CURRENT RECOMMENDATIONS FOR FERTILIZING CITRUS

REITZ: FERTILIZING CITRUS 175 supplied with fertilizer phosphorus and were injured slightly more. Rates and timing had no effect. All the observations reported here indicate that fertilization practices have a relatively small effect on cold tolerance of citrus when obvious deficiencies are absent. In general, they support the conclusion drawn by Law less (2) after the 1940 freeze that "there is no magical virtue in any single element" insofar as imparting cold protection to the tree. Fer tilization practices that produce a healthy, vigorous tree seem to offer about the maximum protection that can be attained within the tree. However, it seems probable that shifts in fer tilization practices in recent years resulting in more nitrogen and less potassium within the trees and a total disappearance of magnesium deficiency played a positive role in the "less than expected" degree of damage from one of the severest winters in Florida's history. Other factors, such as the drought-induced dormancy prior to the severe December cold and the pre vailing cool weather that delayed the advent of new growth betwen potentially damaging freezes, no doubt had a large part in reducing over-all damage. Summary Visual ratings of the degree of damage were made on trees in about 1,000 fertilizer plots in Orange, Lake, and Pasco Counties following the freezes of 1956-57 and 1957-58. Low tem peratures ranged from 18 to 22 degrees in Pasco County both seasons and in the other counties in the latter season. For the most part, damage to bearing trees in the experiments ranged from none to some degree of defoliation and slight twig damage. Young-tree damage ranged from moderate defoliation to the kill ing of the entire top down into the protective soil bank around the trunk. In general, nutrient status of the tree above deficiency levels had but slight effect on cold tolerance. High rates of nitrogen fertilization had a small but definite tendency to increase the resistance of the tree to freeze injury. High rates of potash had a small but definite ten dency to reduce the cold resistance of the tree. Differential supplies of phosphorus, copper, zinc, and manganese had no effect on the de gree of freeze injury sustained. Young trees, the first year in the field were appreciably less injured when the last ferti lizer application of the season was made in late August than when the trees were fed again in October. Such an effect was not apparent on 2-year-old trees. Young trees, stunted for lack of phosphorus on Lakewood sand, were more severely damaged in each of 2 years than trees adequately supplied with phosphorus. LITERATURE CITED 1. Hume, H. H. The Cultivation of Citrus Fruits. Mac- Millan Press, New York. 1926. Also, Citrus Fruits. MacMillan Press, New York. 1956. 2. Lawless, W. W. Effect of freeze damage on citrus trees and fruit in relation to grove practices. Proc. Fla. Stafe Hort. Soc. 54: 67-74. 1941. 3. Lawless, W. W. and A. F. Camp. Preliminary reports on varieties, fertilizers and other factors as influencing cold resistance in citrus. Proc. Fla. State Hort. Soc. 53: 120-125. 1940. 4. Reuther, W., P. F. Smith, G. K. Scudder, Jr., and G. Hrnciar. Responses of Valencia orange trees to timing, rates, and ratios of nitrogen fertilization. Proc. Amer. Soc. Hort. Sci. 70: 223-236. 1957. 5. Smith, P. F. Effect of phosphate fertilization on root growth, soil ph, and chemical constituents at different depths in an acid, sandy soil. Proc. Fla. State Hort. Soc. 6. Smith, P. F. and W. Reuther. Preliminary report on the effect of nitrogen source and rate and lime level on ph, root growth, and soil constituents in a Marsh grapefruit grove. Proc. Fla. Soil Sci. Soc. 15: 108-116. 1955. 7. Smith, P. F., W. Reuther, and G. K. Scudder, Jr. Effect of differential supplies of nitrogen, potassium, and magnesium on growth and fruiting of young Valencia orange trees in sand culture. Proc. Amer. Soc. Hort. Sci. 61: 38-48. 1953. CURRENT RECOMMENDATIONS FOR FERTILIZING CITRUS TREES IN FLORIDA Herman J. Reitz Florida Citrus Experiment Station Lake Alfred Practical fertilizer recommendations for cit rus should describe a program which will pre vent any nutritional factors either deficien cies or excesses from adversely affecting the health and growth of the trees or yield and quality of the fruit. The only completely sound recommendation will be one firmly based on research. Out of all the new research information, only a part can be applied directly to practical problems such as fertilizing citrus trees. Anoth-

176 FLORIDA STATE HORTICULTURAL SOCIETY, 1958 er part will consist of fundamental information which has no apparent practical application. Although of no immediate use to citrus grow ers, this fundamental information is extremely useful to other scientists in their continuing search for new and improved solutions of prac tical problems. Citrus growers cannot wait for all the re search work to be completed sometime in the future. They must adopt an immediate course of action. Research workers share that position they cannot wait until they obtain all the facts, but must at all times give the in dustry the benefit of all possible information and judgment. Therefore at present, general recommendations for citrus fertilizer practices are based not only upon research but also upon known successful commercial practice. Historical At first, the research groups concerned with citrus fertilization made piecemeal recommen dations on specific subjects, but the problem of fertilization became too complex for that approach. In 1943 a unified approach to the complexity of fertilization was made by Camp (2). This program coordinated the practical applications of research information available at that time. A second concerted step in the development of fertilizer recommendations was marked by the publication in January, 1954, of Florida Agricultural Experiment Station Bulletin 536 (11) under the title, "Recommended Fertili zers and Nutritional Sprays for Citrus." These recommendations were prepared in 1953, af ter industry groups had expressed a need for an up-to-date and comprehensive statement on recommended fertilizer practices. The nu trition groups at the Citrus Experiment Sta tion and at the U. S. Horticultural Field Sta tion conferred with industry groups, after which the recommendations were drawn up. General acceptance of the program offered in Bulletin 536 was very rapid, a result entirely expected since large acreages of citrus were and had been for some time on the newly recommended program. Recently the supply of Bulletin 536 was exhausted, so that the need to revise and reissue the fertilizer recommen dations was presented. During 1958, a second series of conferences on fertilizing citrus was sponsored by the Cit rus Station. The nutrition groups at the Citrus Station and at the U. S. Horticultural Field Station at Orlando jointly proposed a revised set of recommendations. These were offered for criticism to production management groups, to industry groups, and to the teaching and extension personnel of the College of Agricul ture of the University of Florida. The manu script has now been submitted for publica tion by the Florida Agricultural Experiment Station. Highlights Of New Recommendations No drastic changes in recommendations are to be found in the 1958 version compared with the 1954 version of Bulletin 536. Most of the balance of this paper will touch briefly on those points in which notable changes appear in the new version. Nitrogen.- A new feature of the fertilizer recommendations consists of a differential rec ommendation for nitrogen on oranges and on grapefruit. Current thinking of the research agencies would indicate that the figure of 0.4 pound nitrogen per box of fruit per year continues to be a satisfactory level of nitrogen for oranges, but that a figure of 0.3 pound nitrogen per box per year is more realistic and satisfactory for grapefruit. This differentiation is based largely on unpublished experimental work. Results obtained from continuation of the experiment described and discussed by Smith and Reuther (17), a nitrogen rate ex periment conducted by the Citrus Experiment Station terminated in 1956 with results still unpublished, and a small, supplementary ex periment conducted at the Citrus Experiment Station tend to support the lowered figure for grapefruit. The differential further seems to be indicated in work currently being con ducted in the Indian River Area, where maxi mum yields of grapefruit have been obtained from use of nitrogen at rates lower on a per box basis than necessary for maximum yield of oranges. It is believed that grower practice parallels this line of thought, although possi bly because grapefruit has been recently a less profitable crop than oranges. A second innovation in the nitrogen recom mendation is the suggestion of maximum ap plication rates of 300 pounds nitrogen per acre per year in central Florida and 150 pounds nitrogen per acre per year in Indian River

REITZ: FERTILIZING CITRUS 177 groves. These ceilings would apply to all va rieties. Ceilings are based upon yield records obtained in the experiments cited above as well as upon other considerations. One of these is the notation by Ford, Reuther, and Smith (5) that high to excessive nitrogen applications have led to reduction in concentration of feeder roots in soils. It is doubtful that yield can be improved in even the most productive groves by applications of nitrogen greater than the amounts suggested as ceilings. Phosphorus.- In 1954 is was recommended that phosphorus be omitted from the fertilizer applied to old bearing groves with a history of phosphorus fertilization. The new recom mendations provide that phosphorus be applied to young bearing groves in amounts equal to that removed in the crop, and that application to young trees be reduced. A few of the facts upon which the phosphorus recommendation is based are presented below. Most of the virgin soils commonly planted to citrus do not contain large amounts of phos phorus (10, 13), and probably require phos phate when first planted to citrus. However, some soils are sufficiently supplied that no ap plication of phosphate fertilizer are necessary (9, 12). Many investigations have shown that fertilizer phosphorus accumulates in soils such as used in Florida for citrus (13 3, 10, 12, 13, 14, 19). This accumulation is in the form of iron or aluminum phosphates and calcium phosphates, the proportion depending partly on soil ph (1, 15). Phosphorus in these com pounds may move downward slowly but ap preciably in the soil, particularly if the sur face soil has been allowed to remain at low ph levels (1, 3, 10, 14, 19). Only after many years would phosphorus move out of the root zone of citrus trees. Availability to citrus trees of the accumulated soil phosphorus has been proved by growth tests and by use of radio isotope techniques (1, 14). Thus it is apparent that past fertilizer practice has in many groves created a reserve of available soil phosphorus. The only case of phosphorus deficiency de scribed in the literature of Florida citrus oc curred on muck soils in Broward County (6, 20). This deficiency was characterized by higher soluble solids and acidity in the juice of fruit, thick fruit rind, high fruit drop, and low fruit grade and yield. Some evidence exists that high phosphorus in the soil may at times have unfavorable ef fects. Increasing the amount of phosphorus applied has resulted in: (a) lower soluble sol ids concentrations in the juice of the fruit (6, 12, 20); (b) inhibited root growth (4, 16, 19); and (c) lessened cold hardiness (16). Research has not yet determined the opti mum soil phosphorus level as measured by chemical methods of soil testing. In the ab sence of a reliable soil testing method, the fol lowing program that appears reasonable has been devised: Application of phosphorus (P2O5) is recommended in young tree and young bearing tree fertilizers, that is, for trees up to 10 years of age, at a rate equivalent to & the rate of nitrogen application, For trees 10 years old and older, it is believed adequate to apply only the amount of phosphate equal to that removed by the crop. This amounts to about 20 pounds P2O5 per acre per year in many bearing groves (7, 18). A convenient method of applying this would be to apply phosphorus (calculated as P2O5) every fourth year at a rate equivalent to )k that of nitrogen. During the three ensuing years no phosphorus need be applied. Other equivalent programs could be devised. Potassium.- No important change in the ratio of nitrogen to potassium was considered advisable at this time although a slight modi fication was adopted. Five years ago, potassi um was recommended at 0.35 pound per box of fruit per year compared with 0.4 pound ni trogen. It is now considered desirable to bring the ratio between these elements to an even 1:1 level. This does not constitute in the minds of the research groups a change in recommen dation but merely reflects the feeling that the ratio of 8:7 left the erroneous impression of a higher degree of precision in making this recommendation than actually could be at tained from the data available. Therefore the ratio has been modified in order to bring the potassium level into better perspective. Noted above was the recommendation for lower nitrogen rates for grapefruit than for oranges. This differential in conjunction with the 1:1 ratio of nitrogen to potash results in a net reduction of potash application to grape fruit paralleling that of nitrogen. Magnesium.- Recommendation of a slight reduction in the use of magnesium was the

178 FLORIDA STATE HORTICULTURAL SOCIETY, 1958 consensus. While the old recommendation gave a range of 0.2 to 0.4 pound MgO per box of fruit, the new recommendation contains a range of 0.1 to 0.3 pound per box of fruit. This is partly a reflection of the upward re vision in ph recommendation from a range of 5.5 to 6.0 to a range of 5.5 to 6.5, based primarily on use of dolomite. It is also based on the generally high magnesium content of leaves found in a recent survey of 168 com mercial Valencia groves (8). It is quite proba ble that the appearance of traces of magnesium deficiency leaf patterns may be adequate warning to a grower to increase the magnesi um level in his fertilizer program, and that in the absence of deficiency symptoms, no fear need be felt concerning an inadequate mag nesium program. Manganese, Zinc, and Copper.- The major change in recommendations concerning man ganese reflects the feeling of the research groups that reliance upon visual leaf symptoms is a safer practice than has been thought in the past. No evidence is available to indicate that the appearance of mild, transient man ganese leaf deficiency symptoms is of any economic importance, and in absence of such symptoms manganese applications can be omit ted. No chelated forms of zinc or other new methods of zinc application have proven ade quate and practical for all situations, so that major reliance must continue to be placed on foliage sprays for satisfying the zinc require ment of citrus trees. The 1954 recommendation that copper ap plication be omitted in groves with 50 pounds of copper or more in the surface six inches of soil has proven satisfactory, and no change was made. Boron.- The boron recommendation in 1954 unfortunately contained an inconsistency in that the amount of boron recommended in fer tilizers was small compared with the amount recommended in foliage sprays. It is still not entirely clear what the optimum boron level is, but for a maintenance program reasonable leaf analyses will probably result from the ap plication of boron as B2Oa in % of the year's fertilizer at rates of about 1/40 that of nitro gen. This will result in application of boron roughly equivalent to the amount applied as borax (36.5 percent B2O3) at % pound per 100 gallons in one foliage spray per year. Any other program that will provide comparable amounts of boron per acre during the course of a year would be satisfactory. If reason exists to suspect a boron deficiency, a program con sisting of 1/20 the nitrogen analysis in % of the annual fertilizer or one spray containing 1 pound borax per 100 gallons would be ade quate. Leaf and Soil Analyses.- No additional rec ommendations for routine leaf or soil analyses could be made at this time. Soil ph and total copper determinations continue to be useful as recommended in 1954. Other analytical procedures are useful primarily as research tools. Future Possibilities Most of the changes in recommendations which can be endorsed at the present time have been mentioned briefly in the paragraphs above. There is no delusion among the re search groups that these recommendations con stitute either an entirely satisfactory or a per manent set of recommendations. Substantial improvements still are possible, and current experimental work promises to yield the in formation upon which these can be based. New developments which appear to be most hope ful are the further refinement of nitrogen rate recommendations for specific varieties and specific market outlets, phosphorus recommen dation on the basis of soil analysis, differen tiation of potassium recommendation on the basis of varieties as well as use for which the fruit is intended, and more effective and re liable methods of applying zinc to the soil in order to avoid the necessity of zinc foliage sprays. Some of these developments seem im minent, but in the writing of general fertilizer recommendations, conservatism is required to the extent that some of these innovations, while very promising, could not be included. It is the intention of the research agencies that at the earliest time justifiable on the basis of newly obtained information, these recom mendations will again be revised and modern ized. LITERATURE CITED 1. Bryan, O. C. The accumulation and availability of phosphorus in old citrus grove soils. Soil Sci. 36: 245-259. 1932. 2. Camp, A. F. A resume of feeding and spraying citrus trees from a nutritional standpoint. Proc. Fla. State Hort. Soc. 56: 60-79. 1943.

SPENCER AND SELHIME: NEW MITICIDES 179 3. Collison, S. E., and S. S. Walker. Loss of fertilizers by leaching. U. of Fla. Agr. Expt. Sta. Bull. 132, pp. 1-20. 1916. 4. Ford, H. W. Root distribution of citrus trees. Fla. Agr. Expt. Sta. Ann. Rept., p. 207. 1957. 5. Ford, H. W., Walter Reuther, and Paul F. Smith. Effect of Nitrogen on root development of Valencia orange trees. Proc. Amer. Soc. Hort. Sci. 70: 237-244. 1957. 6. Forsee, W. T., Jr., and J. R. Neller. Phosphate re sponse in a Valencia grove in the eastern Everglades. Proc. Fla. State Hort. Soc. 57: 110-115. 1944. 7. Koo, R. C. J., and J. W. Sites. Mineral composition of citrus leaves and fruit as associated with position on the tree. Proc. Amer. Soc. Hort. Sci. 68: 245-252. 1956. 8. Koo, R. C. J., H. J. Reitz, and J. W. Sites. A survey of the mineral nutrition status of Valencia orange in Florida. Fla. Agr. Expt. Sta. Tech. Bull. In press. 1958. 9. Leighty, R. G., J. R. Henderson, and R. E. Caldwell. Soil associations of Hillsboro County, Florida. Fla. Agr. Expt. Sta. Circular S-28. 1951. 10. Peech, Michael, and T. W. Young. Chemical studies on soils from Florida citrus groves. Fla. Agr. Expt. Sta. Bull. 448. 1948. 11. Reitz, H. J., C. D. Leonard, J. W. Sites, W. F. Spencer, I. Stewart, and I. W. Wander. Recommended fertilizers and nutritional sprays for citrus. Fla. Agr. Expt. Sta. Bull. 536. 1954. 12. Reuther, Walter, F. E. Gardner, P. F. Smith, and W. R. Roy. A progress report on phosphate fertilizer trials with oranges in Florida. Proc. Fla. State Hort. Soc. 61: 13. Rogers, L H., O. E. Gall, L. W. Gaddum, and R. M. Barnette. Distribution of macro and micro elements in some % a\ (?{y&eninsular Florlda- F'a- Agr. Expt. Sta. Tech. Bull. J41. 1939. 14. Spencer, W. F. Distribution and availability of phos phates added to a Lakeland fine sand. Soil Sci. Soc. of Amer. Proc. 21: 141-144. 1957. 15. Spencer, W. F. Phosphatic complexes in the soil Their effect on retention and availability of nutrient cations Fla. Agr. Expt. Sta. Ann. Rept., p. 193. 1952. 16. Spencer, W. F. The effects of phosphate and lime applications on growth, root distribution, and freeze injury of young grapefruit trees. Proc. Fla. State Hort. Soc. 71: 1958. In press. 17. Smith, P. F., and Walter Reuther. Preliminary report on the effect of nitrogen source and rate and lime level on ph, root growth, and soil constituents in a Marsh grape fruit grove. Proc. Soil. Sci. Soc. of Fla. 15: 108-116. 1955. 18. Smith, P. F., and Walter Reuther. Mineral content of oranges in relation to fruit age and some fertilization practices. Proc. Fla. State Hort. Soc. 66: 80-85 1953 19. Smith, P. F. Effect of phosphate fertilization on root growth, soil ph, and chemical constituents at different depths in an acid sandy Florida citrus soil. Proc. Fla. State Hort. Soc. 69: 25-29. 1956. 20. Young, T. W., and W. T. Forsee, Jr. Fertilizer experi ments with citrus on Davie mucky fine sand. Fla. Agr. Expt. Sta. Bull. 461. 1949. FOUR YEARS OF EXPERIMENTS WITH NEW MITICIDES AND INSECTICIDES Herbert Spencer and Allen G. Selhime USD A Agricultural Research Service Orlando Since 1954 we have tried out 13 new ma terials for control of citrus red mites1 (purple mites), citrus rust mites,2 purple scales,3 and whiteflies.4 The materials were Thimet, Chlorobenzilate, Hercules AC 528, malathion, demeton (Systox), Trithion, Kelthane, Guthion, zineb, maneb, Chipman R-6199, Chipman R-6200,5 and Tedion. In most of the experi ments they were used in combination sprays, but if there was indication of* control of several pests, the material was also used alone. Most of the work was done in the Norman G. Platts grove of Temple orange trees near Fort Pierce, where copper, zinc, and man ganese are always used in the postbloom spray, and usually also parathion and wettable sul fur. There the red mites and purple scales are quite a problem. The timing of the experi mental sprays followed the cooperative spray schedule, the first spraying being the postxpanonychus citri (McG.) 3 Phyllocoptruta oleivora (Ashm.) 3 Lepidosaphes beckii (Newm.) 4 Dialeurodes eitri (Ashm.) and citrifolii (Morg.) 5 O,O-diethyl S-(2-diethylamino)ethyl phosphorothionate (ptoluene sulfonate). bloom and the second the summer, usually in July. With the most effective combinations no other applications were needed, but in 1955 and 1956 the grove was dusted with sulfur for rust mites late in the fall. In all experiments the new miticides or in secticides were compared with each other and with a standard control recommended in the spray schedule. For red mites this was either oil emulsion at % or 1% gallons, ovex 50WP at 1 pound, or Aramite 45WP at % pound per 100 gallons of spray. For rust mites it was wettable sulfur 5 pounds per 100 gallons. Re sults of these comparisons are tabulated be low, the conclusions being based on immediate control and the time before reinfestation. At different times during the four years these miticides were combined with the scalicides oil emulsion, parathion, and malathion and supplements basic copper sulfate, neutral zinc, and neutral manganese, and were found to be compatible. The materials and combina tions were safe for the trees. Excellent immediate control of red mites was obtained with Thimet, Hercules AC-528, demeton, Trithion, Kelthane, Chipman R-6199^ and Chipman R-6200, and the period of pro tection against reinfestation exceeded that from