EFFECT OF NITROGEN AND POTASSIUM FERTILIZATION ON YIELD AND FRUIT QUALITY OF VALENCIA ORANGE ON CALCAREOUS SOIL

Transcription

1 12 FLORIDA STATE HORTICULTURAL SOCIETY, 1959 EFFECT OF NITROGEN AND POTASSIUM FERTILIZATION ON YIELD AND FRUIT QUALITY OF VALENCIA ORANGE ON CALCAREOUS SOIL Herman J. Reitz and Robert C. J. Koo Florida Citrus Experiment Station Lake Alfred Fertilization practices along the Indian River are determined primarily by custom, as few experiments have been reported on the use of major fertilizer elements. This paper presents results of a single field experiment conducted for six years with orange trees on a typical calcareous soil of the Indian River citrus area. The variables were three rates each of nitrogen and potassium. These studies were made to determine the effects of the treatments on tree appearance, yield, and quality of fruit, and upon leaf and soil analyses in order to provide facts useful in solving practical fertilization problems. Methods The experiment was conducted on Merritt Island, near Orsino, in a Valencia orange grove on sour orange rootstock planted on single beds in The grove was selected as typical of those on Florida calcareous soils following a survey made in part for this pur pose (3). The site was almost flat and the grove showed reasonably good uniformity in tree size and soil characteristics. The soil was classified in Parkwood series and had a very dark brown surface horizon. Limerock was encountered at a depth of one to two feet in a few parts of the grove. The surface soil at 0 to 6-inch depth averaged 13.9 per cent moisture equivalent, 4.7 per cent calcium car bonate, 206 pounds exchangeable potassium per acre, ph 7.0, exchange capacity 21.2 M.E. per 100 grams, and 5.4 per cent organic matter. Plots consisting of four trees complete ly surrounded by at least one-half of a buffer row were then set up. s were ran domized in four replicate blocks. The experiment consisted of three levels of nitrogen and three levels of potassium applied in all of the nine possible combinations of rates. The basic over-all fertilizer (N1K1) contained the minimum rates of both nitrogen (Nl = 1.2 lb. N/tree/year) and potassium (Kl z= 0 lb. K20/tree/year) used in the ex periment, and consisted of two equal applica tions of 732 pounds of commercial mixed fertilizer per tree per year. Sufficient additional nitrogen as sodium nitrate was ap plied in two equal applications to the two higher nitrogen treatments to bring the rates to 1.8 lb. N (N2) and 2.4 lb. N (N3) per tree per year. The intermediate and high levels of potassium consisted of 2.4 lb. (K2) and 4.8 lb. (K3) K20 per tree per year derived from 60 per cent muriate of potash. The first differ ential fertilizer application was made in May 1953 and the experiment was terminated with the picking of the mature crop in April Other details of the experimental plan have been previously published (2). Results Leaf analysis and tree appearance. Some effects of the main treatments upon leaf ana lysis are shown in Fig. 1. There was a signifi cant difference in leaf nitrogen content two months after initiation of the treatments, ac companied by marked yellowing of the foliage on low nitrogen plots. Both effects persisted through most of the experiment but unaccount ably became indistinct in the final year of the experiment. By contrast, differences in leaf potassium content developed slowly and were NITROGEN EFFECTS-POTASSIUM EFFECTS YEAR OF SAMPLING Fig. 1. Effects of nitrogen and potassium application rates on analysis of spring-flush leaves sampled in July of each year.

2 REITZ AND KOO: NITROGEN & POTASSIUM 13 not statistically significant until the third year. No distinct leaf symptoms of potassium de ficiency ever appeared. The higher nitrogen treatments decreased the concentration of potassium in leaves by a significant margin. Both high potassium and low nitrogen decreased the magnesium concen tration in leaves significantly and to a suffi ciently low level to produce marked magne sium deficiency symptoms in the leaves. Soil analysis. Soil samples were taken in December 1958, two weeks before making the final application of fertilizer, and results of analyses for certain factors are given in Table 1. Exchangeable potassium averaged about the same in the medium potassium plots at the beginning and the end of the experi ment, while the low potassium treatments lost and the high potassium plots gained approxi mately 100 pounds exchangeable potassium per acre in the 0 to 6-inch depth. The large difference in soil sodium content was due to the use of sodium nitrate as a source of nitro gen. Yield. Differential nitrogen fertilization had an immediate significant effect upon yield (Fig. 2) that continued throughout the dura tion of the experiment. Actual usage of nitro gen per box of fruit produced during the last three years of the experiment was as follows: Nl, 0.41 lb. per box; N2, 0.52 lb. per box; and N3, 0.58 lb. per box. Potassium fertiliza tion had no statistically significant effect upon yield by individual years until the last year of the experiment. In the last year, the actual yield difference due to the low potassium treatment exceeded that due to nitrogen. Value of crop for fresh fruit. A summary of the main treatment effects upon some ex ternal fruit characteristics is given in Table 2. Fruit size increased as potassium fertilization increased, but decreased with increasing nitro gen application. Most of the fruit except the small fruit produced in some years on nopotash plus high-nitrogen plots was of desir able size for fresh fruit. NITROGEN EFFECT POTASSIUM EFFECT YEAR OF PICKING Fig. 2. Effect of nitrogen and potassium application rates on cumulative yield of fruit in successive years. Yields are quoted in field boxes, each having a vblume of 2.2 bushels. Table 1. Soil ph and exchangeable bases extracted from soil samples taken December PH Lb. Ca per acre Lb. K per acre Lb. Na per acre 61 f-12" 6"-12"

3 14 FLORIDA STATE HORTICULTURAL SOCIETY, 1959 Table 2. Effects of main treatments on some external characteristics of the fruit. Weighta Grams/fruit U. S. No. 1 Gradeb Yr. Avg. Boxes per 3-yr. U.S.I Tree total athree-year average. bnon-degreened fruit. The proportion of fruit grading U.S. No. 1 was determined in each of the last three sea sons in the Citrus Experiment Station pack inghouse. High nitrogen resulted in reduced proportion of No. 1 fruit in 1957, due pri marily to green color. The data do not bring out the true difference between the treat ments, as the fruit from low nitrogen plots actually made a superior No. 1. In contrast to the generally poor fruit color of 1957, fruit color in 1958 and 1959 was very good through out the block, presumably due to weather con ditions, and there was no effect of treatments on grade of fruit. Growers who use high nitrogen programs will probably have at least occasional years when poor color will be an important obstacle to high pack-out. Potas sium rates had no consistant effect on grade of fruit. When yield data are combined with pro portion of U.S. No. 1 grade (Tables 2 and 3), it is seen that increasing the nitrogen applica tion gave little increase in total amount of high grade fresh fruit, and that the differences in amount of U.S. No. 1 fruit due to rate of potassium applied were almost entirely de pendent on the total crop yield. Value of the crop for processing.-the main effects of treatment upon characteristics of the extracted juice are shown in Table 3. High Main Effect Table 3. Effects of main treatments on juice quality (average of crops picked 1957, 1958, 1959). Soluble Acidity Ratio Juice Total Yield Solids - % as Brix- % by Boxes/tree Brix Citric Acid Weight per,year Total Solids Pounds/tree per year a

4 REITZ AND KOO: NITROGEN & POTASSIUM 15 nitrogen very slightly reduced and high potas sium very slightly increased the concentration of soluble solids in the juice. The nitrogen effect on acidity was relatively constant each year while the potassium effect increased with successive years. The ratio of soluble solids to acidity (commonly used as an index of maturity) was in all years higher in the fruit from low nitrogen and low potassium plots. Since the effects of the treatments on indi vidual juice characteristics were small, the total production of soluble solids per tree de pended mostly on the yield of fruit per tree (Table 3). Discussion The experiment described above demon strated the effects of applying various amounts of nitrogen and potassium to citrus trees under these conditions. The nitrogen rate of 1.2 lb. per tree per year resulted in production of re latively good quality fresh fruit, at an actual nitrogen expenditure of 0.41 lb. per box. Due to lowered yield and, at times, impaired tree condition, this amount would be considered the minimum by most growers. The 2.4-lb. rate resulted in actual nitrogen usage averaging 0.58 lb. per box, with reason to believe higher yield could have been attained with higher nitrogen usage. The no-potassium rate failed to maintain fruit production longer than five years, dur ing which the potassium content of the sur face six inches of soil declined about 100 lb. per acre. The intermediate potash rate of 2.4 lb. per tree per year maintained yield and also soil potassium content. This rate resulted in fruit production at an expenditure of 0.63 lb. KoO per box of fruit per year. Higher potash usage was not beneficial in any respect. The ill effects of high potassium fertiliza tion on fruit quality reported on acid soil (4, 5) were not found in this experiment. On acid soils, low in calcium, high application of po tassium leads to high concentration of potas sium in leaves with resultant poorer fruit quality. On calcareous soils, the naturally high calcium levels impede the absorption of potas sium even when large amounts of potassium are present in the soil. Therefore in this ex periment high potassium applications did not lead either to high leaf potassium content or to lower fruit quality. There remains the practical problem of translating the results in the experimental grove to groves in similar situations. Two general methods are possible: first, through leaf or soil analysis; or, second, by direct adop tion of the rates shown experimentally to be successful. Nitrogen leaf analysis (Fig. 1) did reflect the treatments in general, but has many draw backs in practical application. The actual diff erences in leaf analysis due to treatment were comparatively small in relation to sampling errors; the levels fluctuated considerably from year to year; and the analysis is expensive to perform. No better way to determine opti mum nitrogen usage on an individual Indian River Valencia grove seems to be available than to begin a program of nitrogen usage between 0.4 and 0.6 lb. N per box per year and adjust upward or downward in relation to tree appearance and the price structure for different qualities of fruit. Three bases for choosing a potassium appli cation rate may be considered. Soil analysis reflected potash application reasonably well and over a 5/2-year period changed to a measur able degree. Unfortunately, it is not known whether the change in soil potash content changed slowly and regularly or whether it changed irregularly from year to year. How ever, there is at present no basis for extending the results obtained from the experiment to other locations. In fact, soil potassium analyses vary so markedly from grove to grove in the Indian River area (1), that it appears virtual ly impossible to set up a single set of soil potassium analysis standards for the area. Leaf analysis for potassium has distinct possibilities for practical usage. Decreased yields were obtained from plots receiving no potassium when leaf analysis ranged from 0.56 to 0.84 per cent K. This presumably is a broad range into which leaf potassium analy sis should not be allowed to fall. Very close interpretation of leaf analysis results should be avoided due to errors in sampling and analysis, unexplainable year to year fluctuations, and effects of other elements on potassium content of leaves. High leaf potassium values are difficult to obtain and hence of little conse quence on calcareous soils. Leaf potassium analysis is reasonably stable from year to year, fairly easy to sample for, and not difficult to determine in the laboratory.

5 16 FLORIDA STATE HORTICULTURAL SOCIETY, 1959 For most growers, potash rates based on ratio to nitrogen will continue to be the most used guide. Results of this experiment give support for the fertilizer recommendations made for calcareous soils in Florida Agricul tural Experiment Station Bulletin 536A. In ad dition, the results do indicate greater difficulty in maintaining adequate leaf potassium at high rather than low nitrogen. Therefore relatively higher potassium mixtures (at least 5 K20 to 4 N) should be used with high nitrogen appli cation rates than would be necessary with low nitrogen rates. At low nitrogen rates, it may be inferred that a ratio of 1 N to 1 K20 would be adequate, although this exact ratio was not included in the experiment at the low nitrogen rate. Finally, fertilizer practice in these situations must be adjusted to produce fruit of greatest market value. Growers whose market demands highest quality fresh oranges may find it neces sary to limit fertilizer application rates to mini mum amounts. At the other extreme, growers who simply aim to produce the greatest num ber of pounds of soluble solids per tree will seldom be ill advised to fertilize heavily and aim for greatest possible total fruit yield. Summary Three levels of nitrogen and three levels of potassium fertilization were applied to bearing Valencia orange trees on a typical calcareous soil in the Indian River area of Florida over a six-year period. The higher levels of nitro gen fertilization resulted in dense, dark green foliage compared with the low levels, while potassium fertilization had little effect upon tree appearance. Both low nitrogen and high potassium fertilization resulted in noticeable magnesium deficiency symptoms in the foliage. Yield of fruit was significantly reduced by low nitrogen from the second year of the ex periment, but low potassium did not signifi cantly influence yield until the sixth year of the experiment. Both high nitrogen and high potassium increased the acidity of the juice and delayed maturity. High nitrogen increased the amount of green color on the fruit. Increasing nitrogen tended to decrease fruit size, while increasing potassium tended to increase fruit size, with indications that the potassium effect was of greater influence. Other effects on fruit quality were of minor significance. LITERATURE CITED 1. Peech, Michael, and T. W. Young Chemical studies on soils from Florida citrus groves. Fla. Agr. Expt. Sta. Bull Reitz, Herman J., and Robert C. J. Koo Effect of nitrogen and potassium fertilization on yield, fruit quality, and leaf analysis of Valencia orange. Proc. Amer. Soc. Hort. Sci. In Press. 3. Reitz, Herman J., and Wallace T. Long Mineral composition of citrus leaves from the Indian River area of Florida. Proc. Fla. State Hort. Soc. 65: Reuther, W., and P. F. Smith Relation of fertilizer treatment to fruit quality of Valencia oranges. Proc. Fla. State Hort. Soc. 64: Sites, John W., and Edward J. Deszyck Effect of varying amounts of potash on yield and quality of Valencia and Hamlin oranges. Proc. Fla. State Hort. Soc. 65: CURRENT STUDIES ON THE EFFICIENCY OF EQUIPMENT FOR THE APPLICATION OF PESTICIDES TO CITRUS TREES IN FLORIDA J. R. King, P. J. Jutras and W. L. Thompson Florida Citrus Experiment Station Lake Alfred Equipment for the application of agricultural pesticides is equally as important as the pesti cides. During recent years failure to obtain satisfactory pest and disease control in tops of trees with effective pesticides has indicated a need for better equipment for spraying the tops of citrus trees. Since 1938, application Florida Agricultural Experiment Station Journal Series, No equipment and methods have been evaluated periodically (1, 2, 3, 4, 5, 6, 7, 8, 9), and, al though improvements have resulted, tall trees with interlocking branches are still difficult to spray. An additional study of this problem has been needed and preliminary results are reported here. The purpose of this study is twofold: (1) to measure the ability of sprayers to obtain uniform and thorough application of spray materials to all parts of citrus trees; and, (2) to improve the application of sprays to Florida citrus by modification of existing machines