IPIBulletin 4

2nd revised edition

International Potash InstituteBasel/Switzerland 1999

IPI-Bulletin No. 4

Fertilizing for High Yield

CITRUS

2 "d revised edition

Y. Erner, Ph.D.Dept. Citriculture, AROThe Volcani Center, Israel

A. Cohen, Agric. Eng., M.Sc. Agri.Dead Sea Works Ltd. (Emeritus)

H. Magen, M.Sc.Potash Marketing DivisionDead Sea Works Ltd.

g @ International Potash InstituteP.O. Box 1609CH-4001 Basel/Switzerland1999

© All rights held by: International Potash InstituteSchneidergasse 27P.O. Box 1609CH-4001 Basel/SwitzerlandPhone: (41) 61 26129 22/24Telefax: (41) 61 261 29 25E-mail: ipi@iprolink.chWebsite: www.ipipotash.org

Layout: Sandrine Nguyen, IPI, Basel, Switzerland.

Printing: Imprimerie Brinkmann, Mulhouse/France.

Contents

Page

i. Introd uction ................................................................. 71.1. Production trends ......................................................... 71.2. Consumption trends and outlook .......................................... 9

2. Climate, water and soil requirements ................................ 92.1. C lim ate .................................................................... 92.2. W ater ...................................................................... 102 .3 . S o il ........................................................................... 1 1

3. Crop management ...................................................... 123.1. Soil cultivation and weed control ..................................... 123.2. Irrigation ................................................................... 133.3. Fertigation ................................................................. 133.3.1. Introduction ................................................................. 133.3.2. A dvantages .................................................................. 143.3.3. Fertilizers ................................................................. 143.4. Foliar spray ................................. . . ..................... 15

4. Citrus nutrition ......................................................... 164.1. Nutrients removal ....................................................... 164.2. Nitrogen nutrition ....................................................... 174 .2.1. Function ................................................................... 174.2.2. Symptoms of N deficiency ................................................ 184.2.3. Effect of N on yield ........................................................ 184.2.4. Effect of N on fruit quality ................................................ 194.2.5. Nitrogen sources ......................................................... 204.2.6. T im e of application ........................................................ 204.3. Phosphorus nutrition ...................................................... 2 14 .3.1. Function ................................................................... 2 14:3.2. Phosphorus requirements ............................................... 214.3.3. Symptoms of P deficiency .............................................. 214.3.4. Effect on yield ........................................................... 224.3.5. Effect on quality ................................................ ....... 224 .3.6 . T ox icity ..................................................................... 234.3.7. Phosphorus fertilization .................................................. 23

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4.4. Potassium nutrition ......................................................... 234.4.1. The role of potassium in plant physiology ............................ 234.4.2. Potassium requirements .................................................... 244.4.3. Indications of K deficiency .............................................. 254 .4 .4. K rem oval ................................................................... 254.4.5. K effect on tree growth ................................................ 254.4.6. K effect on yield ............................................................ 254.4.7. Effect of K on fruit quality ................................................ 274.4.7.1. External and internal fruit quality ....................................... 274.4.7.2. Fruit splitting ............................................................. 274 .4 .7.3. Fruit size ..................................................................... 284.4.7.4. Storage properties .......................................................... 30

4.4.8. Potassium fertilization .................................................. 304.4.8.1. Soil factors ............................................................... 304.4.8.2. C ultivar - rootstock ......................................................... 304.4.8.3. Sources of potassium 3....................................................... 04.4.8.4. Methods of application .................................................... 31

4 .5 . C alciu m ..................................................................... 314 .6 . M agnesium .................................................................. 3 1

4 .7 . S u lphu r ....................................................... ......... .. 32

4.8. Micro-nutrients deficiencies .............................................. 33

4.8.1. B oron ........................................................ . . ........ 334.8.2. C opper .................................................................... 334 .8.3 . Iron ....................................................................... ..334 .8.4. Z inc ....................................................................... 344.8.5. M anganese ............................................................... 344.8.6. M olybdenum ................................................................ 35

5. L eaf ana lysis ............................................................... 35

5.1. Factors influencing leaf nutrient content ............................ 355.1.1. Species, variety, rootstock ................................................ 355.1.2. Interaction between elements ............................................ 36

5.2. Sam pling for analysis ...................................................... 37

5.3. Methods of analysis ..................................................... 385.4. Leaf analysis standards ................................................ 405.5. Interpretation of leaf analysis ......................................... 40

6. Fertilizer recommendations in selected countries ................. 41

6 .1. A rgentina .................................................................... 4 16.2. B razil ..................................................................... 42

6 .3 . C alifo rn ia .................................................................... 4 5

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6.4. C hina ...................................................................... 456.5. Florida ..................................................................... 456.6. India .................................................................... .4 66.7. Israel ...................................................................... 496 .8. M orocco ..................................................................... 50

7. References ............................................................... 50

8. Appendix: Visual symptoms of nutrient deficiency ................ 57

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1. Introduction

Citrus is grown in more than 80 countries all over the world and is one of themost widely produced fruits, Total production exceeds that of all others,including bananas and deciduous fruits. In 1996 world production reached93.7 million tons and is expected to increase further (Table 1). It is aneconomically important crop in terms of employment and makes a significantcontribution to farmers income.

Table I. World citrus production from 1980 to 1996 (ton).

Citrus type 1980 1985 1990 1996

Oranges 39,993,280 40,686,860 49,466,900 59,558,270Tang., Mand., 8,501,715 9,856,073 13,005,350 15,954,490Clement., SatsumaLemons and Limes 5,161,761 6,288,361 7,258,396 9,103,760Grapefruit and Pomelo 4,523,036 3,817,633 4,078,931 5,003,501Citrus Fruit Nes 2,838,805 3,249,884 3,833,973 4,128,575Total 61,020,577 63,898,811 77,643,550 93,748,596

Source: FAO database on the Internet. Production from 1996.

1.1. Production trends

Production grew by 6% p.a. during the 1960s and by 3% in the 1970s;between 1980 and 1996 total production increased by 50%. The increase since1985 was mainly in developing countries (+30%) While production indeveloped countries declined in absolute terms. Developing countries nowaccount for 65% of global production compared with 50% a decade earlier. Itis a labour intensive crop mostly required for the picking operations.The rate of growth has differed between geographical areas. There has beenmuch growth in South America, mainly in Brazil, and also in Mexico, Cuba,Argentina and Uruguay. In the Near East, Egypt and Turkey have shown anincrease while Morocco in North Africa has achieved substantial growththough the area has remained constant. In Asia the most spectacular growthoccurred in China where production increased nearly five times in the eightiesto reach 5.5 million tons in 1989-90. Most of the production in China andother Asian countries is taken up by the domestic markets (Table 2).In the developed countries, there has been a decline in Israel and Japan andthere was a significant, though temporary, decline in Florida (USA) followingsevere frosts late in the eighties. In Spain production has remained steady forthe last few years.

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Table 2. Production and export of main citrus producers (10' MT).

Country Oranges Tangerines* Lemons** Grapefruit***Production Export Production Export Production Export Production Export Production

Total 59,558 4,436 15,954 1,995 9,104 1,268 5,003 1,132 89,619Argentina 758 84 462 27 713 111 202 36 2,135Australia 436 98 71 32 5 25 564Brazil 21,811 114 760 495 1 62 23,128China 2,257 16 5,992 115 216 8,395Egypt 1,608 42 475 330 13 2 2,415Cuba 275 16 18 2 261 90 570Greece 850 257 85 16 125 32 10 1,070India 2,000 1,700 70 3,770Iran 1,600 60 630 655 3 58 2,943Israel 380 200 130 30 27 3 404 108 941Italy 1,597 122 450 30 545 56 1 2,593Japan 136 1,199 6 1,335Korea Rep. 600 600Mexico 3,555 375 4 1,000 169 240 5,170Morocco 972 253 403 161 20 3 1,398Spain 2,153 1,361 1,414 1,108 436 324 28 4,031S. Africa 735 350 22 62 38 162 38 959Turkey 700 90 450 116 325 141 50 46 1,525USA 10,635 569 500 33 948 137 2,465 496 14,548

* including mandarins, clementines and satsumas; ** including limes; *** including pomelo.

Source: FAO database on the Internet, Production from 1996, Export from 1995.

1.2. Consumption trends and outlook

Oranges and tangerines are still the most important varieties accounting for80% of total output. Oranges also account for more than 80% of the citrusprocessed by the industry followed by grapefruit (8%) and tangerines andlemons (5% each, Table 3). Juices are the main processed products, frozenconcentrated orange juice (FCOJ) being the most important item ininternational trade.Total fresh fruit consumption increased by 2% in the 1980s but has declinedon a per capita basis. A major factor of this decline in developed markets hasbeen the higher relative price of fresh citrus compared with other fruits such asbananas, apples and grapes.There is some concern about the longer term market outlook for fresh andprocessed citrus as there seems to be a possibility of.over supply around theyear 2000, while some major markets like western Europe are already over-supplied in winter. The growth in demand will remain stronger in developingcountries although consumption will be greater in the high income markets.

Table 3. Major producers and exports of orange juice concentrated.

Country Production 10' MT Export

Brazil 961 1146Israel 34 21Morocco 9South Africa 20Spain 16USA 1120 88

Source: FAO database on the Internet.Production from 1996, Export from 1995.

2. Climate, .water and soil requirements

2.1. Climate

Citrus is an evergreen, cold-sensitive plant and low temperature is the mainfactor restricting its geographical distribution. Thus climate largely determinesits geographical distribution and the crop is more or less confined to a bandbetween 40'N to 40'S latitude. Because of variations in climate and altitudethis band becomes broader or narrower in different parts of the world.Conditions in the tropics are not optimal for production of high qualityoranges and mandarins. Most major citrus growing areas are concentrated,therefore, between 250 and 35' north and south of the equator.

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Temperature is the most important climatic determinant in citriculture. At oneextreme, frost may be a threat to whole orchards or may restrict yield; at theother, high temperatures may endanger fruit-set and interfere with overall treeperformance, presumably by excessive respiratory losses. Within the non-lethal range temperatures are also the most significant climatic factor, withclear quantitative effects on most physiological processes. A temperature dropbelow 1 0°C may cause limitations to growth and productivity.Day length is a predominant factor in plant development and photo-morphogenetic effects have been recorded in numerous plant species. But thesearch for daylength effects in citrus has not revealed any striking responses.Rooted citrus cuttings behave as quantitative short day plants with respect toflowering at a mean temperature of 21.5°C. However, at higher or lowertemperatures, day-length does not affect flowering. Stem elongation inseedlings was enhanced under long days, as in most other higher plants.Light intensity is an important determinant of plant growth and activity. Shadeunder the canopy may be so extreme that only 0.5% of the total irradiationreaches ground level. Such conditions do not permit normal leaf activity andboth vegetative and reproductive growth are completely inhibited. This willinevitably lead to abscission and die back of branches. On the other hand, full,high-intensity sunlight is also inhibitory for vegetative growth. Partial (up to50%) shading promotes vegetative growth as is the practice in citrus nurseries.On the other hand, flowering is more abundant in the well lit parts of thecanopy.The citrus tree, being evergreen has no resting period and requires a supply ofwater throughout the entire year. The appropriate choice of rootstock canameliorate the effects of drought and frost and other marginal conditions.Temperature requirements: Minimum I 0C-Optimum 20°C-Maximum 35°C.

2.2. Water

In many growing regions where there is today insufficient or unsuitabledistribution of rainfall, the supply of water becomes a limiting factor inmodem citrus cultivation. Hence, irrigation is common in citrus growing areas,except in the humid tropics. Higher fruit yields are obtained in irrigateddistricts than in the best rainfed areas.Chapman (1968) considered that a content of 500-700 ppm total soluble saltsin the irrigation water poses a danger of salt damage to the foliage. Intensivefertigation reduces the risk of salt damage to the tree by reducing mineraldeficiency stress and giving better tree growth (Dasberg et al., 1991).Several authorities recommend using the chloride content as an index of watersalinity. In Israel, a good correlation was found between the chloride (meq V')

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and total salt (as conductivity, in terms of dS m') content of water, and theaccumulation of chloride in the soil profile - the tolerance limit for sensitivecitrus rootstock was found at 10 meq I chloride in the saturation extract(Yaron, 1969).Citrus is a salt-sensitive plant in which the accumulation of Cl and Na in thetree can be related to a specific toxic effect. Sodium chloride occurs in manysoils and irrigation water and chloride is a component of many commercialfertilizers. Therefore, chloride is rarely reported as mineral deficiency in citrustrees. On the contrary, salinity problems related to chloride have been reportedmost often in arid and semi-arid regions when the amount of water appliedwas insufficient for leaching the chloride below the root system.Chloride content in leaves has long been used as an index oftolerance/sensitivity to salinity. It should be emphasised that rootstocks differconsiderably in their ability to take up minerals. Cleopatra mandarin is knownas the most tolerant of chloride in irrigation water but not tolerant to sodiumuptake which can be harmful to the scion. Rootstocks tolerant to chloride arein the following order: Cleopatra > Rangpur > Sour orange > Lemons >Trangpur > 812 (Sunky x Benecke) > swingle Citrumelo > Citrange >trifoliate. Rootstocks of mandarin types are sensitive to sodium (includingCleopatra) (Sagee, 0. and Shaked, A., pers. comm., Castel, 1987).

Table 4. The chloride hazard in irrigation water for citrus in Israel.

Soil textureEC dS m' Cl (meq I') Sandy Loamy Clay1200 <6.0 + - + - + -1200-1500 6.0-7.5 + - + - + +1500-1750 7.5 -9.0 +- +- +++1750-2250 9.0-15.0 + - + + . . . .

+ - = no danger or very low risk; + + = low risk; ± . . = medium risk;++ ++ = dangerous.

2.3. Soil

Citrus plants have a shallow root system; their nutrient absorption capacity israther low due to the limited number of root hairs. For this reason, relativelylight, well aerated rich soils are preferred. Heavy soils are less suitable forgrowing citrus because of poor aeration. The difficulties posed by marginalun-aerated, heavy soils can be overcome by planting on high (at least 50 cm)beds.

II

The main citrus areas are situated in deep well-drained sands, sandy oams,

loams and clay oams. Sands and sandy oams with good physical propertiesare preferred.As regards soil reaction, citrus will grow within the range of pH 4-9. In sandy

soils with low buffer capacity, plant nutrient seems to be more available at pH

values between 5.5 and 6. Regulating the pH value, in acidic soil, by gypsum,

ground limestone and dolomite improves fertilizer efficiency and increases theyield.

3. Crop management

3. 1. Soil cultivation and weed control

Ploughing or deep cultivation should be avoided. Moreover, the trend of "no-

till" in which very little soil cultivation is practised is preferred in most of the

citrus growing areas. Soil cultivation to control weeds should be as shallow as

possible, in order to avoid damaging the roots. In rainy areas with steep slopes,weeds are used to avoid soil erosion.In some areas, planting legumes during the rainy season will reduce erosion.

Green manuring plants must be worked into the soil and buried before the end

of the rainy season.

The gains from ground cover plants, according to Jones & Embleton (1973),are:- Increase of the soil organic matter.- Lowering soil temperature- Control of erosion and nitrogen losses.- Better availability and distribution of soil nutrients.

Weed competition for water and nutrients is most important in subtropical arid

zones. Severe weed competition may reduce yield and inhibit young tree

development. Care is needed in the use of herbicides since temperature,rainfall, wind, soil type and stage of development of weeds affects herbicide

efficiency. It is best to aim for complete weed eradication throughout the year

through clean cultivation or by means of herbicides. To achieve a clean area,

herbicide should be applied before and after the rainy season. Ground cover by

maintaining a grass sward or the sowing of green manure crops can be

effective.Generally the inter-cultivation of cash crops should be avoided; though it may

be tolerated for small growers on young citrus plantations. In this case the

fertilizer application should be increased accordingly.

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3.2. Irrigation

Ackerman (1938) stated the optimum water requirements to be 1900-2400 mmper annum, and that the absolute minimum should be 1270 mm, whileexperiments done in the last 10 years indicated that the minimum can be aslow as 1000 mm per annum, including rainfall. A dramatic economy of waterhas been made possible by water use efficiency-management, control devicesand by reducing the amount of leaching. The quality of water plays animportant part because citrus shows little tolerance to salts; it may benecessary to apply some additional water to leach out the salt from the rootzone.

Irrigation requirements are calculated by the evaporation measured in a ClassA pan. A crop irrigation factor of 0.5 to 0.8 is used to calculate the amount ofwater given to an area unit of mature orchard.

Irrigation methods used are surface, furrows, sprinklers, micro jets and drip.The trend of partial wetting irrigation is becoming more popular due to ahigher irrigation efficiency.

3.3. Fertigation

Fertigation is defined as the application of solid soluble or liquid fertilizersthrough pressurised irrigation systems, thus applying irrigation water andnutrients at the same time. The development of Micro Irrigation System (MIS)including drip, mini-sprinklers micro jets was a major factor in the promotionof modem fertigation in many countries..

3.3.1. Introduction

The development over the past 20 years of modem irrigation systems with

partial wetting of the root zone have made fertigation a popular practice

especially because it also makes possible the injection of nutrients at accurate

quantities and rates through the water for defined periods in each irrigation.Experiments done with partial wetting of the root zone showed that in order to

get high yield and good quality, up to double the amount of the macro-elements applied by MIS should be given (Bielorai et al., 1984). This positiveand significant response to the fertigation method is due to better uptake and a

reduction in minerals losses beneath the root system. The increment of nutrient

application is designed to compensate for the limited supply of nutrients in thepartially wetted soil volume.

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3.3.2. Advantages

* Uniformity of fertilizer application in the soil volume receiving water;* Better efficiency due to improved mobility and availability of nutrients in

the wetted root zone;* Fertilizers can be applied according to the uptake curve of the plant;* Reduced soil compaction - elimination of heavy mechanical tools;* Fertilizers can be applied at a "multi -dose" way with no labour involved.

3.3.3. Fertilizers

Fertilizers for fertigation use should have the following properties:* High solubility;* Quick dissolution in irrigation water at ambient temperature;* Non-clogging mineral and bacterial insolubles;* High nutrient content in the saturated solution;" No negative reaction with the irrigation water (precipitation, pH, EC...)Single and multiple nutrient fertilizers suitable for fertigation are listed inTable 5 (Magen, 1995).

Table 5. Fertilizers suitable for fertigation in their solid and saturated solutionsat 100C (500F).

Nutrient Compound Nutrient content N-P 2O5 -K20 contentin solid fertilizer in saturated solution

(100 C)

N 1. Urea 46-0-0 21.3-0-02.* Ammonium nitrate 33-0-0 21.4-0-03. Ammonium sulphate 21-0-0 8.8-0-0

P I.** Phosphoric acid 0-61-0P& N 2.* Mono ammonium 12-61-0 2.8-13.8-0

Phosphate (MAP)P& N 3.* Di-ammonium 18-46-0 8.2-20.6-0

phosphate (DAP)K 1.* Potassium chloride 0-0-60 0-0-14.9

2. Potassium sulphate 0-0-50 0-0-4.6K& N 3. Potassium nitrate 13-0-46 2.4-0-8.1K& P 4. Mono potassium 0-52-34 0-7.7-5.2

phosphate (MKP)"Fertigation Grade" (with very low insoluble compounds).

** Liquid product.

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There are two current methods to inject fertilizers into the irrigation systems:

(I) by pressure differential using solid or liquid fertilizers in a by-pass tank;(2) using only liquid fertilizers with a vacuum pump (Venturi) or displacement

pump.

These methods differ in their price, ease of operation, discharge rates,automation, etc.

Table 6. Irrigation and fertigation recommendations of young citrus in Israel.

N K2OAge Irrigation Amount per day (liter tree-') Total g tree -' g tree -'Year Interval day April May June-Nov. m3 ha-' d-1 d-1

1 3-7 3- 5 3- 5 4- 7 1250 0.2 0.22 3-7 6-10 7-10 8-15 2000 0.4 0.43 3-7 8-15 12-20 15-25 4000 0.6 0.64 5-7 10-20 20-25 28-32 5000 1.0 1.0

Remarks:a. 5 th year and on - fertigation according to adult trees.b. Water: 1. At planting - 100 liter tree-' should be applied.

2. Trifoliate, Troyer, Citromelo and Rangpur rootstocks aresensitive to excess water; therefore use of the tensiometers formonitoring soil moisture is recommended.

c. Fertilizers: 1. During years 1-4, apply over the whole irrigation season.2. N:K 2 0 ratio should be 1:1 and 1:0.6 in light and heavy soils,respectively.3. Phosphorus will be given according to soil test.

3.4. Foliar spray

Generally speaking, foliar spray is mainly used for emergency treatment oftrees with micro-element deficiencies (chapter 4.6.), increasing fruit size,decreasing peel disorders and to apply hormones. In some cases of over-production (Murcott, Willking and other mandarin types), NPK can be appliedas a foliar spray as emergency treatment to prevent tree collapse. The use offoliar spray has been also suggested for the prevention of groundwaterpollution (Embleton, personal communication). Spraying potassium nitratetogether with auxins has been found to increase fruit size and decreasesplitting (Erner et al., 1993, Lavon et al., 1992). Timing for spray was foundto be best 6-8 weeks after full bloom.

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4. Citrus nutrition

Citrus trees require for their growth and for fruit production large quantities ofmineral nutrients which are found partly in the soil. Such large quantities ofnutrients have to be replaced in order to maintain the soil fertility level and topermit the continuous production of high yields.However, in order to achieve an intensive production of fruits, it is necessaryto follow, simultaneously, all the practices aimed at improving the soilproductivity - mainly the maintenance of a good physical soil condition,irrigation, drainage and phytosanitation.

4.1. Nutrient removal

According to various sources (Table 7), one ton of oranges would remove1.18-1.90 kg of N, 0.17-0.27 kg of P, 1.48-2.61 kg of K, 0.36-1.04 kg of Caand 0.16-0.19 kg of Mg. Average ratio: N:P 20:KO = 3:1:5.Among nutrients removal, calcium is the most important mineral in vegetativeparts while potassium is the dominant mineral in fruits.

Table 7. Nutrients removed in kg per ton of citrus fruit.

Author N P K Ca Mg

Smith and Reuther (1953) 1.29 0.20 1.87 0.36 0.18Chapman (1968) 1.18 0.27 2.61 1.04 0.19Labanauskas and Handy (1972) 1.85 0.17 1.79 0.78 0.17Golomb and Goldschmidt (1981) 1.85 0.18 1.48 1.02 0.16Feigenbaum et al. (1987) 1.90<

The quantities of nitrogen and potassium found in the fruit increase steadily upto maturity; consequently, they are absorbed regularly during the entiregrowing season and should be supplied accordingly. The phosphorus andmagnesium contents increase during early development of the fruit and thenremain constant. Lastly, calcium is absorbed only during the first third of thegrowing period of the fruit. Its translocation is limited due to wax formation inevaporative organs (stomata) which limits xylem movement.Varieties bearing fruit in an alternate sequence ("ON" and "OFF" year),present a significant difference in their mineral removal.More potassium and phosphorus is removed in the fruit of "ON" tree of easy-to-peel varieties, like Wilking mandarin, than the totals contained in otherparts of the tree (Golomb and Goldschmidt, 1981). Nitrogen, magnesium andcalcium in fruit were found to comprise one half, one third and only 6%

16

respectively of the totals for these elements in the tree. Clearly nutrientremoval in harvested fruit is very considerable, emphasising the importance ofproper fertilization to produce commercial yield.Fertilization should supply not only the nutrients removed by the harvestedfruit but also those needed for vegetative growth and the quantities lost byerosion and leaching. Growers should avoid loss by leaching and erosion or atleast reduce it to a minimum.Fertilization programmes based on nutrient removal and soil analysis alone arenot always successful in ensuring the correct amount and relative proportionsof plant nutrients. This is demonstrated by the frequent occurrence ofdeficiency symptoms even though fertilizers may have been used.

4.2. Nitrogen nutrition

4.2.1. Function

Nitrogen is a major constituent of many organic compounds, includingproteins, amino acids, chlorophyll, alkaloids and others. As such it plays asignificant role in determining the growth and yield of the tree. In citrus,nitrogen occurs mainly in organic compounds with only traces of ammoniaand nitrate N. Nitrogen is absolutely essential for vegetative growth and fruitproduction. The foliage which represents only 7.3% of the total weight of thetree, contains 22.4% of the total nitrogen (Table 8).

Table 8. Dry matter and nitrogen content of different vegetative organs in'Shamouti' oranges (Feigenbaum et al., 1987).

Plant part dry weight N

% of totalLeaves 17.3 22.4Fruits 13.3 20.4Stem and branches 55.4 41.8Roots 24.1 15.4

In an experiment with Marsh grapefruit, the number and size of leaves andtheir longevity on the trees were all increased by N application but there wasno significant interaction between N rate and timing of application (Smith,1969).

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4.2.2. Symptoms of N deficiency

a) Development of the plant is slowed down, and the formation of new shootsand new leaves is reduced, followed by reduction in flowering and fruitproduction.

b) Appearance of pale-coloured leaves, less chlorophyll, followed byyellowing of leaves.

c) Severe deficiency will lead to leaf abscission and die-back of branches.

4.2.3. Effect of N on yield

Wherever citrus is grown commercially nitrogen is usually applied. Forconsistent production and high yields of fruit, nitrogen fertilization is alwaysnecessary.

Mature trees seem to require from 100-300 kg N ha"' depending onenvironmental factors of growth, irrigation system and yield; the higher theproduction the more nitrogen is needed. Rates above 250 kg N ha' areconductive to inefficient use of nitrogen although, with nitrogen applied bylow volume irrigation (fertigation), there may be further yield increase up toover 300 kg N ha'. A yield of 40 M.T. per ha, would remove about 50 kg of N(Smith, 1966a).

The yield response of citrus to increasing N rate seems to take the form of aMitscherlich curve. Smith (1966a) reported an increase of 12% in yield ofMarsh grapefruit with increasing rate of nitrogen from 0.5-0.7 Kg per tree. InBrazil, 250g N per tree resulted in an increase of 32% in yield, whereas 500gN per tree gave no further increase (Rodriguez and Moreira, 1969). In Israel,750-900g of N per tree in low volume fertigation still increased yield withsignificant reduction of N movement beyond the reach of the root system(Bielorai et al., 1984; Bravdo et al., 1992). A recent review on nitrogenfertilization in citrus showed that 200 kg N ha' applied annually is sufficientto sustain good citrus yields and tree development (Dasberg, 1987). On theother hand, in Israel, application of nitrogen over 200 kg ha' by fertigation in3 separate experiments, showed an increase of nitrate concentration in leavesand positive correlation with yield (Figure 1). Experiments with "5N labelledfertilizer showed that the highest N uptake rate occurred during fruit-settingand that uptake was very slow in the winter. Nitrogen reserves in the oldertissues played an important part in the development of new leaves and flowersin the spring (Feigenbaum et al., 1987; Kato, 1986).

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too 300.4- Bet -Dogan

,- Nordia

a..- Emek Hefer

80 .

6C Z

---- zo a260 1

aZ

40

1O0 -1

20> 40 0

0 I I #

0 100 200 300 400N kg ho"

Fig. I. Effect of fertigation on nitrate accumulation in leaves and yield.

4.2.4. Effect of N on fruit quality

While the effect of N on the yield is quite straightforward, its effect on fruitquality is much less certain. Jones and Embleton (1968) reported that summer-applied N reduced fruit quality of dessert grapefruit. A later summary byEmbleton et al. (1973a, b), stated that rate and timing of N application affectedfruit quality, mainly in rind appearance and that the effect differed betweenforms of N fertilizer. This was confirmed by Legaz et al. (1992) who foundthat rind thickness and % acidity increased with nitrate whereas maturity ratiodecreased. Work in Australia (Shorter and Cripps, 1970) showed thatexcessive use of N had an adverse effect on rind texture, increasing rindthickness and reducing the juice content of oranges.In Florida, the response of Temple oranges to increasing the rate of nitrogenresulted in reduced fruit size, lower soluble solids and acid content of juice.The peel of fruits from higher N plots were distinctly greener and coarser thanthose produced on lower N plots (Calvert, 1970). In Israel, high nitrogen levelsadversely affected colour and increased peel thickness (Bielorai et al., 1984).Smith (1966a) showed that grapefruit and oranges manured with organic Nhad lower soluble solids and acid contents than fruit grown with a mixture ofinorganic N sources. In addition, Sites (1951), among others, mentioned thatthe application of ammoniacal N increased the soluble solids content inoranges as compared with nitrate fertilization. This effect is of economicimportance in areas where citrus is grown mainly for juice.

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4.2.5. Nitrogen sources

Citrus can utilise various forms of inorganic and organic nitrogen and theirconversion products present in the soil. The major forms of inorganic nitrogenin soil are nitrate (NO3 ) and ammonium (NH4). Though citrus trees absorbmore ammonium than nitrate in water culture experiments, nitrate nutritionwas better than ammonium for tree growth (Yokomizo and Ishihara, 1973).Recently, ammonium nitrate fertilizer was found significantly superior asregards yield increase over ammonium sulphate, urea or calcium nitratefertilizers (Lavon et al., 1995a).Alternative sources of nitrate are: calcium nitrate, Chilean sodium nitrate,potassium nitrate and ammonium nitrate. Ammonium sources are: ammoniumsulphate, mono-ammonium phosphate, di-ammonium phosphate andammonium nitrate. Ammonium ions derived from ammoniacal fertilizers mayreduce pH of soil due to: 1) nitrification process, and 2) absorption ofammonium by the tree. Therefore, total ammoniacal fertilization should beminimized in low pH soils.Urea is usually considered to behave as an ammonical nitrogen source.Synthetic urea is commonly used as a source of N; it is relatively cheap andcan be applied on the soil or as foliar spray. It might contain a smallpercentage of biuret which has some toxic effects in citrus. Spray grade ureashould not contain more than 0.25% biuret and 2.5% is the maximum for soilapplication (Smith, 1966a).Organic materials such as animal manures and plant residues are important asnitrogen sources. Long-term experiments in Israel showed no advantage oforganic manure over other nitrogen sources (Bar-Akiva, 1965) and the use offarmyard manure has been almost completely eliminated from citrus orchardsin Israel unless there is a specific problem like excessive boron.

4.2.6. Time of application

The efficiency of N uptake does not seem to be greatly affected by the timingof application, but leaching is a major factor leading to low efficiency.Splitting the N supply into 3-6 equal applications did not increase the averagecontent of leaf N over the concentration that resulted from one singleapplication (Reuther et al., 1957). Embleton and Jones (1969) reported that asingle application of N in the winter resulted in better orange and grapefruitquality than split or total application at other times of the year. On the otherhand, a recent experiment in Israel comparing fertigation with broadcastapplication to mature 'Shamouti' orange trees showed a significant advantageof fertigation (Lavon et al., 1995a). Measurements of nitrate in sandy loamsoil, one month after fertigation, showed negligible levels of nitrate. The

20

spring flush following nitrogen applied in the previous season showednitrogen levels in leaves to be correlated with the amount of N applied byfertigation; an adequate level of nitrogen during the spring and summer leavesa reserve sufficient for the next year's flush without any need for winternitrogen application. Heavy rain, low root activity and uptake, might causenitrogen losses and underground water pollution (Erner et al., 1983).Split applications with almost every irrigation are recommended for youngtrees, especially with low volume systems, to enhance tree growth and earlyproduction (Ener, personal information).

4.3. Phosphorus nutrition

4.3.1. Function

Phosphorus, taken up in much lower quantities than N and K, is however animportant nutrient for citrus. P is a component of nucleo-proteins, enzymesand lecithin. It performs an essential part in respiration, photosynthesis(Fredeen et al., 1989) and in the formation of reproductive organs (Wallace etal., 1966) such as fruits and seeds (Chapman, 1968; Marschner, 1995).

4.3.2. Phosphorus requirements

Citrus trees have a fairly low requirement for P. A ton of fruit contains onlyabout 200 g and an average yield of 40 tons will remove about 8 kg ha -'. Thisdemand is met by most of the citrus soils and the fertilization rates of citruscrops used in the major areas. Except for South Africa and some parts ofFlorida, application rates of P are quite low (Embleton et al., 1973a,b; Smith,1966a). Some varieties (e.g. grapefruit) frequently tend to be low in leaf P andapplication of phosphorus or poultry manure increased yield and Pconcentration (Bar-Akiva et al., 1968).

4.3.3. Symptoms of P deficiency

P deficiency is rarely found in citrus, even in unfertilised orchards. Deficientleaves lack lustre, appear a poor green to bronze colour and are also small insize. The tree sheds the leaves from young shoots and takes on an unhealthyappearance. Growth and fruiting are reduced; early shedding of fruits and lowyield occur frequently (Chapman, 1968; Embleton et al., 1973). The leaves ofP-deficient trees had a low P content and abnormally high N concentration.Phosphorus application led also to a decrease of Cu, and to increase in Ca andMn, in leaves (Bar-Akiva, 1968).

21

4.3.4. Effect on yield

In California, Embleton et al. (1973b) reported that numbers of orange fruitharvested and volume yield can be increased by increasing phosphorus level inthe leaves. Phosphorus applied at the rate of 4.35 kg P205 per tree increasedthe yield of lemons up to 60%. Embleton et al. (1952) checked the effect ofphosphate fertilizers on Valencia oranges and found that application of 4 kg ofP20 5 as superphosphate resulted in the highest yield (increase 50 kg per tree).A slight increase in fruit size can be expected as leaf phosphorus increasesfrom 0.10 to 0.15%. On the other hand, in South Africa, phosphorus andnitrogen fertilization over a period of 10 years showed 57% increase in yieldand fruit size against a negligible effect with N only. Some explanation for thisuntypical result is that most citrus soils of South Africa are deficient inphosphorus. In Florida, the same effect was observed on Valencia oranges byForsee and Neller (1944). In Brazil, application of about 140 kg P205 ha -' to P-deficient soils increased the yield (Cantarella et al., 1992).Heyman-Herschberg (1956) found indications of a yield response in one groveof Shamouti oranges and noted a fruit size and quality effect similar to thosereported by others. In several groves the use of P decreased the yield,apparently because of an antagonistic depression of N uptake.

4.3.5. Effect on quality

De Cicco et al. (1988) found that high phosphorus concentration in the fruit ofNavelina orange increased splitting; a high K:P ratio produced a thicker peelwhereas a low ratio decreased the peel thickness. Lavon et al. (1995b) foundthat foliar spray of mono-potassium phosphate significantly increased the Pconcentration in the leaves while soil application in most cases had little effect.Moreover, the spray improved fruit quality, decreased peel thickness andincreased juice content. The effect of P on fruit quality is quite consistent(Smith, 1966a; Chapman, 1968; Embleton et al., 1973b) and may besummarised as follows:

a) Fruit size is not affected or slightly decreased.

b) Both de-greening and accentuated re-greening have been noted.

c) Rind thickness is decreased.

e) Soluble solids content is not affected or slightly increased.

f) Juice acidity is consistently reduced.

g) Vitamin C concentration is lowered in the juice.

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4.3.6. Toxicity

There is no record of direct P toxicity in citrus. Rates higher than 150 kg ha'reduced the quantity of feeder roots in the top soil (Smith et al., 1963). Thereare several reports that the toxicity of high rates of superphosphate isassociated with the high acidity induced in the soil solution followingapplication. In some cases the toxicity is attributed to the activation ofaccumulated Cu at low pH. Iron Chlorosis, Zn deficiencies and Cu deficiencyhave all been associated with heavy P fertilization (Smith, 1966a; Embleton etal., 1973b).

4.3.7. Phosphorus fertilization

The need for phosphate fertilizers on P deficient soil should not be neglected.In order to avoid production of coarse fruits (high potassium and nitrogen andlack of phosphorus), the P dressings should be increased as the trees age.The pH of the soil should be taken into consideration in the choice of Pfertilizers. If pH is lower than 6.5, preference should be given to ground rock-phosphate or to basic slag. Superphosphate should be used on neutral oralkaline soils. P fertilizers, when applied on the soil surface, were lesseffective than ploughed into a depth of 40-60 cm, or when applied as asolution with the irrigation system (fertigation). More recently, foliar spray of20-20-20 (N-P 2O5-K20) (Reuveni et al., 1983) and of mono-potassium-phosphate (Lavon et al., 1995b) increased P concentration in leaves, treegrowth and improved fruit quality.Phosphorus is easily fixed by iron and aluminium compounds under acidconditions, as in the soils of tropic regions. Leaching of phosphate is lowcompared with other nutrients. With the use of continuous heavy dressings,large reserves of P can be built up in the soil, sometimes impairing theavailability of other nutrients.

4.4. Potassium nutrition

4.4.1. The role of potassium in plant physiology

Potassium is the most important cation not only in regard to its content in thefruit but also with respect to its physiological and biochemical functions suchas:

a) Enzyme activation; a most important function of K in plant growth.b) Cell division and growth of young tissues.c) Synthesis of carbohydrates, proteins and oil.d) Transport of sugars through the phloem using ATPase as a source of

energy.23

e) Water use: uptake of water by the roots and regulation of transpiration.f Higher tolerance to stress conditions due to drought, salinity, frost and

diseases.g) Regulating ionic balances in the tree.

4.4.2. Potassium requirements

Normal vegetative growth of citrus can occur under a wide range of K contentin the leaves (Smith, 1966a). Various authors found that vegetative growth isnot affected by K within the range of 0.40 to 2.40% in 4-7 month old leaves ofValencia and Navel oranges. Uptake rates are low in winter and highest inspring and summer.Some results on the N and K content of mature citrus trees and theirdistribution are shown in Table 9. These data are adapted from Dasberg (1988)who showed that the total amount of K in the tree is somewhat lower than theamount of N. However, K is more abundant in the fruit.Large amounts of N and K are stored in the trunk and branches. Drasticchanges in the K content of different organs occur with biennial bearing. Kmoves from leaves, trunk and roots to the developing fruits in the "On" year.It is essential to apply potassium fertilizers to replace K removed in the fruit,to improve fruit quality and to maintain soil productivity.

Table 9. Nitrogen and potassium distribution in mature citrus trees.

Grapefruit* Wilking** Shamouti***"On" " Off' High Low

Plant parts N K N K N K N N

g tree-Total 2061 1920 858 490 j 811 305 2379 2072

percentFruits 6 7 32 52 - - 20 11Leaves 18 19 14 2 26 23 22 20Trunk and 33 37 44 39 52 55 42 44branchesRoots 43 36 10 6 22 22 15 25

Tree age 19 years old (Barnette et al., 193 1).** Tree age 15 years old (Golomb and Goldshmidt, 1981).*** Tree age 20 years old (Feigenbaum etal., 1987).

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4.4.3. Indications of K deficiency

a) Retardation of growth and leaf size.b) Excess leaf drop in the spring following flowering.c) Foliage loses chlorophyll and looks bronzed or yellowish.d) Dieback of weakened twigs.e) Twisting, curling, puckering and cupping of leaves.0 Rind texture smooth and peel thin.g) Fruit creasing and splitting.

4.4.4. K removal

Significant amounts of potassium are removed in the fruit, a point which meritsattention especially following years of heavy crops when action should be takento restore the K status before any deficiency symptoms appear in the tree.According to various sources, one ton of oranges exports an average of 2.5 kgK2O, corresponding to 125-250 kg ha-' according to yield potential.Potassium distribution in "Shamouti" orange fruit was described in detail byGolomb (1983): Highest K concentration was found in the stem-end (1.49% ofdry matter), gradually decreasing towards the equator (0.54%) and increasingtowards the stem (0.84%). Pulp contains an average K concentration of 1.05%.

4.4.5. K effect on tree growth

In experiments with Pineapple orange on rough lemon planted on Lakelandfine sand, Koo and Reese (1971) observed a reduction of tree growth whenany one of the major nutrients P, K, Ca or Mg was omitted from the fertilizerand that fruit production was directly related to tree size.Deszyck et al. (1958) and Reese and Koo (1975) measured several maturecultivars of orange trees in two K experiments. They found that K-deficienttrees (< 0.4%) were smaller and also had smaller leaves.

4.4.6. K effect on yield

The effect of K on size and quality of fruits is observable soon afterapplication but response jn terms of yield is slower. Reports of yield responsesoriginate mainly from long-term experiments.Embleton and Jones (1973) reported the effect of combined NK application toValencia orange in a 9-year experiment. The results of the last 4 years showthat the higher NK treatment increased the size and yield of fruits per tree.Boman (1995) found that the highest gross packed value ($ ha-') were obtainedfrom high KNO, fertigation during both 1993 and 1995 seasons (Table 10).The returns from high KNO plots averaged about 20% more than the customarybroadcast treatment, during the 2 seasons that fruit size was measured.

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Table 10. Gross packed value for grapefruit.

kg tree -' Fruit number box-'Treatment N 1i N, N, N,

K, 158 164 147 153K , 168 185 138 165

Increased fruit production from K fertilization has been reported up to leaf Kcontents of 1.5 to 1.7% in Florida, Brazil and Australia (Chapman, 1968; Koo,1985; Malavolta, 1992). Du-Plessis and Koen (1988) in South Africa obtainedsignificant yield increases by K application starting in the second season,while fruit size was only affected in the fourth season. They also observed thatmaximum yield did not necessarily ensure maximum income in an area knownfor small fruit size. In the case of maximum yield the optimal N:K ratio wasbetween 2.4 and 3.0 with leafN higher than 2.1% and K higher than 0.8%. Formaximum fruit size the ratio was from 1.6 to 2.2 with N higher than 1.8% andK higher than 0.9%.Bazelet et al. (1980) in a long term experiment showed increases in leaf Kfrom K application with clear effects on the soil K status but failed to showany significant effect on yields and fruit size. Table 11 with data from a salinityexperiment by Dasberg (1988) shows clear effects of K application on leaf Kcontent while the effect on yields and fruit size is significant in alternate years.

Table 11. Effect of K fertilization on leaf K, yield and fruit size of Shamoutioranges during 4 years.

Treatment 1984 1985 1986 1987

Leaf K concn. (%)- K 0.45 0.60 0.44 0.51+ K 0.64 0.85 0.67 0.87Sign. + + + +

yield (ton ha -')-K 69 57 71 67+K 77 54 86 72Sign. + - +

fruit weight (g)-K 181 211 187 172+K 193 256 197 222Sign. - + - +

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4.4.7. Effect of K on fruit quality

In contrast to the relatively small response of the vegetative parts to potassium,the fruit may be affected throughout the range from severe K deficiency to Kexcess.

4.4.7.1. External and internal fruit quality

The need for maintaining K levels within the optimum range is especiallyimportant in relation to external aspects of fruit quality. Excessively high Klevels will result in large fruits with coarse, thick rind and poor colour. Toolow K will result in small fruit, not acceptable in the fresh fruit market or forexport, though with thin rinds and good colour. Creasing and splitting areoften associated with small fruits having thin, weak rinds (Koo, 1961; Calvert,1969; Bar-Akiva, 1975). Potassium fertilization will usually reduce theincidence of these disorders in the following years but has no effect on windscar, russet or scab. Embleton and Jones (1973) emphasise the profit obtainedby K fertilization on Valencia oranges. NK fertilizer decreased the percentageof creased fruits as compared with N alone, and the highest return wasobtained with the high rate of NK fertilizer.Increasing potassium beyond sufficiency levels will affect internal quality. Ithas long been known that application of K will increase per cent acid, vitaminC in juice. This will lead to increase in the ratio solids to acid in the juice.Moreover, the increase of peel thickness by the K will decrease per cent juicein fruit.

The general effect of K on fruit quality is summarized as follows:

I. Fruit size is strongly increased especially at lower starting level.2. The fruits are greener.3. The rind thickness and the peel is coarser (except for lemon).4. The juice percentage is decreased (except for lemons).5. The yield of soluble solids per hectare is decreased slightly.6. The acidity of the juice is consistently increased.7. The ratio of soluble solids to acids is reduced.8. The vitamin C content is increased.

4.4.7.2. Fruit splitting

Fruit splitting is a serious problem in several citrus cultivars such as Navel andValencia oranges, Nova and Murcott easy-peelers. De Cicco et al. (1988)found that a higher phosphorus concentration in the fruit of Navelina orangeincreased fruit splitting; a high K:P ratio produced a thicker peel whereas alow ratio decreased the peel thickness. Koo (1961) showed that it is possible to

27

reduce the occurrence of splitting in Hamlin oranges by increasing the Kcontent in the leaves. Similar results were reported by Bar-Akiva (1975) for'Valencia' orange on sour orange rootstock.Lavon et al. (1992), using a Florida hybrid (Clementine X Orlando) named"Nova", investigated the effects of nutritional and hormonal sprays indecreasing fruit splitting (Table 12). It was found that two sprays of potassiumnitrate at 3% to 5% applied in June and August decreased fruit splitting by20%. Potassium nitrate was sprayed alone or combined with 2,4-D (2,4 di-chloro phenoxy acetic acid) at a concentration of 20 ppm. Two spraytreatments including K and 2,4-D also increased the yield per tree by nearly 50per cent. There was an indication that this treatment reduced the percentage ofcreased fruits. In some cases fruit splitting might be the result of creasing, aserious peel disorder of 'Valencia' orange, 'Nova' mandarin and others. It hasbeen found that foliar application of KNO, reduced creasing significantly(Bar-Akiva, 1975).

Table 12. Reduction of fruit splitting in "Nova".

Treatments Yield No. of Fruit Splittingkg tree' fruits tree -' weight, g %

Control 35 b 404 b 94 a 52 a2,4 D 20 ppm 43 ab 433 a 99 a 40 abPotassium nitrate 3% 57 a 539 a 109 a 35 bPotassium nitrate 3% + 49 ab 447 a 104 a 35 b2,4 D 20 ppm

4.4.7.3. Fruit size

The consumer preference for large-size fruit has become a major concern ofproducers of fresh fruit for export and local consumption. Embleton et al.(1973a) reported that an increase in potassium usually increased fruit sizethroughout the range of leaf potassium values found in the field in California.Over the range 0.3 to 0.7% K in the leaf, an increase in potassium has a strongeffect on increasing volume yield through the size and number of fruit; it alsodecreased pre-harvest drop. Over the range 0.7 to 1.7% K the effect was lessmarked. When leaf K concentration is below 0.7%, one foliar spray of 5%KNO with 20 ppm 2,4-D can increase fruit size by 8-25% over the control(Table 13, Erner et al., 1993). The best time of application was found to be 6-8weeks after flowering. Potassium concentration in leaves was increased by ca-0.2% (DW) over control, after 5% KNO3 foliar spray application. Moreover,KNO was found superior over the potassium sulphate.

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Table 13. Effect of potassium and 2,4-D on fruit size and internal quality of'Shamouti' orange, spray: 1-6 June, 11-6 July,111-6 August harvest February.

Treatment Concentration pH Packed boxes* Acid TSS T:A

3.5 No. % % ratio

Control .-- 12.3 c 100 1.34 b 10.2 a 7.6 abKNO 3 + 2,4-D 5%+20 ppm HNO 3 15.1 a 123 1.54 a 10.6 a 6.9 bK2SO4 + 2,4-D 4.2% +20 ppm -- 14.3 ab 116 1.47 ab 10.7 a 7.2 abKNO3 + 2,4-D 5% + 20 ppm UP50 13.6 b 110 1.47 ab 10.8 a 7.3 ab2,4-D 20 ppm HNO 3 13.8 b 112 1.34 ab 10.6 a 8.0 ab2,4-D x 2 20+20 ppm HNO3 13.8 b 112 1.42 ab 10.6 a 7.5 ab2,4-D x 3 20+20+20 ppm HNO 3 13.8 b 112 1.40 ab 11.0 a 7.9 ab2,4-D 40 ppm HNO3 13.4 b 109 1.34 b 10.8 a 8.1 a

* Number of packed boxes achieved by 1000 distributed fruits.

4.4.7.4. Storage properties

The keeping quality of fruit in storage may also be influenced by K nutritionwhich reduces the incidence of stem-end rot (Diplodia natalensis P. Evans)and green mould (Penicillium digitatum Sacc.).

4.4.8. Potassium fertilization

It is not possible, nor is it advisable, to issue fertilizer recommendations tocover a wide range of soil and climatic conditions. To produce maximumyields of high quality one must consider the natural fertility of the soil, usingsoil and leaf analysis as well as the history of practices and production records.The importance of balanced fertilization must be kept in mind. Varieties thattend to have high acid concentration, or varieties with a preference for lowacidity, should be kept at a relatively low level of potassium nutrition. On theother hand, K nutrition of 'Valencia', 'Nova', 'Murcott' trees is more to berecommended as it reduces creasing and splitting.

4.4.8. 1. Soil factors

The effectiveness of K applied to the soil varies widely with soil type. Highpotassium uptake by citrus trees has been found in acid, sandy soils in humidregions such as Florida (Koo, 1985). K availability decreases at low soilmoisture content, high Ca and Mg concentration and high fixing capacity.Large amounts of K fertilizer must be applied for several years before anyresponse is observed. Foliar applications of potassium nitrate are moreeffective under such conditions (Embleton et al., 1973b; Erner, personal).

4.4.8.2. Cultivar - rootstock

There is little difference in K uptake between commonly used rootstocks butthere are differences between cultivar scions. In orange and grapefruit, high Kmay have adverse effects on juice and peel .In Hamlin and Shamouti orangesK can improve the fruit size should this be required. 'Valencia' on sour orangerootstock has apparently a very low affinity for K uptake in comparison withother stock-scion combinations. In mandarins and hybrids, K will increasefruit size without any adverse effect on juice acidity, rind colour or texture.

4.4.8.3. Sources of potassium

Potassium chloride and potassium sulphate are equally suitable sources of Kfor citrus which has an excluding mechanism for chloride (Smith, 1966a).

30

However, where soil or irrigation water salinity is a problem, potassiumsulphate is preferred. The double salt sulphate of potash magnesia is widelyused in areas where magnesium deficiency occurs.Foliar application of potassium nitrate is more efficient for quickly increasingleaf K and curing K deficiency. Potassium sulphate can also be used but thenitrate is absorbed better. Bar-Akiva et al. (1972) successfully used 15-20litres of a 4% solution per tree. Sprays at 10% concentration of KNO 3 at 3000-5000 I ha -' did not cause any leaf burn. Foliar spray of mono potassiumphosphate at 5% is a very efficient source of K.

4.4.8.4. Methods of application

a) Before planting by deep ploughing or by placement in the planting hole.b) Annual dressing before the main rainy season.c) Application through the irrigation system but not on the foliage.d) Foliar spray at the appropriate concentration according to the compound.

4.5. Calcium

Citrus trees contain more Ca than any other cation. It plays a part in theregulation of nutrient absorption (sodium, potassium and magnesium). Soilsgenerally contain sufficient calcium to satisfy crop requirements. Irrigationwater and commercial fertilizers contain an appreciable amount of availablecalcium. A strictly nutrient deficiency is unlikely to occur in citrus grownunder normal conditions.On calcareous soils citrus trees have a high Ca content, which may induce lowK and Mg content. Under these conditions the availability of Ca, Zn, Mn andB may present a problem (Malavolta, 1962). Citrus is also sensitive to soilacidity at pH below 5.0. This can cause reduction of plant growth as a result ofroot system damage by excess of Al 3 and H'. Application of dolomiticlimestone or calcareous limestone plus a soluble source of Mg showed anincrease in yield after some time of up to 200% over the control (Anderson,1987). Similar results were reported by Quaggio et al. (1992) for liming acidsoil in Brazil with 'Valencia' orange on rangpur lime rootstock.The application of calcium amendments may be necessary for raising the pHon acid soils, also for improving the physical properties of heavy soils or soilswith high sodium content.

4.6. Magnesium

Magnesium is a constituent of chlorophyll and pigments; it is found in theleaves and shoots of citrus trees in higher proportion than in other parts of theplant. A deficiency of magnesium results in poor green pigmentation and

31

chlorotic aspect of the leaves. In citrus the deficiency symptoms first appearon the older leaves as bronzing, followed by the formation of a green wedge atthe base of the leaves caused by the fading of the chlorophyll in thesurroundings.Magnesium deficiency symptoms are very common in citrus orchards onhighly acid soils with low Mg content. Dolomitic limestone has been used as asoil amendment and source of Mg for citrus trees. Maximum yield wasattained when exchangeable Mg in the soil and leaf Mg were higher than 0.9meq per 100 cm' and 0.35%, respectively (Quaggio et al., 1992).In the past, low grade fertilizer materials such as ashes, dolomite, kainite andmanures were sources of magnesium. Soluble Mg is supplied mainly bymagnesium sulphate and potassium magnesium sulphate. Embleton and Jones(1959) obtained long-lasting results by large applications of magnesiumsulphate to the soil. On heavy soil, even this treatment has not beensatisfactory. Magnesium sulphate in foliage sprays has proved useless in allcitrus growing areas, whereas, magnesium nitrate sprays are more efficient butmust be applied frequently (Embleton and Jones, 1959).Bar-Akiva (1969) found magnesium nitrate superior to Mg sulphate.Magnesium nitrate, at the rate of 5000 litre per ha of 1.6% solution or 500 litreper ha of 10% solution was used successfully. An aerial spray of 50 1magnesium nitrate (55%) in 200 1 water per ha also succeeded in increasingleaf Mg content in Valencia oranges without causing damage to fruit or leaves.Ermer et al. (1984) reported that band application of MgCI2.6H 20 under thetree canopy of 'Shamouti' orange trees significantly increased leaf Mg and Clconcentration. In spite of high Cl concentration in the leaves, no visible toxicitysymptoms were observed. Moreover, foliar spray of Mg(NO,) 2.6H 20 resultedin increases of only 0.02% to 0.05% in leaf Mg content, requiring repeatedspraying at least once a year, depending on the severity of the Mg deficiency.

4.7. Sulphur

Sulphur is an essential nutrient and is a constituent of the amino acids cysteineand methionine and hence of proteins. Both of these amino acids areprecursors of other sulphur-containing compounds such as co-enzymes andsecondary plant products. Sulphur is often provided as sulphate in manycommercial fertilizer formulations and therefore is rarely deficient for citrustrees under field conditions (Smith, 1966b). On the other hand, the increaseduse of fertilizers free of S may cause deficiencies to occur more widely.A drastic decrease in chlorophyll content of leaves is a typical feature ofsulphur deficiency, a symptom very similar to that of N deficiency, but Ndeficiency is more common.

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4.8. Micro-nutrient deficiencies

4.8.1. Boron

Boron deficiency may occur both on acid and alkaline soils; several exampleshave been reported in Florida, Zimbabwe and Israel. Toxicity may show upwhen this element is present in irrigation water at a concentration higher thanI ppm, or at high B content in soil. Poultry manure can sometimes reduce thetoxicity effect of boron. A boron content in leaves below 20 ppm in dry matterindicates deficiency, which may be corrected by spraying 5-15 kg of borax perha.Rootstocks tolerant to boron are: Macrophylla > Sour orange > trifoliate.(Sagee and Shaked, personal communication; Castel, 1987).

4.8.2. Copper

Copper deficiency is frequently found on sandy, acid soils. Over-Liming andheavy applications of phosphorus may induce copper deficiency. Excesscopper may cause iron chlorosis by inhibiting the uptake of iron (Table 14).

Table 14. Cu level in orange leaves (Malavolta et al., 1962).

Deficient Normal Excess Source

ppm of dry weight4 4-10 15 Chapman et al., 19474 6-16 23 Reuther & Smith, 1954

10-20 -- Anonymous, 1958

Copper deficiency is corrected by adding cupric oxide to the soil at 10-15 kgha-' , or by soil application of copper sulphate. A quicker response is achievedby spraying a solution of copper sulphate on the foliage (Malavolta et al.,1962). When copper-based fungicides are routinely used, Cu deficiency isunlikely to occur.

4.8.3. Iron

Iron deficiency is a serious problem and is difficult to correct on calcareoussoils. This deficiency is also found on poor acid, sandy soils where excess Zn,Mn and Cu have an antagonistic effect on Fe uptake. High rates of P and K arealso conducive to iron deficiency (Stewart and Leonard, 1952). In some cases,iron chlorosis may be reduced or corrected by reducing the rate of irrigationwater and by improving drainage and other factors of the root systemenvironment.

33

The treatment of iron deficiency in citrus is achieved by reducing the pH ofcalcareous soil. Soil application of iron sulphate must be at a high rate to showresults. More effective is to apply iron chelate at the rate of 15-20 g per treedepending on tree size. In calcareous soils Fe-EDDHA has been proved to beeffective (Davenport, 1983) but may appear expensive for mature trees.Nevertheless, with rootstocks like Troyer on calcareous soils, small amountsof chelate should be applied at least for the first 3 years. Leaf spray of Fe-sulphate with L-77 surfactant proved to be effective in lemons (Horesh andLevy, 1981).

4.8.4. Zinc

Zinc deficiency occurs in all citrus growing areas; it is the most frequentlyencountered deficiency in citrus. Foliar spray of zinc compounds, such as Znsulphate, Zn oxide and Zn oxy-sulphate are currently used to correct orprevent this deficiency.In Florida, soil applications of Zn compounds proved to be satisfactory, butwere ineffective under neutral or alkaline conditions (Smith, 1966a). Bar-Akiva et al. (1971) developed the carbonic anhydrase assay for assessing Zndeficiency in leaves and found it compared favourably with assessment byvisual symptoms and/or Zn concentration in the leaves. This methodaccurately identified the deficiency and assessed the degree of recoveryfollowing spraying; it was more sensitive than leaf analysis.

4,8.5. Manganese

Manganese deficiency is a persistent problem on calcareous soils. So far, nosoil treatment has been found satisfactory. Mn deficient plants, under long-term stress in nutrient solution, absorbed three-fold in nitrate and two-fold inwater uptake (Lerer and Bar-Akiva, 1979). Moreover, high and even un-normal nitrate concentration in leaves (Shamouti) was highly correlated withMn deficiency.Neutral manganese sulphate, manganese oxy-sulphate, manganese oxy-manganese dioxide are all effective when applied in good time as sprays. InCalifornia, under dry climatic conditions, the use of neutral manganesesulphate is preferred, whereas, in Florida, manganese oxide replaced sulphatebecause it leaves less residue on the foliage (Smith, 1966a). Easy-to-peelvarieties tend to have manganese deficiencies which can be corrected withMnSO4 foliar spray at 0.2%.

34

4.8.6. Molybdenum

Molybdenum is required only in very small quantities by citrus thoughdeficiency symptoms have been observed under field conditions in Florida. Itis corrected by applying a leaf spray solution containing 30-100g of sodium orammonium molybdate in 400 litres (Smith, 1966a).

5. Leaf analysis

The solution of nutritional problems in citrus requires a correct evaluation ofthe mineral requirement under different conditions and the application ofbalanced fertilization. Leaf analysis has been proven as a useful method for theevaluation of nutrient uptake from the soil over an extended period and ofnutrient absorption through the leaves following foliar applications.This method is useful in evaluating the extent of any deficiency detected incitrus groves or in detecting nutritional disorders before the appearance oftheir visual symptoms. The soil test generally used, in addition to the problemof choosing a suitable extractant for. each soil nutrient, does not provide'enough information about the actual uptake of each nutrient by the trees andits distribution in the different organs. Generally, leaf analysis is preferred tosoil analysis for most nutrients. Lately, juice analysis has become, for someextend, a useful technique in certain respects, e.g. chloride. Toxicity, surfacecontamination is excluded and easy sampling and handling (Gallasch, 1992).However, the fruit is not a good representative for the elements and yet nocritical norms have been developed for guidelines.

5.1. Factors influencing leaf nutrient content

5. 1. 1. Species, variety, rootstock

Embleton et al. (1973a) reported that leaf nutrient content was influenced byboth rootstock and scion; there were differences between species or varietiesgrown on the same rootstock and between rootstocks carrying the same scion.For instance, nitrogen and phosphorus contents in Marsh seedless grapefruitwere lower than in Valencia orange. Washington Navel leaves have a highercontent of N, P and K and a lower content of Ca than Valencia oranges(Reuther and Smith, 1954; Jones and Embleton, 1968; Bar-Akiva et al., 1972).Smith (1966) maintained that the differences in mineral composition of leavesassociated with rootstock or variety in citrus did not justify the establishmentof different standard values. On the other hand, significant differences in leafmineral composition between the most important rootstocks (sour orange andsweet lime) and between varieties on the same rootstock were reported inSouth Africa, Morocco and Israel.

35

Bar-Akiva et al. (1972) found that sour orange rootstock induced low P and Kand high Ca levels in several varieties tested; rough lemon induced a low Mglevel in all the scions. When comparing old clone and nucellar type varietieson the same rootstock, N, Ca, K and Na tended to be lower, and Mg higher inthe nucellar trees. Thus, when a commercial leaf analysis service was introducedto offer fertilizer guidance to citrus growers in Israel, different standard valueswere suggested for the various combinations of scion and rootstock.

5.1.2. Interaction between elements

Except in cases of extreme deficiency or excess, it is necessary to consider theeffect of the nutrients on one another when evaluating the nutritional status ofcitrus. Increasing or decreasing one element in the fertilizer supply nearlyalways affects the status of other elements. This is particularly true on light orsandy soils of low buffering capacity. Some trends of these interactionsmentioned by Smith (1966a) and Embleton et al. (1973a) are summarised inTable 15.

Table 15. General effect of an applied element on the mineral composition ofcitrus leaves.

Element Elements measured in leavesadded N P K Ca Mg Cu Zn Mn B

N + - +0 0 +P 0 + - + + + 0K - 0 + - 0 0 0 -Ca + 0 + 0 ? ? ? ?Mg 0 0 - + - + + 0Cu 0 0 + 0 0 + - - 0Zn 0 0 + - - - + - 0Mn 0 0 + 0 - - 0 + 0B 0 - + - 0 0 - +

Increased concentration is indicated by +; decrease by -; no consistent effectby 0; and uncertain response by ?

The following are the major inter-nutrient interactions:

a) Nitrogen-phosphorusPhosphorus is a weak antagonist of N, but N has a strong effect on P. Leaf Poften depends more on leaf N than on the supply of available P. It is virtuallyimpossible to have excess of both N and P in the same leaf.

36

b) Nitrogen-potassiumThese two elements respond differently to the same treatments. Factors thatlead to a high N status tend to lower the K status and vice versa. This applieswhen ammonium is the source of nitrogen. In contrast, potassium nitrateincreased both potassium and nitrogen contents in leaves. Under low calciumconditions high N and high K may appear in the same leaf.

c) Nitrogen-magnesiumThese ions tend to be synergistic, especially with non-basic N fertilizers suchas urea, anhydrous ammonia or ammonium nitrate. Heavy rates of K tend tohamper this effect.

d) Calcium-nitrogenThis interaction is not fully understood. Under acid soil conditions Ca supplyaffected neither leaf Ca nor N; on calcareous substrate, leaf Ca was markedlyincreased and leaf N drastically reduced. This has been confirmed by results ofsurveys. On the other hand N rate appears to have little effect on leaf Ca.

e) Potassium-calciumThere is strong antagonism between these elements. High levels of both K andCa are not found in the same leaf.

I) Potassium-magnesiumK is a strong antagonist of Mg but Mg is a weak competitor against K (Erneret al., 1984). On a high Ca and Mg substrate the antagonistic effect of K islowered by Ca.

g) Macro and micro-elementsN can reduce sulphur and boron in leaves and is considered to be one of thebest treatments available to counter the risk of boron excess. Applying Preduces the concentration of copper and zinc in the leaves. Large applicationsof manure and phosphorus to a sandy loam soil have been found to inducesevere copper and zinc deficiency but this can be corrected with sprays ofcopper and zinc.

h) Nitrate-chlorideRecently, nitrate was found to reduce accumulation of Ct in grapefruit leavesunder saline conditions (Levy et al., 1999).

5.2. Sampling for analysis

N, P and K contents of the leaves vary greatly according to their age, but theirvalues remain almost constant between 4 and 7 months of age. Work inMorocco and Israel showed that the position of the leaves on the tree,especially their orientation, has a marked influence on the mineral content ofthe leaf (Praloran, 1955; Heyman-Herschberg, 1956). With this in mind, it is

37

generally recommended to sample leaves at random from spring flush, aged 4-7 months, situated 0.80-1 in from the ground. Alternative criteria apply to theselection of position for taking samples: (1) leaves from non-fruiting terminalswhich are the most reliable source since they are easy to select and comprisethe majority on the tree; (2) leaves from behind the fruit on fruiting terminals,which indicate well the effect of fruiting on leaf nutrient status and representthe same leaf age. In the U.S. the sampling is made around the trees, whereas,in Israel and Morocco, leaves from one side of the trees (North) are selected.Some authors recommend sampling 10-20 leaves per tree on 5-25 trees for oneorchard of each variety; others prefer sampling fewer leaves on a greaternumber of trees.Clearly, results will vary according to the method of sampling chosen andthere is as yet no model conversion equation to enable comparison betweendifferent methods used in different parts of the world. In any case it isimportant that the sampling procedure should be standardised and strictlyobserved to ensure consistent results.

5.3. Methods of analysis

Two different methods of leaf analysis are currently used in commercialpractice. The most common method is the determination of total concentrationof the appropriate element after total acid digestion or ashing. The othermethod, which was developed in Israel (Bar-Akiva, 1974), measures the waterextractable quantities of the different elements. The advantage of the latter tsthat it avoids ashing or digestion, is easy to use and cheaper than the standardmethod. Nitrogen is expressed as nitrate in ppm, potassium concentrations areclose to but not less than 90% and phosphorus about 60% of the valuesobtained by ashing.It has recently been suggested that analysis of fruit juice might offer analternative to leaf analysis in some circumstances; it would circumvent thepossibility of surface contamination and sampling and handling are easy(Gallasch, 1992). However, the fruit does not truly reflect the status of eachand every element and as yet no critical norms have been developed asguidelines. It has been found useful in testing for chloride toxicity.Knudsen et al. (1981) suggested using X-Ray fluorescence using dry leafpowder. This would be cheaper than the water extractable system but needsvery precise standardisation if it is to be used commercially. Analysis of leafsap has been used for annual vegetables and has been tried for tree cropsincluding citrus in the last few years.

38

Table 16. Standards for classification of the nutrient status of orange trees based on concentration of nutrient elements in4 to 7 month old, spring-cycle leaves from non-fruiting terminals.

Element and chemical Dry matter Deficient Low range Optimum High range Excesssymbol basis less than range more than

Nitrogen (N) % 2.2 2.2 to 2.4 2.5 to 2.7 2.8 to 3.0 3.0

Phosphorus (P) % 0.09 0.09 to 0.1 0.12 to 0.16 0.17 to 0.29 0.30

Potassium (K) % 0.7 0.7 to 1.1 1.2 to 1.7 1.8 to 2.3 2.4

Calcium (Ca) % 1.5 1.5 to 2.9 3.0 to 4.5 4.6 to 6.0 7.0Magnesium (Mg) % 0.20 0.20 to 0.29 0.30 to 0.49 0.50 to 0.70 0.80Sulfur (S) % 0.14 0.14 to 0.19 0.20 to 0.39 0.40 to 0.60 0.60Boron (B) ppm 20 20 to 35 36 to 100 101 to 200 260

Iron (Fe) ppm 35 35 to 49 50 to 120 130 to 200 250?Manganese (Mn) ppm 18 18 to 24 25 to 49 50 to 500 1000Zinc (Zn) ppm 18 18 to 24 25 to 49 50 to 200 200

Copper (Cu) ppm 3.6 3.7 to 4.9 5 to 12 13 to 19 20Molybdenum (Mo) ppm 0.05 0.06 to 0.09 0:10 to 1 2 to 50 100?

Sodium (Na) %* -- less than 0.16 0.17 to 0.24 0.25

Chlorine (CI) % ? ? less than 0.2 0.3 to 0.5 0.7Lithium (Li) ppm * -- less than I I to 5 12

• These elements are not known to be essential for normal growth of citrus.

? Indicates lack of information regarding value.

U)J

5.4. Leaf analysis standards

Standard values of leaf analysis have been established by different approaches:a. Sand and water culture studies.b. Surveys of commercial orchards to obtain leaf analysis values and ranges.c. Comparison of mineral status in normal trees with those showing specific

symptoms of deficiency.d. Calibration of standards in long-term field fertilizer experiments, 10 or 20

years may be required to obtain reliable results.e. Finally, tree condition, yield level and fruit quality must be taken into

account when assessing leaf content standards for optimal yield and quality(Embleton et al., 1973a; Du Plessis et al., 1992).

The data in Table 16 were drawn up by Smith (1966b) from numeroussources; the ranges are narrower than those proposed by Reuther and Smith(1954). Other standards, applicable to local conditions and using varioussampling methods, have been proposed in different countries.

5.5. Interpretation of leaf analysis

Interpretation of the data obtained from leaf analysis should take into accountvarious external and interionic relationships. For example, low leaf Mn isassociated with very high NO, level (Lerer and Bar-Akiva, 1979). Also, highK levels induce low Mg content in leaves.Bar-Akiva and Gotfried (1972) found that the response of Valencia orangetrees to N fertilization was reflected more clearly by leaf nitrate than by leafnitrogen. Field experiments have often demonstrated good yield responses to Pfertilizer with no corresponding increase of leaf P content (Rodney andSharples, 1961) but this could be due to surface absorption of P applied inorgano-phosphorus pesticides.Leaf analysis may give no indication of sodium toxicity induced byaccumulation of Na in the roots the physiological effect of which is reducedgrowth and fruit production. Analysis of soil and root hairs is needed toindicate whether this is the case. The situation may well be improved byreviewing the irrigation programme and making the necessary changes.Possible interionic relationships must always be considered. Sometimes thecorrection of a deficiency can be achieved simply by lowering the supply ofanother element found to be in excess. Several experiments in Israel showedthat chicken manure significantly reduced boron toxicity.Ortuna et al. (1971) suggested that investigation of the ratios between leafconcentration of various elements would give a good indication of nutrientstatus of citrus trees. They found that salinity had a considerable effect on leaf

40

mineral content. In recent years, emphasis has again been placed on ratiosbetween elements which are known to interact with each other e.g. N/K ratiofor fruit size and yield (Du Plessis and Koen, 1988). The DRIS approach isbased on the assumption that the optimum ratio between any two elements canbe established if sufficient data are available. This method should not be usedon its own but is useful as an additional tool for interpretation.It has been suggested that the study of the action of enzymes systems in leavesmight be a better approach to determine the nutrient status of trees - forexample, carbonic anhydrase is a good indicator of Zn status; nitrate reductaseactivity showed good correlation with the productivity of grapefruit (Bar-Akiva, 1969).

6. Fertilizer recommendations in selected countries

The following gives a brief survey, mainly in the form of tables, of standardrecommendations for citrus in various countries where citrus is an importantcrop.

6.1. Argentina

The Fertilizer Manual for Citrus (Melgar and Ronco, 1992) indicates theaverage requirements for young and mature trees.

Table 17. NPK recommendations for mature Orange trees in two maingrowing areas.

Bella Vista ConcordiaYear N P205 K20 MgO N P205 K20 MgO

g tree-

5 450 250 450 70 200 120 200 307 550 350 650 100 280 140 280 50

9 700 450 800 150 350 180 350 70

11+ 900 550 1000 200 450 220 450 100

Remarks: for low or high foliar values increase or decrease the doses by 15%;for deficiency or excess increase or decrease by 30%.

The rates specified in this table apply to medium leaf nutrient status. If leafconcentration is low, increase by 15% as appropriate; if high decrease by 15%.In the case of deficiency or excess adjust the rates up or down by 30%.

41

Table 18. Mixed fertilizer of 15-15-15-3 for young trees.

Year No. of applications kg tree'

2 4-5 0.40-0.503 4-5 0.70-0.904 3-4 0.80- 1.005 3-4 0.90- 1.20

Table 19. Application schedule for fertilizer.

Time of application N P K Mg

percent of total dosesOctober - November 45 - 50 *March - April 40 80 50 100August 15 20 -*

Spray if necessary.

6.2. Brazil

Technical Bulletin No. 100 ed. 2. Editors: Bernardo van Raij; HeitorCantarella; Jose Antonio Quaggio; Angela Maria Cangiani Furlani (1996)gives fertilizer recommendation for the state of Sao Paulo.

General notes:* When lime is applied to increase base saturation, magnesium should be

kept at a level of at least 9 mmol dm3 .* P can be given in a single application between from July to August. N and

K should be given in 4 split applications, between September and March.In the two foregoing tables, N levels are those determined by leaf analysis, Pand K by soil test.

Table 20. Recommendations for young trees.

Age N P20 5' K20*(years) g plant' annum

0-1 80 0 0- 201-2 160 0-160 0- 802-3 200 0-200 0-1503-4 300 0-300 0-2004-5 400 0-400 0-300

• According to soil test.

42

Table 21. Recommendations for various cultivars.

N level in leaves, g kg' Resin P level, mg dm-' Exchangeable K, mmole dm"3

Expected >23 23-27 28-30 >30 0-5 6-12 13-30 30< <0.7 0.7-1.5 1.6-3.0 >3.0

Yield, t ha" kg ha' of nutrientLemon

<16 60 50 40 30 50 40 20 0 6 20 20 0

17-20 70 60 50 40 70 50 30 0 10 70 40 0

21-30 100 80 60 50 90 70 40 0 14 90 50 10

31-40 140 120 100 70 130 100 50 0 19 130 70 20

41-50 160 140 120 90 160 120 60 0 24 170 100 30

>50 200 160 130 100 180 140 70 0 27 190 120 40

Tangerine & Murcott<16 70 60 50 40 50 40 20 0 7 50 20 0

17-20 80 70 60 50 70 50 30 0 8 60 40 0

21-30 110 90 70 60 90 70 40 0 11 80 50 10

31-40 160 130 100 90 130 100 50 0 16 110 70 20

41-50 200 170 140 110 160 120 60 0 20 140 100 30

>50 230 190 150 130 180 140 70 0 22 150 120 40

Oranges and Lima Tahiti

<16 90 70 60 40 50 40 20 0 60 40 30 0

17-20 100 80 70 50 70 50 30 0 70 50 40 0

21-30 140 120 90 60 90 70 40 0 90 70 50 0

31-40 190 160 130 90 130 100 50 0 120 100 70 0

41-50 240 200 160 110 160 120 60 0 160 120 90 0

>50 260 220 180 130 180 140 70 0 180 140 100 0

In Valencia orange, reduce K application by 20%.* In Oranges and Lima Tahiti, avoid the last K application when leaf K >19 g kg'.

." Table 22. The range of essential elements in 4-7 old month old leaves of the main citrus varieties.

Varieties N P K Ca Mg Sg kw'

Sweat orange in Sichuan 27-32 1.4-1.7 7-15 32-55 2-5 /Satsuma mandarin 30-35 1.5-1.8 10-16 57-70 2.7-3.4 2.8Citrus grandis in Fujian 25-31 1.4-1.8 14-22 20-30 3.2-4.7 /'Ponkan' mandarin 27-33 1.2-1.5 10-18 23-27 2.5-3.8 /Navel orange and 24-26 1.2-1.6 7-11 30-55 2.5-6 2-3summer orangeCitrus Junos 25-27 1.2-1.6 12-17 30-45 3-5 2-4

Varieties Fe Mn Zn Cu B Mo

pg g-1

Sweat orange in Sichuan 60-170 20- 40 13- 20 4- 8 40-110 /Satsuma mandarin 50-100 25-100 25-100 4-10 / 0.1-0.5Citrus grandis in Fujian 60-140 15-140 24- 44 8-17 15- 50 /'Ponkan' mandarin 50-140 20-150 25- 50 4-20 I /Navel orange and 60-120 25-200 25-100 5- 6 31-100 0.1-3summer orangeCitrus Junos 50-120 25- 49 25- 49 5-12 36-100 0.01-1

6.3. California

Soils are clay to sandy oams relatively more fertile than Florida.Nitrogen: In general, high rates of N fertiliser are used.Example of application under average conditions: 100-150 kg ha-' until

optimum leaf value is achieved.Phosphorus: nominal application, 2-3.5 kg tree year' .

Potassium: Usually 1.2 to 3 kg KO per tree for two consecutive years alongthe tree drip line.Alternative: Foliar sprays of 15 kg KNO3 in 100 gallons of water each yearwhen needed.Micronutrients: Foliar sprays with Zn, Mg, Mn and sometimes B and Mbwhen needed.

6.4. China

Recommended rates of N, P and K for highest yield and fruit quality in matureSatsuma mandarin trees were: 1,170 kg N, 360 kg P205 and 1,170 kg K20 per

tree per year (personal communication Zhihong Cao, Academia Sinica).Recommendations are based on leaf diagnosis and soil tests, enzyme activity

and the DRIS. The optimum nutrients standard for the main citrus varieties in

China are shown in Table 22 and the range of soil nutrient contents in orchardsfor fruit production of 22.5 t ha- are given in Table 23.

Table 23. The range of essential nutrient elements in soil of main citrusproducing regions in China (yield up to 22.500 kg/ha).

Citrus orchards N P KVg g-,

Aging orchard >150 2.8-3.3 25- 42High yield orchard 223-229 41-70 83-116Guangxi orchards 70-133 6-16 66-130

6.5. Florida

Fertilizer guidelines Koo et al. (1984) give recommendations in terms of bothsoil and foliar application.

45

Table 24. N rates and number of applications - non bearing trees.

No. of applicationsYear kg tree-' year "' dry fertigation

I 0.70-0.14 6 102 0.13-0.25 5 103 0.20-0.40 4 10

Table 25. N rates and number of applications - bearing trees.

Oranges Grapefruit Other No. of applications

kg N ha dry fertigation

50-90 50-70 50-90 3 10

Remarks: 1. Orlando tangelos: up to 110 kg N- tree-' and up to 140 kg N forHoney tangerines (Murcott).2. N and K rates are adjusted according to leaf trends.3. P, Ca, Mg, pH monitored by soil analysis.

6.6. India

Recommended fertilizer rates for citrus in the state of Maharashtra are givenby H.L.S. Tandon in: Fertilizer recommendations for horticultural crops: Aguide book, 2nd edition (1991), Fertilizer Development and ConsultationOrganisation, New Delhi.

Table 26. Fertilizer recommendations in young and bearing trees.

Annual fertilizer (g tree -lAge N P,05 K,0

Early years 100 0 0Bearing age •1000 100 200

Other recommendations are found in: Citriculture in India by Randhwa andSrivastava (1996), Hindustan Publishing House, New Delhi.

46

Table 27. General fertilizer recommendations for Mandarins.

Age N P205 K20 FYM

(years) kg tree -'

1 0.035 0.135 0.015 ---

2 0.120 0.120 0.078 6.0

3 0.270 0.270 0.180 10.0

4 0.400 0.270 0.400 15.0

5 0.550 0.370 0.550 20.0

6 and onward 0.550 0.370 0.550 30.0

Table 28. General fertilizer recommendations for all citrus varieties in the

state of Assam.

Age Ammonium sulphate Super-phosphate Potassium sulphate(years) kg tree -'

1 0.23 0.14 0.09

2 0.45 0.28 0.18

3 0.68 0.45 0.23

4 0.91 0.45 0.45

5 1.14 1.36 0.68

6 1.14 1.36 0.68

7 1.37 1.36 0.91

8 1.60 1.59 1.14

9 1.83 1.70 1.25

10 2.16 2.00 1.47

11 2.39 2.27 1.81

12 2.62 2.50 1.81

13 3.29 2.80 2.00

14-24 3.63 3.10 2.27

25< 4.54 4.54 2.72

47

Table 29. Manure and fertilizer schedule recommended for 'Coorg' Mandarin (by age).

Material Application 1 2 3 4 5 6<period kg tree-'

Cattle manure Feb.-Mar. - 5 10 15-20 20-25 25-30CAN / Amm. Sul. Mar.-Apr. 0.1 0.225 0.6 0.6 0.6Super phosphate April * 0.225 0.45 0.5 0.6 0.7MOP April * 0.065 0.14 0.2 0.4 0.4CAN / Amm. Sul. June * 0.35 0.7 0.5 0.8 1.0Super phosphate June * 0.45 0.9 0-8 0.8 0.8MOP June --- 0.065 0.15 0.2 0.3 0.3CAN / Amm. Sul. Sept.-Oct. * 0.225 0.45 0.4 0.6 0.7Super phosphate Sept.-Oct. 0.225 0.35 0.5 0.5 0.5MOP Sept.-Oct. ---- 0.065 0.15 0.2 0.3 0.3

Actual nutrient received per year (kg plant')N -- 0.13 0.27 0.4 0.5 0.6P205 -- 0.14 0.27 0.37 0.4 0.47K20 -- 0.09 0.19 0.4 0.6 0.6Dolomite Jan. - Feb. -- 1.20 2.4 4.5 5.6 5.6

* A mixture of 1: 1:0.5 prepared by mixing Calcium Ammonium Nitrate (CAN), Single Super Phosphate and Muriate ofPotash (MOP).

6.7. Israel

Most of the fertilizer for citrus is applied by fertigation.

Table 30. Fertigation of young trees with micro-sprinklers.

Soil Interval Liter day' Total N

Year type day April May June-Nov m 3 ha" g tree'

day"

I light 3-4 5 5 7 1250 0.2

heavy 7 3 3 3-5 730 0.15

2 light 3-4 10 10 12-15 2500 0.4

heavy 7 6 7 12 1800 0.3

3 light 3-5 15 20 25 4300 0.6

heavy 7 7 12 25 3450 0.5

Remarks: a. Years 1-4, the fertilizers should be applied over the whole

irrigation season.b. N:K 20 ratio should be 1:1 in light and 1:0.6 in heavy soils.

c. Phosphorus applied according to soil tests, lowest of 10 ppm by essay.

d. When irrigating with recycled water N and K should be calculated to

complete the nutrients required including those in the irrigation water.

Recommendations for bearing orchards are based on results of leaf analysis.

Table 31. Recommendations for N in kg ha" .

Varieties StatusLow Desirable High

Shamouti, grapefruit, Lemon, Valencia 200 120-180 100

Clementine, Temple 200 150-180 120

Ortannique, Mineola, Murcott 250 150-200 120

Table 32. Recommendations for fertilizers in kg ha -'.

Nutrients Varieties Deficient Low Desirable

P205 All 120 60 0

K20 300 180 0

Mg(NO3)2 50 0 0

MnSO4 8 0 0

Remarks: Valencia on Sour Orange: if there was no response to potassium

over 5 years, spray potassium nitrate at 160 kg in 4000 liter ha'.Magnesium should be sprayed in April-May; in case of severe deficiency,repeat on the young vegetative growth.Manganese should be sprayed in May-June.

49

6.8. Morocco

Recommendations for fertigation with a standard 1:0.3:1.2 (N:P:K) solutionare in the range:

N: 140-180 kg ha-'P205 : 30- 50kgha-I

K20: 150-200kg ha -'

Good results are obtained with ammonium nitrate + MAP + MOP or SOP.Foliar contents, soil salinity and pH should be monitored regularly.

7. References

Achituv, M. and Bar-Akiva, A. (1973): Nitrogen accumulation induced byphosphorus deficiency in citrus plants. Scientia Horticulturea 1:251-262.

Anderson, C.A. (1987): Fruits yield, trees size and mineral nutritionrelationship in 'Valencia' orange trees as affected by liming. J. PlantNutrition 10: 1907-1916.

Anonymous (1958): The mineral nutrition of citrus. International PotashInstitute, Berne.

Bar-Akiva, A. (1965): Does organic manure necessary for citrus groves?Hasadeh 46: 437-439 (in Hebrew).

Bar-Akiva, A. (1969): Methods of diagnosing nutrient deficiencies in citrus.Proceedings of the 7 th Colloquium - International Potash Institute, pp. 160-167. Berne.

Bar-Akiva, A. (1974): Nitrate estimation in citrus leaves as a means ofevaluating nitrogen fertilizer requirement of citrus trees. Proc. Int. Soc.Citriculture 1:159-164. Murcia, Spain.

Bar-Akiva, A. (1975a): Effect of foliar application of nutrients on creasing of'Valencia' oranges. Hort Science 10: 69-70.

Bar-Akiva, A. (1975b): Effect of potassium nutrition on fruit splitting in'Valencia' orange. J. Hort. Sci. 50: 85-89.

Bar-Akiva, A., Hiller, A.V. and Patt, J. (1972): Effect of rootstocks, old cloneand nucellar scion on the mineral composition of citrus tree leaves. Hort.Sci. 47: 73-79.

Bar-Akiva, A., Gotfried, A. and Lavon, R. (1971): A comparison of variousmeans of testing the effectiveness of foliar spray for correcting zincdeficiencies in citrus trees. J. Hort. Sci. 46: 397-401.

Bar-Akiva, A. and Gotfried, A. (1972): Effect of nitrogen and potassiumnutrition on fruit yield and quality and leaf mineral composition ofValencia orange trees. Agrochimica 15: 127-135.

50

Bar-Akiva, A., Tal, D. and Hirsh, J. (1969): Use of aerial sprays for correctingmagnesium deficiency in orange groves. Exp. Agric. 5: 339-342.

Barnette, R.M., De Busk, E.F., Hester, J.B. and Jones, W.W. (1931): Themineral analysis of nineteen years old Marsh seedless grapefruit tree.Citrus Ind. 12: 5-6.

Bazelet, M., Feigenbaum, S. and Bar-Akiva, A. (1980): Potassium fertilizerexperiment in a Shamouti orange grove. Pamph. 220. ARO. The VolcaniCenter, Israel.

Bielorai, H., Dasberg, S. and Erner, Y. (1985): Long-term effects of partialwetting in a citrus orchard. Proc. 3 rd Inter. Drip/Trickle Irrigation Congress.Fresno, CA, USA, 2: 568-573.

Bielorai, H., Dasberg, S., Emer, Y. and Brum, M. (1984): The effect offertigation and partial wetting of the root zone on production of'Shamouti'oranges. Proc. Int. Soc. Citriculture. 1: 118-121. Sflo Paulo, Brazil.

Boman, B.J. (1995): Effect of fertigation and potash source on grapefruit sizeand yield. Dahlia Greidinger Int. Symp. on Fertigation. pp. 55-66. Haifa,Israel.

Bravdo, B., Salomon, E., Erner, Y., Saada, D., Shufman, E. and Oren, Y.(1992): Effect of drip and microsprinkler fertigation on citrus yield andquality. Proc. Int. Soc. Citriculture 2: 646-648. Acireale, Italy.

Calvert, D.V. (1969): Spray application of potassium nitrate for citruscalcareous soils. Proc. Int. Soc. Citriculture 3: 1587-1597. Riverside, CA,USA.

Calvert, D.V. (1970): Response of 'Temple' oranges to varying rates ofnitrogen, potassium and magnesium. Fla. State Hort. Soc. 83: 10-15.

Cantarella, H., Quaggio, J.A., Bataglia, O.C. and van Raij, B. (1992):Response of citrus to NPK fertilization in a network of field trials in SaoPaulo State, Brazil. Proc. Int. Soc. Citriculture 2: 607-612. Acireale, Italy.

Castle, W.S. (1987): Citrus Rootstock. Eds. R.C. Rom and R.F. Carlson.Rootstocks for Fruit Crops. pp. 361-399. John Wiley and Sons, Inc.

Chadha, K.L. (1969): What nutrition citrus requires. Indian Horti. 13(2): 23-27.

Chapman, H.D. (1968): The mineral nutrition of citrus. In: The CitrusIndustry. Ed. Reuther, W. The Citrus Industry 2: 127-289. Riversideedition, Univ. of Calif. Berkley, CA, USA.

Chapman, H.D., Brown, S.M. and Raynee, D.S. (1947): Effects of Kdeficiency and excess on orange trees. Hilgardia 17: 619-650.

Dahlia Greidinger (1995): International Symposium on Fertigation. Technion -Israel Institute of Technology Haifa, Israel 26 March - 1 April 1995.

Dasberg, S. (1987): Nitrogen fertilisation in citrus orchards. Plant Soil 100: 1-8.

Dasberg, S. (1988): Nitrogen and potassium requirment of Citrus. Proc. Int.Soc. Citriculture 2: 625-632. Tel Aviv, Israel.

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Dasberg, S., Bielorai, H., Haimowitz, A. and Erner, Y. (1991): The effect ofsaline irrigation water on 'Shamouti' orange trees. Irrig. Sci. 12: 205-211.

Davenport, T.L. (1983): Importance of iron to plant grown in alkaline soil.Proc. Fla. State Hort. Soc. 96: 188-192.

Davis, F.S. and Albrigo, L.G. (1994): Citrus. Crop production science inhorticulture 2. Cab International, Wallingford, Oxon OX 10 8DE UK.

De Cicco, V., Intrigliolo, F., Ippolito, A., Vanadia, S. and Guiffrida, A. (1988):Factors in Navellina orange splitting. Proc. Int. Soc. Citriculture 1: 535-540. Tel Aviv, Israel.

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8. Appendix

Visual symptoms of nutrient deficiency

Lcaes sho\N ing sxmptoms of nutrient deficiencies were collectedby Dr. Y. liner and I . Macen

fP!utc o I

I caves showingsyiptoms ofpotassullt (K)deticienicy

I/laL \o 2

ea ves ithInafileslum MILt

5.7

J'/utc %V o

I eaves showingsy mptomls ot

boron (B)deficiency

Plate No 4

I.eaves showingsymptoms of iron(:e) deficiency

58

u/ute No, 5

Leaves showinasymptoms of zinc(Zn) deficienc

Leaves x ithmanganese (Mn)deficiency

50