March 1984 Bulletin 536D. Recommended Fertilizers and Nutritional Sprays for Citrus R. C. J. Koo, Editor Agricultural Experiment Stations Institute of Food and Agricultural Sciences University of Florida, Gainesville F. A. Wood, Dean for Research in cooperation with the U.S. Department of AgricultureRECOMMENDED FERTILIZERS AND NUTRITIONAL SPRAYS FOR CITRUS ‘Compiled by R. C. J. Koo, C. A. Anderson, Ivan Stewart, D. P. H. Tucker Citrus Research and Education Center, Lake Alfred D. V. Calvert Agricultural Research and Education Center, Fort Pierce and H. K. Wutscher USDA Horticultural Station, Orlando Acooperative contribution from the IFAS Agricultural Research and Educa- tion Center, Lake Alfred; the IFAS Agricultural Research Center, Fort Pierce; and the U.S. Horticultural Station, U.S. Department of Agriculture, Orlando,CONTENTS INTRODUCTION BEARING ORANGE AND GRAPEFRUIT TREES Time of Fertilization a Amount of Fertilizer to Use Methods of Application . : Elements to be Included in the Fertilizer Program Nitrogen ........ : Phosphorus Potassium ‘Magnesium ‘Manganese Copper Zine Boron .... Molybdenum tron. Caleium Sulfur .. pH Control. : Fertilizer Programs Mixed Fertilizers Straight Materials... _ MANDARINS, HYBRIDS, AND ACID FRUITS «...1......... 13, Dancy ameuavrninaerees 1B Temple 1B Murecott . 13 Orlando * 4 Lemons and Persian (Tahiti) Limes e 4 NON-BEARING AND YOUNG BEARING CITRUS TREES ....... 14 RESETS IN ESTABLISHED GROVES . 15 SOIL AND LEAF ANALYSES 16 Soil Analysis i 16 Soil pH . 17 Soil Calcium: 17 Soil Magnesium . 7 Soil Phosphorus 18 Soil Copper 18 Leaf Analysis... 18 Leaf Nitrogen... 20 Leaf Phosphorus 20 Leaf Potassium 20 Leaf Calcium ... 20 Leaf Magnesium ... 21 Manganese, Zine, Iron, Copper, and Boron. a iiiADDITIONAL CONSIDERATIONS IN FERTILIZER PRACTICES 21 Soil Amendments for Young Trees ........0+0++ 21 Controlled Release Fertilizers ..........0.s0ss 22 Rootstock-Nutrition Relationship ..........1., etn Foliar Application of Macronutrients rely eal Fertigation ........ ae i {eB Some Fertilizer Sources for Macronutrients ....... 23 RECOMMENDED READING ivRECOMMENDED FERTILIZERS AND NUTRITIONAL SPRAYS FOR CITRUS INTRODUCTION Most Florida citrus is grown on sandy soils which are inherently low in natural fertility. Such soils have a low exchange capacity and retain only small amounts of applied exchangeable plant nutrients against the leaching action of rainfall and irrigation. Other applied plant nutrients may be retained in varying degrees of availability in the soil by other mechanisms. For these reasons, citrus trees must be fertilized with some elements abundantly and regularly and with others less frequently to obtain high production of good quality fruit. Citrus will thrive under a wide range of nutrient levels, and it is impossible to outline a single fertilization program that is optimum for all conditions. A wide selection of programs is now being used; many of these programs result in high yields of good quality fruit. ‘The programs outlined here recognize the existence of differences in grove conditions, including soils, rootstocks, scions, tree age, pre- vious fertilization, irrigation, insect and disease control, and other factors. The amount and quality of the fruit crop in any one year are not determined by the ratio of elements or the rates used in any single fertilizer application or even the previous year’s program. They are influenced by production practices, previous crops, and environmen- tal conditions of the past several years. Research in the field of citrus nutrition make it desirable to revise recommendations periodically. Guidelines given in this publication are intended primarily for orange and grapefruit trees on sandy, well-drained soils which are low in organic matter, such as Astatula and Apopka sands. For fine textured and calcareous soils of the flatwoods and lowlands, and specialty fruit varieties, modifications are suggested. ORANGE AND GRAPEFRUIT TREES Time of Fertilization Research and grower experience have shown that only two applica- tions of fertilizer are necessary each year under normal conditions onwell-drained soils. Applications may be made in fall or winter (November to February) and in late spring (May and June), Timing of fertilizer applications is less important than the total amount applied per year. The method chosen, however, should be followed consist- ently, as frequent changes may adversely affect the bearing habits of the tree. Trees planted on coarse, shallow soils, because of the restricted root systems may require three to four applications a year. More frequent applications are sometimes needed in years following hurricane or freeze damage. Amount of Fertilizer The optimum amounts of some elements that can be profitably applied per acre per year have been established through controlled experiments. Soil and leaf analyses, when combined with a knowl- edge of past fertilizer practices, can improve the precision of the fertilizer program. The quantities recommended in this bulletin (Table 1) are based on production performance in terms of field boxes (about 90 pounds of fruit). These rates provide ample supplies of nutrients for new growth, possible yield increases, variations due to rootstock, and scion variety differences. Grapefruit and oranges re- quire about the same amount of fertilizer per acre. Nitrogen is used as a basis on which to calculate fertilizer rates. ‘The rates recommended in Table 1 are applicable in most cases, but both young bearing and extremely high yielding groves may require special consideration. The lowest amounts recommended are for healthy, young, bearing groves still developing productive capacity through tree growth. This is also for older low-producing groves. Neither of these types of groves should receive less than the mini- mum amounts in Table 1, regardless of the yield, because these quantities are ordinarily necessary to maintain trees in good condi- tion. The intermediate ranges in Table 1 cover the normal variation in grove productivity, Fertilization of close planted groves should be guided by fruit production on a “per-acre basis” (Table 1). Fertilizer may be used more efficiently in high density than in low density plantings, but the quantity applied should be based on fruit produc- tion rather than tree spacings. Excessive rates are wasteful and may reduce yield. Trees that have lost substantial proportions of their canopies due to freezes, severe pruning, or water damage will need less total fertilizer applied at more frequent intervals. Trees that have only been defoliated — by freezes, hurricanes, or pests and diseases —may temporarily need more fertilizer. Rates and timing may need to be adjusted to moisture supply, including both rainfall andTable 1. Pounds of nitrogen fertilizer to be applied to furnish annual nitrogen requirement of orange and grapefruit {tees under normal conditions, Nitrogen Fertlizer'Ye ‘Nitrogen a Needed 15.5% 0 335% N 45.0% N Production Orange Grapefruit Orenge _Grapetrult_ Orange Grapefruit Orange Grapetrult Boxesncre ‘Pounds par Acre <200 100 20 es 405 300 225 220 165 ‘300 120 20 775 580 360 270 265 200 00 160, 120 1030 75 420 360 365 265 500 200 150 1200 965 600 450 445 335 600 240 180 1580 1160 75 538 535 400 700 280 210 1605 1355 5 20 620 485 >a00 300 240 1935 1880 85 670 665 500 “Wtogenseededis based on 0.4 pound per bo offaifr fanaa srd0.8pourd pr bon prepara rostcasesdorct se essthan 1O0poundser ‘mor than 300 pounds cf nifogen for erang ard not ess than 90 pounds or more than 240 pods for great per See pet year "he fewes gvon should be dviged bythe nanbr of epplcatons por yer lo ean pounds pa appicatonirrigation. No set rules will ever eliminate the need for skill and insight on the part of the grove operator. Methods of Application In mature, bearing, non-bedded groves, the choice of application equipment is not critical. Uniform distribution of fertilizer material is desirable. In bedded groves, fertilizer placement should be directed toward the beds where root systems are concentrated, rather than in the water furrows, Separate application of fertilizer materials is as satisfactory as mixtures, but the cost factor should be considered. Application directly to the soil by dry or liquid distributors, in dilute concentrations through irrigation systems, or in granules by aircraft, are all effective, but each method has its own economie and practical problems. Elements to be Included in the Fertilizer Program Research has shown that citrus requires 15 chemical clements for satisfactory growth and production. Twelve of these must come from the soil or from applied fertilizers, soil amendments, or foliar sprays. ‘These 12 elements are nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), manganese (Mn), copper (Cu), zine (Zn), caleium (Ca), sulfur (S), boron (B), iron (Fe), and molybdenum (Mo). All 12 of these essential elements are applied to some Florida citrus groves. Carbon (C), hydrogen (H), and oxygen (0) are provided by air and water. Chlorine (Cl) is generally believed to be required for plant growth, but no deficiency has ever been observed in citrus, Nitrogen Nitrogen is the key fertilizer clement affecting yield. It also has a pronounced effect on the growth and appearance of the tree and fruit quality, Because it leaches readily from the soil, little reserve is accumulated, and regular applications are required to maintain an adequate supply to the tree. Many sources of nitrogen are satisfactory for citrus, provided pH control is practiced and no harmful contami- nants (such as biuret or perchlorate) are present in the nitrogen material. Optimum rates of nitrogen application have not been clearly estab- lished for orange and grapefruit despite numerous experiments that have been conducted on both well and poorly drained soils. For high production, mature trees seem to require from 100 to 300 pounds of nitrogen per acre per year (Table 1). Under current cultural prac- atices, it is questionable whether rates above 180 to 200 pounds are justified except under the most favorable conditions, Fruit production inereases were usually limited to 3% to 5% when more than 200 pounds of nitrogen per acre was applied. Growers should consider fruit price and cost of nitrogen to decide if the increased nitrogen use is justified (Fig. 1) 500 — 450 Wi « ° < a w 400 x ° oO z 50 é 3 zB s a a00 No Irrigation « a E a fz uw 250 50 100 150 200 250 NITROGEN ( POUNDS/ACRE ) Fig. 1. Yield response of Valencia orange to nitrogen fertilization with and without irrigation aMost nonirrigated Florida groves need no more than 150 pounds of N per acre per year under normal rainfall, and it seems fairly safe to conclude that few irrigated Florida groves are apt to show benefit from more than 250 pounds N per acre per year. Exceptional groves may respond to more than this, but in most groves, yield may be reduced by excess nitrogen. On fine textured soils, in the Indian River area, use the same rates and a maximum of 150 pounds of total N per acre per year for fresh fruit and slightly more for processed fruit is desired. Nitrogen in- fluences both external and internal fruit qualities. Use of high nitro- gen rates is frequently associated with small, coarse, green fruit. Where good color and size are of special importance, for the fresh fruit market, less nitrogen may be desirable. Since nitrogen will increase both the sugar and acid contents of juice, the use of 150 to 200 pounds of N per acre per year is probably optimum for processed fruit. Phosphorus Phosphorus is usually applied as ammoniated phosphate, normal, or triple superphosphate. Phosphorus from fertilizer applications accumulates in most peninsular Florida soils in available forms. After many years of fertilizer use, most older groves accumulate adequate amounts of available phosphorus in the soil. Thus, mature citrus trees (over 20 years of age) growing on soil which has been fertilized with phosphorus for several years usually do not need annual phosphorus applications. The need for phosphorus can be determined by soil test for the amount of available phosphorus. Leaf analysis also has some value, especially in cases of pronounced deficiency. Occasionally some phosphorus should be applied to replace that removed by the crop. The average fruit crop removes only about 20 pounds P,O, per acre. This need may be met by using 400 pounds of 20% superphosphate per acre or one application of 500 pounds of 10-10-10 mixture every four years. Phosphorus levels are inherently low in many flatwood soils. Phos- phorus should therefore be included in the regular fertilizer program until sufficient reserve is accumulated. The need for further phos- phorus can be determined by soil and leaf tests. Potassium Potassium, normally supplied as muriate of potash (potassium chloride) or sulfate of potash (potassium sulfate), is required annu- ally. Apply the same quantity of potash (KO) as nitrogen, up to a 6maximum of about 250 pounds per acre. On calcareous soils, it may be desirable to apply 25% more potash than nitrogen, especially to varieties which tend to produce small fruit. Excessive potash fertilization of ‘Valencia’ orange grown on acid soils may produce large, coarse, and poorly colored fruit; and in grapefruit, highjuice—acid levels. Insufficient potassium, regardless of soil type or citrus variety, usually results in small fruit and in- creased incidence of fruit splitting, creasing, and plugging. Foliar applications of potassium nitrate have been used ex- perimentally and commercially with varying degrees of success where soil application were not effective. The usual rate is 30 pounds per 100 gallons of spray. Since it may burn the fruit at certain times of the year, discretion should be exercised in using such sprays. Magnesium Magnesium is usually applied as sulfate in mixed fertilizers. Mag- nesium oxide is generally equivalent to sulfate in availability, but some oxide sources are less available than others. Formulators may encounter difficulty with magnesium oxide unless the granular form is used. Citrus trees can obtain magnesium from the dolomite (a combination of magnesium and calcium carbonates) commonly used for pH control. If sulfur is used regularly in pest control and dolomite is used to maintain the soil pH at 6.0 or above, magnesium may not be needed in the fertilizer. If leaf magnesium deficiency symptoms occur, magnesium sulfate or oxide should be applied in the fertilizer, and the rate of magnesium (Mg) should be increased up to 30% of the nitrogen rate until symptoms are no longer present in mature leaves of subsequent flushes. These symptoms often occur in seeded varieties, or in trees grown on caleareous soil or where calcitic lime- stone is used for pH control. For most seedless varieties, magnesium Table 2. Pounds of copper, zinc, manganese, and boron compounds to equal the standard dosage" per 500 gallons of water. Metallic Content Shown on Label (%) Compound 5.0 205 48-50 52-56 60 78-80 Copper 75.0 asd 8.5 7.0 at _ ae Zine 24.0t _ — 93 — 1 6.2 Manganese 75.0 — 85 7.0 63 = — Boron - 1.25 _ tas ate ~ = “The standard dosage (metallic content per §00 gallons) is 3.75 pounds Cu and Mn, 5.0 pounds for Zn, and 0.25 pounds B. {This rate is equal to 2.0 gallons of 5% liquid formulation which will provide 1.0 pound of Zn.at 15% of the nitrogen rate is sufficient, Leaf analysis (see subsequent section) offers an excellent guide to magnesium status, particularly when the level is above the visible deficiency range. If leaf analysis shows more than 0.50% (Table 5), magnesium applications may be omitted until slight symptoms of magnesium deficiency appear, or until leaf analysis shows a low magnesium level. Soil tests for mag- nesium may be used to supplement leaf analysis. A soil magnesium level of 100 to 160 pounds per acre seems desirable depending on the extracting solutions used. Foliar sprays using 7.0 gallons of magnesium nitrate solution (7% magnesium) or equivalent per 500 gallons are effective in correcting ‘Mg deficiency symptoms. Manganese Maganese may accumulate in available form in acid soils as a result of repeated applications when soil pH is maintained in the recommended range, so additional manganese is not required in many groves. Manganese applications may be omitted when there are no deficiency symptoms. Groves showing mild, transitory symp- toms of manganese deficiency on young foliage are common in Flor- ida, but these symptoms are not related to reduced yield or fruit quality. For groves on acid soils where symptoms persist in mature leaves, apply 7 to 10 pounds of Mn (as MnSO,) per acr per year in the fertilizer or use a manganese foliar spray. Insoluble manganese sources are very sparingly available to citrus trees unless incorpo- rated thoroughly into the rooting zone of the tree. On calcareous or heavily limed acid soils, manganese should not be used in the fertilizer, but applied asa foliar spray. Use 3 to5 pounds of manganese per 500 gallons or its equivalent as neutral manganese materials (Table 2). Manganese sulfate is somewhat more effective as a foliar spray than insoluble manganese materials, but all give satisfactory results. Copper Copper sulfate or copper oxide are the usual forms of copper applied in fertilizer. Most of the applied copper accumulates in the topsoil. Excess copper depresses root and top growth and can induce iron chlorosis. No copper applications are needed on soils containing approximately 50 pounds or more of total copper per acre. Sufficient copper usually will be applied in the first 10 years after planting on new grove land if the fertilizer contains about 0.2 unit of Cu. Excessive amounts of copper have accumulated in many groves,with some being found to contain over 600 pounds of total copper per acre in the top 6 inches of soil. Acid sandy soils which contain more than 100 pounds of total copper per acre in top 6 inches of soil are potentially phytotoxic. The pH of such soils should be maintained above 6.5, and no further copper should be applied in the fertilizer. Fungicidal copper sprays should be reduced or avoided if possible. Somewhat higher levels of copper can be tolerated in caleareous and organic soils without injury to the trees. Apply no copper in the fertilizer to groves over 10 years old, unless copper deficiency symptoms occur, Omit copper from the fertilizer on any grove which receives copper fungicidal sprays. Do not use copper in fertilizer for young trees replanted on old grove sites. Copper sprays are effective, produce quick results, and should be used to correct visible symptoms of copper deficiency. In addition, they can provide control of greasy spot, scab, and melanose. Zinc Zinc sulfate or zinc oxide are the usual forms of zinc used. Zine should be applied as a nutritional spray during the post-bloom period when deficiency symptoms appear. Annual foliar spraying with zine is necessary only when zinc deficiency symptoms persist in the foliage. Correction of severe deficiency may require more than one spray per year, but rates lower than specified in Table 2 may be adequate in old groves. Zinc deficiency is frequently one of the symp- toms associated with blight, but correction of zine deficiency will not control blight. Zinc applications in the fertilizer do not give rapid correction of zine deficiency. Massive applications incorporated by cultivation into the surface of acid soils (on the order of 200 to 400 pounds zinc sulfate or 100 to 200 pounds zinc oxide per acre) may bring about the correc- tion of zinc deficiency, but the results may not be apparent for about a year. Correction with the oxide will be slower than with sulfate. Soil applications may last for several years. Soil applications are rarely economically practical. Boron Boron is usually applied as borax or other soluble borates. Care should be exercised in the use of boron, since there is a relatively narrow range between deficient and toxic levels, Boron may be ap- plied either in the fertilizer or in nutritional sprays, but both should not be applied in the same year. As a maintenance program, apply boron in the fertilizer at an annual rate equivalent to 1/300 of thenitrogen rate, This amount may be applied by including borax in either one or all fertilizer applications made during the year. For maintenance spray applications, 1.25 pounds of soluble borate, con- taining 20% B, per 500 gallons may be used (Table 2). Where defi- ciency symptoms are present, double the amounts suggested. Boron use on arsenated grapefruit trees has a tendency to offset arsenic-induced peel gumming and fruit malformation, but use of borax or copper with arsenic will reduce the effectiveness of the arsenic application. Molybdenum Foliar sprays are recommended to correct molybdenum deficiency, which is commony known as yellow spot. Symptoms usually appear in late summer and are occasionally severe in trees on grapefruit rootstock. Spray trees showing mild yellow spot leaf symptoms with 5.0 ounces of sodium molybdate per 500 gallons; in several cases, spray with 10.0 ounces. Molybdenum sprays should be applied in spring or summer. Sprays applied in October or later may not result in regreening of the yellow spots, but will prevent the occurrence of the symptoms the following summer. Molybdenum sprays should be applied only to groves showing yellow spot. Oceurrence of yellow spot is usually associated with acid soils. Prevention can be achieved through liming. Soil applications of molybdenum salts have not been effective in correcting molybdenum deficiencies. Iron Iron chelates, ethylenediamine tetraacetic acid and its hydroxy- ethyl form (FeEDTA and FeEDHAOH), may be used to correct: iron deficiency. On acid soils apply about % ounce of actual iron per tree. For example, application of this amount of iron requires about 14 ounces per tree of a chelate containing 5% iron. Half of this amount is sufficient to correct mild cases of iron chlorosis. The iron chelate may be applied to the soil around the chlorotic trees or may be mixed with fertilizer. It may require 6 to 8 weeks before any greening takes place. On calcareous soils (pH above 7.0) where the above compound is ineffective, soil application of technical sodium ferric ethylene- diamine di-(o-hydroxyphenylacetate), (Sequestrene 138-Fe) at the rate of ¥% to % ounces of iron (Fe) per tree is recommended. These amounts are equivalent to 8 to 12 ounces of chelate containing 6% iron. The 8-ounce rate should be used for bearing trees of ordinary 10size and the 12-ounce rate for unusually large bearing trees. When applied in early summer, these treatments will usually correct severe iron deficiency on calcareous soils within 2 months, Chelate is much less effective when applied in the winter. Sequestrene 138-FE is expensive, so it is recommended that it be applied separately by hand to only those trees on calcareous soils with rather extensive iron deficiency symptoms. Because of the fineness of this iron chelate powder and the small amount required per tree, it should be mixed with enough sand, grove soil, or other inert filler so that about 5 pounds of material is applied per tree. Care must be taken during the application of iron chelate while fruit is on the trees, because chelate dust may severely burn fruit on which it settles. A dust-free form of the chelate is recommended if it is applied with a fertilizer distributor. Use caution in applying iron chelate to young trees, because there is danger ofsevere damage from over-dosage. Sequestrene 138-Fe is subject to photo-decomposition. It should be incorporated into soil. Iron sulfate has not been satisfactory for correcting iron chlorosis on either acid or calcareous soils. Foliar sprays of iron, regardless of formulation, are not recommended. Calcium Most newly planted acid soils in Florida do not contain sufficient calcium for optimum growth of young trees. However, sufficient amounts for nutritional purposes are generally supplied in dolomitic or calcitic limestone, calcium nitrate, and normal superphosphate. Irrigation water from deep wells is another source of calcium. Soil tests are useful in monitoring the calcium level in grove soils. Sulfur Sulfur applied for mite control, sulfates used in fertilizer mixtures, and sulfur compounds in irrigation water more than meets the nutri- tional needs of citrus trees. On calcareous soils sulfur can be used to lower soil pH. PH Control It is recommended that the pH of acid soils in bearing groves be maintained between 6.0 and 7.0. In young non-bearing groves on acid soils, liming material should be applied as necessary to gradually raise the pH to 6.0. As groves mature, accumulation of several ele- ments in the soil is common. Copper is the most important of these, aLand it has accumulated to a potentially toxic level in some groves. ‘Therefore, with increasing age of the groves, some increase in soil pH level is desirable. Ifthe soil is known to contain 100 pounds or more of copper per acre in the top 6 inches, the pH should be maintained between 6.5 to 7.0, regardless of tree age. Either calcitic or dolomitic limestone may be used to adjust pH. Calcitic limestone acts more quickly, but the effects of dolomitie limestone are more persistent. Dolomitic limestone will supply mag- nesium in groves where the pH has been raised through routine applications, Either liming material will raise the soil pH above 7 if used in excessive quantities. High soil pH will interfere with absorp- tion of phosphorus, potassium, magnesium, and micronutrients. All applications of liming material result in a pH response, although at times this response may be delayed because of dry weather or concealed by subsequent application of acidic fertilizers. Finely ground limestone and oolitic aragonite react more rapidly than the coarse limestone materials. The amount of liming material needed depends on the pH change desired and the buffering capacity and organic matter content of the soil, the amount of acid- or base-forming fertilizer applied, and the amount of sulfur used for pest control. No specific recommendations can be made to meet the needs of all groves. Five or more tons per acre may be required to change materially the pH of highly organic soils. such as peat and mucks. Most irrigation water from deep wells is alkaline and has a pH between 7.0 and 8.0. Continued use of such irrigation water will raise the pH of the soil. Fertilizer Programs Mixed Fertilizers The amount and analysis of fertilizer must be considered in select- ing a fertilizer program. For example, 1 ton of 16-0-16 mixture contains the same amount of plant food as 2 tons of 8-0-8. High analysis mixtures have the advantage of being less bulky with atten- dant lower handling and transport costs. On the other hand, it is not always advisable to use the highest analysis fertilizer available, because the physical condition of the mixture may make spreading difficult or unsatisfactory. Generally, it will be advantageous to use a fertilizer as high in analysis as the manufacturer can deliver to the grove in good physical condition for satisfactory spreading. 12Straight Materials Under certain conditions, it may be more economical to apply individual fertilizer materials separately rather than as mixtures. The two methods are equally satisfactory provided similar amounts of essential elements are applied in both cases. Liquid fertilizers for soil application have the same value to the trees as dry fertilizers of the same guaranteed analysis and contain- ing the same fertilizer chemicals. The choice between liquid and dry fertilizers should be made on the basis of relative over-all cost of the fertilizer program. MANDARINS, HYBRIDS, AND ACID FRUITS Dancy Nitrogen requirements of Dancy tangerine are similar to that of oranges. Higher rates will not appreciably increase yield, but will increase green color of the fruit and acid content of the juice, thus delaying maturity slightly. Potassium rates higher than those re- commended for oranges apparently will not increase juice acid con- tent or green color as in Valencia oranges, but there is no evidence that higher rates are advantageous in commercial groves. Temple Follow the recommendations for oranges in Table 1. A ratio of 1:1 for N and K,0 is recommended. Since Temple is a fresh fruit variety, avoid excessive fertilizer rates, especially of nitrogen. Murcott This variety usually bears large crops in alternate years. It encoun- ters extreme nutritional stress in heavy bearing years because of the large requirement of the crop for nitrogen and potassium. During these heavy bearing years, Murcotts have responded to heavy nitro- gen and potassium fertilization, up to 400 pounds of N and KO per acre, with about half the amount applied between September to December in several applications during the ripening of the fruit. Lighter applications, in proportion to the crop set, should be made in the off-bearing years. The ratio of elements should be the same as for oranges. Mechanical and chemical thinning during heavy fruit set years may increase fruit size and reduce the incidence of tree collapse from overbearing. 13Orlando Surveys and grove observations indicate that Orlando tangelos are more susceptible to nitrogen deficiency than sweet oranges. Yellow- ing of leaves is frequently observed in the fall and winter, and leaf loss due to nitrogen deficiency can be excessive. Somewhat heavier nitrogen rates of 200 to 300 pounds N per acre per year divided into three or more applications will help alleviate this problem. Lemons and Persian (Tahiti) Limes Limited research on acid sandy soils in the southern Ridge section indicates lemons and limes respond best to fertilizer rates about 25% higher than for oranges. Bearing groves, even during the early years, benefit from application of about 250 pounds N per acre per year. Similar rates are recommended for caleareous soils, A nitrogen:pot- ash ratio of 1:1 for limes and 1:1.25 for lemons is suggested. Mareotted (air layered) limes are particularly susceptible to mag- nesium deficiency and conventional applications of soluble magne- sium may fail to correct deficiency symptoms. This can readily be overcome by supplementing soil applications with foliar sprays using 7.0 gallons of magnesium nitrate containing 7% magnesium or equivalent solution per 500 gallons. NON-BEARING AND YOUNG BEARING CITRUS TREES ‘This section primarily concerns the fertilization of groves up to 7 years of age, planted in previously uncultivated soils. The soil condi- tions found in such cases are different from those found in heavily fertilized old-grove soils. Uncultivated soils generally contain very low levels of all essential elements. Therefore, young trees in pre- viously unfertilized soils should receive regular applications of most essential elements. Many fertilizer formulas can be satisfactory; however, the most desirable formulas should contain adequate amounts of all elements essential to good tree growth. The following ratio of elements will satisfy the requirements of young, non-bearing citrus trees under most conditions: N-1, P05-1, K,O-1, Mg-1/5, Mn-1/20, Cu-1/40, B-1/ 300. A fertilizer mixture such as 8-8-8-1.6-0.4-0.2-0.025 will contain the elements in this ratio and order. Higher analysis mixtures are generally more economical and may be used, but they are more difficult to apply uniformly to the roat zone. Care should be taken to avoid root damage due to excess salt concentrations in localized areas brought about by uneven distribution. Spread fertilizer evenly in a 3-foot diameter circle the first year. Avoid placing fertilizer against 14the trunk. After the first year, the fertilizer area should be steadily enlarged with each application. A good rule is to cover an area twice the diameter of the tree canopy. Thus, for a tree with a 2-foot canopy, apply the fertilizer unformly over a circle 4 feet in diameter. A suggested schedule of fertilization for a grove planted during the dormant season is shown in Table 3. The rate and number of applica- tions of fertilizer should be based primarily on tree age and tree condition. Trees should be fertilized about every 5 to 6 weeks during the growing season of the first year. The frequency of application may be reduced in succeeding years. Rates of application the first year should be relatively low because of the limited root system of the tree, but it should be increased sharply in succeeding years. Fertilizer applications should be omitted between October 1 and February 1 for the first year or two to reduce the possibility of inducing untimely growth flushes in the winter. On acid soils, an application of dolomitic limestone should be made before planting, so it will influence the pH level during early tree growth. The pH should be tested and adjusted annually to attain the recommended levels. Emphasis the first 5 years should be in promoting vigorous tree growth, and the quality of the crop should be secondary. From the third to seventh year, the trees may come into commercial bearing but still benefit from a complete fertilizer. This is the period when the program should be changed to that for bearing trees. Nutritional sprays may be used when needed. Table 3. Suggested fertilization of young citrus trees up to 7 years of age. Number of Pounds per Application Applications per Tree* Years in Grove each Year (Range) First 56 0.75-1.25 Second 45 1.75-2.25 Third 34 3.0-4.0 Fourth 34 3.5-4.5 Fifth 3-4 4.0-5.0 Sixth 34 4.55.5 Seventh 34 5.0-6.0 “Use 8-8-8-1.6-0.4-0.2-0.025 mixture or equivalent. RESETS IN ESTABLISHED GROVES Trees in replant areas or individual replants in mature groves should receive a young tree fertilizer mixture containing mostly 16nitrogen, potassium, and magnesium. There should be an adequate supply of phosphorus and micronutrients in the soil which are not readily leached from fertilization of the mature trees. A mixture of 8-2-8-2 or equivalent is suggested. Resets in established groves should receive the supplemental fertilizers in addition to the routine mature tree program. Nutritional sprays are warranted only if de- ficiency symptoms appear SOIL AND LEAF ANALYSES Soil and leaf analyses provide an opportunity for more precise control of tree nutrition, permit the detection of symptomless det- rimental conditions in the soil or tree, confirm the nature of visible symptoms, and may permit greater precision in the fertilizer program. If'soil and leaf analyses are to be of value, they must be carried out precisely according to standardized procedures. These procedures are somewhat difficult, but the importance of adhering to them cannot be overemphasized. If proper sampling, analytical procedures, and in- terpretation are not followed, soil and leaf analyses will prove expen- sive and misleading. Soil Analysis When properly used, soil analysis can serve as a useful guide in improving the lime and fertilizer programs for a particular grove. Tests are especially informative when continued over a period of years so that trends can be followed. Soil tests for pH, calcium, magnesium, phosphorus, and copper are useful, Soil tests for the more readily leached elements, such as N and K, are of little value, since it is very difficult to establish a correlation between yield responses and levels of the fertilizer elements in the soil. ‘The reliability of soil analyses depends upon an accepted method of collecting the soil samples. Each soil sample should consist of a composite of about 16 soil cores taken to a depth of 6 inches from an area at the dripline of the tree (the outer edge of the tree canopy). One core should be collected from the dripline of each of the 16 different trees scattered throughout the area of the grove represented by that particular sample. Separate samples should be taken for each part of the block that is visibly different in tree or soil characteristics. Where both soil and leafanalyses are utilized routinely, the area from which soil samples are taken should correspond to those areas where leaf samples were collected. Regardless of the apparent uniformity of a plock, no one sample should represent an area larger than about 20 acres. Thus, an 80-acre grove would require a minimum of four soil 16samples. On large acreage, indicator blocks may be used. Soil analysis can sometimes serve as a diagnostic tool in identify- ing localized problems. In such cases, a minimum of two samples should be collected: one sample should be from the problem area and the other from an adjacent apparently normal area. Samples should be collected according to a regular schedule. If samples are to be collected once a year, the best time would be in late summer after the rainy season and prior to fall fertilization. Each sample should be air-dried, screened, and thoroughly mixed before analysis Interpretation of soil analysis results is not possible without con- sideration of extraction procedure used. Soil pH Soil pH of every citrus grove on acid soils should be determined annually, A 1:1 volume of soil and distilled water should be used. The mixture should be allowed to stand 30 to 60 minutes with occasional stirring, before reading the pH with a meter. Color tests are not satisfactory for pH determinations. Soil Calcium Experimental data are not yet adequate to indicate the optimum calcium level in soils of different exchange capacities. Using a neu- tral extractant such as ammonium acetate buffered at pH 7, a soil calcium level of 500 to 700 pounds of Ca per acre (about 850 pounds for Double Acid extraction procedure) on soils similar to Astatula fine sand seems desirable. Soils having a higher exchange capacity would have a higher minimum requirement for calcium, The use of gypsum to supply calcium is not recommended. If soil calcium analyses are to be effectively utilized, determinations should be made as regularly as for soil pH measurements. Soil Magnesium Soil magnesium is usually determined along with soil calcium. Using the neutral ammonium acetate extracting solution for cal- cium, a soil magnesium level of 100 pounds per acre seems desirable. The equivalent level with Double Acid extracting solution would be 160 pounds per acre. Acaleium: magnesium ratio of 7:1 seems desirable. A ratio of 10 or higher indicates potential magnesium deficiency and may need a magnesium supplement in the fertilizer. Heavily dolomited soils usually have a low calcium-magnesium ratio. 7Soil Phosphorus For acid sandy soils, there are sufficient data correlati performance with soil test values for phosphorus to make. tests useful supplements to general recommendations for fertilization. : Soil samples may be analyzed by any of the four methods listed in Table 4. Soils that test higher than the levels given probably do not need immediate phosphate additions. Soils that test lower than these amounts should receive some phosphate, either in the regular fertil- izer program, as recommended for young bearing trees, or from a separate application of superphosphate. Since phosphate is fairly stable in the soil, soil phosphorus deter- minations need to be made only every few years. Soil Copper Acid sandy soils should be tested for excessive value over 100 pounds of total copper per acre ind phytotoxicity, and the soil pH needs to be maintained Total copper is extracted by hot digestion in a mixt trated nitric and perchloric acids Table 4, Adequate phosphorus test values for soil sz groves on acid sandy soils. Extraction Method” Acid ammonium acetate Bray P; Bray P2 Mehlich | (Double Acid) “The acid ammonium acetate extracting solution is appre nium acetate adjusted to pH 4.8 with acetic acid. The Bray ammonium fluoride and 0.025 N hydrochloric acid, The | Nammonium fluoride and 0.1 Nhydrochloric acid. The: (0.05 W HCI and 0.025 N H2SO,) has not been calibrates: correlated to leaf P uptake. The preceding values for mined from field experiments with agronomic crops on $2 Florida. izer rate and source experiments, and numerous su 18conducted in commercial citrus groves. A considerable amount of information has been accumulated in this way, making it possible to establish acceptable levels of the various elements in leaves. The main usefulness of leaf analysis lies in the establishment of trends in the nutritional status of the trees resulting from certain fertilizer practices over a period of years. These trends can best be detected by a regular annual leaf analysis program. Leafanalysis can be helpful in determining the cause of localized or general production problems, The importance of proper sampling of leaves cannot be overempha- sized. Analysis of leaf samples is expensive, and an improperly selected leaf sample may provide misleading rather than helpful information. The standard leaf sample consists of at least 100 4- to 6-month-old spring flush leaves from at least 20 trees taken from nonfruiting twigs. Thus, leaves should be collected between July and September. ‘The area represented by one sample should be of uniform general tree appearance, of a single variety and rootstock, under one fertilizer program, and usually not larger than 20 acres. Leaf samples should be delivered to the analytical laboratory promptly in fresh condition if they require washing (see next paragraph), If the leaves must be mailed, ship them in a plastic bag promptly after picking. Leaves that do not require washing may be air-dried promptly after picking and then shipped. Values for iron are valid only if the leaf samples are individually and thoroughly washed with a detergent solution and rinsed in deionized or distilled water before drying. Leaves that have been sprayed with copper, zinc, or manganese should not be analyzed for these elements even if washed, as it is impossible to eliminate in- terference from surface contamination. If the levels of an element found by leaf analysis are near the extremes of the ranges given in Table 5, the fertilizer program should be adjusted accordingly. Leaf analyses in the low to deficient range in Table 5 may be associated with visible symptoms of deficiency or reduced yield. Leaf analyses in the high to excess range usually are associated with wastefully high rates of fertilization, and may indi- cate the potential for reduced yield. The mineral content of leaves of young trees is usually higher than that of mature trees because of more frequent fertilization and continuous growth of new flushes. The relationship of mineral composition of leaves to fruit quality varies with different elements. High quality fruit may be associated with high or low leaf levels, depending on the effect of the individual elements. These relationships are discussed below 19Table 5. Leaf analysis standards for citrus based on 4 to 6-month-old spring-cycle leaves from nonfruiting terminals. Element Deficient Low Optimum N (%) <22 222.4 25-27 23.0 P (%) <.09 09-41 12.16 >30 K (%) <7 1 1.21.7 >24 Ca (%) 7.0 Ma (%) <.20 20-29 30-49 >.80 Cl (%) z 2 05-.10 >20 Mn (ppm) =< 17 18-24 25-100 >500 Zn (ppm) S17 18-24 25-100 >300 Cu (ppm) <3 34 5-16 > 20 Fe (ppm) <35 36-59 60-120 >200 8 (ppm) <20 21-35 36-100 >250 Mo (ppm) — <.05 06-09 10-1.0 >5.0 NOTE: See text for method of taking samples of leaves. Leaf Nitrogen Nitrogen concentrations below 2.2% (Table 5) are generally associ- ated with pale green or yellow foliage. Low nitrogen levels may result in premature drop of older leaves, resulting in thinner canopies Where good fruit color is of special importance, as in the fresh Sait trade, the lower portion of the optimum range for leaf nitrogen i= preferred. Nitrogen will increase both soluble solids and acid con- tents of the juice. For maximum yield of processed fruit, i may Be desirable to maintain leaf nitrogen at the upper limit of the optimum range. Leaf Phosphorus The performance may be satisfactory over the entire range given mm Table 5. Values below 0.09% usually indicate deficiency. but the effects of higher values are not very significant and are dificult to interpret. Low leaf phosphorus is usually associated with hich acid content in fruit. Leaf Potassium Production may be satisfactory over the optimum range given When fruit size is too large or juice acidity too high. it may prove advantageous to maintain leaf potassium near the lower limits ofthe optimum range shown in Table 5. Likewise, when fruit size becomes too small or acidity too low year after year; leaf potassium level should be raised to near the upper limit of the optimum range Quite often a compromise is necessary, in which case the mid-range is 20satisfactory. Creasing, plugging, and fruit splitting are frequently associated with low potassium levels. Leaf Calcium There is a wide range of satisfactory leaf calcium levels. No par- ticular significance need be attached to the analysis values as long as they fall within the range given. Leaf calcium increases with leaf age. Leaf Magnesium The leaf symptoms of magnesium deficiency are readily identified. Leaf magnesium levels of less than 0.3% in 4- to 5-month-old leaves often show deficiency systems later in the year especially on seeded varieties with heavy fruit crop. Leaf magnesium levels above 0.3% have only minimal effects on fruit production and quality. Leaf Manganese, Zinc, Iron, Copper, Boron The leaf deficiency symptoms of manganese, zinc, and iron are readily identified. Their occurrence on a few twigs and branches gives adequate warnings of approaching deficiency levels. Uneconomically high levels may be detected by leaf analysis Copper deficiency symptoms probably will not appear at the lower limit given in Table 5. Levels above 20 ppm copper indieate toxicity or contamination by foliar spray. There is little correlation between leaf and soil copper. Soil analysis is a better indicator of high copper status. The leaf symptoms of boron deficiency are poorly defined, but those of toxicity are quite characteristic, producing yellowing of leaves with brown gum spots on both leaf surfaces. Leaf analysis is useful in identifying groves near deficient or toxic boron levels. ADDITIONAL CONSIDERATIONS IN FERTILIZER PRACTICES ‘This discussion concerns practices directly or indirectly related to citrus fertilization, In some cases, enough research data are available to warrant recommendations. In others, discussions are based on the best information available and no recommendations are made. Soil Amendments for Young Trees The incorporation of soil amendments in tree planting holes to increase the water and nutrient holding capacities of the soil has 21been tried with varying degrees of success. The frequency of wateran= in newly planted trees can be reduced with the addition of clay and other amendments. The amendments should be mixed well with the soil at the time of planting. The quantity of amendments used will depend on the waier holding and exchange capacity of the compound in question. The following amendment rates are suggested: Attapulgite clays 10-16 Ibs per tree Soft rock phosphate 15-20 lbs per tree Vermiculite 3 Ibs per tree The use of sludge and other organic compounds has been tried by growers. No specific recommendation can be made because of the wide variations in the composition of the compounds, Broadcast ap- plications of amendments are not feasible because of the large quan- tities of materials required. Controlled Release Fertilizers Most controlled release fertilizers presently available are nitrogen carriers, although formulations involving other elements are also under evaluation. Controlled release fertilizers supply plant nutri- ents to the trees for longer periods, thus reducing leaching losses. Their use makes possible a reduction in the frequency of fertilizer applications to young trees. Limited research data preclude recommendations for their use on mature bearing or nonbearing trees. Tree performance and economic considerations should determine the usefulness of controlled release fertilizer. Rootstock-Nutrition Relationship Limited controlled studies and field observations showed that trees on certain rootstocks are more susceptible to mineral deficiencies than others. For example, trees on Carrizo and Troyer citrange root- stocks are quite susceptible to manganese and magnesium deficien- cies on both acid and calcareous soils. Other rootstocks which pro- mote magnesium deficiency are C. macrophylla and Severinia. Trifoliate orange and trifoliate hybrid rootstocks are very susceptible to iron deficiency, especially on calcareous soils. Lime marcotts (air layers) are susceptible to magnesium deficiency. To correct these deficiencies, separate applications of the elements in question are usually necessary, in addition to regular fertilizer applications. Foliar Application of Macronutrients Foliar applications of nitrogen, phosphorus, potassium, calcium, and magnesium are not usually recommended because citrus leaves 22are inefficient in absorbing nutrient elements in any appreciable quantities. In most cases, soil application of macronutrients are more economical and effective. There are, however, circumstances where nutrient elements are fixed in the soil and not readily available to the plants. Under these conditions, supplemental foliar applications may be beneficial. Urea, polyphosphate, potassium, and magnesium ni- trates have been used with varying degrees of success. Only urea with a biuret content of less than 0.25% should be used to avoid biuret toxicity, Fertigation Application of fertilizer materials through irrigation systems is referred to as fertigation. This practice has potential savings in fertilizer application cost and offers increased flexibility in schedul- ing. Fertilizers can be applied through overhead sprinkler and under canopy low volume irrigation systems. Certain factors should be considered when fertilizer materials are applied through sprinkler or low volume irrigation systems because of differences in the two types of irrigation systems. For fertilization through permanent overhead sprinkler irrigation systems, both liquid or fine-textured suspended fertilizers are suit- able. Foliage should be wetted before any fertilizer is applied and should be rinsed free of fertilizer salts before the irrigation cycle is completed. A normal fertigation operation consists of 1 hour to wet the leaves, 1 hour to apply the fertilizer materials, and 4 to 6 hours to rinse off the fertilizer salts from the leaves. Fertilization through low volume irrigation systems involves dif- ferent guidelines with coverage being the prime consideration. Fer- tigation should not be attempted with irrigation systems designed to cover only a small portion of the tree rooting area, because insuf- ficient nutrients will be absorbed. Fertigation of newly planted trees with some low volume irrigation systems is often wasteful, as too large an area is covered. To minimize clogging of emitters, only liquid fertilizer should be used with low volume irrigation systems. The water application rate during the fertigation operation should be sufficient to bring the soil moisture content in the root zone to field capacity. Excess water will result in leaching of nutrients below the root zone, No specific recom- mendation can be made because of the differences in soil types, tree planting patterns, and irrigation system design. Some Fertilizer Sources for Macronutrients Some of the fertilizer compounds in common use are listed in Table 6 for reference purposes. 23‘Table 6. Sources of some ferilizer materials. Nuivient Content % Material N POs Ko ca a Properties inorsane ‘Ammonium nate 30 = 3 & = ‘krmonium sata 200 = = E Gatcum nivate 155 200 Sodium rivate eo = iNivato of soda potash 150 Potassim nivate 130 = = Diamant phosphate 180 460 Superohosphate = 200 200 eS Tipe superphosphate = 60 ea = Fock phosphate = 28-35 25.35 = Hegre Se = = = 1020 Sula ofpolash-magnesia = = = ioe Magrosim oxide = = = ps um carbonate = = as 30 of posh = = = = lo of potashOrganic res" 460 = - = Casior pomace 80 W 5 = = Catton Seed meal 0 25 18 z = ‘Guano, Poru 130 80. 20 = = ‘Activated sludge 50 32 - = = Organi" 270 = = = = Lut ‘Ammonium nitrate 210 = = ze = ‘Aninydrous ammonia 20 = = = s LUree-ammonvum nitrate 320 = = = = Magnesium nitrate 70 = 2 = 66 Polyphosphate 100 340 = = = Controtied release nitrogen Suilur coated urea (SCU) 3237 = - Niroform 380 = = = = Isobutyidene diurea (IBDU) 310 = Quich’y Slowly Slowly Moderately ‘Siowly Siowiy Crystlization 5G Moderately 320 a6 Release time 36 months 6-12 months 3-12 months “Urea and orgarorm are syntote organics. To avid Bwattcly ony ure wih ow Burt coriart shoud be usedRECOMMENDED READING Publications on Fertilizing Florida Citrus Anderson, C.A. 1966, Effects of phosphate fertilizer on yield and quality of Valencia oranges. Proc. Fla. State Hort. Soc. 79:36-40. ____. 1968. Effects of gypsum as a source of calcium and sulfur on tree growth, yields, and quality of citrus. Proc. Fla. State Hort. Soc. 81:19-24. ___—. 1970. Dolomitic limestone as a source of magnesium for citrus. Proc. Soil and Crop Sci. Soc. of Fla. 30:150-157. . 1971, Effects of soil pH and calcium on yields and fruit quality of young Valencia oranges. Proc. Fla. State Hort. Soc. 84:4-10. —____. 1980. The generation of soil acidity from agricultural fertilizers and chemicals. The Citrus Industry 61(8):8-13, _ 1982. Comparison of several zinc foliar treatments for correction of zine deficiency in citrus trees. Proc. Soil and Crop Sci. Soc. of Fla. 41:169- 171. ____. 1982. Persistance of accumulated copper in soils of citrus groves. Proc. of Fla. Fertilizer and Lime Conf. 12:6-15. 1983. Comparison of Mehlich Land Bray I soil tests for phosphorus in citrus groves. Proc. Soil and Crop Sci. Soc. of Fla. 42:146-149. Anderson, C. A., and L. G. Albrigo. 1977. Seasonal changes in the rela- tionships between macronutrients in orange leaves and soil analytical data in Florida. Proc. Int, Soc. Citriculture 1:20-25. Anderson, C. A., and D. V. Calvert. 1967. Effects of phosphate fertilizer on young citrus trees on flatwood soils. Proc. Fla. State Hort. Soc. 80:19-25. Anderson, ©. A., and F. G. Martin. 1969, Effects of soil pH and calcium on the growth and mineral uptake of young citrus trees. Proe, Fla, State Hort. Soe, 82:7-12. Calvert, D. V. 1969. Effects of rate and frequency of fertilizer applications on growth, yield and quality factors of young Valencia orange trees. Proc. Fla. State Hort. Soc. 82:1-7 ____. 1970. Response of Temple oranges to varying rates of nitrogen, potassium and magnesium. Proc. Fla. State Hort. Soc. 83:10-15. 1978. Response ofMarsh’ grapefruit trees in the Indian River area to potassium application—yield and fruit quality. Proc, Fla. State Hort. Soc. 86:13-19. Calvert, D. V., R. R. Hunziker, and H, J. Reitz. 1962, A nitrogen source experiment with Valencia oranges on two soil types in the Indian River area. Proc. Fla. State Hort. Soc. 75:77-82. Calvert, D, V,, and H..J. Reitz. 1964. Effects of rate and frequency of fertilizer applications on yield and quality of Valencia oranges in the Indian River area. Proc. Fla. State Hort. Soc. 77:36-41. 26. 1966, Response of citrus growing on calcareous soil to soil and foliar applications of magnesium. Proe. Fla. State Hort. Soc. 79:1-6. Calvert, D. V., and R. C. Smith. 1971, The correction of potassium deficiency of citrus with KNOs sprays. Jn Proc. of Symposium on Fertilizer-Pesticide Combinations. Agri. and Food Chem. Soe. 20:659-661. Calvert, D, V., E. H. Stewart, R. S. Maneell, J.G. A. Fiskell, J. , Rogers, L. Hallen, Jr., and D. A. Graeiz. 1981. Leaching losses of nitrate and phos. phate from a spodosol as influenced by tillage and irrigation level. Soil Crop Sei. Soc. Fla. Proc. 40:62-71. Campbell, C. W., and P. G. Orth. 1968. Effects of three years of differential nitrogen and potassium application on Tahiti lime yield and leaf analysis on Rockdale soil in Florida. Proc. Fla. State Hort. Soc. 81:336-340. Cooper, T.,Jr., A. H. Krezdorn, and R. C. J. Koo. 1969. The nutritional status of the Orlando tangelo (Citrus paradisi Macf. ‘Duncan’ X C. retaculata Blanco ‘Dancy’. Proc. Fla. State Hort. Soc. 82:34-38, Ford, H. W., I. Stewart, and C. D. Leonard. 1954. The effect of iron chelates on root development of citrus. Proc. Amer. Soc. Hort. Sei. 63:81-87. ley, J. R, 1977. Leaf tissue analyses in the fertility program. Proc. Fla. State Hort. Soc. 90:17-18. Jackson. 1981. Plant food requirement for freezer damaged citrus. Proc. Fla. Fertilizer and Lime Conf. 11:15-18. Koo, R. C. J. 1962, The use of leaf, fruit and soil analysis in estimating potassium status of orange trees. Proc. Fla. State Hort. Soc. 75:67-72. . 1967. Effects of soil amendments on soil moisture and growth of young orange trees. Proc. Fla, State Hort. Soc, 80:26-32, 1971. A comparison of magnesium sources for citrus, Proc. Soil and Crop Sei. Soe. Fla. 31:137-140, . 1979. The influence of N, K, and irrigation on tree size and fruit production of ‘Valencia’ orange. Proc. Fla. State Hort. Soc. 92:10-13. ——.. 1980. Results of citrus fertilization studies. Proc. Fla. State Hort. Soc. 93:33-36. Koo, R.C.J.,and A. A. McCornack, 1965. Effects of irrigaton and fertilization on production and quality of Dancy’ tangerine, Proc, Fla. State Hort. Soc, 78:10-15. Koo, R. C. J., and R. L. Reese. 1971. The effects of omitting individual nutrient elements from fertilizer on growth and performance of Pineapple orange. Proc. Fla, State Hort. Soc. 84:11-16. Koo, R. C.J., and R. L, Reese. 1972. A comparison of potash sources and rates for citrus. Proc. Fla, State Hort. Soc. 85:1-5. 1976. Influence of fertility and irrigation treatments on fruit quality of Temple’ orange. Proc. Fla. State Hort. Soe. 89:49-51 ———. 1977. Influence of nitrogen, potassium and irrigation on citrus fruit quality. Proc. Int. Soc. Citriculture I.34~38. 27Koo, R. C. J., H. J. Reitz, and J. W. Sites. 1958. A survey of the mineral nutrition status of Valencia’ orange in Flordia. Fla. Agric. Expt. Sta. Tech. Bull. 604. Koo, R. C.J. and T. W. Young. 1969. Correcting magnesium deficiency of limes grown on calcareous soils with magnesium nitrate. Proc. Fla. State Hort. Soe. 82:274-278, Koo, R. C.J., T. W. Young, R. L. Reese, and J. W. Kesterson. 1973. Responses of ‘Bears’ lemon to nitrogen, potassium and irrigation applications. Proe. Fla. State Hort. Soc. 86:9-12. Leonard, C. D. 1969. A comparison of soil and spray applications of four manganese sources for control of manganese deficiency in Valencia orange trees. Proc, Fla. State Hort. Soc. 82:12-20. Leonard, C. D., and D. V. Calvert. 1971. Field tests of new iron chelates on citrus growing on calcareous soils. Proc. Fla. State Hort. Soc. 84:24-31. Leonard, C. D., and F. H. Myer. 1973. Zinc oxide and liquid zinc chelate sprays and leaf dips for correction of zine deficieney in citrus. Proc. Fla. State Hort, Soc. 86:4-8. Leonard, C. D., and I. Stewart. 1952. Correction of iron chlorosis in citrus with chelate iron. Proc. Fla. State Hort. Soe. 65-20-24. Leonard, C. D., and I. Stewart. 1955. Longevity of chelated iron treatments applied to citrus trees, Proc. Fla. State Hori. Soc. 68:59-64. 1959. Soil application of manganese for citrus. Proc. Fla. State Hort. Soc. 72:38-45. 1960, Yellow vein in citrus. Proc. Fla. State Hort. Soe. 73:69-79. Leonard, C. D., I. Stewart, and G. Edwards. 1958. Soil application of zinc for citrus on acid sandy soil. Proc. Fla. State Hort Soe. 71-99-06. Leonard, C. D., I. Stewart, I. W. Wander. 1961. Comparison of ten nitrogen sources on Valencia. Proc. Fla. State Hort. Soc. 74:79-86. Reese, R. L., and R. C. J. Koo. 1974. Responses of ‘Hamlin,’ ‘Pineapple’ and “Valencia’ orange trees to nitrogen and potash applications. Proc. Fla State Hort. Soc. 87:1-5. . 1976. Influence of fertility and irrigation on yield and leaf and soil analysis of ‘Temple’ orange. Proc. Fla. State Hort. Soc. 89:46-48. Reitz, H.J. 1956. Timing fertilization of citrus in the Indian River area. Proc. Fla. State Hort. Soc. 69:58-64. Reitz, H.J. and R, C.J. Koo, 1959, Effect of nitrogen and potassium fertiliza- tion on yield and fruit quality of Valencia orange on calcareous soil. Proc. Pla. State Hort. Soc. 72:12-16. Reitz, H. J., and T. W. Long. 1952. Mineral composition of citrus leaves from the Indian River area of Florida. Proc. Fla. State Hort. Soc. 65-32-38. Smith, P. F. 1966. Yield expectancy and the basis of citrus fertilization Proc. Fla. State Hort. Soc, 79:115-119,——. 1966. Leaf analysis of citrus. In Nutrition of Fruit Crops. N. F. Childers, ed. Horticultural Publications. Rutgers—The State University, New Brunswick, N.J., pp. 208-228. . 1967. Effect of soil placement, rate, and souree of applied zine on the concentration of zinc in Valencia orange leaves. Soil Sci. 103:209-212. . 1969, Effects of nitrogen rates and timing of application on marsh grapefruit in Florida. Proc. st Int. Citrus Symp. 3:1559-1567. . 1969, Nitrogen stress and premature leaf abscission in citrus. Hort- Science 4:326-327. 1970. A comparison ofnitrogen sources and rates on old high-yielding Valencia orange trees in Florida. J. Amer. Soc. Hort. Sci. 95:15-17. 1971. Effects of time of application of N and K and of N rate on performance of nucellar Valencia orange trees on two stocks. J. Amer. Soc. Hort. Sci. 96:568-571. 1971. A comparison of Murcott and Valencia trees grown on four different stocks in nutrient solutions with three potassium levels. Proc Fla. State Hort. Soc. 84:1—4. 1972. Studies with zinc sources as soil additives in an orange groveon acid sandy soil. Proc. Fla. State Hort. Soc. 85:6-9, Smith, P. F., G. K. Scudder, Jr., and G. Hrnciar, 1968. A comparison of nitrogen sources, rates, and placement on the performance of Pineapple orange trees. Proc. Fla. State Hort. Soc, 81:25-29. ——— 1969. Nitrogen rate and time of application on the yield and quality of Marsh grapefruit. Proc. Fla. State Hort. Soc. 82:20-25. Spencer, W. F. 1954. A rapid test for possible excess of copper in sandy soils. Fla. Agrie. Expt. Sta. Bull. 544. 12 pp. Spencer, W. F. 1963, Phosphorus fertilization of citrus, Fla. Agric, Expt. Sta. Bull. 653. Stewart, I. 1963, Chelation in the absorption and translocation of mineral elements. Ann. Rev. Plant Physiol. 14:295-310. Stewart, I., and C. D. Leonard. 1952. The cause of yellow tipping in citrus leaves. Proc. Fla. State Hort, Soc. 65:25-27. 1952. The chemistry of metal chelates and their application in agri- culture. Proc. Soil Sci. Soe. Fla. 12:17-20. ———.. 1958. Correction of molybdenum deficiency in Florida citrus. Proc. Amer, Soc. Hort. Sci. 62:111--115. —__—.. 1953. Molybdenum—the 12th addition to the list of citrus nutrients, Citrus Magazine 15:35-38 January), ———- 1957. Use of chelates in citrus production in Florida. Soil Sei. §4:87— 97. ——. 1963. Effect of various salts on the availability of zinc and manganese to citrus. Soil Sci, 95:149-154, 29Stewart, I, C. D. Leonard, and G. J. Edwards. 1955. Factors i absorption of zine by citrus. Proc. Fla. State Hort. Soc. 6 Stewart, I, C. D. Leonard, and I. W. Wander. 1961. Comparison: rates and sources for ‘Pineapple’ orange. Proc. Fla. State Hort 79. Stewart, |., and T. A. Wheaton. 1965. A nitrogen source and re Valencia oranges. Proc. Fla. State Hort. Soc. 78:21-26. Stewart, 1., T, A. Wheaton, and R. L. Reese. 1968. Murcott collls nutritional deficiencies. Proc. Fla. State Hort. Soc. 81:15—1 Stricker, J. A., and R. P. Muraro. 1981. Determining N fertilization for citrus. Proc. Fla. State Hort. Soe. 94:8 Tucker, D. P. H., and C. A. Anderson. 1972. Correction of stunting on fumigated soils by phosphate application. P Hort. Soc. 0-12. Wutscher, H. K. 1973. Rootstocks and mineral nutrition of e Int. Citrus Short Course. L. K. Jackson, A. H. Krez i Fruit Crops Department, IFAS, Univ. of Fla. Gaines ——.. 1978. Citrus tree rootstocks as a means of copin nutrient deficiencies and toxicities (Texas and Flo : Tree. The Int. Dwarf Tree Assn., Michigan State Univ. 11:28-30. —__—.. 1979. Minor element use in citrus. Citrus and Veg. 49-52. Young, T. W., and R. C. J. Koo. 1967. Effects of fertilization on Persian limes on Lakeland fine sand. Soc. 80:337-342, 1972. Effects of magnesium nitrate sprays on n ime trees on caleareous soils. Proc. Fla. State Hort. S The assistance of the Florida Agricultural Research Ins information and review is appreciated. 30This public document was promulgated at an annual cost of $1,484 or a cost of 21.2¢ per copy to inform Florida citrus growers of the most efficient and economical methods of fertilizing citrus groves. All programs and related activities sponsored or assisted by the Florida Agricultural Experiment Stations are open to all persons regardless of race, color, national origin, age, sex, or handicap, ISSN 0096-607X