR

    ica

    (Atla

    ntic

    zone

    )N

    ,P,K

    ,Ca,

    Mg,

    Mn,

    Zn

    Bor

    nem

    isza

    etal

    .,19

    85;A

    lvar

    doet

    al.1

    994;

    Ara

    yaet

    al.,

    1994

    7.E

    gypt

    (Asw

    an,B

    ehei

    ra,T

    ahri

    r)N

    ,P,F

    e,M

    n,Z

    n,E

    l-Fo

    uly

    etal

    .,19

    84;S

    alem

    etal

    .,19

    95;

    8.In

    dia

    (nor

    thw

    est,

    nort

    heas

    t,so

    uth,

    cent

    ralr

    egio

    n)N

    ,P,C

    a,M

    g,Fe

    ,Mn,

    Zn

    Aw

    asth

    ieta

    l.,19

    84;D

    hatt,

    1989

    ;Sri

    vast

    vaet

    al.,

    2001

    ;Sri

    vast

    ava

    and

    Sing

    h,20

    069.

    Iran

    (Jir

    off

    valle

    y)Z

    n,M

    n,C

    uR

    ao,1

    993

    10.

    Isra

    el(N

    egev

    ,Sin

    ai,J

    orda

    nva

    lley)

    Ca,

    Mg,

    Fe,Z

    nSh

    aked

    and

    Ash

    kena

    zy,1

    984;

    Hor

    esh

    etal

    .,19

    8611

    .It

    aly

    (Sic

    ily,C

    alab

    ria,

    Bar

    asili

    cata

    )N

    ,K,M

    g,C

    uPe

    nnis

    i,19

    7512

    .Ja

    pan

    (Shi

    zuok

    a,E

    him

    e,K

    anag

    awa)

    N,P

    ,K,M

    g,Z

    nTa

    kats

    ujia

    ndIs

    hiha

    ra,1

    980;

    Koz

    aki,

    1981

    ;Wad

    aet

    al.,

    1981

    13.

    Ken

    ya(R

    iftv

    alle

    y)N

    ,P,B

    ,Fe,

    Zn,

    Cu,

    Mn,

    Mo

    Kim

    ani,

    1984

    206

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  • 14.

    Kor

    ea(J

    eju

    Isla

    nd)

    N,P

    ,K,C

    a,M

    g,S,

    Cu,

    Zn

    Kim

    etal

    .,19

    69;M

    oon

    etal

    .,19

    8015

    .M

    oroc

    co(S

    ouva

    lley)

    Fe,M

    n,Z

    nPe

    nkov

    etal

    .,19

    7916

    .N

    epal

    (Dha

    nkut

    a,L

    amju

    ng,G

    orkh

    a),

    B,M

    g,C

    u,C

    a,Z

    nG

    upta

    etal

    .,19

    89;T

    ripa

    thia

    ndH

    ardi

    ng,2

    001

    17.

    Paki

    stan

    (Pun

    jab)

    K,Z

    n,B

    Haq

    Izha

    ret

    al.,

    1995

    18.

    Sier

    raL

    one

    (Sie

    rra)

    N,P

    ,K,C

    a,M

    g,Z

    nH

    aque

    and

    God

    frey

    ,197

    619

    .Sp

    ain

    (Val

    enci

    a,Se

    ville

    ,Mur

    cia,

    Cat

    ania

    )N

    ,P,K

    ,Ca,

    Mg,

    Fe,M

    n,Z

    nM

    ajor

    ana,

    1960

    ;Hel

    linet

    al.,

    1988

    20.

    Tha

    iland

    (Kor

    atPl

    atea

    u)C

    a,M

    g,P

    Zn

    McC

    all,

    1965

    21.

    Tri

    nida

    d(C

    arib

    bean

    area

    )M

    g,Z

    n,M

    nW

    eir,

    1969

    ;197

    122

    .T

    urke

    y(I

    zmir

    ,Aeg

    ean

    regi

    on)

    Ca,

    Mg,

    Fe,Z

    nE

    rciv

    an,1

    974;

    Saat

    cian

    dM

    ur,2

    000

    23.

    USA

    (Flo

    rida

    ,Cal

    ifor

    nia,

    Texa

    s)N

    ,P,K

    ,Fe,

    Mg,

    Zn,

    Mn,

    Cu,

    B,M

    oK

    oo,1

    982;

    Zhu

    and

    Alv

    a,19

    93;T

    ucke

    ret

    al.,

    1995

    ;Zha

    nget

    al.,

    1997

    24.

    Ven

    ezue

    la(C

    arab

    obo)

    N,P

    ,Ca,

    Mg,

    Zn

    Pint

    oan

    dL

    eal,

    1974

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  • 208 A. K. Srivastava and S. Singh

    orchards and from adjacent virgin soil in several districts of South Africaindicated no differential accumulation or depletion of any nutrient, and no pHsoil change was observed in growing citrus for 20 years or more (Bredell andConradie, 1975). The performance of Balady lime, Cleopatra mandarin, andSour Orange seedlings evaluated at various soil pH values showed a reductionin growth by 9.8, 25.4, and 40.1% at soil pH 6.0, 7.0, and 8.0, respectively(Shawky et al., 1980). Canopy of Satsuma mandarin at soil pH 4.0 was observedas half of trees growing at soil pH 5.0, and attributed low soil pH to heavy N-fertilization (Yuda, 1985). However, the effect of copper (Cu) on the growth ofHamlin orange trees was more pronounced at soil pH range of 5.56.0 than athigher or lower soil pH regimes (Alva et al., 1995). Double acid extractable-Ca(2721249 mg kg1) and soil pH (5.86.2) under blighted trees were higherthan Ca (112532 mg kg1) and pH (4.85.7) under healthy trees representingsix major citrus belts of Florida, USA (Wutscher, 1989).

    Soil Salinity

    Damage to citrus crop due to excessive accumulation of salts is well recognizedin many parts of the world growing citrus, especially in semi-arid and aridregions where saline water is largely used for irrigation. Citrus trees are quitesensitive to excess salts (Bielorai et al., 1988; Srivastava and L. Ram, 2000), andtolerance to soil salinity is correlated with its ability to restrict the entry of toxicions [sodium (Na), chlorine (Cl), and boron (B)] into roots and onward transportto shoots. Visible symptoms of salinity include: leaf bronzing, defoliation,leaf chlorosis similar to iron induced chlorosis, small leaves, small fruits, anddie-back of young twigs (Cole, 1985). Goell (1969) described salinity boundchlorosis in Eureka lemon leaves into five groups as: 0) dark green and healthyleaves, 1) light green color and slight bronzing, 2) beginning of chlorosis andyellowing of margins and tips, 3) pronounced chlorosis on margins and tipswith blotching, 4) wholly chlorotic leaves with necrotic spots on margins andtips, and 5) advanced chlorosis in form of necrosis on margins and tips coupledwith interveinal necrotic spots. Maas (1992) observed a decrease in Satsumayield at the rate of 13% for each 1.0 dS m1 increase in EC of saturated soilextract beyond the threshold EC limit of 1.4 dS m1. However, increasingsalt stress delayed fruit maturation, but had little effect on quality of Valenciaorange (Cerda et al., 1990; Dasberg et al., 1991; Nieves et al., 1991a; 1991b)and a slight increase in TSS and TSS/acid ratio in Shamouti orange (Bieloraiet al., 1988).

    The methods of combating salinity consists of: identifying and screeningcitrus rootstocks against their reaction to salinity (Singh et al., 1997), use ofmicro-irrigation systems (Singer et al., 1995), chemical amendments (Zekriand Parsons, 1990), and breeding rootstocks for salinity tolerance. The Cl andNa+ exclusion behavior is widely used in rating the citrus rootstocks against

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  • Citrus Decline: Soil Fertility and Plant Nutrition 209

    salinity. Grieve and Walker (1983) observed that trifoliate orange possesseda better Na+ exclusion property than limes and mandarins, which are betterClexcluders. The common rootstocks such as rough lemon, trifoliate, andtroyer citrange are considered salt sensitive rootstocks. While Rangpur lime,Cleopatra mandarin, and Citrus macropylla are salt tolerant rootstocks (Singhet al., 1997). It is often observed that selections made for tolerance at earlygrowth stage proved to be not as tolerant during the later growth stages or atother salinity concentrations (Shannon, 1979) in order to harvest the sustainableproduction.

    Acidity

    Ironically, soil acidity is the major production constraint but the worlds topcitrus production is obtained from those soils (Srivastava and Singh, 2001a).Soil acidity is defined as a soil system having proton-yielding capacity during itstransition from a given state to a reference state. The soil acidity is partitionedinto pH-dependent acidity and exchange acidity collectively known as totalacidity. The growth of some sensitive citrus species is adversely affected atexchangeable-Al as low as 15 mg kg1 and hazard to more tolerant speciesincreased at > 510 mg kg1 (Robinson, 1989). Based on the response ofcitrus rootstocks to varying levels of Al+, the relative tolerance of rootstockswas rated as: Cleopatra mandarin > Rough lemon > Sour orange > Swinglecitrumelo > Carrizo citrange with respective optimum Al concentration of 163,93, 89, 85, and < 50 m (Lin and Myhre, 1991).

    Calcareousness

    Calcareousness is characterized by the presence of calcium carbonate (CaCO3)which has relatively high solubility, reactivity, and alkalinity. Its dissolution insoil produces high solution bicarbonate ion (HCO3) concentration bufferingthe soil in the pH range of 7.58.5. Lime-induced chlorosis is known to beone of the oldest forms of decline due to immobilization of micronutrientsavailable in soil. The primary factor associated with Fe-chlorosis in soils rich inCaCO3 emerges from the effect of HCO3) in reducing Fe-uptake and onwardtranslocation to leaves. However, it is still not clear about the mechanisminvolved in Fe-inactivation in chlorotic leaves.

    Carbonates have their own inherent particle size which affect the availabil-ity of nutrients and modify soil texture in highly calcareous soils dependingupon their origin (Yaalon, 1957). Presence of CaCO3 in soil is classified intopedogenic (geogenic) and non-pedogenic (lithogenic) forms, the former beingmore active, hence not desirable from nutrient availability point of view. Theorigin of CaCO3 and their influence further confirm their properties in a cross

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  • 210 A. K. Srivastava and S. Singh

    section of Vertisol types (Srivastava and Singh, 2002; 2003). Although CaCO3rich nodules are common in smectite-dominant Vertisols (Srivastava and Singh,2001a; 2002), no significant difference was observed with respect to CaCO3under healthy (28.0142.0 g kg1) versus declining trees (22.0112.0 g kg1).Presence of non-pedogenic (geogenic) CaCO3 contributed substantially to soilnutrient pool under healthy trees compared to declining trees. Strong associ-ation of CaCO3 nodules with micronutrient-containing minerals (nontronite,suponite, and suconite), later these micro-nutrients released into soil solutiondue to combined influence of argillo-pedoturbation and high rate of manuringcollectively improved the available pool of nutrients in soil to act as a stimulanttowards improved performance of trees. Common occurrence of citrus blighton shallow soils above limestone in Florida (Cohen, 1980), Uruguay, Argentina(Wutscher et al., 1977), Cuba (Wutscher et al., 1983), and Brazil (Rodriguez,1985) supported the possible involvement of nature of soil as a contributoryfactor.

    The use of salt tolerant rootstock holds a greater potential to counter theadverse effect of soil salinity or calcareousness, which is genetically controlled.Earlier studies (Shaked et al., 1988; Gallasch and Dalton, 1989; Sagee et al.,1992) have shown the response of citrus rootstocks to soil calcareousness.However, limited studies are available to suggest the rootstock alternativeswhich can withstand high soil CaCO3. The effect of soil texture on the responseof citrus rootstock seedlings to CaCO3 showed the better performance onclay soil (47% clay, 39% silt, and 14% sand) with 33% CaCO3 than on siltyclay soil (21% clay, 19% silt, and 60% sand) containing only 23% CaCO3(Sagee et al., 1992). In another study, Sagee (1996) demonstrated that thetolerance of citrus rootstock to CaCO3 is governed by the ability of rootstockto lower the pH of the medium. Screening of six rootstock seedlings againstcalcareousness (Rough lemon, Cleopatra mandarin, Rangpur lime, Carrizo,Troyer citrange, and Trifoliate orange) suggested Cleopatra as most tolerantrootstock (El-Otmani, 1996). In countries where Poncirus trifoliata and roughlemon rootstocks are commercially used, they are exposed to intolerance oncalcareous soils (Castle, 1987). These rootstocks might face twin problem withregard to salinity as well as calcareousness.

    Nutrient Constraints

    Soil fertility is that component of productivity which primarily deals with nu-trient supplying capacities of soil to the plant. The occurrence of multi-nutrientconstraints contributing to citrus decline like condition is widely reported.Consequently, the impact of nutrient management on improvement in orchardproductivity is widely acclaimed (Ghosh and Singh, 1993; Zhou et al., 1996;Li et al., 1999; Srivastava and Singh, 2003; Srivastava and Singh, 2005).

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  • Citrus Decline: Soil Fertility and Plant Nutrition 211

    It is often observed a kind of secondary transformation of decline-likesymptoms. In such cases, nutrient constraint may not be necessarily involved.Higher K associated lower concentration of Fe, Mn, and Zn is common inthe rhizosphere soil of blighted trees than those of healthy trees (Pavan andWutscher, 1993). Comparison of soil properties under etiolated versus normalcitrus trees showed a significantly higher pH, exchangeable Ca2+, and Mg2+

    (7.7, 146.1 mg kg1, and 110.9 mg kg1) under etiolated trees than the cor-responding values (5.3, 514.1 mg kg1, and 82.9 mg kg1) of the soils undernormal trees (Cheng and Zeng, 1991). Differences in soil fertility in sweet or-ange orchards of Agra region (India) revealed no significant difference betweenhealthy and chlorotic trees. However, the leaf analysis values established Zn-deficiency induced chlorosis due to antagonistic effect of Fe- on Zn-availability(Singh and Tripathi, 1985).

    Changes in cation-anion ratio and water soluble nutrients in soil were pro-posed to be promising approaches to explain citrus blight syndrome (Wutscherand Hardesty, 1981). Mean exchangeable-Ca2+ was observed to be higher (33.8mg 100 g1) under healthy trees (Table 4) compared to declining Nagpur man-darin trees (29.2 mg 100 g1) established on Entisols, Inceptisols, and Vertisols.The nutrient mining-linked fertility depletion in surface (Srivastava and Singh,2004a) and sub-surface soil (Dass et al., 1998; Reddy et al., 2003) is by andlarge responsible for citrus decline in Deccan plateau of India.

    THRESHOLDS OF SOIL PROPERTIES

    The central element for optimization of soil properties is the optimum value,best regarded not as a constant, but having a range of values associated withhighest crop yield. The idea has become firmly established that the optimumvalue is rather a variable quantity, but the current models of soil fertility takelittle or no notice of this fact (Medvedev, 1990). Larson and Pierce (1991)suggested that minimum data set consisting of soil attributes namely, nutrientavailability index, total organic carbon, labile organic carbon, particle size,plant available water content, soil structure, soil strength, rooting depth, pH, andelectrical conductivity must be considered for suitability appraisal. For subsoilappraisal, Koga (1972) advocated the basic factors affecting the productivity ofSatsuma mandarin orchards, as non-capillary porosity and bulk density, whichalong with soil depth can be used a criteria to decide whether a soil can beadapted for Satsuma production in addition to methods of soil management.

    Shishov and Kapshuk (1984) suggested that reddish brown soils of theTripolitanian coastal plain of USSR are considered suitable for Citriculture, ifthe following criteria are met: soil depth more than or up to 60 cm, CaCO3 lessthan or up to 812%, EC less than or up to 5.0 mmhos cm1, and gypsum lessthan or up to 30%. Ko and Kim (1987) observed that high yielding orchardshad high mineral content, but low in available P. Available N and K levels were

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  • Tabl

    e4

    Com

    pari

    son

    ofso

    ilph

    ysic

    o-ch

    emic

    alpr

    oper

    ties

    unde

    rhe

    alth

    yan

    ddi

    seas

    edsw

    eeto

    rang

    eor

    char

    dsof

    Mar

    athw

    ada

    regi

    on,M

    ahar

    asht

    ra,I

    ndia

    EC

    CaC

    O3

    Exc

    hang

    eabl

    eca

    tions

    (me

    100

    g1)

    Tre

    est

    atus

    pH(d

    Sm

    1)

    (%)

    Ca2

    +M

    g2+

    K+

    Na+

    Hea

    lthy

    8.0

    0.16

    7.3

    33.8

    12.6

    1.2

    8.0

    Dis

    ease

    d7.

    90.

    187.

    329

    .213

    .61.

    36.

    4Si

    gnifi

    canc

    eN

    SN

    SN

    S3.

    1N

    SN

    S0.

    98So

    ilav

    aila

    ble

    nutr

    ient

    s(m

    gkg

    1)

    NP

    KC

    aM

    gFe

    Mn

    Cu

    Zn

    BM

    oH

    ealth

    y12

    6.0

    13.2

    173.

    90.

    1911

    2.7

    10.6

    8.2

    2.8

    22.6

    0.41

    0.11

    Dis

    ease

    d11

    0.3

    10.5

    151.

    00.

    1210

    8.5

    7.4

    5.9

    2.8

    16.1

    0.28

    0.09

    Sign

    ifica

    nce

    10.2

    2.4

    10.2

    0.04

    NS

    1.1

    0.9

    NS

    5.2

    0.04

    NS

    Tota

    llea

    fnu

    trie

    nts

    (%)

    Tota

    llea

    fnu

    trie

    nts

    (ppm

    )N

    PK

    Ca

    Mg

    FeM

    nC

    uZ

    nB

    Mo

    Hea

    lthy

    2.01

    0.13

    1.58

    2.59

    0.32

    95.6

    63.5

    5.9

    22.1

    29.8

    0.34

    Dis

    ease

    d1.

    830.

    091.

    732.

    210.

    3075

    .453

    .75.

    517

    .721

    .20.

    30Si

    gnifi

    canc

    e0.

    110.

    030.

    240.

    38N

    S10

    .19.

    8N

    S4.

    12.

    8N

    S

    Sho

    win

    gnu

    trie

    ntde

    clin

    e.So

    urce

    :Sri

    vast

    ava

    and

    Sing

    h,20

    04a.

    212

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  • Citrus Decline: Soil Fertility and Plant Nutrition 213

    high in those orchards practicing cultivation of mid-season cultivars in Jejucounty of Korea with average values of soils as pH 5.7, organic matter 8.9%,and exchangeable K, Ca, Mg, and N levels of 1.4, 6.7, 2.3, and 0.2 me 100 g1

    soil, respectively. Other studies with reference to highest fruit yield of Satsumamandarin (1012 tons ha1) suggested the values in soil as: 3149 mg 100 g1

    exchangeable K, 5254 mg 100 g1 Mg, and 197223 mg 100 g1 Ca withCa: Mg 2.53.0, Mg:K2O 3.04.0, and Ca:K2O 8.010.0 in Western Georgia(Beridze, 1987).

    Evaluation of various citrus growing soil series for citrus production incentral Taiwan revealed the suitability of 27 soil series and 5 great groups.Orchards established on dark grey colluvial soils registered the highest fruityield of 28.1 tons ha1 and lowest yield of 16.1 tons ha1 on yellow soils.In terms of soil series, the highest fruit yield of 39.6 tons ha1 and 16.1 tonsha1 was observed on grey yellow colluvial and yellow earth colluvial soils,respectively (Lay and Wang, 1997). Soils maintaining exchangeable-Ca and-Mg collectively above 5065% of CEC showed no response of Ca- and Mg-application in Valencia orange and rough lemon (Aso and Dantur, 1971). Thisis in contrast to other study (Quaggio et al., 1992) which showed that maximumyield of Valencia orange on Rangpur lime rootstock was obtained at the soilexchangeable Mg level constituting more than 10% of total cation exchangecapacity.

    Aso and Bustos (1981) based on different citrus regions of Argentinareported the appearance of hidden signs of Mg-deficiency on soils havingexchangeable Mg2+ less than 0.8 me 100 g1. Hidden hunger signs are alsoproduced due to high K (K/Mg >4.0) or Ca (Ca/Mg > 7) content. Nunez-Moren and Valdez-Gascon (1994) observed average values of different soilproperties as: 1.1 dS m1 ECe, 6 me L1 water soluble Na, 4 me L1 Ca, 1.2me L1 Mg, and exchangeable sodium percentage 4.6 me L1 for the orchardshaving high productivity of 162 kg per tree. These values were different atlower productivity level of 48 kg per tree as: 3.8 dS m1 ECe, 12 me L1 watersoluble Na, 16 me L1 Ca, 3.8 me L1 Mg, and 7.3 me L1 exchangeablesodium percentage.

    Poor growth of Nagpur mandarin in central India (Nilangekar and Patil,1982) and Kinnow mandarin in northwest India (Brar et al., 1986) was attributedto presence of high clay, silt, CaCO3 in subsoil. Deep tillage has met with mixedsuccess as a method of ameliorating subsoil, a physical limitation to root growth.In many cases, benefits are transient and variable (Eck and Unger, 1985). Thisin large parts stems from not understanding the basic factors influencing theeffectiveness of the modification and the subsequent maintenance of structuralstability. Attempting to ameliorate a physical problem without correcting anunderlying chemical cause is a common mistake. An example is the failureof deep ripping of subsoil without ameliorating sodicity (Rengaswamy et al.,1992). Some of the citrus growing soil types (Orthents and Psamments) inValencia, Spain are so nicely managed that the fruit yield of > 35 tons ha1

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  • 214 A. K. Srivastava and S. Singh

    on an average is common even with limitation of sub-soil CaCO3 high as37.839.9% and soil pH 7.98.1 (Hernando, 1969).

    RESPONSE OF AMENDMENTS

    The depth of soil to be modified is a function of both economic and rootfactors, and possibly depth to a drainable layer. The amendments in formof lime/dolomite in acid soil conditions and gypsum/phosphogypsum insodic/saline soil conditions have proved effective beyond doubt, to raise thesoil productivity potential. Number of studies (Davitadze, 1991; Nemec andLee, 1992; Liu, 1993; Panzenhager et al., 1999) in the past reported the fa-vorable response of citrus to lime and deep tillage or dolomite combined withorganic matter and NPK (Chen et al., 1997). Quaggio et al. (1998) reported bestresponse of phosphogypsum (4 tons ha1) + dolomite (3 tons ha1) incorpo-rated into surface 20 cm soil of Oxisol soil type with reference to improvementin the base saturation and reduction in soil pH under Valencia sweet orange.The moderate lime application (2.5 kg per tree) in a red soil growing 1117-year-old trees of Satsuma mandarin, increased the fruit yield by 13.229.2%over untreated trees, besides improvement in fruit quality (Liu, 1993). Anotherstudy by Meng et al. (1991) showed an increase in yield of Satsuma mandarinby 37.2% as a result of lime application at the rate of 2.5 tons ha1 in redsoil containing 1.021.13% organic matter, 0.0380.045% N, 0.0300.034%phosphorus pentoxide (P2O5), and 0.881.02% dipotassium oxide (K2O).

    Surface application of 400 kg ha1 quick lime or 520 kg ha1 magnesiumcarbonate (MgCO3) soil pH increased at 025 cm soil depth, in an orchardwith highly acidic but regular application for 4 years is needed to notice sub-soil changes with regard to increased nutrient availability (Obreza, 1990) withapplication of CaCO3 and CaSO4 (200800 mg Ca kg1 soil) and reducedthe Cu-phytotoxicity (Alva et al., 1995) in sandy siliceous hyperthermic AericHaplaquods. Addition of Ca on the other hand to saline irrigation water helpedsour orange seedlings to tolerate sodium chloride (NaCl) toxicity by reducingaccumulation of Na+ and Cl in the leaves (Zekri and Parsons, 1990). Shi-mogori et al. (1980) observed that application of 400 kg dolomite ha1 per yearled to an extra fruit yield of about 300 kg 10 acres1 per year in the 6 years ofexperiment.

    DECLINE MANAGEMENT AND FERTILIZER TREATMENT

    Effect of nutrients on plant growth and development has been studied for over350 years since the experiments of van Helmont in 1648 (Epstein, 1972). Ex-citing progress has been made in the past to develop and improve diagnostictechniques of identifying nutritional constraints and accordingly, the fertilizer

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  • Citrus Decline: Soil Fertility and Plant Nutrition 215

    management strategies have changed from time to time. Multiplicity of meth-ods and techniques currently available for determining nutrient requirementemphasizes the importance attributed to an awareness of fertilizer require-ments.

    The widely used approaches to fertilizer recommendations are: the defi-ciency correction philosophy (originates from nutrient constraint-based cropresponse through nutrient additions to the point of maximum economic yield),maintenance concept (aims to maintain soil fertility level slightly above thepoint of maximum economic yield), and nutrient removal or balanced philoso-phy (emphasizes the return to the soil what is removed by the crop to maintainproductivity, but often over-recommends nutrient need, since it does not takeinto account for the soils ability to supply available nutrients to the plants overtime). An optimum supply of nutrients is, therefore, aimed to meet two primeconditions: i) all nutrients should be available in quantities which exclude thepossibility of absolute deficiency or excess, and ii) the proportion of all thenutrients should be such as to exclude any deficiency as no nutrient worksindependent to each other.

    The fertilizer requirement of citrus depends whether the purpose is to growthe crop (pre-bearing stage) or feed the crop (bearing stage). Based on theseobjectives, two types of fertilization viz., corrective and preventive are usuallyadopted. According to Gallasch (1992), an optimum fertilizer program is onein which the cost of each unit of fertilizer applied is at least covered by an extrareturn through fruit yield obtained in both, the short and long term life of acitrus orchard.

    A successful nutrient management program in citrus can be separatedinto four major components (Obreza, 2003). These are: monitoring, programdevelopment, application, and evaluation. Monitoring can be qualitative (vi-sual observations of orchard performance in terms of growth and yield) orquantitative (laboratory-based analysis of soil or leaf samples). In the programdevelopment, the factors like type of fertilizer sources, the rate, timing, andfrequency are considered. The application phase concentrates on methods offertilizer application e.g. basin application, foliar spray or fertigation, etc. Fol-lowing fertilizer application, the evaluation step determines the crop responsethrough improvement in tree growth, fruit yield, and quality. Nutrient manage-ment can become a complex task, if all the factors affecting the efficiency offertilizer use are considered. Therefore, relative sensitivity of citrus to variousnutritional factors is of utmost importance. The sensitivity of citrus trees toshortage or excess of individual nutrients differs greatly. For example, Mn-deficiency does not affect the production as much as N-deficiency or excessof B affects fruit quality more than an excess of Mg. Likewise, related tosoil fertility changes, a decreasing level of exchangeable K may be less of aproblem than a large decrease in organic carbon because K may be replacedby weathering mineral or inorganic fertilization, whereas improving organiccarbon to the original level is cumbersome. Diagnosis and recommendation

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  • 216 A. K. Srivastava and S. Singh

    integration system (DRIS)-based leaf or soil analysis in relation to growth oryield performance can provide the information on the relative importance ofdifferent nutrients.

    Nutrient management program for citrus trees is often based on nutrientremoval of fruits (Quaggio et al., 1996). The knowledge of the nutrient distri-bution in trees is important to establish sound nutrient management programsfor citrus production. Earlier studies (Smith, 1966; Chapman, 1968) discussedthe mineral composition of citrus trees published between the 1930s and the1960s. These authors reported results of several chemical analyses of importantcomponents of citrus tree biomass, which allowed a broad understanding ofamounts of proportion and distribution of nutrients in the various compartmentsof the plant. The distribution of the total tree dry weight (%) was observed as:fruit 30.3, leaf 9.7, twig 26.1, trunk 6.3, and root 27.8. Calcium made up thegreatest amount of nutrient in the citrus tree (273.8 g per tree), followed by N(234.7 g per tree), and K (181.5 g per tree). Other macronutrients collectivelycomprised about 11% of the total nutrient content of trees. The contents ofvarious nutrients in fruits (kg ton1) were: N 1.20, K 1.54, P 0.18, Ca 0.57, Mg0.12, sulfur (S) 0.09, B 1.63 103, Cu 0.39 103, Fe 2.1 103, Mn 0.39 103, and Zn 0.40 103. Total contents of N, K, and P in the orchard cor-responded to 66.5, 52.0, and 8.3 kg ha1, respectively, which were equivalentto the amounts applied annually by fertilization (Mattos et al., 2003). Thereis an agreement that Ca, N, and K are the dominant constituents of citrus treebiomass. While, P, Mg, and S represent a smaller proportion (10%) followedby micronutrients (

  • Citrus Decline: Soil Fertility and Plant Nutrition 217

    Various models like quadratic plateau model (Obreza et al., 1993; Srivastavaand Singh, 2001b; 2002; 2003), DRIS-based norms as discussed earlier, DRISderived FTOVAL (Sautoy, 1992; Woods and De Villiers, 1992), simulationmodels (Jones, 1998), fertilization program model (Gallasch, 1992), and twostochastic dynamic optimization models (Feinerman and Voet, 1995) suggesteddifferent methods of arriving at sound citrus fertilization program under diversegrowing conditions. These decision support systems can provide informationon site specific irrigation scheduling, nutrient requirement, seasonal variationin nutrient uptake, and nutrient budgeting through annual orchard specificnutrient recommendations; eventually to reduce production cost and negativeenvironmental impacts.

    Expert system for citrus fertilization (SEFEAG) proposed by Resina et al.(1992) is an advanced prototype aimed to study the citrus nutritional problemsand maximizing the effect of fertilizers. The system identifies the causes ofnutrient excess or deficiency on the basis of information collected in the field(visual analysis) and using data on orchard history, values of leaf and soilanalysis obtained from interfaced data bank or from interactive interviews. In asubsequent publication, Basile et al. (1992) described the data base required forcitrus fertilization which can be used in SEFEAG consists of cultivar features,production characteristics, cultural techniques, technological standards, andresults of leaf/soil/water analysis. With this data bank using SEFEAG, it ispossible to improve fertilizer management for individual farm to establishyield and quality standards for each cultivar in different areas to compare andevaluate the relationship between cultural techniques and orchard performancein various locations. Chiriatti and Plant (1996) proposed a prototype, casebased reasoning (CBR) system for fertilizer application management, adoptingthe case based planning technique and a planning system in cooking domain.Although not yet in a form suitable for field implementation, the prototypeprovides an insight into how CBR system can be used to provide decisionsupport in nutrient management program.

    Optimum Foliar Applied Nutrients

    The foliar fertilization is better than conventional soil fertilization under theconditions: i) soils having an acute shortage of nutrient supply, ii) nutrientsimmobilized on account of unfavorable soil conditions, iii) under soil nutrientimbalances i.e. having an unfavorable influence on root absorption for anoptimal growth, and iv) restricted nutrient uptake through the plant roots.Studies in the past applying foliar sprays of urea (2831 kg N ha1) in Valenciaorange (Albrigo, 2000), multiple application at 1% urea in Codoux clementinemandarin (El-Otmani et al., 2002), 10% potassium chloride (KCl) in Eurekalemon (Qin et al., 1996), 5% potassium nitrate (KNO3) with 1820 ppm 2,4-Din Shamouti orange (Erner et al., 1993), and 5% KNO3 in Valencia orange

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  • 218 A. K. Srivastava and S. Singh

    (Koo et al., 1984) demonstrate that only two nutrients, N and K amongstmacronutrients are effective through foliar application.

    Foliar sprays of micronutrients are more popular and, therefore, fre-quently used. A large variation exists with regard to foliar recommendationof micronutrients-based chelates viz., Fe-ethylenediamine-di-o-hydroxyphenylacetic acid (EDDH) (0.1%) for Valencia orange (Zude et al., 1999), Fe-polyflavonoid (1%) for Verna lemon (Fernandez-Lopez et al., 1993), Fe (50ppm)manganese sulfate (MnSO4) (5 ppm)Zn (75 ppm) for WashingtonNavel (Hassan, 1995), MnSO4zinc sulfate (ZnSO4) (0.15% each) for Thomp-son Navel (Razeto et al., 1988), ferrous sulfate (FeSO4)copper sulfate(CuSO4) (0.25% each)ZnSO4 (0.5%) for Coorg mandarin (Desai et al.,1991), ZnSO4 (0.60 g l1)MnSO4 (1.2 g l1) for Valencia orange (Garcia-Alverez et al., 1986), Zn-EDTA (0.4%)Cu-EDTA (0.2%) for Kinnow man-darin (Sharma et al., 1999), and boraxmagnesium sulfate (MgSO4) (0.2%each)ZnSO4 (0.1%) for Jiaogan mandarin (Wang, 1999).

    Optimum Soil Applied Nutrients

    Response of fertilizer applied through soil on growth, yield, and quality ofdifferent citrus cultivars is well recognized under different agroclimatic citrusregions (Ghosh et al., 1989; Tachibana and Yahata 1996). Contrary to foliarfertilization, soil application of macronutrients is more efficacious. The opti-mum requirement of macronutrients for different commercial citrus cultivarssuggest: 475 g N, 320 g P2O5, 355 g K2O per tree for Satsuma mandarin inTurkey (Koseoglu et al., 1995), 240 g N, 40 g P2O5, 100 g K2O ha1 for DancyTangerine in Spain (Pedrera et al., 1988), 1.4 kg N, 1.08 kg P, 1.1 kg K per treefor acid lime (Chundawat et al., 1991), 4001200 kg N, 200 kg P2O5 ha1 forKinnow mandarin in India (Sharma et al., 1993), 120 kg N, 150 kg P, 75 kg S,6 kg Cu, 0.8 kg molybdenum (Mo), 5.0 kg Zn ha1 for Neck Orange in Korea(Lim et al., 1993), 1.02 kg N, 0.58 kg P2O5, 0.55 kg K2O per tree for Satsumamandarin in Georgia (Liu et al., 1994), 200 kg N, 140 kg P, 210 kg K ha1 forPera sweet orange in Brazil (Cantarella et al., 1992), 0.5 kg N, 0.5 kg P2O5, 1.0kg K2O ha1 for Grapefruit in Greece (Androulakis et al., 1992), and 1.5 kgurea, 0.25 kg superphosphate, 1.25 kg potassium chloride, 1.1 kg magnesiumsulfate, 0.10 kg zinc sulfate per tree for Jincheng orange in China (Yin et al.,1998).

    The researchers even today are not unanimous on the efficacy of soil versusfoliar fertilization with reference to micronutrients. Elevating Zn concentrationonly in the tops of Zn-deficient plants with foliar sprays partially restored thenormal root growth, but clearly was not as effective as the roots absorbingZn directly from in soil (Swietlik and Zhang, 1994). The micronutrient-basedZn chelater complexes are poorly or not at all absorbed by plant roots, asdemonstrated through water culture studies (Chaney, 1988). Soil application

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  • Citrus Decline: Soil Fertility and Plant Nutrition 219

    of micronutrient, e.g., Zn from ZnSO4 is fixed in the surface soil, while thechelated-Zn remain soluble and get distributed evenly throughout the soil,as evident from 46-times higher uptake of Zn from Zn-EDTA than ZnSO4on sandy soils (Parker et al., 1995). The studies carried out world over have,therefore, shown some diversity in optimum doses of micronutrients establishedthrough long term field experiments. These include: Fe-citrate (2.6- 6 mgkg1)-MnSO4 (1.33 mg kg1) for Satsuma mandarin (Liu and Nan, 1996),(292 g Fe-292 g Mn-315 g Zn-EDTA ha1) for Valencia orange (He et al.,1998.), MnSO4 (483 kg per tree)-ZnSO4 (304 g per tree) for Valencia orange(Garcia-Alvarez et al., 1986), and Zn-EDTA (30 g per tree) for WashingtonNavel orange (Swietlik, 1996). The combination of two methods is also oftenused consisting: e.g. ZnSO4 K2SO4 (0.5%-foliar spray)-K2O as potassiumsulfate (K2SO4) (210 g per tree, -soil application) for Kinnow mandarin (Singhet al., 1989) and ZnSO4-iron sulfate (FeSO4)-MnSO4 (50 g per tree each,-soilapplication)+ (0.50%-foliar application) for Sathgudi sweet orange (Devi et al.,1996).

    Site Specific Nutrient Management

    Large variations in tree canopy and subsequently, the tree-to-tree yield dif-ference, are common in many of the large size citrus orchards. Knowing therequired nutrients for all stages of growth, and understanding the soils abilityto supply those needed nutrients are critical to profitable crop production. Therecommendations on fertilizer application may not, however, produce the samemagnitude of yield response when practiced in an orchard of large area, becauseof its inability to accommodate variation in soil fertility status. Slight changesin the nature of soil, local climate, and agronomic practices etc. may seriouslyaffect the nutrient utilization capacity of the plant.

    The conventional long term fertilizer trials (Tiwari, 2002) revealed that: i)omission of limiting macro- or micronutrient leads to its progressive deficiencydue to heavy removals; ii) sites initially well supplied with P, K, or S becomedeficient when continuously cropped using N alone; and iii) fertilizer ratesconsidered optimum still resulted in nutrient depletion at higher productivitylevels, if continued, become sub-optimum rates. There is a strong necessity tokeep overall nutrient balance in relation to total crop load. Application of asingle rate of nutrients may result in over-application of nutrients at some sitesand under-application at other sites, often lead to reduced FUE. Under suchcircumstances, site specific nutrient management is adopted in big orchardsrequiring variable precision application as per soil variability so as to improvethe orchard efficiency (average yield of specified trees in relation to averageorchard yield) in ultimate terms.

    With new advances in technology, grid sampling for precision citricultureis increasing. The first step in the process is to divide large fields into small

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  • 220 A. K. Srivastava and S. Singh

    zones using a grid. Next, a representative location within the grid is identifiedfor precision soil sampling. Grid sampling is integrated into global positioningsystem (GPS) based soil sampling and nutrient-mapping that in turn uses ageographic information system (GIS) to identify spatial variation in productiv-ity and accordingly, employ variable rate technology for fertilizer applications(Schumann et al., 2003; Zaman et al., 2005).

    Variable Rate Fertilization

    It is one of the most effective techniques for rationale use of fertilizers executedby matching the fertilizer rate with tree requirement on a per tree size basis. Sitespecific management of 17-year old Valencia grove (2980 trees) in Floridausing automated sensor system equipped with differential global positioningsystem and variable rate delivery of fertilizers (135170 kg N ha1 per year)on a tree size basis (0240 m3 per tree), achieved a 3840% saving in granularfertilizers cost. While, conventional uniform application rate of 270 kg N ha1per year showed that trees with excess nitrogen (>3%) had canopies less than100 m3 with lower fruit yield and inferior quality (Zaman et al., 2005). Inanother long term experiment, the large fruit yield difference of 30.2 and 48.9kg per tree initially observed on shallow soil (Typic Ustorthent) and deep soil(Typic Haplustert) in an orchard size of 11 ha, reduced to respective fruit yieldof 62.7 and 68.5 kg per tree with corresponding fertilizer does (g per tree)of 1200 N-600 P-600 K-75 Fe-75 Mn -75 Zn-30 B and 600 N-400 P-300 K-75 Fe-75 Mn 75 Zn-30 B, suggesting the necessity of fertilizer applicationon variable rate application for rationality in fertilizer use (Srivastava et al.,2006).

    Analysis of tree size of 3040 trees space of 40-acre grove showed a skeweddistribution with 51.1% trees having 25100 m3 per tree size classes and amedian size of 82 m3 per tree. At a uniform fertilization rate of 240 kg N ha1

    per year, the leaf N concentration of 12 trees with different canopy sizes thatwere randomly sampled in the grove showed optimal levels (2.42.6%) in thelarge trees and excess levels (> 3%) in the medium to small trees (Tucker et al.,1995). From the regression line, trees with excess N had canopies < 100 m3

    per tree, and constituted 62% of the grove. Under such conditions, variablerate fertilization can, therefore, save production costs, reduce N leaching, andincrease yields per variable acre (Schumann et al., 2003). A 30% saving ingranular fertilizer cost was estimated for this Valencia grove if variable Nrates were implemented on a per tree basis ranging from 129 to 240 kg N ha1

    per year. For comparison purposes, the eastern half of the grove received thefull uniform rate of 240 kg N ha1 per year. No fertilizer was allocated byspreader to skips or resets of one-to-three year age. Due to a very restricted rootsystem, new resets should be fertilized individually, usually by hand (Tuckeret al., 1995), ensuring that the granules are accurately placed adjacent to the

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  • Citrus Decline: Soil Fertility and Plant Nutrition 221

    tree. Application of variable fertilizer rate technology in this grove saved innitrogen equivalent to the 32 to 43% reduction of N rates achieved through useof fertigation and foliar sprays of urea (Lamb et al., 1999).

    Fertigation

    Low water(WUE) and fertilizer-use-efficiency (FUE) are amongst the majorproduction related constraints (Germana, 1992; Srivastava and Singh, 2003).Flood irrigation in tree basin is widely used in citrus orchards, but it has severaldrawbacks in terms of losses through conveyance, percolation, evaporation,and distribution, yet without much adverse impact on growth, yield, and fruitquality (Shirgure et al., 2001a). In light of growing scarcity of water and poorWUE under basin irrigation, micro-irrigation based fertigation has gained wideapplication in citrus orchards.

    Fertigation (application of nutrients through irrigation) has produced betterresults in improving the tree growth, fruit yield, quality, the reserve pool of soilnutrients, and consequently the plant nutritional status (Zhang et al., 1996;Shirgure et al., 2001b). Besides the better mobility of nutrients, fertigation hasbeen shown to have several advantages over broadcast application of granularfertilizers (Willis et al., 1991) with respect to growth response (Koo, 1979),nutrient uptake (Koo, 1980), effective placement of nutrients and flexibility inapplication frequency (Ferguson and Davies, 1989), development of uniformroot distribution in wetted zone, an important pre-requisite for better FUE (Alvaand Syvertsen, 1991; Zhang et al., 1996), fruit yield (Koo and McKornack,1965), fruit quality (Bowman, 1996), and fertilizer savings (Srivastava andSingh, 2003).

    Other studies showed far superior results with fertilizers applied throughdrip irrigation (fertigation) in Spain (Legaz et al., 1981), central India (Shirgureet al., 2001a; 2001b) and in Arizona (USA) using microsprinklers over basalfertilizer application under flood irrigation (Weinert et al., 2002). Zhang et al.(1996) evaluating the effect of fertigation versus broadcast application of watersoluble granular fertilizer on the root distribution of 26-year-old White Marshgrapefruit trees on sour orange rootstock, showed 94% of the root density in thetop 030 cm depth with soluble granular fertilizers. These observations supportthe earlier observations that shallow depth of wetting and delivery of nutrientsin fertigated production systems, results in most of the roots concentrated insurface soil (Alva and Syvertsen, 1991; Zhang et al., 1998).

    Koo (1984a; 1984b) while describing the importance of ground coverage oforchard floor by fertigation reported that the treatment having 37% coverage ofground and 82% of canopy area produced fruit yield higher than the broadcastfertilizer treatment covering 100% of ground surface and 53% canopy area.These observations suggest the importance of canopy coverage for high nutrientuptake efficiency and higher yield. Response of six year-old Hamlin orange

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    to fertigation frequency using 324 to 464 g N per tree showed nitrogen uptakeefficiencies ranged from 24 to 41% of N applied, but no effect of fertigationfrequency on the amount of N taken up by the trees, was observed whenfertigation frequency increased from 12 to 80 times a year (Syvertsen andJifon, 2001). Alva and Obreza (1998) earlier found that 18 split fertigationapplications through microsprinklers under the trees increased the fruit yieldwith fertigation than equivalent rates of granular fertilizer treatments due togreater nutrient uptake efficiency.

    Alva et al. (2003) studied the comparative response of 32 months-old non-bearing Hamlin orange trees on a Candler fine sand (Typic Quartzipsamments)using three methods of fertilization namely, fertigation (FRT), controlled re-lease fertilizers (CRT), and water soluble granular fertilizers (WSG) at tworates, high and low fertilizers rates. Total N content in tress which received thehigher fertilizer rates were 82.3, 70.2, and 41.4 g per tree for the FRT, CRF, andWSG sources, respectively. The corresponding values for the low- fertilizerrate treatments were 38.6, 50.4, and 28.4 g per tree. However, the proportionof total N partitioned to leaves was greater for WSG than for the CRF and FRTsources at both the fertilizer rates. Similar observations were made through theresponse of 25 yr-old Hamlin orange in Highland county with varying N rates(112180 kg ha1) and fertilizer management practices (WSG, CRF, and FRT).Spring flush leaf N content increased with increasing N rates decreased in theorder of FRT > WSG > CRF (Paramasivam et al., 2000). Other studies byHe et al. (2003) involving CRF (1 application per year), FRT (15 applicationsper year), and WSG (3 applications per year) showed no response of fertilizersources either on fruit yield of grapefruit or leaf nutrient composition on ArenicGlossaqualf soil.

    Irrigation at 20% depletion of available water content (AWC) combinedwith fertilizer treatment of 500 g N + 140 g P + 70 g K per tree per year pro-duced a significantly higher fruit yield per cubic meter of canopy in addition tohigher nutrient status and fruit quality compared to other treatments involvingirrigation either 10% depletion or 30% depletion of AWC with 600 g N + 200g P + 100 g K per tree per year in 14-yr-old Nagpur mandarin (Citrus retic-ulata Blanco) on an alkaline calcareous Lithic Ustochrept soil type (Shirgureet al., 2003; Srivastava and Singh, 2003). Field experiments on response ofpre-bearing acid lime plants to differential N-fertigation versus circular bandplacement (CBP) method of fertilizer application showed superiority of formerover latter treatments. The higher leaf N, P, and K with 80% fertigation over100% N through CBP further demonstrated that saving of N up to 20% isattainable (Shirgure et al., 2001c). Experiments carried out by Garcia-Petillo(2000) demonstrated 50% higher leaf N content with 64% higher yield oncumulative basis in fertigation treated trees compared to conventional methodof fertilization. These studies provided strong support in favor of fertigationbeing better than conventional basin or flood irrigation with broadcast methodof fertilizer application.

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  • Citrus Decline: Soil Fertility and Plant Nutrition 223

    Cropping system Nutrient requirement Soil fertility

    Nutrient recommendation

    INM

    * Organic manures * Biofertilizers* Crop residues

    * N, P, K & S* Fe, Mn, Cu, Zn, Mo & B

    Soil pH Low High

    Lime Gypsum

    Agricultural

    Available fertilizers

    Integration Environmental

    Legislation

    Nutrient management plan

    Figure 2. Schematic plan for simplified form of INM.

    INTEGRATED PLANT NUTRIENT MANAGEMENT

    Renewed emphasis is being given on the importance of integrated soil produc-tivity management strategies and technologies for enhanced and sustainableagricultural production systems. Over the last few years, the concepts of in-tegrated nutrient management (INM) and integrated soil management (ISM)have been gaining acceptance, moving away from a more sectoral and input-driven approach. The advocates the careful management of nutrient stocks andflows in a way that leads to profitable and sustained production. Integrated soilmanagement not only emphasizes the management of nutrient flows, but alsohighlights the other important aspects of the soil complex, such as maintainingorganic matter content, soil structure, moisture and biodiversity. Still more at-tention is needed to integrate soil biological management as a crucial aspect ofsoil fertility management.

    Fertilizer recommendation using INM approach embodies a strategy for theeconomic use of fertilizers, taking into account a number of modifying factors(Figure 2). Two important modifying factors are soil type (texture and pH) andcrop requirement, upon which, the role of INM-based components viz., organicmanures, biofertilizers, and inorganic chemical fertilizers vary, and collectivelyfulfill the twin requirement as nutrient source and soil amendment as well.

    Integrated Soil Management

    The use of conservation practices like trench planting on Ultisols (Lu et al.,1997), ridging (Coetzee, 1995), flat bottom trench (Muldabaev and Zaitsev,

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    1989), and half moon terraces (Chanukvadze, 1990), have proved highly effec-tive in enhancing the productive life of citrus orchards on. These conservationpractices warranted zero tillage or permanent sod in citrus orchards that helpedin regulating soil temperature regimes to benefit citrus orchards in the hillyterrain (Cary, 1981).

    Cover crops, on the other hand have proved equally effective in improvinginfiltration, microbial biomass, P-uptake, and suppressed weed growth (Huang,1998) in addition to protection against freeze (Santinoni and Silva, 1995).Huang (1998) based on 20 years of experimentation in hill side citrus orchardsestablished on red soil types (Alfisols) observed suitable intercrops as: Indiancowpea (Vigna unguiculata), groundnut (Archies hypogea), soybean (Vignaradiata), vetch (Vicia sativa), and Chinese milk vetch (Astragalus sinicus).Based on eight-years of experimentation, Luo et al. (1992) showed yellowclover (Melliotus officinalis) as a promising green manure crop for citrus thatadded 7.512.0 tons ha1 green biomass supplying 36.758.8 kg N, 3.76.0 kgP, and 23.237.2 kg K ha1 into the soil.

    Cultivation of soil (4349% sand and 2330% clay) with Acanthus mollisand Amaranthus retroflexa in spring liberated good amount of tied N, improvingyield of mandarin from 66.0 to 80.7 tons ha1 (Pisa and Fenech, 1990). Anexcellent performance of Satsuma mandarin in Sichuan basin was observed oneast, west or south facing mountain slopes (30%) with 1 m depth of cultivatedsoil in pH range of 5.57.3 and organic carbon content of 2.53.0% using80100 kg organic manure, 0.500.80 kg urea and 0.501.00 kg complexfertilizer per tree per year (Xong and Zhou 1997).

    Integrated Nutrient Use

    Many studies viz., 400 g N-150 g P-300 g K-FYM 25 kg per tree in Khasimandarin (Ghosh and Besra, 1997), 800 g N-300 g P-600 g K per treeneemcake 15 kg per tree in sweet orange (Tiwari et al., 1997), 150 kg N-120 kgP-80 kg K-5 tons FYM ha1 in Meyer lemon (Beridze, 1990), 600 g N-200 gP-300 g K-25 kg FYM per tree in Nagpur mandarin (Huchche et al., 1998),and 100 kg N-20 tons rice straw ha1 in Satsuma mandarin (Tachibana and Ya-hata, 1996), have shown experimentally that application of inorganic fertilizersin combination with organic manures has proved superior over conventionalinorganic fertilization.

    Improving soil quality is one of the important core problems for sus-taining productivity on long term basis. The quality index of citrus growingmeadow soils (Umbric Fluvisols: pH 7.58.2 and base saturation 85100%)was observed higher (11.2) than the quality index (6.8) on argillaceous redsoils (Haplic Acrisols: pH 6.57.6 and base saturation 8892%) in Jiangxiprovince of China (Wang and Gong, 1998). However, delinking the concept ofsoil quality from productivity, the former with the rationale that it is determined

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    by the efficiency in the use and management of resource inputs whereas thelatter is related to a set of intrinsic soil properties capable of addressing thesustainability more effectively. More research is, hence, needed to delineatethe critical limits of soil properties beyond which quality of soil environmentis severely and irretrievably jeopardized.

    The organic matter level in the soil is important to help maintain an activepopulation of microorganisms in soil to promote organic matter mineralization.The soil physical condition promotes the absorption of nutrients by plant roots.These effects of soil organic matter are usually considered indicators of thesustainability of soil management system. One of the major advantages to bederived from long term fertility trials is that they enable soil organic matterchanges to be monitored. Norms need to be established for different soils to beused as an index of sustainability of soil management system. The principles ofhow to maintain nutrient and organic matter level, preserve soil structure, andavoid erosion are now well understood. It is an utmost necessity to quantify therate of change for different soils and climates so that the soil changes can bemodeled and lined to the crop performance.

    The introduction of site specific practices in citrus orchards, for the si-multaneous analysis for spatial variability in soil properties and tree-to-treeperformance within an orchard using a number of information managementtechnologies such as remote sensing, GIS, GPS, and VRAT may provide betterdecision support tools to develop model orchards and ideotype trees. In thesecond step, the frequent use of these tools as a part of precision citriculturerequires to be popularized to identify sectorial occurrence of citrus decline, andaccordingly develop the averting mechanism. The possibility of exploring cropregulation through soil fertility management to produce fruits throughout theyear holds an equally good promise, to compensate the loss in production onaccount of unsustainability in production.

    Improving native soil fertility by exploiting the utility of mycorrhiza needsrevisiting since it has helped in uptake of diffusion of limited nutrients suchas (a) P, Zn, Cu, Mn, and Fe by the host plants (Srivastava and Singh, 2002).However, studies pertaining bioinoculation vis-a-vis soil fertility changes incitrus orchards are very limited.

    FUTURE RESEARCH

    Analysis of various components of sustainability of specific citrus cultivar ina given citrus belt, demonstrates that climate and soil are the two decisivecomponents of successful citriculture. However, the clues for the sustainabilityof citrus can further be drawn from the success stories accrued through some ofthe infamous commercial citrus belts like, Florida (USA), Bet Dagan (Israel),Jeju plateau (Korea), Nile valley (Egypt), New South Wales (Australia), Koratplateau (Thailand) etc. representing an humanized art of citrus cultivation on

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    soils those having inherently poor nutrient supplying capacity. Developmentof threshold norms for various soil properties including the drainage criteria inrelation to fruit yield offers a better understanding on qualitative relationshipbetween constraints-based soil type and crop response to help to exploit theproductivity potential of soil.

    Suitability of different concepts of soil fertility assessments viz., sufficientlevel of available nutrients (based on Mitsherlich equation) and modified fur-ther as critical level concept (Cate and Nelson, 1965), basic cation saturationratio (Bear et al., 1945), fertility capability classification (Boul et al., 1975),quantitative soil fertility evaluation (Janssen et al., 1990), phosphate absorp-tion coefficient (Egashira et al., 1990), and polarity coefficient (Tavdgiridzeand Putkaradre, 1991) need to be explored for an effective production linkedsoil fertility evaluation and subsequently, devising the land use planning as perland capability criteria. The role of substrate dynamics to engineer rhizospheresoil through microbes displaying their ability in growth promotion as well asantagonism against soil borne diseases need to be made more of a regular prac-tice. At the same time, use of open field hydroponics in countries like Australiaand South Africa, has opened new vistas of commercial citriculture withoutgiving due weight to the soil as a medium of growth. Such an option shouldbe continuously tested and scrutinized under different citrus-based productionsystems.

    Development of compaction is often regarded as increase in soil bulkdensity, and physico-chemical criteria are often used to measure it. Thesecriteria in many respects are conditional and comparative, and do not providesufficient information needed for a quantitative evaluation of phase interactionand physical consequences. There is a real need for a theory and methods ofevaluating compaction and monitoring physical condition of soils to preciselyrelate with production response of citrus.

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