sugarcane: physiology, biochemistry, and functional biology (moore/sugarcane) || mineral nutrition...

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Chapter 5 Mineral Nutrition of Sugarcane Graham Kingston SUMMARY An understanding of the mineral nutrition of sug- arcane is required for development of sustainable nutrient management plans. Sustainability is dis- cussed in terms of productivity and profitability for producers, including cane quality and mainte- nance or enhancement of soil fertility, while min- imizing offsite effects on the environment. The chapter explores the role of major and minor nutri- ents in the physiology of sugarcane and the con- sequences of nutrient excess for physiology, crop quality, and the environment. Soil and leaf analy- sis are introduced as tools to support development of sustainable crop nutrition. The chapter con- cludes with a discussion of opportunities and risks associated with genetic interventions in nutrient management. INTRODUCTION An understanding of plant nutrition and the potential for interactions between nutrients and soil conditions is of paramount importance to agronomists, plant and crop physiologists, and soil scientists for interpretation of data, implementa- tion of sustainable nutrient management practices, and planning new research. Basic information on general plant nutrition is available from texts such as Gauch (1972) and Marschner (1986), while information specific to sugarcane can be found in Anderson & Bowen (1990), Calcino (1994), Ruiz (1995), Bruce (1999), an anonymous author (1999a), Schroeder et al. (2007), Garcia et al. (2003), and Dinardo-Miranda et al. (2008). This chapter reviews knowledge on nutrition of sug- arcane, drawing heavily on recent research from the Australian sugar industry but also incorporat- ing contemporary experience from industries in South Africa, Brazil, Hawaii, and Florida. Introductory concepts in plant nutrition for improved nutrient management are considered along with roles of mineral nutrients, the symp- toms of deficiency/excess, and interactions of nutrients with soil and nutrient management. INTRODUCTORY CONCEPTS IN PLANT NUTRITION Production focus for plant nutrition Traditional approaches to crop nutrition included a strong emphasis on crop productivity and incor- porated a focus on balanced nutrition, economic yield, and product quality. Balanced nutrition implies that all essential nutrients are present in the growth environment at levels not limiting to growth and maturation of the crop (avoidance of deficiency). Excessive application of some nutri- ents usually has one or more negative effects on the agro-economic system. For example, unbalanced nutrition can affect product quality (excessive nitrogen can depress sucrose levels in sugarcane and also elevate undesirable colorants in raw sugar [Chapman et al. 1996]); Sugarcane: Physiology, Biochemistry, and Functional Biology, First Edition. Edited by Paul H. Moore and Frederik C. Botha. C 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc. 85

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Page 1: Sugarcane: Physiology, Biochemistry, and Functional Biology (Moore/Sugarcane) || Mineral Nutrition of Sugarcane

Chapter 5

Mineral Nutrition of Sugarcane

Graham Kingston

SUMMARY

An understanding of the mineral nutrition of sug-arcane is required for development of sustainablenutrient management plans. Sustainability is dis-cussed in terms of productivity and profitabilityfor producers, including cane quality and mainte-nance or enhancement of soil fertility, while min-imizing offsite effects on the environment. Thechapter explores the role of major and minor nutri-ents in the physiology of sugarcane and the con-sequences of nutrient excess for physiology, cropquality, and the environment. Soil and leaf analy-sis are introduced as tools to support developmentof sustainable crop nutrition. The chapter con-cludes with a discussion of opportunities and risksassociated with genetic interventions in nutrientmanagement.

INTRODUCTION

An understanding of plant nutrition and thepotential for interactions between nutrients andsoil conditions is of paramount importance toagronomists, plant and crop physiologists, and soilscientists for interpretation of data, implementa-tion of sustainable nutrient management practices,and planning new research. Basic information ongeneral plant nutrition is available from texts suchas Gauch (1972) and Marschner (1986), whileinformation specific to sugarcane can be foundin Anderson & Bowen (1990), Calcino (1994),Ruiz (1995), Bruce (1999), an anonymous author

(1999a), Schroeder et al. (2007), Garcia et al.(2003), and Dinardo-Miranda et al. (2008). Thischapter reviews knowledge on nutrition of sug-arcane, drawing heavily on recent research fromthe Australian sugar industry but also incorporat-ing contemporary experience from industries inSouth Africa, Brazil, Hawaii, and Florida.

Introductory concepts in plant nutrition forimproved nutrient management are consideredalong with roles of mineral nutrients, the symp-toms of deficiency/excess, and interactions ofnutrients with soil and nutrient management.

INTRODUCTORY CONCEPTS INPLANT NUTRITION

Production focus for plant nutrition

Traditional approaches to crop nutrition includeda strong emphasis on crop productivity and incor-porated a focus on balanced nutrition, economicyield, and product quality. Balanced nutritionimplies that all essential nutrients are present inthe growth environment at levels not limiting togrowth and maturation of the crop (avoidance ofdeficiency). Excessive application of some nutri-ents usually has one or more negative effects on theagro-economic system. For example, unbalancednutrition can

� affect product quality (excessive nitrogen candepress sucrose levels in sugarcane and alsoelevate undesirable colorants in raw sugar[Chapman et al. 1996]);

Sugarcane: Physiology, Biochemistry, and Functional Biology, First Edition. Edited by Paul H. Moore and Frederik C. Botha.

C© 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.

85

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86 Sugarcane: Physiology, Biochemistry, and Functional Biology

� affect nutrient uptake and utilization (highcalcium and magnesium in soil interferewith potassium nutrition [Duvenage & King1996]);

� affect trends in long-term soil fertility (highlevels of nitrogenous fertilizers enhance ratesof soil acidification [Aitken et al. 1998]); and

� have adverse effects on productioneconomics.

A nutrient is considered essential if thegrowth/reproductive cycle of the plant cannot becompleted in the absence of that element. Theliterature generally reports 13 essential elementsfor plant growth (Fig. 5.1). These are subdividedinto the six macronutrients, within which nitro-gen, phosphorus, and potassium are regarded asprimary nutrients and calcium, magnesium, andsulfur as secondary nutrients. This subdivisionis made largely on the quantitative acquisition ofnutrients by plants, rather than the functional sig-nificance of the elements (Table 5.1).

Seven elements–copper, zinc, manganese, iron,boron, molybdenum, and chlorine–make up themicronutrient group. Elements having a synergis-tic or indirect beneficial effect on plant growthare termed beneficial nutrients. Silicon currentlyhas this status for sugarcane, as there are reports inthe literature of beneficial effects on yield of sugar-

cane, without identifying a physiological functionfor the element (Kingston 1999; Korndorfer &Benedini 2000; Berthelsen & Noble 2001). Siliconis considered essential for some species of siliconaccumulators such as Equisetum spp. or horsetails(Chen & Lewin 1969) and certain wetland species(Takahashi & Miyake 1977).

Sustainability focus for plant nutrition

The traditional production focus on plant nutri-tion has expanded in recent years to a greaterfocus on sustainability of production and ecolog-ical resources. Production and sustainability con-cepts in plant nutrition should not be regarded asmutually exclusive. Sustainable systems thereforehave three major elements (Anonymous 1991):� Management must result in profitable crop

production.� Management must be directed toward main-

taining the productive capacity of naturalresources, e.g., maintaining soil fertility.

� Management must minimize effects of theproduction system on the ecology of sur-rounding areas.

The interest of the community in the manage-ment of natural resources in conservation areas andagricultural zones raises a major issue in regard to

13 Essential plant nutrients

6 Macro-nutrients

3 Secondary nutrients3 Primary nutrients

NitrogenPhosphorusPotassium

Silicon may be abeneficial macro-nutrient

CalciumMagnesiumSulfur

Copper, Zinc, IronManganese, BoronMolybdenumChloride

7 Macro-nutrients

Fig. 5.1. Essential and beneficial nutrients for sugarcane.

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Mineral Nutrition of Sugarcane 87

Table 5.1 Nutrients (kg/ha) contained in sugarcane biomass and corresponding yield of cane culms(tc/ha) for four agro-climatic regions in Australia (collated from Calcino 1994) and for all of Hawaii.

kg/ha in Biomass

NutrientWet

tropicsIrrigated

dry tropicsCentralregion South Hawaii

N 122 154 123 150 280P 17 37 18 23 56K 208 276 203 260 560Ca 27 55 41 51 224Mg 18 57 35 52 –S 25 47 25 48 –Cu 0.07 0.11 0.09 0.09 –Zn 0.46 0.59 0.38 0.37 –Fe 7.32 5.65 6.93 8.61 –Mn 3.91 1.90 3.64 2.66 –tc/ha 93 119 84 92 224

Source: Hawaii, L. Santo personal communication.

the economics of managing to perceived standardsof sustainability or to rational standards basedon rigorous scientific investigation. The challengefor scientists and producers is to turn commu-nity needs and pressures into more efficient use ofresources and enhanced profitability.

Maintaining soil fertility

Sustainability of agricultural ecosystems dictatesthat soil fertility is defined in its broadest senseto include chemical, physical, and biologicaldimensions because of opportunity for interac-tions among these dimensions. Chemical fertilitydepends on organic matter, cation status, capacityto store and release nitrogen, phosphorus, sulfur,trace elements, and soil acidity. Physical fertilitydepends on soil texture and organic matter, whichinfluence both water holding capacity and chemi-cal fertility. Soil structure and its stability are alsoimportant components of physical fertility. Bio-logical fertility refers to the suites and balance ofmicro and macrofauna in soils. Biological fertil-ity has major implications for mineralization oforganic matter and root health and therefore feed-back links to availability and accessibility of waterand nutrients from soil.

Loss of organic matter due to tillage and parti-tioning of nutrients into the harvested productcan lead to soil acidification. Removal of basic

cations from the field in the harvested product alsocontributes to soil acidification. The quantities ofnutrients in sugarcane biomass (Table 5.1) showthe crop has a high demand for nutrients. Withapproximately 50% of these nutrients exportedfrom the field in the harvested crop, Noble et al.(2000) showed that the above-mentioned processremoved ash alkalinity from soil at a rate equal to1.5 kg calcium carbonate/ha/100 tonnes of caneyield. Thus, capacity of soil to supply the variouselements must be combined with a rational nutri-ent management plan to avoid critical degradationof soil reserves and/or over supply of nutrientsfrom soil and fertilizers. Such management plansshould be based on concepts of critical or thresholdvalues in soil and plant analysis. The critical valueis associated with a defined level of relative yield,usually 90 or 95%. The threshold value is thatabove which no further yield response is obtainedfrom input of the nutrient in question.

PRIMARY NUTRIENTS

NitrogenRole of nitrogen

Nitrogen is

� a major constituent of nucleic acids, proteins,and enzymes; there are four atoms of nitrogenin each molecule of chlorophyll;

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88 Sugarcane: Physiology, Biochemistry, and Functional Biology

� important to meristematic activity;� taken up as NO3

− and NH4+ ions, but NO3

−is reduced to NH4

+ by nitrate reductasebefore nitrogen is incorporated into plantstructures;

� an element that leaves can acquire from NH3

gas (Denmead et al. 1993);� stored in vacuoles of cells;� a promoter of tillering (Das 1936; Wood

1968) and suckering (Stanford 1963; Salter &Bonnet 2000).

Symptoms of nitrogen deficiency

Nitrogen deficiency affects older leaves before theyoung leaves due to remobilization of scarce Nresources from older tissues to the actively grow-ing tissue. Nitrogen-deficient leaves become lightgreen to yellow in color (Color Plate 5.1) witheffects on growth of the entire plant reflected inthin and stunted culms and a reduced number oftillers. Yield is affected before chronic leaf symp-toms become evident. Leaf analysis is a reliabletool to measure plant nitrogen status.

Consequences of excess nitrogen in sugarcane systems

Excess nitrogen levels in sugarcane can lead tolower sucrose concentration in the culm. Thisis mainly an effect on sucrose % fresh weightthrough the effect of nitrogen on hydration lev-els in culms (Muchow & Robertson 1994). Highnitrogen regimes are also associated with increasedlevels of reducing sugars (glucose and fructose) injuice (Das 1936). Industry experience shows boththese effects are most severe for cane harvestedearly in the milling season.

High levels of applied nitrogen are often asso-ciated with a tendency for sugarcane to lodge. Itis not clear whether this observation is due to thehigh levels of biomass often associated with higherrates of nitrogen fertilizer or an effect of nitrogenon culm structure. High rates of nitrogen fertil-izer can change the balance of hormones in cereals,with increased cytokinin production enhancingstem elongation and lodging potential (Marschner1986). This effect has not been investigated forsugarcane.

The nitrogen in the plant not required forimmediate metabolic use is stored mainly in sug-arcane culms as amino acids, of which asparagineis the dominant form (Chapman et al. 1996; Keat-ing et al. 1999). These amino acids can react withreducing sugars during heating in the milling pro-cess to produce high molecular weight phenoliccompounds in the Maillard reaction (Reyes et al.1982). During storage of raw sugar, these com-pounds form melanoidin colorants, which lead tomarketing difficulties because of increased refin-ing costs for color removal. Amino nitrogen levelsin sugarcane juice can be measured by wet chem-ical or near infrared spectroscopic methods and apreliminary critical range of 220–250 �g N/mLof juice has been suggested (Keating et al. 1999).

Loss of nitrogen to ground and stream waterhas major ecological significance in terms of effecton quality of water for human consumption andthe balance of riverine and benthic flora. Suchloss of nitrogen from the production systemhas economic significance for sugarcane grow-ers. The potential for loss of nitrogen by leach-ing is increased when application rates exceed thenitrogen demand of the crop and the capacity forabsorption into the organic nitrogen pool in soil(Keating et al. 1997). Split applications are verysuccessful for reducing leaching losses in sandysoils in Florida (Rice et al. 2008), where 200 kgN/ha may be applied in up to five splits of ammo-nium nitrate

Utilization of nitrogen by sugarcane

Uptake of nitrogen should be assessed in rela-tion to its utilization for production of biomassand commercial yield. The uptake of nitrogendepends on:

� Crop class (Meyer & Wood 1994; Wood et al.1996). Plant cane produces more biomass/kgof accumulated N than does ratoon cane.

� Genotype. There is general recognitionof differences between genotypes and therequirement for different nitrogen fertiliza-tion to produce optimum yield and cane qual-ity. So-called vigorous canes can produce tar-get yields with lower inputs and uptake ofnitrogen than less vigorous canes. Wood et al.

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Mineral Nutrition of Sugarcane 89

(1996) reported that the cultivar Q117 hada lower biomass:N ratio than did the vigor-ous Q138, when the two cultivars were grownunder the same non-N limiting conditions.

� Age and biomass accumulation. The rate ofnitrogen accumulation by biomass is mostrapid during the period 50–120 days aftercrop initiation and is substantially complete180–200 days after initiation of ratoon andplant crops, respectively (Wood et al. 1996).Biomass accumulation above 3000 g m−2

occurred with little additional accumulationof nitrogen. Sugarcane is a 365-day crop andthe propensity described above for acquisi-tion of nitrogen early in the growth periodcan interact with yield-limiting factors dur-ing the major culm growth period to producelower yield efficiency ratios (kg biomass/kgN uptake). It is for this reason that Keatinget al. (1999) stress the need for amino-N lev-els in cane culms to be interpreted againstknowledge of crop growing conditions as wellas total nitrogen supply before making judg-ments about the fertilizer nitrogen regime.

Yield efficiency decreases at higher nitro-gen rates as luxury consumption allows nitrogenuptake rate to increase more rapidly than doesbiomass accumulation. Most of the excess nitro-gen is stored in culm tissue as the amino acidasparagine (Chapman et al. 1996; Keating et al.1999). Sugarcane, with yield efficiency ratios of250–586 kg/kg N, is regarded as a relatively effi-cient user of acquired nitrogen compared to maizeand sorghum, which support ratios of 102 and99 kg/kg N, respectively (Muchow & Robertson1994). The long growth period of sugarcane rela-tive to that of cereal crops, and sugarcane’s accu-mulation of biomass late in the growth cycle withlittle additional nitrogen required, clearly con-tribute to these species differences in yield effi-ciency for nitrogen.

Stanford and Ayres (1964) considered 475–525 kg biomass/kg N as an optimal range for20- to 24-month cane in Hawaii; this translatesto 394–422 kg biomass/kg N for the biomass asso-ciated with 12-month cane (Muchow & Robert-son 1994). Conversion of this Hawaiian biomass

figure to a fresh culms (cane) basis suggests pro-duction of 833–909 kg cane/kg N, or by inversion1.1–1.2 kg N/tonne cane, which is in close agree-ment with an independently developed estimatorfor Australian conditions (Keating et al. 1997).The Brazilian sugar industry is renowned for lowinputs of nitrogen fertilizer. However, it appearsthat approximately 1 tonne of cane may be pro-duced for each kilogram of total nitrogen, whenall inputs of nitrogen (fertilizer, nitrogen fixation,legumes, filter mud, and vinasse) are recognized inrelation to cane yields. Nitrogen is rarely appliedto sugarcane grown on the highly productive mucksoils in Florida, as nitrogen mineralized from theseorganic soils generally meets the crop demand ofnitrogen (Rice et al. 2008)

Acquisition of fertilizer nitrogen by sugarcane

There are numerous reports showing sugarcane isinefficient in recovering applied fertilizer nitrogenin the year of its application, with values rangingfrom 6–45%, but with values of 23–40% beingmore common (Wood 1972; Chapman et al. 1992;Keating et al. 1993; Tung-Ho et al. 1993; Cour-tillac et al. 1998; Kingston & Anink 2008). Thebulk of nitrogen acquired by sugarcane is there-fore derived from mineralization of organic poolsin the soil, with potential for a contribution frombiological fixation mechanisms.

Some of the low efficiency associated withrecovery of fertilizer nitrogen in the crop year ofapplication is associated with loss processes (leach-ing, denitrification, and volatilization). However,fertilizer nitrogen makes an important contribu-tion to maintaining the nitrogen status of soilorganic matter for subsequent mineralization andcrop uptake (Wood 1972; Keating et al. 1993;Courtillac et al. 1998). Keating et al. (1993) foundsimilar amounts of 15N labeled nitrogen in above-ground parts of the crop as in the soil and roots ina crop at 6 months of age, while Courtillac et al.(1998) found 30–40% of the fertilizer nitrogen wasimmobilized in vertisol soil.

Management of nitrogen nutrition

Fertilizer nitrogen management for sugarcanetraditionally was directed at an economic yield

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90 Sugarcane: Physiology, Biochemistry, and Functional Biology

optimum derived from rates of nitrogen experi-ments conducted in different agro-climatic zones:South Africa (Du Toit 1957; Meyer et al. 1986),Hawaii (Stanford & Ayres 1964), Australia (Chap-man 1994), and Brazil (Penatti et al. 1997). Organicmatter status of soil is now used to refine fer-tilizer nitrogen inputs in South Africa (Meyeret al. 1986), Australia (Schroeder & Wood 2001),Brazil (Penatti et al. 1997), and Florida (Riceet al. 2008), as nitrogen mineralization capacityis related to soil organic matter status. Routinesoil tests for soil nitrate levels have not receivedwide acceptance in the sugar industries across theworld, as the 12-month growth period of sugar-cane allows greater potential for gradual contribu-tion from mineralization than would be the case in120-day crops.

Nitrogen content of the top visible dewlap(TVD) leaf (Schroeder et al. 1999) or leaves 3–6(Clements 1959) is a sensitive indicator of nitro-gen status of sugarcane. Leaves should be sam-pled during periods of active growth in the sum-mer, usually between 3 to 5 months of age; cropsshould not be stressed from moisture excess ordeficit, from pests, or from disease and shouldbe at least 6 weeks removed from previous appli-cation of fertilizer. The range of nitrogen valuesconsidered critical, marginal, and/or adequate forgood growth has been established for several of theinternational sugar industries (Anderson & Bowen1990; Reuter & Robinson 1997). A single criticalvalue of 1.8% nitrogen in dry matter of the TVDlamina at 3–5 months of age is used in Australia,whereas for South Africa, values range between1.6 and 1.9% nitrogen for 3–9 months of age indifferent climatic zones. Lower critical values areapplied as cane moves toward the end of the suit-able window in crop age for sampling.

Sources and application of nitrogenous fertilizer

Criteria and recommendations for management ofnitrogen nutrition in several sugar industries arereported by Calcino (1994), Ruiz (1995), Bruce(1999), an anonymous author (1999a), Schroederet al. (2007), Garcia et al. (2003), and Vitti et al.(2008). The range of materials used to supplynitrogen to sugarcane is summarized in Table 5.2.

Phosphorus

Role of phosphorus

Phosphorus is

� a constituent of nucleic acids and is thereforecrucial to cell division and heredity transfer.Its role in cell division highlights the impor-tance of phosphorus for growth of root sys-tems, tillering, and shoots.

� required for energy-rich bonds (ADP andATP). Therefore, carbon dioxide assimila-tion depends on phosphorus assimilation.

� taken up as HPO4= or H2PO4

− ions, but isnot reduced in the plant. Phosphorus can bejoined with OH− ions to create sugar phos-phates. Phosphorus helps in maturation ofcrops by allowing biomass dilution of highconcentration of nitrogen in plant tissues.

Symptoms of phosphorus deficiency

Phosphorus deficiency is first observed in olderleaves. Leaf blades can turn dark-green to blue-green, often with reddish or purple tinges on mar-gins and tips of leaves (Color Plate 5.2). Thesecolors are associated with enhanced formationof anthocyanin pigments, while the darker greencolor is due to greater relative effects of phos-phorus deficiency on reducing leaf expansion thanon inhibiting chlorophyll development in youngleaves. Leaves are thinner, narrower, and shorterthan normal and may appear more erect than nor-mal. Deficient leaves then turn yellow and die fromthe tips and margins. Culms are short and thin.Tillering is poor. Cane from phosphorus-deficientfields produces juice low in phosphorus and insome cases phosphoric acid may be added duringthe milling process to assist formation of calciumphosphate flocs to improve juice clarification.

Consequences of excess phosphorus

Excessive rates of phosphorus fertilizer willincrease soil reserves of phosphorus (Chapman1972, 1985). Consequently, use of phosphorus fer-tilizer cannot be justified economically where soil

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Mineral Nutrition of Sugarcane 91

Table 5.2 Sources of nitrogen for sugarcane and their average N% from manufactured fertilizers andwaste materials, with brackets indicating typical ranges of % N in waste materials.

Nitrogen source Common term % Nitrogen

FertilizerAnhydrous ammonia Anhydrous 82Urea Urea 46Diammonium phosphate DAP 18–19Monoammonium phosphate MAP 12.6Ammonium nitrate Nitram 34Calcium ammonium nitrate Cal-Am, CAN 27Ammonium sulfate Sulfate of ammonia 20.2Sulfur fortified urea Urea - S 40High analysis mixtures Various Various

Waste materials (after Barry et al. 1998)Filter mud Mill mud, Cachaza 1.5 (0.8–2.2)Filter mud/ash mixtures Mud/ash, Cachaza/ceniza 0.6 (0.4–0.8)Mill ash Ash, ceniza 0.15 (0.1–0.5)Vinasse Vinasse/dunder (0.05–0.8)Sewage sludge Sewage sludge 3.1 (0.5–6.6)

analysis shows phosphorus levels that are well inexcess of the threshold for a response. A policyof maintenance fertilizing should be implementedfor soils closer to the yield response threshold, withapplications of 20-50 kg P/ha varying in accor-dance with biomass removal (Table 5.1).

Phosphorus is not susceptible to loss by leach-ing (Coale et al. 1993) and loss of particulate formsduring soil erosion represents the most seriousloss mechanism for sugar industries. Loss of par-ticulate phosphorus to the environment is per-haps the most serious nonproduction consequenceof excessive use of phosphorus and its accumu-lation in soil. Elevated levels of phosphorus infresh water bodies are associated with certain algalblooms, while in marine locations, there is con-cern of not only increased algal growth but alsoadverse effect of phosphorus on the structure ofcorals (Kinsey 1990).

Concern has been expressed that excessive useof phosphorus fertilizers and subsequent high lev-els in soil might have an effect on uptake and uti-lization of zinc, as reported by Burleson and Page(1967), Boawn and Brown (1968), and Trier andBergmann (1974) for species including flax andmaize. There is field evidence of induced cop-per deficiency in sugarcane associated with useof broadcast superphosphate at 1.25 t/ha, butno such evidence for zinc. The reported interac-

tions between copper, zinc, and phosphorus couldbe associated with several mechanisms, includingbiomass dilution from the phosphorus response insoils where the trace element supply is marginal,the cations associated with phosphorus fertilizersperhaps inhibiting uptake of the trace elements,or the tendency of phosphorus to enhance sorp-tion of trace elements on soil particles (Loneraganet al. 1979). No relationship could be establishedbetween concentrations of zinc in leaf tissue of sug-arcane across a wide range of soil phosphorus andzinc values, even where zinc values were approach-ing deficiency in Australia (B.L. Schroeder per-sonal communication).

Phosphorus in soil

Phosphorus reserves in soils are mainly unavail-able for plant uptake because they are held inorganic, inorganic, or chemically sorbed forms.Only a small proportion of the total soil phos-phorus is therefore available for plant uptake.Phosphorus availability is very dependent on soilpH, with increasing fixation as calcium phosphatesabove pH 7 and as iron and aluminum phosphatesbelow pH 5.5. Maintenance of soil in the pHrange 5.5–6.5 is therefore important for optimiz-ing phosphorus availability to sugarcane. Lossesfrom the profile are more likely to be associated

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92 Sugarcane: Physiology, Biochemistry, and Functional Biology

with soil erosion rather than leaching, because ofthe relative immobility of soil phosphorus.

In addition to pH effects, fixation or sorp-tion of phosphorus is enhanced by the presenceof certain clay minerals (kaolonite -rich in ironand aluminum oxides and allophane) and humus-aluminum complexes. Information on phosphorusfixation (the phosphorus desorption index or PDI)is used to modify phosphorus fertilizer advice forsugarcane in South Africa (Anonymous 1999a;Meyer 1974; Meyer & Dicks 1979) and with aphosphorus sorption or buffer index (PBI) in Aus-tralia (Wood et al. 2003).

Roots may contact only 1–3% of the soil sur-face area in the top 15–20 cm of soil (Anonymous1999b) therefore adequate phosphorus is requiredto ensure good root growth for uptake of waterand other nutrients. The extent of root systems,especially in compacted interspaces in sugarcanefields, and opportunity for fixation of phospho-rus highlights the benefits of banded applicationof phosphorus to maximize plant uptake, particu-larly if phosphorus levels are marginal.

Management of phosphorus nutrition: soil analysis

Various extractants are used across sugar indus-tries to index plant-available phosphorus in soil.This practice causes difficulty in comparing criti-cal values and subsequent fertilizer recommenda-tions. While the theoretical justification for appli-cation of a particular method can be debated, it ismost important to have the methodology linked toplant response via field calibration experiments.The temptation should be avoided to convertcritical values derived in this manner to criticalvalues for another analytical method by a regres-sion function across a range of soils. Thisprocedure is particularly dangerous for soilphosphorus as extractants vary from sulfuric,hydrochloric, and acetic acids, through sodiumbicarbonate, to water and anion resin strips. Theacidic extractants have widest application to acidicsoils (mimicking rhizosphere conditions of sugar-cane); bicarbonate extractants are most relevantto alkaline soils, with water extraction provinguseful for short growing period vegetable cropswith a high requirement for available phosphorus

Table 5.3 Materials used to supply phosphorus tosugarcane.

Phosphorus source % Available P % Total P

FertilizersSuperphosphate 8.5 9.1Triple superphosphate 17.7 19.2Diammonium phosphate 20.0 20.2Monoammonium phosphate 21.9 22.0Rock phosphate 1.1 16.0Blood & bone 5.1 6.0High analysis mixture Various VariousPhosphoric acida 21.5 21.5Ammonium polyphosphatea 16 16

Waste materials (after Barry et al. 1998)Filter mud, Cachazab 0.5–1.4Sugar mill ash, Cenizab 0.04–0.3Sewage sludgeb 0.3–2.8

aThese materials can be used in drip irrigation systems.bPhosphorus from these sources may be available slowly.

(Andries & McCray 1998). Critical values can alsobe influenced by soil:extractant ratios and extrac-tion time. Critical values are reported on a weightbasis (mg P/kg soil) in Australia and South Africa,whereas a volumetric basis is used in Brazil (mgP/dm3 soil) and the United States (mg P/L soil,sometimes also converted to kg P/ha in soil).

Sources and application of phosphorus

The range of products used to supply phosphorusto sugarcane is summarized in Table 5.3. Choice ofphosphatic product usually depends on price/kgof phosphorus, but this can vary if there are sig-nificant savings in logistics of application of alter-native products. For example, triple superphos-phate is usually the cheapest form of phosphorus,but savings in application costs by using mixedfertilizers based on monoammonium phosphate(MAP) or diammonium phosphate (DAP) makesthese the favored sources of phosphorus for theAustralian sugar industry. Rock phosphate is avaluable source of phosphorus, but slow availabil-ity suggests it is most suited to a maintenance role,rather than immediately supplying plant-availablephosphorus in responsive soils where phosphorusis in low supply. The change from single super-phosphate to high analysis phosphorus fertiliz-ers in Australia resulted in the revised need for

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Mineral Nutrition of Sugarcane 93

active management of calcium, sulfur, and zincnutrition, as previous use of superphosphate atrates for maintenance of phosphorus (20 kg P/ha)also applied 44 kg Ca/ha, 22 kg S/ha, and traceamounts of zinc.

Sugar mill filter mud is a valuable source ofplant-available phosphorus (Table 5.3). However,the low dry matter content (30%) limits economicapplication to a relatively compact zone aroundsugar mills. Composting is used in Brazil to reducemass and volume of filter mud to 55 and 75%,respectively, of their original states before bulktransport to the field for application, generally toplanting furrows at approximately 20 wet t/haby purpose-built applicators (C. Penatti personalcommunication). Fresh filter mud is used at 60–90 wet t/ha when applied to the interspaces ofratoon crops in Brazil, or broadcast at 120–150wet t/ha prior to the plough-out operation. Useof bulk transport vehicles to also apply fresh filtermud to Australian cane fields resulted in broadcastrates of 100–150 wet t/ha, which supplies enoughphosphorus for six crops of sugarcane (Chapman1996); however, recent government regulation ofphosphorus application rates has led to develop-ment of machinery for application of filter mud inbands at rates as low as 50 wet t/ha (Markley &Refalo 2011).

High-quality forms of soluble phosphatic fer-tilizers (phosphoric acid or ammonium polyphos-phate) are required if phosphorus is applied byfertigation in drip irrigated fields.

Recommendations for application of phospho-rus are summarized in relation to critical soiltest values in Table 5.4 for several sugar indus-tries. Critical and optimal values for soil phos-phorus reflect different analytical methods, butrecommendations for application of phosphorusto plant and ratoon cane are quite similar acrossindustries.

Care should be exercised when comparing ratesfor application of phosphorus reported in sugarindustry publications, as values reported for theUnited States and Brazil are usually in kg P2O5/haunits, while those for Australia and South Africaare in kg P/ha units. This consideration alsoapplies to elements such as potassium, calcium,and magnesium. Reporting fertilizer assays in theoxide form have no relevance to mechanisms orelemental forms for uptake by plants, presenceof the elements in soil, or calculations relatedto cation exchange capacity. Rather, the oxidereporting appears to be a relic of the days whenchemical assays relied on analysis of ashed mate-rials, rather than modern techniques where ele-ments are extracted by chemicals before assaysare completed by colorimetry, inductively coupledplasma scan, or atomic absorption spectroscopy.

Potassium

Role of potassium

Potassium is acquired by roots from the soilsolution as the K+ ion and is found mainly in

Table 5.4 Recommendations for application of phosphorus fertilizer to plant and ratoon crops of sugarcane in relation tosoil test results.

Crop Australia South Africa Hawaii Florida Brazil

Critical soil <10 mg/kg (BSES) <11 mg/kg(Truog)

10 mg/L (Olsen) <14 kg/ha(Bray 2)

<10 mg/dm3

(anionexchange resin)

Optimal soil >20 mg/kg (BSES) >31 mg/kg(Truog)

>30 mg/L (Troug) –

Plant cane kg P/ha 0–80 0–50 0–100 0–36 0–53Ratoon cane kg P/ha 0–80 0–80 a 0–36 0–13Comment Rate depends on

soil test valueand PBI

As Australia butup to 160 kgP/ha with highPDI

Rate depends onsoil test value;P split if>50 kg P/ha

Rate depends onsoil test value

Rate depends onsoil test value

Note: Names in parentheses identify assay method.aLittle ratoon cane grown in Hawaii due to 24-month plant crop.

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94 Sugarcane: Physiology, Biochemistry, and Functional Biology

water-soluble forms in the cytoplasm. Potassiumis required for

� activation of the 50 enzymes involved in syn-thesis of starch;

� protein synthesis in the bonding of transferRNA to ribosomes;

� photosynthesis in maintaining the necessarypH gradient for ATP synthesis;

� osmoregulation, especially in stomata, andplaying a key role in uptake of water and theefficient use of water by plants;

� electron balance in cells;� stimulation of phloem transport of sugars.

Some growers of sugarcane and their advisorshave associated use of high levels of potassium fer-tilizer with increased accumulation of sugar andhigher sucrose % in cane culms. This conclu-sion is not supported by research and may be theresult of erroneous interpretation of classic workof Hartt (1969). This work, with 14C labeled car-bon dioxide, showed adequate potassium supplyenhanced translocation of sucrose from leaves toculms when compared to cane not supplied withpotassium. Thus, it is important to distinguishbetween effects of potassium deficiency and suffi-ciency on sucrose accumulation.

Symptoms of potassium deficiency

Symptoms of potassium deficiency (Color Plate5.3) appear first in the older leaves due to itstranslocation to actively growing tissue. Lowerleaves develop a deep yellow to orange color, withtips and margins becoming necrotic (scorched).Interveinal necrosis may develop from the areasof red discolorations. Young leaves may remaingreen. This coloration can be used to distinguishpotassium deficiency from salinity stress, wherethe tips and margins of young leaves becomescorched. Growth is restricted and culms are thin.Potassium-deficient plants have reduced resis-tance to disease (Anonymous 1999c).

Consequences of excess potassium

Sugarcane has the capacity for luxury consump-tion of potassium from natural soil fertility or frompotassium in fertilizers, vinasse (dunder), or sugar

mill ash (Leverington et al. 1965). Apart fromthe direct adverse effect of excess potassium onproduction economics, the major effect of excesspotassium is on increased ash in cane juice andraw sugar (Leverington et al. 1965; Irvine 1978;Kingston & Kirby 1979; Clarke 1981; Kingston1982a). Increased ash in juice reduces the recov-ery of sugar crystal (Irvine 1978; Clarke 1981).Potassium plays an important role in osmoreg-ulation in higher plants (Marschner 1986), andpotassium and ash levels in juice are inflated byincreased salinity in the root zone, be it from salinesoils (Kingston 1982a, 1982b) or elevated elec-trolyte levels in irrigation water (Kingston & Kirby1979).

Potassium in soil

As most of the potassium in soil is held in the latticeof 2:1 clay minerals such as montmorillonite andillite, there can be substantial differences betweenthe total and readily available potassium content inmost soils, except those with very sandy texture,depending on the balance of 2:1 and 1:1 clay min-erals. Nonexchangeable potassium in the clay lat-tice is released into exchangeable and soil solutionpools during the wetting and drying cycles thatcontribute to long-term weathering of minerals.While the latter two pools can be replenished fromnonexchangeable potassium, a fertilizer programbased on soil analysis is essential for avoidance oflong-term depletion of the clay lattice reserve andenhanced destruction of the clay mineral. This isparticularly relevant for sugarcane, where luxuryconsumption of potassium and the long growthperiod place additional stress on reserves of soilpotassium. Luxury consumption of potassium bysugarcane makes it extremely difficult to increasesoil potassium reserves through application of sol-uble potassium fertilizers.

The availability of soil and fertilizer potassiumto sugarcane can be affected adversely in soilswith potassium selective clay minerals, allowingsuch soils to fix potassium within the clay lat-tice (Wood & Meyer 1986). Higher applicationsof potassium fertilizer are required for potassiumfixing soils in order to generate levels of exchange-able and soil solution potassium similar to those

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Mineral Nutrition of Sugarcane 95

encountered in soils with less capacity to fixpotassium.

Availability and uptake of potassium are con-strained by high levels of calcium and/or magne-sium in soil (Evans 1959; Humbert 1963, 1971;Wood & Meyer 1986; Santo et al. 2000). Highlevels of calcium and magnesium in the soil solu-tion favor uptake of these elements by mass flowof the transpiration stream thus interfering withpotassium uptake by diffusion across an elec-trolyte gradient between soil and roots. Donald-son et al. (1990) suggested that potassium nutritionneeds special attention when base saturation, mea-sured by the ratio (Ca+Mg)/K, is greater than20 (when the units of measurement are mg/kgof soil for cations). Long-term use of magnesicirrigation water has elevated magnesium levels insoils in Hawaii. This has resulted in visual symp-toms of potassium deficiency, even though nor-mal amounts of potassium fertilizer were applied(Santo et al. 2000).

Release of potassium from clay minerals isinhibited by low soil temperature in winter andspring (Leverington et al. 1962; Donaldson et al.1990) and by high moisture levels in soils domi-nated by 2:1 clay minerals (Donaldson et al. 1990).The latter authors recommend a higher thresholdvalue for soil potassium for base saturated soilswith more than 40% clay and where irrigationgenerally does not allow drying and cracking of thesoil. The higher threshold recognizes that potas-sium is released from the exchange complex whensoil is dried for analysis and that the conventionalthreshold will not allow sufficient potassium in soilsolution for optimal growth of sugarcane in cooland wet soils. For best results potassium should beapplied soon after harvest for cane grown on 2:1clay soils, particularly during cool and wet condi-tions (Donaldson et al. 1990).

Management of potassium nutrition

Soil analysisThe extended length of the sugarcane growing sea-son, combined with the dynamic balance betweennonexchangeable and exchangeable potassium insoil, raised doubt about the value of measurementsof the latter fraction alone for predicting need for

potassium fertilizer (Haysom 1971; Hunsigi & Sri-vastava 1981). This led to dual assessment of soilpotassium (Haysom 1971; Wood 1985; Moody &Bell 2006). Exchangeable potassium is indexedwith relatively mild extractants such as waterin electro-ultrafiltration, 1N ammonium acetate,1N ammonium chloride, or 0.02N hydrochloricacid; nonexchangeable potassium is indexed witha more aggressive 1N nitric acid (Haysom 1971)or tetraphenylborate (Moody & Bell 2006).

Large differences in the exchangeable andnonexchangeable potassium values reflect areserve of potassium, which can be released tothe exchangeable pool. Such reserves are usuallyassociated with potassium held in the lattice of 2:1clays or by organic matter. The absolute amountof nonexchangeable potassium must be consid-ered along with the rate of its release to exchange-able forms and availability to plants (Martin &Sparks 1985; Schroeder & Wood 2002). The latterauthors showed the rate of release of nonexchange-able potassium was influenced by soil texture andclay mineralogy.

Leaf analysisLeaf analysis is another useful tool for manag-ing potassium nutrition of sugarcane. Thresholdvalues reviewed by Anderson and Bowen (1990)highlight the variation in index tissue and cropage used for assessing potassium nutrition. Thereis quite good agreement between indices basedon the third or top visible dewlap leaf for SouthAfrica (1.05%) and Australia (1.11%) for cane of3–5 months of age (Meyer 1981; Calcino 1994).The low critical value of 0.62% for Brazil (Ander-son and Bowen 1990) may be an effect of samplingcane at 6 months of age, as the optimal range was0.62–1.45%. Threshold values less than 1.05% areproposed for leaf potassium for winter cut cane inSouth Africa, sampled at 2–5 months of age in theOctober to January period on base saturated 2:1clays (Donaldson et al. 1990; Duvenhage & King1996).

Potassium fertilizer recommendations for Aus-tralia, South Africa, Hawaii, Florida, and Brazilare summarized in Table 5.5 in relation to soil cri-teria. The high rates of potassium indicated forHawaii are required for high cane yield (224 t/ha)

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96 Sugarcane: Physiology, Biochemistry, and Functional Biology

Table 5.5 Recommendations for application of potassium fertilizer to sugarcane, in relation to chemical and physicalanalyses of soil.

Australia South Africa Hawaii Florida Brazil

Threshold soilexch. K

<0.24 (cmolc/kg) <0.17 (cmolc/kg) <0.35 (cmolc/kg) 139 (kg K/ha) <1.2–2.3(cmolc/dm3)

Plant cane kgK/ha applied

0–100 <30% clay; 0–175>30% clay; 0–200>40% clay; 0–250 for

base saturatedconditions

0–375 0–233 0–116

Ratoon cane kgK/ha applied

0–120 <30% clay; 0–175>30% clay; 0–200>40% clay; 0–250 for

base saturatedconditions

0–375 0–233 0–108

Comments Rate modified bysoil analysis forexch. K andnonexch. K

Rate modified by soilanalysis for exch. Kand nonexch. K

Rate modified bysoil analysisand K inirrigation water

Rate modified byexch. K in soil;139 kg K/hacritical valuefor all soil testsfor ratoon ≥3

Rate modified byexch. K in soil

over a 2-year period, while the upper range ofapplications in South Africa is applied to clay soilswith high base saturation and capacity to fix potas-sium.

Sources of potassiumFertilizer and waste materials used to supplypotassium to cane fields are summarized inTable 5.6. Potassium chloride is a popular choiceof product in most sugar industries because itsprice is lower than that of potassium sulfate. The

Table 5.6 Materials used to supply potassium tosugarcane.

Potassium source % Potassium

FertilizerPotassium chloride (muriate of potash) 50.0 w/wPotassium sulfate (sulfate of potash) 41.0 w/wPotassium nitrate 38.3 w/wPotassium magnesium nitrate 18.0 w/w

Waste materials (after Barry et al. 1998)Vinasse–Brazil 0.25–0.42 w/vVinasse–Australia (reboiled) 2.24 w/vVinasse–Australia (bio-dunder) 3.00 w/vSugar mill ash, Ceniza 0.9 (0.07–2.3) w/wFilter mud, Cachaza 0.4 (0.1–0.9) w/wSewage sludge 0.8 (0.2–1.9) w/w

Note: Figures in brackets represent typical ranges of % K inwaste materials.

solid form is used most widely, but liquid for-mulations are used successfully in some areas ofthe industry in Sao Paulo (Brazil). Distillery waste(vinasse, dunder, or bio-dunder) is an importantsource of potassium in Brazil and several dis-tricts in Queensland, where ethanol distilleries arelocated in sugar districts. The potassium concen-tration of the vinasse depends on distillery feedstock (cane juice or molasses) and industrial pro-cessing to concentrate effluent. Distillery wastecan be enriched with urea solution (Chapman et al.1995) or anhydrous ammonia to provide a mixednitrogen-potassium fertilizer.

More recently, the latter nitrogen enriched bio-dunder has been fortified with sulfuric acid tomeet the sulfur requirements of sugarcane. Useof excessive rates of vinasse may lead to the devel-opment of saline areas, particularly in lower land-scape positions in areas receiving < 1200 mmrainfall/year. Long-term use of excessive rates ofvinasse may also result in an imbalance of soilcations, particularly in subsoils.

Potassium in irrigation water, particularlyground-waters, can represent a significant sourceof plant-available potassium and should be con-sidered in nutrient management plans. For exam-ple, irrigation waters drawn from below the illiticalluvia of the Burdekin Delta in Australia and the

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Mineral Nutrition of Sugarcane 97

Indus Valley in Pakistan consistently show potas-sium concentrations of 0.12–0.18 mmol/L, whichwould supply 47–70 kg K/ha, respectively, in each10 ML/ha of irrigation water.

SECONDARY NUTRIENTS

Calcium

Role of calcium

Calcium is

� required for cell division;� required for activity of only a few enzymes;� counters soil acidity and the toxic effects of

aluminum and manganese;� found primarily in cell walls as calcium pec-

tates and is also bound to the plasma mem-brane as exchangeable calcium.

Calcium therefore has low mobility in plants. Itplays a critical role in stabilizing and strength-ening cell walls and regulating membrane per-meability. Calcium-deficient cells have a greaterleakage of low molecular weight compounds, lead-ing to higher respiration than where calcium isadequately supplied (Bangerth et al. 1972). Theeffects of calcium on permeability of cell wallsalso influence the uptake of other nutrients.

Symptoms of calcium deficiency

The relative immobility of calcium after incor-poration in plant cells usually leads to thefirst observation of symptoms of deficiency inyounger leaves, where minute chlorotic spots laterdarken and coalesce into reddish-brown spots withnecrotic centers. Severe deficiency causes youngerleaves to become hooked and the spindle leafappearing necrotic at the tip and leaf margins(Color Plate 5.4). Leaf growth is reduced to givea fan-like appearance to plant tops (Color Plate5.4, right). Older leaves become pale green, oftenwith yellow mottling. Culms are thin and tapered,particularly for onset of calcium deficiency later inthe growth period. Root growth is poor.

Consequences of excess calcium

Excess calcium rarely affects plant growth. Cal-cium will precipitate from the soil solution as cal-cium carbonate if soil pH is greater than 8.3, butthis is usually a consequence of geomorphologyrather than over-liming. Availability of the traceelements copper, zinc, and iron decline if soil pHis greater than 7.5 and deficiency of these ele-ments has been observed in areas where bulk limewas deposited prior to spreading at recommendedrates in cane fields. Calcium, along with magne-sium, can interfere with uptake of potassium fromnaturally high base soils (Wood & Meyer 1986).

Management of calcium nutrition

Calcium in soilCalcium occurs in the soil as Ca++ ions on thesoil exchange complex and in the soil solution, ascalcium carbonate in high pH soils, or as gypsumin arid zone soils and the lower profile of acid sul-fate soils which are formed on pyritic marine mudsand contain iron sulfides. Calcium is important formanagement of soil acidity (see the section titled“Management of aluminum toxicity and soil acid-ity”). Calcium plays an important role in devel-opment of good soil structure through van derWaal’s forces whereby its relatively small ionicradius allows closer association and flocculation ofclay particles for formation of aggregates. This iscontrasted with situations where sodium, with itsrelatively large ionic radius and exceeding 9% ofcation exchange capacity (sodic soils), can result indispersion of clay particles and loss of soil struc-ture.

Calcium deficiency arises from weathering ofacidic tropical and subtropical soils and removalfrom soil in the harvested products.

Soil analysis is a reliable tool for indicatinga need to apply nutritional calcium to sugar-cane. Data in Figure 5.2 (Kingston & Aitken1996) depict the typical shape of the cane yieldresponse to soil calcium. Yield loss is dramaticbelow the critical value (0.55 centimole charge /kgsoil, i.e. cmolc/kg), and yield does not respondto available calcium above the threshold value of1.5 cmolc/kg. There is remarkable similarity incritical and threshold values of soil calcium (and

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98 Sugarcane: Physiology, Biochemistry, and Functional Biology

0.00

20

40

Rel

ativ

e ca

ne

yiel

d (

%)

60

80

100

0.5 1.0 1.5 2.0 2.5 3.0

Soil Ca 0–25 cm (cmolc/kg)

y = 98.12 – 61.44*exp (–6.91*x), r2 = 0.89

3.5 4.0 4.5 5.0 5.5 6.0

Fig. 5.2. Response of cane yield to exchangeable calcium in the surface 25 cm of soil. Kingston & Aitken 1996.

magnesium) used across sugar industries, eventhough different extractants are used, for exam-ple:

� Brazil: Centro de Tecnologia Canavieira rec-ommendations allow for a combined cal-cium plus magnesium threshold level of 1.8cmolc/kg for 97% relative yield (Anony-mous 1993). Calculations based on volumet-ric data of Orlando Filho and Rodella (1987)and Benedini (1988) show threshold levels of0.77 cmolc/kg for calcium and 0.31 cmolc/kgfor magnesium, which gives a total of 1.08cmolc/kg.

� Australia: Critical and threshold values of0.55 and 1.5 cmolc/kg, respectively, are usedfor calcium (Calcino 1994; Kingston & Aitken1996), while critical and threshold values of0.1 and 0.25 cmolc/kg, respectively, are usedfor magnesium. This is a combined thresholdof 1.75 cmolc/kg for (Ca+Mg).

� South Africa: A threshold value of 0.75cmolc/kg is generally applied for soil calcium,but 0.5 cmolc/kg is used only for recent sands

of marine origin. The magnesium thresholdis 0.21 cmolc/kg (B.L. Schroeder personalcommunication).

� Hawaii: Volumetric data calculations (L.Santo personal communication) show Hawaiirecommendations are based on thresholds of1.82 and 0.21 cmolc/kg for calcium and mag-nesium, respectively.

Applications of liming materials to meet tar-get values of calcium plus magnesium in Brazil(Anonymous 1993) or calcium in Australia(Kingston & Aitken 1996) have simplified recom-mendations for nutritional calcium. These rec-ommendations are derived from field experimen-tation and the following derived formulae, whichcan be used to calculate lime requirements to sup-ply nutritional calcium for sugarcane to achievethreshold soil values presented earlier:

� Brazil:

t/ha dolomitic lime = 30 − (Ca

+Mg) ∗ 100/10 ∗ (Rel NV lime)

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Mineral Nutrition of Sugarcane 99

where (Ca+Mg) is determined from soilanalysis (0–25 cm) and is expressed asmmolc/dm3 and Rel NV is the neutralizingvalue of dolomitic lime relative to pure cal-cium carbonate (Anonymous 1993).

� Australia:

kg/ha calcitic lime = (1.5 − SCa)

∗500 ∗ BD/(LCa% ∗ 0.85)

where SCa is calcium (0-25 cm) determinedfrom soil analysis and expressed as cmolc/kg,500 is a factor incorporating the depth of soilto be treated (cm) and the equivalent weightof calcium, LCa% is the calcium content oflime, BD is bulk density, and 0.85 an effi-ciency factor for crushed agricultural lime-stone. The formula can also be used to cal-culate calcium requirement from gypsum byinserting the calcium content (LCa%) for gyp-sum and omitting the 0.85 factor, due to thehigher solubility of gypsum than limestone(Kingston & Aitken 2000).

The above-mentioned formulae result in verysimilar outcomes at the field level. For exam-ple, for values of calcium and magnesium of 0.5and 0.2 cmolc/kg, respectively, the Brazilian algo-rithm, after unit conversion, suggests 2.71 t/ha ofdolomitic lime, whereas the Australian approachresults in 2.39 t/ha of calcitic lime fortified with3% magnesium.

Leaf analysisCalcium in leaf dry matter is used to indexadequacy of calcium nutrition. Critical valuesrange from 0.13 to 0.20% (Anderson & Bowen1990; Calcino 1994). Some of this variation isattributable to choice of the index leaf, or leafsheath and varietal differences. Kingston andAitken (1996) proposed a need to examine alloca-tion of variety-specific thresholds, because culti-vars such as Q110 achieve maximum yield on highfertility soils at 0.12% calcium in leaf dry matter,whereas 0.17% appears an appropriate thresholdfor other cultivars. In Guiana, cane grew very littlewhen calcium content fell below 0.13%, and defi-ciency symptoms were prevalent in leaves withcalcium content between 0.05 and 0.1% (Evans1959).

Sources of calciumCalcitic or dolomitic limestones are the most com-mon sources of calcium for sugarcane (Table 5.7).These materials have the dual roles of supply-ing nutritional calcium to sugarcane grown onacidic soils and managing the adverse effects ofsoil acidity (see the section titled “Managementof aluminum toxicity and soil acidity”). Calciumcontent and particle size are important criteriawhen selecting liming products for calcium sup-ply. Good quality crushed agricultural limestoneshould have 100% of particles <3.35 mm and 60%< 0.25 mm (Aitken & Cowles 1993).

Gypsum is an excellent source of nutritionalcalcium and can be used to improve calcium status

Table 5.7 Products that can supply nutritional calcium to sugarcane.

Calcium source% calcium(DM basis)

Neutralizingvalue (%)

Agricultural lime, earth lime (calcium carbonate) 30–40 67–99Burnt lime/quick lime/unslaked lime (calcium oxide) 68 178Hydrated lime/slaked lime (calcium hydroxide) 51 135Dolomite (calcium magnesium carbonate) 15–18.5 60–85Calcitic lime/magnesium blends 29–33 78–110Calcium silicate (slag or wollastonite) 31 65Cement 18–22 107Gypsum (calcium sulfate) 14–18 0Superphosphate 20 –Triple superphosphate 15 –Filter mud, Cachaza 2.3 –

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100 Sugarcane: Physiology, Biochemistry, and Functional Biology

of subsoils (Aitken & Kingston 1998). Applicationof gypsum is recommended for sugarcane landsin Brazil when calcium status and aluminum sat-uration of the 25–50 cm layer are <0.6 cmolc/kgand>40%, respectively (Anonymous 1993). Gyp-sum can also be used to counter undesirable lev-els of exchangeable sodium in sodic soils but hasno effect on soil pH. Gypsum will also managealuminum levels in crop root zones of some soilsthrough the self-liming effect (Reeve & Sumner1972; Sumner et al. 1986).

Calcium silicate slag is emerging as a usefulsource of nutritional calcium during the coman-agement of calcium and silicon nutrition onlight textured or highly leached tropical soils(Berthelsen, Hurney, Kingston, et al. 2001).

Superphosphate supplies nutritional calcium tosugarcane. Prior to the introduction of high analy-sis fertilizer mixtures, superphosphate was widelyused across sugar industries. The trend towardusing MAP and DAP as sources of nutritionalphosphorus led to a greater focus on calcium nutri-tion per se in Australia in the last two decades ofthe twentieth century (Ridge et al. 1980).

Good management of calcium nutrition is acritical part of sustaining soil fertility and cropproductivity. Philosophical issues about manag-ing soil fertility on the basis of calcium nutrition,aspects of soil acidity, or both issues will be dis-cussed in the section titled “Management of alu-minum toxicity and soil acidity.” The outcomeof decisions in this area has implications for thedirections that may be given to plant breeders ormolecular biologists in modifying the genome ofplants for more efficient use of calcium and/ortolerance of soil acidity (also see the section titled“Novel applications of genetic manipulation toplant nutrition”).

Magnesium

Role of magnesium

Magnesium is

� the central atom of the chlorophyll molecule;� required as an enzyme promotor, e.g., Mg-

ATPase is the substrate of most ATPases;

� involved in protein synthesis by bridging ofribosome sub-units;

� mostly used in plants for charge balance, toachieve electroneutrality in the cytoplasm.Some magnesium is also bound in cell wallpectins;

� taken up as Mg++ ions and has much greatermobility within plants than does calcium.Approximately 70% of magnesium in plantscan be removed in aqueous extracts.

Plants acquire magnesium from the soil solu-tion, from exchange sites on clays in soil, and as aleachate from decomposing plant residues. Mag-nesium is more prone to leaching within soil thanis calcium.

Symptoms of magnesium deficiency

Mobility of magnesium within plants causessymptoms of deficiency to appear first in the olderleaves. Younger leaves (spindle to +3 or +4)may remain green. Symptoms develop from smallchlorotic spots to give pronounced orange/brownnecrotic spots (Color Plate 5.5). Magnesium defi-ciency is often called orange freckle and should notbe confused with rust diseases of sugarcane (Puc-cinia melanocephala or Puccinia keuhnii.), whereruptured pustules are evident on the underside ofleaves, or with damage from mites (Acarina spp.).Magnesium deficiency retards growth and tiller-ing is poor.

Consequences of excess magnesium

Excess of magnesium in soil is relatively rarebut can occur for several reasons. Base satu-rated soils derived from dolomite, olivine, biotite,horneblend, or chlorite can have a high propor-tion of bases as magnesium (Wood & Meyer 1986).Waters from aquifers that are recharged througholivine basalt are rich in magnesium and use ofsuch water for irrigation leads to a change in thebalance of calcium to magnesium on the cationexchange complex and an overall increase in thelevel of magnesium in soil solution (Santo et al.2000; Skilton et al. 2000). Soils developed on for-mer marine clays can also contain high levels ofmagnesium (Evans 1959). High levels of

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Mineral Nutrition of Sugarcane 101

magnesium interfere with potassium uptake(Evans 1959; Humbert 1963, 1971; Wood & Meyer1986; Donalson et al. 1990; Duvenhage & King1996; Santo et al. 2000).

The ratio of calcium to magnesium in soil isused in some advisory services as a criterion of soilfertility and plant performance. A ratio of 3:1 ismentioned by Santo et al. (2000) as favoring uptakeof potassium and good soil tilth. A ratio of 7:1 wasconsidered a desirable target in Louisiana (Hall-mark et al. 1997), but no yield benefits of such ame-lioration were demonstrated across three years ofsugarcane cropping. Research in an area irrigatedwith magnesic groundwater, near Bundaberg inAustralia, also failed to demonstrate responses ofsugarcane to manipulation of calcium:magnesiumratios in favor of calcium in either field exper-iments (G. Kingston unpublished data) or inpot experiments (Skilton et al. 2000); however,a response to such management was recorded forcapsicum (red bell peppers) in the pot experiment.

Magnesic/sodic clays are more prone to dis-persive loss of aggregate structure than are cal-cic/sodic clays (Emerson & Bakker 1973).

Management of magnesium nutrition

Soil analysis is used to monitor magnesium statusof cane land and any need for subsequent applica-tion of magnesium fertilizers. A threshold value of0.21 cmolc/kg is used in South Africa and Hawaii,while 0.25 cmolc/kg is used in Australia (Cal-cino 1994; B.L. Schroeder personal communica-tion; L. Santo personal communication). A higherthreshold of 0.31 cmolc/kg for magnesium aloneis reported for Brazil (Orlando Fiho & Rodella1987).

Critical levels for magnesium in leaf tissue rangefrom 0.08–0.10%, with most reports using thelower number (Anderson & Bowen 1990).

The rate of ameliorative magnesium is deter-mined from soil test values. In Australia applica-tions range from 0–160 kg Mg/ha, applied fromlimestone fortified with magnesium oxide, mix-tures of lime and dolomite, or dolomite alone.Brazilian requirements of dolomitic lime aredetermined from the previously mentioned for-mula (see the section titled “Management of cal-

Table 5.8 Products that can supply nutritional magnesiumto sugarcane.

Magnesium source % magnesium

Dolomite (calcium magnesium carbonate) 8–10Magnesium oxide 54–60Magnesium sulfate (hepta- & monohydrate) 9–20Magnesite (magnesium carbonate) 28Blends of lime with magnesic materials 3–8Cement 1.3Filter mud, Cachaza 0.6 (0.4–1.1)Sugar mill ash, Ceniza 0.7 (0.1–1.1)

Note: Figures in parentheses represent typical ranges of %Mgin waste materials reported by Barry et al. (1998).

cium nutrition; Calcium in soil”). Applicationsof dolomite range from 0–2 t/ha under SouthAfrican conditions. Need to apply magnesium israre in Hawaii because of its volcanic provenance.Products suitable for supplying nutritional mag-nesium to sugarcane are summarized in Table 5.8.

Sulfur

Role of sulfur

Sulfur is required

� as a constituent of amino acids, therefore pro-teins, and for production of coenzymes. Cys-tine and methionine production is one of themost important functions of sulfur (Thomp-son 1967);

� is critical for the function of nitrate reductasein the conversion of nitrate into ammonium,prior to incorporation into amino acids;

� for the formation of chlorophyll.

Sulfur is taken up by plants as SO4= ions by

roots and transported in the xylem. Reductionof sulfate ions for incorporation into amino acidsis localized in the chloroplasts of higher plants(Frankhauser & Brunold 1978).

Symptoms of sulfur deficiency

Young leaves show the first signs of sulfur defi-ciency because of the importance of the elementin formation of chlorophyll. Leaves become uni-formly chlorotic and may develop a purplish tinge

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102 Sugarcane: Physiology, Biochemistry, and Functional Biology

(Color Plate 5.6). Leaves are narrower and shorterthan normal and culms are slender, often with astrong taper toward the apex.

Consequences of excess sulfur

Effects of excess sulfur are rare in most soils thatare conventionally managed for plant nutrition.Soils formed on pyritic marine muds contain ironsulfides, which oxidize to release sulfuric acid aftercreation of an aerobic environment by drainage.These soils are known as acid sulfate soils and areconsistent with the acidic and sulfide or pegassesoils used for sugarcane growing in Guiana (Evans1959). Soil pH can be as low as 2.5–3.0 after oxida-tion, and in such conditions aluminum and man-ganese concentrations in the soil solution can betoxic to plants. Acidity and high levels of alu-minum in drainage water has severe environmen-tal implications for the health of fish and mollusks(Sammut et al. 2008). Efflux of acidic drainagewaters can be minimized by not draining thesesoils below the sulfidic layer and by applying lime-stone to the walls of drains and sludge taken fromdrains during cleaning operations (Gardner et al.2000). Acid sulfate soils are usually highly fertilehumic soils and are successfully used for sugar-cane production, provided sulfidic material is notdisturbed and spread within fields.

Management of sulfur nutrition

Sulfur in soilSulfur is added to soil from organic matter, irriga-tion waters, sea spray, and rainfall, which may sup-ply 5–18 kg S/ha/yr or even higher levels whenthe rain dissolves sulfur dioxide from industrialemissions. Sulfur is released from organic mat-ter as SO4

= ions by a process of mineralization.These ions are readily leached, but may accumu-late in subsoils of acidic tropical soils with signif-icant anion exchange capacity. Oxidation of ironsulfides in acid sulfate soils also releases SO4

= ionsinto the soil solution.

Monitoring sulfur nutritionSoil and leaf analysis can be used to monitor thesulfur status of soil and sugarcane to determine

requirement of sulfur fertilizer (Table 5.9). TheSouth African sugar industry also uses the sulfurmineralization potential of a soil, which is a func-tion its organic matter and clay contents (Anony-mous 1999a), for general guidance on possible sul-fur nutrition problems.

Sulfur is recommended in Florida when soil pHis greater than 6.6. The current recommendationis to apply 560 kg S/ha for muck and sandy mucksoils and 340 kg S/ha for mucky sand soils (Riceet al. 2008). Sulfur applications are usually madein the furrow at planting.

Sources of sulfurSulfur is acquired by sugarcane in similar quanti-ties to calcium and magnesium (Table 5.1) andgenerally is not an issue of significance wherecompanion fertilizers contain sulfur (Table 5.10)or where irrigation waters supply maintenancerequirements of sulfur. Mixed fertilizers can befortified with ammonium sulfate when phospho-rus is supplied by the low sulfur products suchas MAP or DAP. Elemental sulfur can be usedfor localized adjustment of pH in the fertil-izer/insecticide application band of neutral toalkaline soils, as well as to supply nutritional sul-fur.

MINOR NUTRIENTS

The minor or trace nutrients are classified as suchon the basis of quantitative acquisition by plants(Table 5.1) not on their relative importance forphysiological function. An adequate supply of allthe minor nutrients is a critical component of anybalanced and sustainable program to manage soilfertility and crop production.

Copper

Role of copper

Copper is required

� for the activity of several enzyme systems,e.g., phenolases and enzymes involved in

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Mineral Nutrition of Sugarcane 103

Table 5.9 A summary of soil and leaf analytical criteria used to manage application of sulfur in the Australian, Hawaiian,and South African sugar industries.

Australia Hawaii South Africa

Soil test mgS/kg

Applied kgS/ha

Soil test mgS/kg

Applied kgS/ha

Soil test mgS/kg

Applied kgS/ha

0–2 25 26 153–7 10>7 0

Leaf %S >0.13 0.2 (sheath) 0.12–0.14Leaf N/S <17 – <17Comment Sulfur mixtures in common

useSulfur deficiencies are rare

due to maritime climate,irrigation, and volcanicactivity

Sulfur application is related tosulfur mineralizationpotential of soil

redox reactions such as ascorbic acid oxidaseand chytochrome oxidases;

� for the formation of chlorophyll;� for lignification of cell walls (Vetter & Teich-

mann 1968).

Copper occurs in Cu metallo enzyme com-plexes. Evidence is mixed in support of acqui-

Table 5.10 Products that can be used for supplying sulfurto sugarcane.

Sulfur source % Sulfur

FertilizersAmmonium sulfate 24.0DAP 2.0MAP 3.0Superphosphate 11.0Triple superphosphate 1.3Gypsum 18.6Phosphogypsum 14.5Copper sulfate monohydrate 17.4Copper sulfate (bluestone) 12.6Zinc sulfate monohydrate 16.8Zinc sulfate heptahydrate 11.0Magnesium sulfate 12.4Elemental sulfur 90–100Fertilizer mixtures Various

Waste materials (after Barry et al. 1998)Filter mud, Cachaza 0.27 (0.12–0.4)Sugar mill ash, Ceniza 0.07 (0.01–0.3)Sewage sludge 1.6 (0.1–4.0)Feed lot manure 0.5 (0.2–0.8)Vinasse (after re-boiling) 0.26

Note: Figures in brackets represent typical ranges of %S inwaste materials reported by Barry et al. (1998).

sition of copper by plant roots as Cu++ ions or asCu chelate (Graham 1981).

Symptoms of copper deficiency

Young leaves show the first symptoms of copperdeficiency because of its effects on chlorophyll for-mation and deposition of lignin. Leaves developa characteristic drooping habit (Color Plate 5.7,left), and feel flaccid or rubbery to the touch. Leavesmay become chlorotic and have small dark greenspots (green islands) distributed along the lamina(Color Plate 5.7, right). Culms are very flexible andgrowth and tillering are noticeably reduced. Cop-per deficiency often occurs variably within fieldsand between adjacent stools of sugarcane.

Consequences of excess copper

Adverse effects of excess copper in the soilhave not been reported for growth of sugarcane.Overuse of copper based fertilizers should beavoided for economic and environmental reasons.Copper can be mobilized from agricultural areasby erosion of soil and organic matter to accumu-late in aquatic sediments and ultimately in filterfeeding species such as shell fish and crustaceans(Topping 1972).

Necrotic spots on leaves of sugarcane sprayedwith copper sulfate solutions can be avoided byensuring the concentration of copper sulfate doesnot exceed 1% in foliar sprays.

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104 Sugarcane: Physiology, Biochemistry, and Functional Biology

Management of copper nutrition

Copper in soilCopper is usually complexed with soil organicmatter so it is not mobile or readily leached, but itcan be lost during erosion events. The supply andavailability of copper to plants is determined byclimate, the level of organic matter in soil, texture,pH, and the presence of aluminum, manganese,molybdenum, and iron. The low mobility of cop-per in soils means that accessibility of the coppersupply in soil is a strong function of root growth,which is conditioned by the nutritional environ-ment and biotic factors such as pests and disease.

Monitoring copper nutritionSoil analysis provides only general guidance onthe likelihood of response of sugarcane to appliedcopper (Calcino 1994). This difficulty is associatedwith poor correlation of plant response with frac-tions of soil copper extracted by various laboratorymethodologies, the high in-field variability of soilcopper measurements, and consequent difficultyin establishing rigorous field experiments.

Leaf analysis can be used to indicate adequacyof copper nutrition. Anderson and Bowen (1990)report critical values in the range of 2–5 mg Cu/kgfor leaf dry matter, with most reports falling in therange 2–3 mg Cu/kg. Levels of 8–20 mg Cu/kgcan be achieved.

Therefore soil analysis, combined with knowl-edge of soil properties, and a history of leaf anal-yses can be used to indicate need to apply copperfertilizer. Leaf symptoms are a very clear diagnos-tic of copper deficiency.

Copper fertilizersCopper deficiency is usually corrected by apply-ing copper sulfate solution to fallow fields or by

directed sprays between rows of cane. Such strate-gies call for application from 2–10 kg Cu/ha inBrazil, Florida, and Australia. These correctivestrategies supply sufficient plant-available copperfor several crop cycles (note biomass removal inTable 5.1). Copper deficiency is rare in Hawaiiand South Africa. A more general occurrence ofcopper deficiency in Australia led to copper fortifi-cation of planting fertilizers, which supply copperfor several crop cycles. Care should be exercisedwhen using copper oxide in the latter role as elec-trostatic action and vibration during transport canlead to settling of copper oxide in bags and conse-quent variable application of copper in the field.Materials used as copper fertilizers for sugarcaneare listed in Table 5.11.

Zinc

Role of zinc

Zinc is

� an essential component of metalloenzymes,e.g., alcohol dehydrogenase, Cu Zn dismu-tase, carbonic anhydrase, and RNA poly-merase. Zinc deficiency therefore impairscarbohydrate metabolism and synthesis ofproteins;

� involved in auxin production, e.g., indole-3-acetic acid, and is therefore important toregulation of growth in plants.

Zinc is acquired by plant roots mainly as Zn++ions and is transported in the xylem as Zn++ ions,or bonded to organic molecules (White et al. 1981).

Table 5.11 Products that can be used to supply copper to sugarcane.

Copper source % Copper Comment

Bluestone (copper sulfate pentahydrate) 25.0 Most common sourceCopper sulfate monohydrate 34.5Copper oxide 80.0Copper chelates 10–14 Best for coatings; can

settle in mixesCopper fortified planting fertilizers VariousCopper fortified superphosphate Various

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Mineral Nutrition of Sugarcane 105

Symptoms of zinc deficiency

Zinc deficiency is typically observed on leaves+3 and older leaves. Symptoms develop as broadbands of interveinal chlorosis (Color Plate 5.8).Veins and leaf margins remain green, except insevere deficiency. There is wide variation betweencultivars of sugarcane in the level to whichsymptoms of deficiency develop. This includesappearance of red fungal lesions, associated withCurvularia brachyspora, on leaves of some sug-arcane cultivars (Calcino 1994). Root and shootgrowth is poor, leading to stunted stools of cane.

Consequences of excess zinc

As with copper, there are no well-documentedreports of the effects of excess zinc on the growthof sugarcane. High concentrations of zinc sulfatesolution can cause necrotic or burn spots on sugar-cane leaves. This can be avoided by not exceeding1% of zinc sulfate in foliar sprays, or by directingmore concentrated solutions to soil along the sideof cane stools.

Management of zinc nutrition

Zinc in soilMost of the zinc in soils is complexed with organicmatter and therefore is not particularly mobile.Sandy and highly weathered soils are most likelyto have suboptimal levels of plant-available zinc.As mentioned in the calcium section, zinc defi-ciency can be induced in localized areas of in-fieldlimestone dumps or by raising pH of weatheredsoils >7.5 by over-liming. Alkaline muck soils ofFlorida and lighter textured alkaline soils in theIndus Valley of Pakistan are prone to deficiency ofsome minor elements, including zinc.

The potential for interference between applica-tion of higher rates of phosphorus and zinc nutri-tion was discussed in the section on effects ofexcess phosphorus. Accessibility of zinc is affectedby root health. Distinct foliar symptoms of zincdeficiency have been observed in plant cane inAustralia when soil zinc levels were just above thethreshold prior to application of Aldicarb for con-trol of nematodes. Symptoms disappeared soon

after the application of the nematicide, presum-ably because of increased access to zinc in soil as aresult of improved root growth (author’s note).

Traces of zinc in superphosphate suppliedmaintenance zinc to cane land when the latterform of phosphorus was the major source of phos-phorus used in Australia. At that time, rock phos-phate came mainly from guano deposits on oceanicislands (G. Kuhn personal communication).

Monitoring zinc nutritionSoil analysis is a guide of the need to use zinc fer-tilizers. As is the case for copper, zinc has variabledistribution in fields, and there are limited calibra-tion data for sugarcane to allow interpretation ofresults produced by the various extractants usedby commercial soil testing laboratories. Resultsproduced by 0.1 M hydrochloric acid extractswere more strongly correlated with yield responsein sugarcane than were extracts from EDTA andDTPA (Reghenzani 1990).

Leaf analysis is regarded as a more reliable indi-cator of zinc nutritional status than is soil analysis.Anderson and Bowen (1990) show critical valuesin leaf dry matter range from 10–13 mg Zn/kg,with an optimal range of 21–25 mg Zn/kg.

Zinc fertilizersSpraying zinc sulfate heptahydtrate (Table 5.12)on foliage or soil is the most economical way toaddress areas of zinc deficiency. Foliar applicationis likely to correct deficiency of zinc only in theyear of application, whereas a soil-based programwill have more enduring prophylactic effects. Zincsulfate heptahydrate should not be used to fortifymixed fertilizers based on MAP or DAP, becausemoisture from its deliquescence will bind fertilizer

Table 5.12 Products used to supply zinc to sugarcane.

Zinc source % Zinc

Zinc sulfate heptahydtrate 22.7Zinc sulfate monohydrate 35.0Zinc oxide 50–80Zinc chelate 9–14Zinc chloride 30.0Zinc fortified planting mixtures 2–3

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106 Sugarcane: Physiology, Biochemistry, and Functional Biology

into a mass unsuitable for flow through applicatormachinery. Zinc oxide can be used to coat mixedfertilizer granules when zinc is to be applied in thefurrow at planting. Applications of zinc to cor-rect deficiency vary from 2.2 kg Zn/ha in Florida,through 5–10 kg Zn/ha in Brazil, to 10 kg Zn/ha inAustralia and South Africa. Products used as zincfertilizers for sugarcane are listed in Table 5.12.

Boron

Role of boron

Boron is required for

� growth of new cells and is therefore criticalto growth of roots, shoots, and leaves. Theapparent association between boron and thetranslocation rate of carbohydrates is indi-rect, as new cells are also carbohydrate sinks(Gauch and Dugger 1954; Whittington 1959);

� maintenance of chloroplasts. Chloroplastsdegenerate when boron is deficient (Lee &Aronoff 1966).

Symptoms of boron deficiency

Boron deficiency is rarely seen in the field. Symp-toms are seen first on the young leaves and aresimilar to those produced by the Fiji Gall Diseasevirus and the fungal disease Pokka Boeng (Fusar-ium moniliforme). The meristem of boron-deficientsugarcane plants is distorted and can die (ColorPlate 5.9). Leaves are distorted as per the for-mer diseases, but boron-deficient plants developtranslucent interveinal watersacks that may exudedroplets of moisture. Leaves are brittle and tipsmay be necrotic. Young plants have an abnormalnumber of tillers.

Consequences of excess boron

Excess boron is toxic to plants. Such occurrencesare rare in cane fields, unless there has been long-term use of boiler ash derived from burning highboron coal. Boron toxicity is extremely difficult tocorrect, but application of lime to elevate soil pHto approximately 7.5 may be beneficial.

Managing boron nutrition

Boron in soilBoron in the soil is derived from organic matterand silicate minerals; marine sediments usuallyhave moderate to high levels of boron. Boron inplant material is relatively insoluble and is releasedslowly from decomposing organic matter. How-ever, plant-available boron is susceptible to rapidleaching from the soil. Boron deficiency can beinduced by liming soil to pH > 7.5–8.0 if soilboron levels are marginal.

Monitoring boron nutritionSoil analysis is not a reliable tool for determiningresponse to boron fertilizers (Calcino 1994). Leafanalysis is thought to reflect more accurately theadequacy of boron levels in plants, with criticalvalues in leaf dry matter ranging from 1–4 mgB/kg (Anderson & Bowen 1990).

Use of boron fertilizersHigh pH and organic soils require regular use ofboron fertilizers in Florida, where boron is appliedat 0.6 kg B/ha for mucky sand/sandy soils or 1.1 kgB/ha for muck soils (Rice et al. 2008). For experi-mental or diagnostic purposes 1.1 kg B/ha can besprayed onto the soil in areas where boron defi-ciency is suspected. Borax (disodium tetraboratedecahydrate at 11.3% B) and Solubor (sodiumoctaborate pentahydtae at 20.5% B) are the mostcommon forms of boron used in agriculture. Boricacid at 17.5% B is another potential source.

Manganese

Role of manganese

Manganese is required for

� activation of enzymes. This is especially thecase for evolution of oxygen from photosyn-thesis in the Kreb’s cycle. Manganese is alsorequired with molybdenum in activation ofnitrate reductase;

� structural elements of metalloenzymes.

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Symptoms of manganese deficiency

Symptoms of manganese deficiency (Color Plate5.10) are first observed in younger leaves becauseof its importance in critical enzyme activity innew growth. Interveinal chlorosis extends fromleaf tips to the middle or lower third of leaves.Older leaves may be frayed, due to separation ofthe lamina in the chlorotic interveinal tissue (ColorPlate 5.10, right). The entire leaf may becomechlorotic in severe cases. Early symptoms of man-ganese deficiency are similar to those of iron, butthe latter extend to the base of the leaf and thewhole plant is often affected.

Consequences of excess manganese

Manganese and aluminum can be toxic to manyplant species when soil pH is less than 5.5. Sugar-cane has high tolerance for both elements. Levelsof manganese up to 800 mg/kg were recorded inthe dry matter of index leaves of high yieldingsugarcane where the soil pH was 4.5, but optimalcalcium nutrition was maintained with gypsum(Kingston & Aitken 1996). Temporary excess ofmanganese over iron can induce severe iron defi-ciency (Color Plate 5.11) or albino shoots (Mar-tin & Evans 1964). These symptoms are similar tothose of Pahala blight produced in water cultureswith Fe:Mn ratios between 1:1 and 1:4, whereasno symptoms occurred with a ratio of 15:1 (Mar-tin 1931). This effect occurs in fields throughoutthe cane sugar world and is usually observed inisolated tillers or stools of young ratoon cane, par-ticularly during cool and damp spring months.These conditions may favor the Fe:Mn imbalancebecause of presence of Mn++ ions from the moresoluble reduced form of manganese. Applicationof iron spray is generally not economical because ofthe sporadic occurrence of the effect within fieldsand across years.

Management of manganese nutrition

Manganese in soilManganese occurs in the soil mainly as relativelyinsoluble oxides, whose solubility is controlledlargely by pH and oxygen tension of soil solutions.

Solubility of manganese increases markedly belowpH 5.5, and therefore deficiency is most commonin alkaline soils. While the organic soils in Floridaare generally acidic, but low in manganese, defi-ciency symptoms are more common on these soilswhere soil pH has been elevated by the spreadingof coralline limestone from drainage ditches andwhere there is only a shallow layer of muck soilover the coralline basement. Manganese is alsoreleased to soil solution under anaerobic condi-tions, and black nodules of manganese oxide insoil profiles are a strong indication of redox con-ditions associated with fluctuating water tables.

Soil analysis has little value for monitoring crit-ical levels of manganese due to seasonal changesin availability associated with fluctuations in aer-ation and the limited availability of data for cal-ibrating soil tests. Leaf analysis is more reliablebecause plant uptake provides an integrated pic-ture of manganese availability. The optimal valuefor manganese in leaf dry matter ranges from 12–400 mg/kg (Anderson & Bowen 1990), but a crit-ical value of 15 mg/kg is used by the Australianand South African industries.

Manganese fertilizersAlkaline muck soils in the Florida sugar industryare probably the major recipients of manganesefertilizers across international cane lands. Man-ganese sulfate (33.2% Mn) is applied prior toplanting in Florida’s muck soils at 17 kg/ha ifpH > 6.0 (Rice et al. 2008). If manganese defi-ciency is only suspected, a foliar application ofmanganese sulfate or manganese chelate may be auseful diagnostic.

Iron

Role of iron

Iron is

� essential for synthesis of chloroplasts;� a constituent of metalloproteins;� a constituent of enzymes (e.g., cytochrome

oxidase for the terminal oxidation step inthe Kreb’s cycle) and is therefore active inoxidation/reduction reactions in plants (e.g.,

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108 Sugarcane: Physiology, Biochemistry, and Functional Biology

ferredoxin and for electron transfer in chloro-plasts, nitrogenase in nitrogen fixation, andnitrate reductase for reduction of nitrate toammonium).

Symptoms of iron deficiency

Young leaves show the first symptoms of iron defi-ciency. Interveinal areas become chlorotic to vary-ing degrees (Color Plate 5.12), but in severe casesthe whole leaf or plant may be affected to the extentof tissue appearing pale yellow or bleached (seealbino shoot in the section titled “Consequencesof excess manganese”). Symptoms can be patchyor affect whole fields and are more likely to occuron alkaline soils.

Consequences of excess iron

No definite effects of iron toxicity have been rec-ognized for sugarcane. Solubility and availabilityof iron increase with soil pH below 5.5. Therefore,increased solubility of iron at low pH is linked tosorption or fixation of phosphorus as sparinglysoluble iron phosphates.

Management of iron nutrition

Iron in soilIron, along with aluminum and silicon, is a majorelement in the earth’s crust, so deficiency generallyis conditioned by availability of iron rather thanits total level. Chelated iron is the dominant formin soil solution for acquisition by plants.

Iron occurs in soil mainly in the oxide form,where oxidation state has a strong influence on soilcolor. Goethite (FeOOH) imparts yellow/brownhues, hematite (Fe2O3) makes soils red, andreduced Fe++ oxides impart blue/grey colors thatare present in wet and anaerobic soils. Iron isbound to clay minerals and organic matter. It issomewhat paradoxical that liming can be used toovercome iron deficiency in acidic organic soils(see Color Plate 5.12, right). In such situations thesmall rise in pH assists with mineralization of ironfrom organic matter, rather than the rise in pHbeing a restriction to availability of iron.

Table 5.13 Products used to supply iron to sugarcane.

Iron source % Iron

Iron chelate 13Iron sulfate monohydrate 30Iron sulfate heptahydrate 20Iron oxides 60–70

Monitoring iron nutritionYoung ratoon crops with limited root systemson high pH soils are most likely to consistentlydemonstrate iron deficiency. Soil tests for iron areunreliable for the same seasonal reasons that affectmanganese analysis. The effect is often transientin cane lands. Characteristic leaf symptoms are anexcellent diagnostic for suboptimal iron in sug-arcane, as the effect is most often seen in youngcane (<3 months) and before routine leaf sam-pling is undertaken (>4 months). Critical valuesfor iron in leaf dry matter range between 5 and 50mg Fe/kg, with optima ranging from 20–600 mgFe/kg (Anderson & Bowen 1990).

Iron fertilizersThe range of iron fertilizers available for sugar-cane is summarized in Table 5.13. A 1% solutionof iron sulfate heptahydrate or iron chelate can beused to correct iron deficiency. Yield responseshave not been recorded in Australia, where symp-toms are usually transient, but iron sulfate sprayshave been used commercially in the Natal Mid-lands regions of the South African sugar industry(B.L. Schroeder personal communication). Ele-mental sulfur at 550 kg S/ha is recommended forpH adjustment to improve availability of iron andsupply nutritional sulfur for Florida’s muck soilsif pH > 6.6 (Rice et al. 2008).

Molybdenum

Role of molybdenum

Molybdenum is an essential cofactor in enzymesystems. Its main function in higher plants isin nitrogen metabolism by activation of nitratereductase and nitrogenase enzymes. The quanti-tative requirement by plants for molybdenum isthe lowest of all mineral elements.

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Symptoms of molybdenum deficiency

Older leaves show the first evidence of molybde-num deficiency. Symptoms are similar to thoseof mild Pokka Boeng disease, with short longitu-dinal chlorotic streaks (Color Plate 5.13) on theapical third of the leaf. Older leaves may die pre-maturely, culms are shorter due to growth restric-tions, and culms are more slender than in nonde-ficient plants.

Management of molybdenum nutrition

Molybdenum in soilMost of the molybdenum in soil is associatedwith iron oxides, with only small amounts boundto organic matter. Most soils contain low lev-els of plant-available molybdenum. Availability ofmolybdenum is enhanced by reducing soil aciditywith lime and is maximized in alkaline soil.

Monitoring molybdenum nutritionSoil testing is not a reliable tool for monitoringcritical levels of molybdenum and few laboratoriespublish critical values. Until recently detectabil-ity was a significant issue for some laboratories.Critical values for leaf dry matter range from 0.05–0.08 mg Mo/kg, with optimal values ranging from0.05–1.34 mg Mo/kg (Anderson & Bowen 1990).

Molybdenum fertilizersCommon forms of molybdenum fertilizer arelisted in Table 5.14. Molybdenum supplementshave been blended into superphosphate becauseof the importance of phosphorus and molybde-num nutrition for cropping systems containinglegumes. A solution of sodium molybdate is a con-venient source of molybdenum if deficiency is sus-pected or general spray mixtures of trace elementsare required.

Table 5.14 Products used to supply molybdenum tosugarcane.

Molybdenum source % molybdenum

Molybdenum trioxide 60.0Sodium molybdate 39.0Ammonium molybdate 54.0Mo fortified superphosphate 0.02

Chlorine

Role of chlorine

Chlorine is

� an enzyme cofactor in the Hill reaction inphotosynthesis;

� a charge compensator and osmoregulatoralong with potassium in plant cytoplasm.

Chlorine has high mobility within plants.

Symptoms of chlorine deficiency

Symptoms of chlorine deficiency are rare in fieldconditions because of the wide spread occur-rence of chlorine in the environment from marineaerosols in rainfall, chloride from irrigation water,and wide spread use of potassium chloride as apotassium source for sugarcane. Rainfall near thecoast contains 7 mg Cl/L and 2000 mm of rainfallwould provide 140 kg Cl/ha (Kingston 1993).

Young leaves are the first to show deficiencysymptoms. Leaves wilt during the day and rehy-drate at night because the daily transpiration ratecannot be maintained if there is insufficient elec-trolyte in cells to sustain the hydraulic gradientfrom soil to root cells. Leaves may be chloroticbut not necrotic.

Consequences of excess chlorine

Excess chlorine in sugarcane is usually associatedwith electrolytes acquired from saline soil or irri-gation water. Symptoms of salt or chloride toxicityinclude premature wilting, followed by necrosis ofmargins and tips of leaves.

BENEFICIAL ELEMENT

Silicon

Role of silicon

Silicon is not regarded as an essential element formost plant species but is generally regarded asbeneficial to plant growth (Epstein 2009). Mecha-nisms for the beneficial effects of silicon generally

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110 Sugarcane: Physiology, Biochemistry, and Functional Biology

are not known at cellular or molecular levels formost species. However, Fawe et al. (2001) showedthat monosilicic acid is essential to expression ofgenes that control production of phytoalexins inthe response of cucurbits to invasion by germ tubesfrom fungi. Similarly, Chain et al. (2009) showedthat silicon is beneficial and for downregulation ofstress response genes in wheat after exposure toBlumeria graminis f. sp. Tritici.

The beneficial effects of silicon are as follows:

� Greater resistance to certain fungal diseasesin rice (Datnoff et al. 1992), sugarcane (Raidet al. 1992), several horticultural species, andsome ornamentals such as orchids;

� Greater resistance to attack and exploration ofsugarcane by the stem boring insects Diatreasaccharalis F.and Eldana saccharina (Elawaldet al. 1985; Kvedaras & Keeping 2007);

� Assistance with transpiration control(Lewin & Reinman 1969; Wong You Cheonget al. 1972) and tolerance of salinity (Lianget al. 2003);

� Competition with phosphate ions for sorp-tion sites in soil (Jones & Handreck 1963),although this has been questioned by Ma &Takahashi (1991);

� Alleviation of manganese toxicity (Clements1965), where a Mn/SiO2 ratio between 30and 50 was considered desirable. The allevia-tion of manganese toxicity results from siliconspreading manganese more evenly throughthe leaf, thus avoiding localized toxic concen-trations (Jones & Handreck 1965; Vlamis &Williams 1967).

Plants acquire silicon from soil only as monosili-cic acid (H4SiO4), which is actively transported inthe xylem (Raven 2001). Silicon is deposited incells as opalized phytoliths and thereafter is notmobile (Sangster et al. 2001). Glassy trichomes onthe epidermis of sugarcane leaf sheaths are formedfrom silicon dioxide.

Symptoms of silicon deficiency

Yellow flecks on young silicon-deficient leavescoalesce and darken to form a bronze freckle only

on the surface of older leaves exposed to the sun(Color Plate 5.14). Parts of both surfaces can beaffected if leaves are twisted. Wong You Cheonget al. (1972) found that exposure to UV-B radiationwas essential to development of deficiency symp-toms. Gascho (1977) reported that symptoms arenot developed in the glasshouse. Symptoms aremost severe on eastern and northern faces of fieldsin Australia. Older leaves can senesce and die pre-maturely.

Consequences of excess silicon

No adverse effects of high or excess silicon havebeen reported for sugarcane. However, sugar-cane belongs to a family of silicon accumula-tors (Marschner 1986) and a tendency to luxuryconsumption has been demonstrated (Gascho &Andries 1974).

Management of silicon nutrition

Silicon in soilSilicon is extremely abundant in the earth’s crust(25–28%) as silicate minerals in rocks and as clayminerals and quartz in soil. Silicon in quartz isrelatively unavailable to plants. The weatheringsequence results in equilibrium between poly-meric silicon and plant-available monosilicic acidin soils. Soil solutions usually contain 1–40 mgsoluble Si/L as monosilicic acid, while concen-trations above 65 mg Si/L cause polymeriza-tion (Savant et al. 1999). Availability of silicon isreduced at pH > 7.0 (Ayres 1966) and by sorptiononto surfaces of sesquioxides in highly weatheredsoils (Brown & Mahler 1987).

Monitoring silicon nutritionThe range of extractants used to index plant-available silicon in soils includes 0.005 M sulfuricacid, 0.01 M calcium chloride, 0.5 M ammoniumacetate (pH 4.8), and 0.5 M acetic acid. Criticalvalues of silicon for response of sugarcane varywith extractant and extraction procedures. Kid-der and Gascho (1977) used a critical value of 100mg Si/dm3 for the ammonium acetate method,while Haysom and Chapman (1975) reported crit-ical values of 100 and 10 mg Si/kg, respectively,

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for sulfuric acid and calcium chloride methods.Berthelsen, Hurney, Noble, et al. (2001) suggested20 mg Si/kg as a threshold value for the CaCl2extract. The sulfuric acid method indexes solu-ble plus slowly soluble fractions of soil silicon,whereas calcium chloride extraction indexes onlythe readily available fraction at the time of sam-pling.

Leaf analysis is also used to assess silicon sta-tus of sugarcane. Response to silicon is likelyin Florida if leaf silicon is <1.0% (Kidder &Gascho 1977), but critical values as low as 0.1–0.2% are reported for Mauritius by Anderson andBowen (1990). Commercial experience in Floridashows that the tentative critical level of 1.0% can-not be maintained economically with calcium sil-icate and that a more conservative target of 0.5–0.6% can be achieved while achieving good yieldresponses (McCray & Mylavarapu 2008). Tissuelevels of silicon as low as 0.09 mg Si/kg havebeen found in Australia in an experiment on asilicon-deficient soil and where application of afilter mud/boiler ash mixture elevated leaf siliconto 0.49% and raised cane yield by 27 tonnes/ha(G. Kingston unpublished data). Results fromreplicated field experiments indicate the criticalvalue for Australian cultivars is 0.55% (Berthelsenet al. 2003).

Yield increases from application of calciumsilicate to sugarcane have been reported fromHawaii, Florida, Brazil, Australia, Mauritius,South Africa, and China (D’Hotman 1961; Ayres1966; Gascho & Andries 1974; Haysom & Chap-man 1975; Korndorfer et al. 1998; Berthelsen,Hurney, Kingston, et al. 2001; Meyer & Keeping2001; Wang & Liang 2001). Therefore, it appearsmanagement of silicon nutrition has the poten-tial to deliver yield benefits across sugar indus-tries once techniques are refined for recognitionof responsive soils, and there is wider capacity toaccess sources of plant-available silicon that pro-vide economic yield responses.

Silicon fertilizersAs indicated previously, the abundance of siliconin the earth’s crust and in soils does not directlyindicate the level of plant-available silicon in theroot zone. This paradigm can also be extended to

many of the siliceous amendments proposed foruse in agriculture. The ultimate availability of sil-icon from amendments is a function of particlesize and reactivity. In general, the finer the mate-rial the more effective it is in supplying silicon torice (Datnoff et al. 1992). Chemical reactivity ofsilicate materials is a measure of capacity to reactwith acidic soil solutions and to release monosili-cic acid. In this regard cationic silicate materialswould be expected to have higher reactivity thanmaterials with higher proportions of amorphoussilica or quartz, in that order. Direct chemicalextraction, indirect chemical extraction after incu-bation in soil, or biological extraction (bioassays)were proposed as techniques for ranking silicatesas sources of plant-available silicon (Savant et al.1999).

Calcium silicate slag, as a waste material fromelectric arc furnace production of elemental phos-phorus, is generally regarded as the standardsource of silicon for sugarcane, because of the con-temporary responses and extensive use in Florida.Approximately 150,000 tonnes of calcium silicateslag has been applied annually to organic and sandysoils in the Florida sugar industry. Calcium sili-cate slags from the steel industry were used toameliorate rice production soils in Japan (Taka-hashi 2002) and are being evaluated for sugarcanein Australia. Major sources of plant-available sili-con are summarized in Table 5.15.

Calcium silicate slag is applied initially at4–6 t/ha in Florida, where experience suggeststhere is sufficient residual value in the slag toallow reduced application rates in subsequent cropcycles (Rice et al. 2008). Calcium silicate also sup-plies nutritional calcium to sugarcane and with aneutralizing value of 65% has the capacity to assistin managing soil acidity (G. Kingston unpub-lished data). This function can be effected with-out the emissions of CO2 that are associated withuse of limestone for this purpose. Sugar indus-tries without economic access to calcium silicateslags can use cement or sugar mill boiler ashas sources of plant-available silicon (Berthelsen,Hurney, Kingston, et al. 2001). The latter prod-uct may contain more than 25% silicon, but only asmall fraction of the total silicon is plant-available.Therefore, such wastes are applied at rates of

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112 Sugarcane: Physiology, Biochemistry, and Functional Biology

Table 5.15 Typical analyses of materials used to supply silicon to sugarcane.

Silicon source % Si % P % Ca % Mg % K

Calcium silicate 17–21 0.87 31.2 0.41 0.67Cement 9.8 0.01 44.9 1.10 0.37Potassium silicate (solution) 24.5 – – – 11.0Sugar mill boiler ash 42.6 0.13 0.4 0.37 0.34Filter mud/sugar mill ash mix 39.1 0.21 0.5 0.31 0.34Thermophosphate fertilizer 13.3 8.15 21.7 8.94 0.66

25–50 dry t/ha, as opposed to rates of 4–6 t/hafor calcium silicate or cement.

TOXIC ELEMENT

Aluminum and acidity

Role of aluminum

Aluminum is neither essential nor beneficial toplant growth. It is toxic to some plant species.Aluminum toxicity causes coralline or stubbedroot growth (Color Plate 5.15), which can restrictaccess to soil moisture and nutrients (Ander-son & Bowen 1990). Elevated concentrations ofaluminum in acidic soil solution can precipitatealuminum phosphates (Jackson 1963), meaningthat phosphorus deficiency is also a consequenceof aluminum toxicity.

Sugarcane has a high tolerance of aluminumcompared to species such as maize and mostlegumes (Hetherington et al. 1986).

Management of aluminum toxicity and soil acidity

Causes of soil acidification were discussed previ-ously in the section titled “Maintaining soil fertil-ity.”

The Brazilian sugar industry relies on applica-tion of dolomitic lime, at rates derived from theCopersucar formula (Anonymous 1993), to main-tain calcium and magnesium status and counteracidity in surface soil. Effects of subsoil acid-ity on calcium status are managed by applica-tion of gypsum, if calcium plus magnesium inthe 25–50 cm zone is <0.6 cmolc/kg and alu-minum saturation is >40% (Anonymous 1993).Zambello et al. (1984) showed 40% aluminum

saturation corresponded to the 91% relative yieldcriterion for sugarcane, across a range of Braziliansoils. Responses in this situation were attributedto increased exchangeable and solution calciumand a decrease in exchangeable and solution alu-minum, the relative effects of which are diffi-cult to separate (Shainberg et al. 1989). Gypsumreduces aluminum saturation through the self-liming effect (Reeve & Sumner 1972; Sumner et al.1986) wherein the sesquioxide ligand undergoesexchange of OH− with SO4

= ions, followed byhydrolysis and precipitation of exchangeable alu-minum as aluminum sulfate.

As reported by Schroeder et al. (1995), the limerequirement to counter aluminum toxicity in theSouth African industry is based on an aluminumsaturation index (ASI), calculated as:

ASI = 100 × Al/100 g clay/(Ca + Mg + K

+ Al/100 g clay)

where all elements are expressed as cmolc(+)/kg.A threshold ASI of 20% applies to most vari-

eties and an ASI > 40% is likely to be toxic toall varieties grown in the acidic Natal Midlandsregion of the South African sugar industry. Thesulfur content of humic soils (>5% organic matterand pH < 5.3) is used to reduce the lime require-ment when managing ASI because of the beneficialeffect of sulfur in countering aluminum toxicity(Schroeder et al. 1993). ASI methodology has beenrefined to improve prediction of ASI after amelio-ration by taking account of inputs of calcium andmagnesium (Anonymous 2001).

Liming recommendations for the Australiansugar industry are not linked to specific aluminumcriteria. Lime is primarily recommended on thebasis of soil calcium status (see the section titled

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Mineral Nutrition of Sugarcane 113

“Management of calcium nutrition”). However, atarget pH range of 5.5–6.2 is also recommendedbecause aluminum solubility is insignificant abovepH 5.5 and because degradation of the soil-appliedinsecticide chlorphyrifos is enhanced above pH 6.2(Chandler 1998). The lime requirement (LR5.5)to achieve pH 5.5 is determined from the formula(Aitken 1999):

LR5.5 = 16.3 + 0.485 (org.carbon %) − 3.05

× pHI

where pHI = initial pH.This calculation is based on high quality lime

(neutralizing value of 98%, Ca of 38%, and 97%of material <0.25 mm). A higher input of lime isindicated if material is coarser or of lower neutral-izing value. If the lime requirement for calciumnutrition is greater than that required to achievepH 5.5, the lime input should be reduced to thelatter value and the additional calcium can be sup-plied from gypsum.

The lime requirement for management of soilacidity in the Hawaiian sugar industry is based onpH buffer curves to achieve soil pH in the rangeof 5.5–6.5 (L. Santo personal communication).

There is an inextricable link between man-agement of calcium and aluminum saturation.The Australian target soil calcium status of 1.5cmolc/kg corresponds to 46% aluminum satu-ration of the effective cation exchange capacityacross six soil types in a humid subtropical regionof Australia (G. Kingston unpublished data). Thisresult is similar to that reported for Brazil by Zam-bello et al. (1984).

The differential tolerance of aluminum toxi-city by sugarcane cultivars as reported for theSouth African sugar industry (Schroeder et al.1995; Anonymous 2001) is not generally recog-nized in other sugar-producing countries. It may,however, be a component of genotype x varietyinteractions reported in results of plant breed-ing experiments. Australian cultivars have beenselected in acidic field environments (pH 5.0–6.0)for several decades, whereas the prime selectionsites in South Africa generally were limed to meetdesirable aluminum saturation criteria.

The effect of soil acidity on manganese andoptions for management were discussed in the“Manganese” section (“Consequences of excessmanganese” and “Management of manganesenutrition”).

Overreliance on the tolerance of sugarcane tothe effects of aluminum and manganese toxic-ity, without regular monitoring of soil pH andsoil base status leads to a decline in soil fertility.Reduced opportunity for successful crop rotationswith legumes or horticultural species, which havemuch lower tolerance of the former metal species,has short- and long-term consequences for landmanagement and property values in some regions.Maintenance of mineral soil pH above 5.5 also hasimplications for improved availability of phospho-rus (less P sorption) and improved mineralizationof nitrogen from soil organic matter.

NOVEL APPLICATIONS OF GENETICMANIPULATION TO PLANT NUTRITION

Introduction

In previous decades, agricultural research indeveloped nations has focused on improved pro-ductivity and profit as the justification for invest-ment in plant nutrition projects. These goals arestill extremely relevant for producers of commod-ity crops, where the ongoing cost price squeezeis a major incentive for improved nutrient useefficiency (increased productivity/unit of appliednutrient).

The potential for inefficient use of nutrientsto negatively affect the off-farm environment is amore recent addition to the imperative to improveboth recovery and utilization efficiency of nutri-ents.

Less developed economies were proposed asmajor beneficiaries of the green revolution, wheregenotypes were developed to deliver improvedyield recovery in response to inputs of relevantnutrients, thereby increasing nutrient use effi-ciency. This strategy seems not to have had broadapplication, in that many subsistence economiescannot afford the critical nutrient/ameliorantinputs required for successful application of the

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114 Sugarcane: Physiology, Biochemistry, and Functional Biology

principles of the green revolution. What opportu-nities might therefore be available for interventionat a genetic level to improve nutrient use effi-ciency in developed and developing agriculturaleconomies?

Sugarcane agronomists are aware of genotype× environment (G×E) effects for nutrient useefficiency, particularly in relation to fine-tuningvariety choice and rates of fertilizer to match soilfertility, e.g., the rate of applied nitrogen after con-sideration of the nitrogen-mineralizing capacity ofthe soil, and choice of varieties for high and lowfertility locations. The interaction is recognizedonly at a gross level, as there is limited knowledgeof the relative significance of recovery and utiliza-tion efficiencies. Sugarcane breeders have begunto isolate some of the effects of nutrition on G×Eeffects, with roles identified for calcium, zinc, andmanganese (Jackson & Galvez 1997). In the past,there have been three major factors that were dis-incentives for breeders of high value crops to focuson nutrient use efficiency:

� the burdensome effect of introducing addi-tional selection characters because this wouldincrease the size of breeding programs;

� the lack of guidance on characters to be iden-tified in parental collections;

� the likely small outcome of such investmentin relation to known strategies for manag-ing nutrients. For example, it is likely to bemore efficient to correct calcium or zinc defi-ciency with management, rather than pur-posely selecting for adaptation to the severeenvironment.

Opportunity for genetic intervention in nutri-ent use efficiency is likely to improve with mat-uration of capacity for genetic manipulation at amolecular level. Genetic markers for nutrient useefficiency may then become valuable tools for plantbreeders. The potential of this tool must be usedwisely. In developed economies, the technologycould supplement balanced nutrition or improveproduct quality and reduce environmental effectsrather than be used as a tool to allow exploita-tion of finite levels of nutrients in soil. The lat-ter approach assumes the exploitative or recovery

efficiency genes will be available in all crops, ifflexibility in species under cultivation is to be pre-served for given environments.

The capacity of this technology to deliver inthe area of resource exploitation and nutrientrecovery efficiency represents a major philosoph-ical dilemma in that soil fertility will continue todecline in economies where nutrients and ame-liorants required for long-term resource main-tenance are not available. Again, for successfulimplementation the technology should not be pro-posed as a panacea, but as a tool whereby produc-tivity can be enhanced with some of the improvedproductivity being devoted to resource mainte-nance.

What are the targets?

The technology for isolating genes encoding nutri-ent transport in plants is quite new (Frommer et al.1994; Smith 1997). Although attempts are beingmade to alter expression of these traits in plants,applications for commercial field crops are not yetavailable. As mentioned earlier, integration of anet improvement into a plant may not be a simpleissue. For some nutrients, increased uptake alonemay not result in increased nutrient use efficiencybecause of potential for expressions of toxicity oradverse effects of luxury consumption.

A wish list for desirable characters in sug-arcane might include overexpression of geneswith inducible promoters for expression in tis-sues involved in targeted processes for uptakeof nutrients. For example, the high levels ofnitrogen and potassium are desirable in leavesfor the “stay green” character and osmoregula-tion, respectively, but high levels are undesirablein harvested culms. Beneficial promoters couldfavor expression in the leaf rather than in theculm, and then to downregulate in the culm inresponse to lower temperature to allow for seasonalripening. There is need to consider both uptakeand assimilation for nutrient and physiologicalbalance.

A focus on nutrient utilization efficiency islikely to produce more immediate outcomes forthe sugar industry than a focus on capacity totolerate lower levels of soil fertility. Improved

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Mineral Nutrition of Sugarcane 115

utilization efficiency could lead to improved juiceand sugar quality through higher productivity perunit of nutrient uptake. Improved efficiency ofutilization of the less mobile elements (calcium,sulfur, and silicon) could have direct physiologi-cal significance.

Down-regulation or variable expression of lowaffinity transporters, which capture nutrients fromhigh external concentrations, could have majorimplications for utilization of nitrogen, potassium,and silicon (nutrients which are subject to luxuryconsumption). Manipulation of down-regulationof nitrogen fixation by endogenous bacteria in sug-arcane could result in significant reductions innitrogen fertilizer inputs. However, the activityof the nitrogen fixation process should also besensitive to down-regulation from the effects oflower temperature or a chemical ripener becauseof the well-established adverse link between ele-vated plant nitrogen status and lower culm drymatter which results in lower sucrose percent freshweight.

There are clear opportunities for novel appli-cations of molecular technology to improve nutri-ent use efficiency. However, it appears that inte-grated knowledge of the functionality of variousoptions for modification in economic plants is notyet available. This knowledge can be obtained bycollaboration among plant physiologists, molecu-lar biologists, and agronomists for ultimate appli-cation to industry through teams of breeders andagronomists, who will combine novel techniqueswith advanced knowledge of balanced and sustain-able nutrient management.

ACKNOWLEDGMENTS

This chapter could not have been produced with-out the generous contributions of agronomistsand soil scientists from several of the interna-tional sugar industries. This input often includedprovision of unpublished data, comments on rec-ommendations, and assistance with interpreta-tion of recommendations into English. Contribu-tions from the following colleagues are gratefullyacknowledged:

� Hawaii: Lance Santo, Hawaii AgricultureResearch Center

� Florida: Mabry McCray and Ron Rice, Uni-versity of Florida, and Nael El-Hout, USSugar Corporation

� Brazil: Jorge Donzelli, Claudmir Pennatti,and Jose Felix Silva Jr, Centro de TecnologiaCanavieira

� South Africa: Jan Meyer, formerly of theSouth African Sugar Research Institute

� Australia: Bernard Schroeder and BobAitken, BSES Limited, and Garry Kuhn,INCITECPIVOT Fertilizers

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