KOO AND CALVERT: MAGNESIUM OXIDES MAGNESIUM OXIDES AS SOURCES OF MAGNESIUM FOR CITRUS1 R. C. J. Koo2 and D. V. Calvert3 Abstract A year study had been made comparing sev eral magnesium oxides as sources of magnesium for citrus on Lakeland and Leon fine sands. The study included magnesium source and rate experiments on bearing nonbearing trees of sev eral scionrootstock combinations. Magnesium sulfates were used for comparison. The data, as determined by leaf and soil analyses, indicated both sulfate and oxide forms of magnesium were satisfactory for citrus. Magnesium content of leaves varied inversely with soil ph regardless of magnesium sources. Trees on 'Cleopatra' mandarin rootstock con sistently showed a higher leaf magnesium con tent than trees of the same variety on the same soil type but on sour orange rootstock. Introduction Magnesium in the sulfate form is a regular component of most fertilizer applied to Florida citrus. During World War II, when magnesium sulfates were in short supply, several water in soluble magnesium compounds were used in citrus fertilizer with variable results (1, 1, 11). Since World War II, magnesium sulfates have been used almost exclusively in citrus fertilizers. Dolomitic limestone, used mainly for control of soil ph has also been a source of magnesium for citrus. Spencer and Wander (1) compared several magnesium sources for citrus on an unlimed Lakeland fine sand with a ph of ap proximately 5. and found magnesium oxide was slightly superior to magnesium sulfate. They theorized that in acid soil, without the addition of liming materials, the magnesium oxide was able to furnish a constant supply of magnesium to the citrus trees. They suggested the results might have been quite different if the magnesium sources had been compared at a higher soil ph level. lthis study was financed in part by a grant from Basic Incorporated of Cleveland, Ohio. 2Associate Horticulturist, University of Florida Citrus Experiment Station, Lake Alfred. 3Assistant Soils Chemist, University of Florida Indian River Field Laboratory, Fort Pierce. Florida Agricultural Experiment Stations Journal Ser ies No. 2517. This paper summarizes the results of several field experiments to evaluate magnesium oxides as a source of magnesium for citrus under a variety of conditions. These conditions included 2 soil series, several scionrootstock combina tions, and 2 methods of application. Experimental Methods Four magnesium source and race experi ments were initiated in 1962 and 1963. A fifth experiment was started in 196 to study the relation between soil reaction and availability of magnesium. The source and rate experiments included 3 nonbearing and 1 bearing tree exper iments. Two of the nonbearing tree experiments (I and II) were located near Fort Pierce on Leon fine sand where 'Pineapple' oranges on 'Cleopatra* mandarins, and sour orange rootstocks, respectively, were planted in 196 on 2row beds. Experiments III and IV were lo cated in central Florida on Lakeland fine sand. 'Marsh* grapefruit on Rough lemon rootstock were selected for both experiments. The trees in Experiment III (nonbearing) were planted in 1961 and those in Experiment IV (bearing) were planted in 192. Treatments in the source and rate experi ments included magnesium oxides and 2 mag nesium sulfates applied at 3 rates. Magnesium oxides included were 1) Magox9 Ag. Grade (91.5 MgO), 2) Magox9 2 mesh (91.5 MgO), 3) Seawater Magnesia ( MgO), and ) Magnesia65 (66 MgO). Magnesium sul fates used included Emjeo (27.5 MgO) and Sulpomag (1 MgO). Because of the limited number of trees available, not every magnesium source was included in all the experiments. Three rates of magnesium were used in these studies. In the nonbearing tree experiments, a bas formula of 6X was used from 1962 to 196 with magnesium rates of 1,, and 7 MgO. In, the base formula was changed to 9212X including magnesium rates of 1, 5, and 9. Annual fertilizer applicaton varied from 6 pounds per tree per year in 1962 to 7.5 pounds in 1963, and 9. pounds per tree in 196 and. The magnesium materials were mixed in the fertilizer and applied in 3 equal applications each year. In the bearing tree ex
FLORIDA STATE HORTICULTURAL SOCIETY, periment, magnesium was applied in separate applications. Magnesium rates ranged from.6, 2., and 3. pounds MgO per tree per year applied in 2 equal applications. All treatments were replicated times in tree plots arranged in randomized block design. A fifth experiment was started in 196 to obtain information on the effects of soil reaction on the availability of magnesium from different sources. Hicalcium limestone was applied at, 2, and tons per acre to a young 'Marsh* grape fruit block on Lakeland fine sand. The liming treatments were repeated in. Magnesium equivalent to the high rate of the nonbearing tree experiments was applied to single 5tree plots without replication. In addition to the 6 magnesium sources named above, dolomitic lime stone was also included. Leaf analysis was used as the principal measure for treatment comparisons. Annual leaf samples were collected from all the experi ments. Soil was sampled before treatment and also in 196 and. Fruit samples were col lected annually from the bearing grove experi ment and analyzed for quality. Fruit production records were taken at the time of harvest each year. Methods of handling leaf, soil, and fruit samples for analysis have been described else where (3). Leaf magnesium was analyzed with a modified EDTA titration method (). Soil magnesium was extracted with a neutral normal ammonium acetate solution and determined by the Clayton yellow method with modifications (6). Results Rate of magnesium application affected the magnesium content of leaves and soils in all experiments. Magnesium treatments had little effects on other elements except in soil reaction. In the bearing grove experiment, magnesium treatments did not influence either fruit produc tion or quality. Only the leaf and soil magnes ium data are reported here. The magnesium content of leaves from the source and rate experiments are summarized in Table 1. ly significant differences due to rate of magnesium application were found in all the experiments except in 2 instances during the first year. There were no significant differences in leaf magnesium content due to magnesium treatments in 1963 from 1 of the nonbearing tree experiments on Leon fine sand (Experiment Table 1. The influence of magnesium sources and rates on leaf magnesium content (Average of replications). No. Treatment source Rate I. 1963 7o 'Pineapple' 'Cleopatra' 196 Nonbearing tree experiments (Leon f.s.) 'Marsh ' gft/r. lemon orange II. Sour orange III. (Lakelai id f.s.) 1963 196 1963 196 7a 7o Bearing tree experiment^ 'Marsh 1 gft/r. lemon IV. ^Lakeland f.s.) 1963 196 : 7. 1. 2. 3. Magox9 Ag. Gr..5...5.6.6.3.33.3. 5. 6. Magox9 2 mesh.5.61.5..5.5.56.6..33 7.. 9. Seawater Magnesia.6.5.6.5.3.5.3.3 1. 11. 12. Emjeo.69.7.5L.27..5.6.61. 13. 1. 15. Sulpomaj*.6.5.5.61 16. 17. 1. Magnesia65.5.5.25.3.33. Significance Source V. Rate ** * it* ** ** ' Treatments 1G to 1 in the bearing tree experiment have only 2 replications and are not included in the statistical analysis. Significance no significane, ^'significant at 57«level, ** significant at 17«level.
KOO AND CALVERT: MAGNESIUM OXIDES 9 II) and the bearing tree experiment on Lake land fine sand (Experiment IV). Sources of magnesium, in general, did not influence the magnesium content of leaves. There were 2 ex ceptions, both of which occurred on Leon fine sand. Significant differences due to magnesium sources were found in 1963 with 'Pineapple' on 'Cleopatra' mandarin rootstock and in 196 on sour orange rootstock. In general, the data were much more variable from the 2 experiments on Leon fine sand than from those on Lakeland fine sand. This was probably due to the variabil ity of the soil as the result of bed construction prior to planting. For example, in the nonbearing tree experiments, the range in soil ph was 5.1 to 7. on Leon fine sand as compared to 5. to on Lakeland fine sand. The magnesium content of leaves was generally lower at higher ph ranges regardless of treatments. The leaf magnesium content in the nonbearing tree experiments was considerably higher than that of the bearing trees. There also seem ed to be a difference in the uptake of magnesium by different rootstocks. 'Pineapple' orange on 'Cleopatra' mandarin showed consistently higher leaf magnesium content than 'Pineapple' on sour orange rootstock on the same soil. Table 2 summarizes the magnesium content of the soils and the soil ph at the to 6 inch depth. As in leaves, the extractable magnesium content of soil was not influenced by source but was affected by the rate of magnesium applica tion. The ph data were not analyzed statistical ly, but higher ph values were observed with higher rates of magnesium application. In gen eral, the soil ph was also slightly higher where magnesium oxides were used. This was more noticeable on Lakeland fine sand than an Leon fine sand. The influence of soil reaction on the avail ability of magnesium from different sources is shown in Table 3. Leaf and soil magnesium contents from all sources decreased with increas ing soil ph. A gradual accumulation of extractable soil magnesium was observed over the years. In, leaf and soil samples were col lected in August, about 2 months earlier than in 196 and, which may also partially account Table 2. The influence of magnesium sources and rates on soil reaction and magnesium content ( to 6" depth average of replications). Treatment source Nonbearing tree experiments Bearing tree experiment1 'Pineapple' orange (Leon f.s.) 'Marsh' gf 'Marsh' gft/r. L. I. 'Cleopatra' II. Sour orange III. (Lakeland f.s.) IV. (Lakeland f.s.) _PH1 PH _Eii MS, PH MR lbs/a Tbs7A Ibs/A 1. 2. 3. Magox9 Ag. Gr. 6. 6. 15 1 223 7 127 25 6.5 36 37 96 6. 6. 72 9 136. 5. 6. Magox9 2 mesh 5.6 19 1 23 5.6 6. 16 152 5.6 29 36 63 11 13 7.. 9. Seawater Magnesia 6.3 6.3 115 165 17 6. 67 15 176 5.5 33 1 6. 6. 7 112 12 1. 11. 12. Emjeo 12 131 122 3 131 16 5.5 19 65 1 6. 6.'7 6 9 123 13. 1. Sulpomag 13 11 6. 2 5 " 15. 6. 163 3 16.. 17. 1. Magnesia65 12 133 199 5.5 2 6 75 6.6 37 1 1 Significance Source Rate n.s ^Treatments 16 to 1 in the bearing tree experiment have only 2 replications and are not included in the statistical analysis..the ph data arc not analyzed statistically.
1 FLORIDA STATE HORTICULTURAL SOCIETY, Table 3.The influence of soil reaction on soil and leaf magnesium contents^ on Lakeland fine sand. Mg Soil Mg ( to 6") Leaf Mg source Liming T/A 196 lbs/a 196 Vo Magox9 Ag. Gr. 9 79 92.5.5.6 79 56 76 53 6 9.59 Magox9 2 mesh 51.5.69 65 95 56.57 Seawater Magnesia 75 1 19.57 3 39 61 53 5 Magnesia65 7 32 11 67 115.5.5. 3 67 7.3 Emjeo 69 16 95.5.75 5 63 59 17.57 Sulpomag 92 61 6 129 1 7 139 12.6.56.75.5 Dolomite 3 2 1 79 36 2 17 6.5.6 lbs/a Soil ph Soil Ca 196 196 Average of all 261 251 256 plots 9 77 35 6.5 7. 7.3 655 1137 175 ^Samples were collected in October 196 and and in August.
KOO AND CALVERT: MAGNESIUM OXIDES 11 for the higher leaf and soil magnesium contents. The slower solubility of dolomite than that of the other magnesium sources at higher soil ph was quite evident, although Magnesia65 was also slow to go into solution. In general, leaf magnesium content was higher in sulfate plots where no lime was added. It should be empha sized that treatments in this experiment were not replicated, but the range in treatments was wide enough to show trends. Discussion The data indicate that both magnesium oxi des and sulfates are satisfactory sources of mag nesium for citrus. The results are in agreement with data reported by Spencer and Wander (1) on unlimited soils. These results are not in agreement with the poor response experienced of some Florida citrus growers who had used magnesium oxides during World War II. Based on the experience of these citrus growers, Camp et ox. (1) stated that magnesium oxide was not a suitable substitute for watersoluble magnes ium sulfate in citrus fertilizer. They explained that magnesium oxide was not sufficiently avail able, especially at a soil ph value above 6. to prevent widespread development of magnes ium deficiency symptoms. The soil ph in the current studies ranged from 5.1 to 7. with the majority of plots 6. or above. Reasons for the difference in response of magnesium oxides in the 19's and 196's are not entirely clear. It could be related to the difference in the nitrogen program between the 2 periods. Several workers (7, 9) have reported higher magnesium content of leaves resulting from increased nitrogen ap plication. The fact that the rate of nitrogen fertilization in Florida citrus groves has more than doubled in the past 25 years (5) may account for some of the differences in the respone of magnesium oxides during the 2 per iods. The nitrogenmagnesium relationship is further substantiated by the fact that under conditions where soil application of magnesium is ineffective, magnesium deficiency in citrus cannot be corrected with magnesium sulfate sprays but can be controlled with magnesium nitrate sprays (2). Among the magnesium oxides, there was little difference between Magox9 and Seawater Magnesia. Magnesia65 seemed to be slightly less available, especially at the higher soil ph ranges. No significant difference was observed between the 2 sulfate forms of magnesium. In clusion of both magnesium sulfate and magnes ium oxide in citrus fertilizer may be desirable. This would provide a quickly available source of magnesium with long residual effects. The influence of soil reaction on the uptake of magnesium by the tree was observed in all the experiments. It was especially noticeable on Leon fine sand in bedded groves where in some plots calcareous clay in the subsoil was disturbed and intermixed with acid sand during the con struction of the beds. Leaf magnesium content decreased as the soil ph values increased regardless of the rates and sources of magnes ium used. The higher calcium content in soils with higher ph values undoubtedly influenced the uptake of magnesium by citrus. Variations in soil ph values had little influence on the extractable magnesium content of soil. This may explain why no significant correlation was found between leaf and soil magnesium contents on Leon fine sand, even though highly significant differences due to rates of magnesium applica tion were found independently in the magnesium content of the leaves and soil. On the other hand, highly significant correla tions were found between the magnesium content of leaves and soils in both experiments on Lake land fine sand. This may be explained by the narrow soil ph range that existed within any 1 experiment. A comparison of the 2 experi ments on Lakeland fine sand indicated that the mean soil ph values from the bearing grove was about 1 unit higher than that of the nonbearing grove but leaf magnesium content was about 3 lower. It was doubtful that the pres ence of the fruit crop in the bearing grove experiment could account for all the differences. The fact that magnesium became less avail able to the tree at higher soil ph ranges regard less of source and rate makes one wonder if the current recommendation () is adequate for citrus grown on calcareous soils. LITERATURE CITED 1. Camp, A. F., et. al. 196197. Citrus nutrition studies. Fla. Agr. Expt. Sta. Ann. Rept. 196: 153; 197: 1. 2. Embleton, T. W. and W. W. Jones. 1959. Correction of magnesium deficiency of orange trees in California. Proc. Amer. Soc. Hort. Sci. 7: 22. 3. Koo, R. C. J., H. J. Reitz, and J. W. Sites. 195. A survey of the mineral nutrition status of Valencia orange in Florida. Fla. Agr. Expt. Sta. Tech. Bull. 6.. Malmstadt, H. V. and T. P. Hadjiioannoul 1959. Rapid and accurate automatic titration method for deter mination of calcium and magnesium in plant material with EDTA titrant. Agr. and Food Chem. 7(6): 12. 5. Mathias, A. F. 196. Changes in fertilizer program
12 FLORIDA STATE HORTICULTURAL SOCIETY, and yields in citrus since 11. Proc. Fla. State Hort. Soc. 73: 912. 6. Mehlick, A.. Improvements in the colorimetric magnesium and ammonium methods with sodium polyacrylate. J. Assoc. Offic. Agr. Chem. 39: 51523. 7. Reitz, H. J. and R. C. J. Koo. 196. Effect of N and K fertilization on yield, fruit quality, and leaf analy sis of Valencia orange. Proc. Amer. Soc. Hort. Sci. 75: 2252.. Reitz, H. J., C. D. Leonard, I. Stewart, R. C. J. Koo, D. V. Calvert, C. A. Anderson, P. F. Smith, and G. K. Rasmussen. 196. Recommended fertilizers and nutritional sprays for citrus. U. of Fla. Agri. Expt. Sta. Bull. 536B. 9. Reuther, W. and P. F. Smith. 195. A preliminary report on the relation of N,K, and Mg fertilization to yield, leaf composition, and the incidence of Zn deficiency in oranges. Proc. Amer. Soc. Hort. Sci. 56: 2733. 1. Spencer, W. F. and I. W. Wander. 196. A compari son of magnesium sources in orange trees. Proc. Fla. State Hort. Soc. 73: 235. 11. Thullbery, H. A. 19. Sources of magnesium. Proc. Fla. State Hort. Soc. 57: 135139. PROPERTIES OF SPRAY OILS IN RELATION TO CITRUS PEST CONTROL IN FLORIDA1 Kenneth Trammel and William A. Simanton2 Abstract Petroleum oils are widely used as pesticides for Florida citrus but sometimes have unedsirable effects on trees and fruit. In a program designed to establish specifications for better spray oils, selected oils were evaluated in field studies to relate various properties to efficiency in controlling armored scales, whiteflies, spider mites, and greasy spot disease. The temperature at which 5 of a rela tively narrowboiling oil distills was found to be the most reliable indicator of pesticidal effici ency. Efficiency increased with 5 distillation temperature (5 DT) in the range of 3 to F (determined at 1 mm Hg by ASTM method D116). Oils with 5 DT below F were considered inefficient. Oils with 5 DT above F gave effective control of armored scales, whiteflies, and spider mites. Greasy spot disease control was directly related to increasing distillation temperatures and was best at F. Hydrocarbon composition of highly refined oils did not appear to be an important factor in pesticidal efficiency. Pest control was not influenced by degree of refinement in the range of 6 to 97 unsulfonated residue. Other properties of oils were not related to pesticidal efficiency. Florida Agricultural Experiment Stations Journal Ser ies No. 255. lthe research on which ths paper is based was sup ported in part by a grant from Esso Research and Engineer ing Company. Oil samples were provided by Humble Oil and Refining Company, Gulf Oil Corporation, Sun Oil Company, and Texaco, Incorporated. 2Assistant Entomologist and Entomologist, respectively, University of Florida Citrus Experiment Station, Lake Al fred. Introduction Petroleum oil is widely used as a pesticide on Florida citrus. Several million gallons are applied annually, mainly during June and July, for the control of scale insects, whiteflies, spider mites, and greasy spot disease, and the removal of sooty mold. In addition to its wide spectrum of pesticidal activity, oil is a good sticker for other chemicals and, compared to most other pesticides, is economical, is safe for the user, has little adverse effect on biological control agents, and its use creates no pesticidal residue problem. Because of the physical mode of action of oil, development of resistance by susceptible pests is unlikely. However, the use of oil has been limited to a short application period in June and July. Improper or excessive use of oil may cause fruit blemishes, excessive leaf and fruit drop, reduced fruit set, poor fruit color and quality, and increased susceptibility of citrus trees to cold weather injury. Spray oils are characterized mainly by dis tillation temperatures, viscosity, molecular weight, hydrocarbon composition, and unsul fonated residue (UR). No specifications for these properties previously have been established for Florida citrus spray oils, primarily because no reliable information was available to relate specific characteristics to performance. In con sequence, the oils used are quite variable. Al though most applications of spray oils give reasonably good pest control without excessive tree damage, some spray oil treatments are not satisfactory. Where problems do occur, details about oil properties, formulation, and applica tion, sufficient to establish a probable cause, are seldom available. Information relating various properties of spray oils to performance on Flor ida citrus is needed.