ANDERSON: EFFECTS OF GYPSUM 9 EFFECTS OF GYPSUM AS A SOURCE OF CALCIUM AND SULFUR ON TREE GROWTH, YIELDS, AND QUALITY OF CITRUS C. A. Anderson University of Florida Citrus Experiment Station Lake Alfred Abstract This paper summarizes the results of 2 gyp sum experiments in citrus groves on acid, sandy soil. The treatments in a young grove of * Par son Brown' oranges consisted of soil applications of waste pond gypsum, a by-product of the phos phate industry, applied at a rate of 3 tons per acre at 6-month intervals over a 2% year period. These applictions reduced tree growth, cold hardiness, and fruit yields without affecting the fruit quality. The treatments in a bearing grove of 'Valenaia' oranges consisted of annual applications of commercial, agricultural-grade gypsum at rates of 0, %,, and 2 tons per acre. After 4 years of treatments, fruit size and internal fruit quality remained unchanged; however, fruit yields decreased progressively with increasing rates of gypsum. The applications of gypsum increased the contents of leaf calcium and sulfur but lowered the contents of leaf potassium, magnesium, nitrogen, and phosphorus. The content of leaf sulfur was in the "excessive" range in both groves. Introduction A number of Florida citrus growers have become interested in soil applications of gypsum (calcium sulfate) as a soluble source of calcium. Some fertilizer and limestone companies now stockpile gypsum for that purpose. In most cases, this gypsum originated from the phosphate industry where it is the major by-product in the production of phosphoric acid. The use of gyp sum on citrus seems to have evolved from 2 rather diverse observations: ) certain agricul tural crops respond favorably to soluble sources of calcium, e.g., peanuts and tomatoes; and 2) calcium is normally present in relatively large amounts in citrus roots, branches, and leaves. Florida Agricultural Experiment Stations Journal Series No. 306. To further stimulate the use of gypsum on citrus, some distributors have emphasized the sulfur content of gypsum. Detailed investigations of the effects of gyp sum on citrus have not been conducted in the past because it did not appear to be a promising area of research. Authentic cases of calcium deficiency in citrus have been observed only rarely anywhere in the world (3,9), which sug gests that citrus is a "strong feeder" for calcium. Reitz, et al (7) advised Florida citrus growers, "Calcium is usually supplied in sufficient amounts for nutritional purposes in either dolo mite or high calcium limestone." Most Florida citrus growers are following recommended lim ing practices. With respect to sulfur, Reitz, et al (7) stated this, "Sulfur in excess of the nutritional needs of citrus is applied in sprays and dusts for mite control and in various sulfates used in fertilizer mixtures." Because of grower interest in gypsum for citrus, grove experiments employing soil appli cations of gypsum were initiated with the objec tive of studying the effects on growth of young trees and on fruit yields and fruit quality of bearing trees. This paper summarizes the re sults of those experiments. Experimental Methods Two gypsum experiments were started in commercial citrus groves in 964. The experi ments were similar in that both were located on Lakeland fine sand having relatively low levels of available soil calcium (300 pounds per acre extracted with neutral ammonium acetate) and reasonably high ph values (ph 6.0 determined in water). The experiments differed in both age and variety of the test trees and in the source of the gypsum. Experiment I, in a 3-year-old grove of Tarson Brown' oranges on sour orange rootstock, consisted of 3 replicates of paired plots receiving 0 or 3 tons per acre of waste pond gypsum applied with a fertilizer spreader over the entire ground area of 8-tree plots. The treatments were repeated every 6 months for 2Y2 years for a total of 5 applications or 5 tons of gypsum. The treatments were applied in April and Au-
20 FLORIDA STATE HORTICULTURAL SOCIETY, 968 gust 964, March and October 965, and March 966. This gypsum, having a moisture content of about 30% and containing various inpurities, was the typical by-product gypsum of the phos phate industry. It was applied as received. The application rates were calculated on this basis, not on a dry-weight basis. Experiment II was located in a 4-year-old grove of 'Valencia' oranges on rough lemon rootstock. The gypsum used in this experiment was commercial, agricultural-grade gypsum. It was applied by hand over the entire ground area of 6-tree plots at rates of 0, %,, and 2 tons per acre. A fifth treatment consisted of an applica tion of calcitic limestone at a rate of ton per acre. All 5 treatments were applied in March or April every year for 4 years. The field design was a randomized block with 4 replica tions. Tree growth was determined at intervals in Experiment I by measuring the size of the tree canopies. (All trees are multi-trunked because of cold injury in 962.) Leaf samples were collected from both experiments for chemical analyses. Fruit yield and quality data were obtained from Experiment I for the 966 crop and from Experiment II for the 964-65, 965-66, and 967-68 crops. Soil samples were col lected to a depth of 36 inches from both experi ments in 968. Results and Discussion Experiment I. The first response of the young 'Parson Brown* orange trees to the ap plications of waste pond gypsum was detected on January 29, 965 0 days after a night of freezing temperatures and severe frost that completely defoliated many trees in colder loca tions in the grove. Weather Bureau records list low ground temperatures in the low and midtwenties for that area that night. Within the experimental area, untreated trees lost 0% of their leaves, whereas trees that had received the 2 gypsum applications the previous year, 964, were 46% defoliated (Table ). This difference just missed statistical significance at a probabil ity level of.05. Although many of the leaves remaining on the trees showed frost damage and discoloration, no unusual nutritional deficiency or toxicity symptoms, nor symptoms of salt burn, were observed. While the trees were being examined to assay the extent of defoliation, numerous dead twigs and small branches were observed in some of the trees. A careful examination of the in dividual trees revealed that the untreated trees contained an average of.0 dead twig or small branch per tree, while the gypsum-treated trees contained 8.9 dead branches per tree. This dif ference was highly significant. The cause of the dieback was not determined, but it was apparent that it had occurred several months previously, perhaps in the late summer or fall of 964. Analysis of leaf samples collected in October 964 and analyzed for calcium, magnesium, and potassium indicated that the gypsum treatments had not yet affected the composition of the leaves, at least with respect to those 3 elements (data not included). In April 965, the first growth response due to treatment was found (Table ). It was shown to be a combination of ) an increase in canopy size of the untreated trees, and 2) an actual decrease in size of the gypsum-treated trees relative to their size in October 964 when both groups of trees were growing equally well. Both treated and untreated trees grew well during the summer of 965 as indicated by the growth data of October 965. Differences due to treatment, however, were still significant as late as June 966. The young 'Parson Brown' orange trees pro duced their first marketable crop of fruit in 966. The plots were harvested in December of that year. (The fifth and final application of gypsum was made in March 966.) The un treated trees produced. boxes per tree and the gypsum-treated trees produced 0.6 boxes per tree (Table 2). This yield difference due to treatment was statistically significant at a prob ability level of.0. Fruit samples were collected from all plots a few days before harvest and tested for several internal fruit quality factors. Since neither the size of the individual fruit nor the internal quality were affected by the gpysum treatments, the data are not included. Analysis of leaf samples collected in Septem ber 966 disclosed that the gypsum treatments had affected the contents of leaf calcium, mag nesium, and sulfur (Table 2). As might have been expected, the increase in leaf calcium was substantial. This in itself probably was not an important physiological change since the con tent of leaf calcium in the untreated trees was also in the "satisfactory range" despite the low levels of available soil calcium in the untreated
ANDERSON: EFFECTS OF GYPSUM 2 Table. Effects of waste pond gypsum on the growth of young 'Parson Brown orange trees. Treatments applied April and August 964, March and October 965, and March 966. Treatments Signif-* Date Check icance Defoliation, % Jan. 965 0.0 46.0 Dieback, branches/tree Jan. 965.0 8.9 Oct. 964 50.4 52.7 April L 965 67.5 4.9 Oct. 965 34.5 20.9 * June 966 4.0 22.0 * +Growth is expressed as percentage increase in canopy size over original size in April 964. * and indicate statistical significance at probability levels of.05 and.0, respectively. indicates nonsignificance at a probability level of.05. plots (2,8). The content of leaf magnesium was very low (below the recommended range of.30 to.45%,) although the distinctive foliar symp toms of magnesium deficiency were not present. The sulfur content of the leaves was strongly affected by the gypsum treatments. The value of.43% sulfur for the gypsum-treated trees is one of the highest values of leaf sulfur ever re ported for citrus in the field. The "satisfactory range" for total sulfur in citrus leaves is between 0.2 and 0.3%; values over 0.4% are considered high; and, in young trees, values above 0.5% have been correlated with depressed tree growth (2,4). The relatively high values of leaf sulfur for both treated and untreated trees in this ex periment may have been due in part to the scion variety-rootstock combination. Both scion and rootstock have been shown to affect the uptake of sulfur, with the 'Parson Brown'-sour orange combination being very efficient (6). The lack of response of leaf fluorine is note worthy since the waste pond gypsum used in this experiment contained from 0.2 to 0.4% fluorine. Furthermore, a significant difference in amount of fluorine was found in samples of feeder roots collected as late as April 968 2 years after the final application of gypsum. Roots of gyp sum-treated trees contained 95 ppm fluorine, whereas those of the untreated trees contained 23 ppm. The presence of a translocation mechan ism in citrus which limits the movement of fluorine from roots to leaves has been demon strated by others (). The leaf samples were not analyzed for any of the essential minor elements. It did not seem necessary since most minor element deficiencies in citrus are accompanied by very distinctive and characteristic foliar symptoms and none of these symptoms were observed at any time dur ing the study. Special care had been taken in checking the trees for symptoms of molybdenum deficiency because the sulfate ion is known to affect the uptake of molybdenum. No signs of "yellow spot" were found. Experiment. None of the treatments af fected the yields of the 'Valencia' orange trees
22 FLORIDA STATE HORTICULTURAL SOCIETY, 968 Table 2. Effects of waste pond gypsum on fruit yields and leaf composition of 'Parson Brown orange trees. Treatments Signif-* Date Check icance Yields, boxes/tree Dec. 5, 966. 0.6 Leaf composition Sept. 7, 966 Ca, % 3.7 4.77 Mg, %.52.8 K, /o.04.7 Na, ppm 872 820 N, % 2.33 2.06 P, %.6.3 0.44..43 F, ppm 3 3 and indicate statistical significance at probability levels of.05 and.0, respectively. indicates nonsignificance at a probability level of.05. during the 964-65 and 965-66 crop years. The trees averaged 4.2 boxes of fruit per tree both years. Unfortunately, yield data were not ob tained for the 966-67 crop. The 967-68 crop, harvested in June 968, was significantly re duced by some of the gypsum treatments (Table 3). From an average of 3. boxes per tree for the untreated trees, yields were reduced to 2.4 and 2. boxes per tree by the - and 2-ton rates of gypsum, respectively. (The relatively low yields for all treatments may be explained in part by the fact that the 'Valencia' grove had no irrigation facilities and drouth conditions were prolonged and severe during the spring of 967 and again in 968.) The yield reductions due to gypsum were not accompanied by any variation in size of the individual fruit nor in any of the internal fruit quality factors that were measured. The mean weight of the in dividual fruit was 25 grams and the fruit contained 53.5% juice, 4.4% Brix,.7% acid, with a Brix-to-acid ratio of 2.34. These values are quite typical of regular bloom 'Valencia' oranges for May 5, 968, as indicated by ma turity test data released by the Florida Crop and Livestock Reporting Service. The 967-68 fruit crop would be expected to be closely associated, physiologically, with the 967 spring-flush leaves. Such leaves were sam pled in October 967 and analyzed for 7 ele ments. The contents of 5 of the 7 elements were significantly affected by at least some of the gypsum treatments (Table 3). The contents of leaf calcium, magnesium, and nitrogen, although responsive to treatment, were within the recom mended "satisfactory ranges" in all cases. The content of leaf potassium was low, even in un treated trees, and was further reduced by the heavier applications of gypsum. It is doubtful, however, whether potassium was a major factor contributing to the variations in yield since the yield reductions werd! not accompanied by re-
ANDERSON: EFFECTS OF GYPSUM 23 Table 3. Effects of agricultural-grade gypsum and limestone on fruit yields and leaf composition of 'Valencia orange trees. Treatments, tons/acre year Date 0 /2 2 Limestone Yields, boxes/tree June 968 3.a 2.9ab 2.4 be 2. 3.0 ab Leaf composition Oct. 967 Ca, % 2.69a 3.09a 3.65 b Mg, %.53a.39 c.35 d K, %.9a.88ab.80 be Na, ppm 742a 736a 780a N, % 2.86a 2.75ab 2.7 be P, %.3a.2a.2a S, %.27a.45 b.58 c 3.90 b.30 e. 78 c 794a 2.6 c.2a.70 d 3.09a.45 b.9a 785a 2.8ab.2a.27a *Within a given row, data followed by the same letter do not differ at a probability level of.05. Table 4. Effects of agricultural-grade gypsum and limestone on the soil ph and soil calcium content of Lakeland fine sand. Sampling date: July 968. Treatments, tons/acre/year Limestone Depth, in. 0 /2 2 Soil ph 0-6 6.2a* 6.3a 6.3a 6.2a 7.0 b 6-2 6.0 b 2-24 5.4 b 24-36 4.8a 4.9a 4.9a 4.9a Soil Ca 0-6 288a 328a 409a 404a 668 b 6-2 86a 3a 89a 77a 9a 2-24 64a 32 b 22 b 74 c 27 b 24-36 39a 59ab 8 b 4 c 74 b *Within a given row, data followed by the same letter do not differ at a probability level of.05.
24 FLORIDA STATE HORTICULTURAL SOCIETY, 968 ductions in size of the individual fruit, an asso ciation of 2 responses commonly brought about by potassium deficiency (8). The response curve of leaf sulfur to increasing rates of gypsum ap peared to be quadratic, i.e., the increase in leaf sulfur was somewhat less than twofold for each twofold increase in rate of gypsum application. Leaf sulfur values of.58 and.70% were found corresponding to reduced yields of 2.6 and 2. boxes per tree, respectively. Analysis of soil samples collected a year after the fourth and final treatment indicated that only the limestone treatments had affected the soil ph (Table 4). Limestone raised the ph relative to that of the untreated soil at all depths to 24 inches., on the other hand, did not affect the ph at any depth. (Precisely the same treatment effects, but at lower ph readings, were found when the ph determina tions were taken in potassium chloride solution of.0 Normal strength.) The gypsum applica tions did not contribute to the supply of soil calcium in the 2 upper soil zones but did increase the content of soil calcium in the 2 to 24 to 36 inch depths. In these 2 lower soil zones, the effects of -ton rates of limestone and gypsum were similar with respect,to soil calcium. In summary, all responses of economic im portance observed in both gypsum experiments were of an undesirable nature. No beneficial responses were detected, such as might have been expected from increased leaf calcium. The un desirable responses to the gypsum treatments appeared to be primarily due to excessively high levels of sulfur within the plants. The content of leaf sulfur in the young 'Parson Brown' trees was one of the highest values ever reported for citrus in the field. The content of leaf sulfur in the bearing 'Valencia' trees was understand ably lower since the application rates were not as high and the scion variety-rootstock combina tion of 'Valencia'-rough lemon is known to be less effective in taking up sulfur. Yield reduc tions without deterioration of fruit quality have been reported by others (5) at sulfur levels very similar to those found in the 'Valencia' experi ment. The effect of excessive sulfur levels in citrus is probably due to an unusually high ratio of sulfate sulfur-to-organic sulfur, result ing in serious derangement of mineral metabo lism (4, 8). It should not be confused with sulfide toxicity, a condition in which plant tissues are poisoned by relatively small quantities of hydrogen sulfide, a strongly reducing compound. The undesirable responses to gypsum were limited to application rates which were, perhaps, unreasonably heavy. The relatively light ap plication rate of Y2 ton per acre per year did not have any detrimental effects; but, and this is also important, neither did it have any benefi cial effects. Therefore, on the basis of these two experiments, soil applications of gypsum, either waste pond or agricultural-grade, cannot be recommended for citrus. Acknowledgments The author expresses his gratitude to Mr. Tom Evans of Lake Alfred and to the Long Realty Company of Tampa, who provided the two groves in which the above-described experi ments were conducted, for their generous co operation throughout this study. LITERATURE CITED. Brewer, R. F. 969. Effect of flourine additions to substrate on naval orange trees grown in solution culture. Soil Sci. 87(4): 83-88. 2. Chapman, H. D.967. Plant analysis values sugges tive of nutrient status of selected crops. In Soil Testing and Plant Analysis, Part II. M. Stelly, Ed. Soil Sci. Soc. Amer., Madison, Wis., pp. 77-92. 3. Chapman, H. D., H. Joseph and D. S. Rayner. 965. Some effectives of calcium deficiency on citrus. Proc. Amer. Soc. Hort. Sci. 86: 83-93. 4. Eaton, F. M. 966. Sulfur. In Diagnosetic Criteria for Plants and Soils. H. D. Chapman, Ed. Univ. of Calif. Press, Berkeley, Calif., pp. 444-475. 5. Jones, W. W., et al. 963. Nitrogen control program for oranges and high sulfate and/or high boron. Calif. Citrog. 48:07. 6. Rasmussen, G. K. and P. F. Smith. 958. Effect of fertilizer rate, roots to ck, and leaf age on level of sulfur in citrus leaves. Proc. Amer. Soc. Hort. Sci. 7: 24-247. 7. Reitz, H. J., et al. 964. Recommended fertilizers and nutritional sprays for citrus. Fla. Agri. Expt. Sta. Bull. 536B, 23 pp. 8. Smith, P. F. 966. Leaf analysis of citrus. In Nutri tion of Fruit Crops. N. F. Childers, Ed. Horticultural Pub lications, Rutgers Univ., New Brunswick, N.J., pp. 208-228. 9. Spencer, W. F. and R. C. J. Koo. 962. Calcium deficiency in field-grown citrus. Proc. Amer. Soc. Hort.Sci. 8: 202-208.