Effects of Phosphate Fertilizer on Young Citrus Trees on Flatwood Soils

Phosphate fertilizer experiments with nonbearing citrus trees were started at planting time in commercial groves on 5 flatwoods soils. The soils included 1) Brighton peat, 2) Sunniland, 3) Keri-Bradenton, 4) Immokalee, and 5) Immokalee-Pomello. Some of the phosphate treatments were applied only once, as broadcast applications at planting time. Other treatments were applied periodically to individual trees in Florida Agricultural Experiment Stations Journal Series No. 2822. the form of an 8-X-8-4 fertilizer mixture con taining 0, 4, 8, or 12% P2O5. On 3 of the 5 soils, tree growth was signifi cantly increased by the phosphate treatments within 2 years; in one case, however, the re sponse appeared to be due to calcium in the phosphate fertilizers rather than to phosphorus, per se. None of the treatments improved tree growth on the 2 calcareous soils, Sunniland and Keri, although the added fertilizers were effec tive in increasing the phosphorus content of the leaves. All of the trees, irrespective of treat ment, grew vigorously on these 2 soils. The effectiveness of the different treatments was evaluated by determining the phosphorus con tent of the leaves. The treatment effects on leaf calcium, magnesium, and potassium contents were also reported. Introduction and Literarure Review Many flatwoods citrus growers indicate that fertilizer programs recommended for citrus 20 FLORIDA STATE HORTICULTURAL SOCIETY, 1967 groves on the Ridge (3, 7, 10) are inadequate or inappropriate for their particular conditions (4). They have reported striking differences in tree survival and rate of growth due to varia tions in soil type. The soils of the flatwoods and marshes do have an extreme range in several important chemical and physical properties (5, 8, 9). With but a few exceptions (2), however, almost all are deficient in native soil phosphorus as indi cated by chemical analyses. For these reasons, a study was initiated to compare and evaluate the effects of phosphate fertilizers on the growth and leaf composition of young citrus trees planted on several typical lowland soils. Phosphate fertilizer experiments with citrus planted on flatwoods soils are limited in number and the results have not been consistent. In one experiment phosphorus increased the rate of growth of 'Hamlin' oranges on sweet orange rootstock planted on Leon fine sand (6). In an other study in the same grove, no growth re sponse due to phosphorus could be detected after 3 years of treatments on 'Valencia' oranges on rough lemon rootstock planted on Scranton fine sand (1). Positive growth and fruit yield re sponses due to phosphate fertilizer were reported for 'Lue Gim Gong* oranges on a Da vie mucky fine sand; however, the role of phosphorus was confounded by the limited availability of cop per (11). Experimental Methods Five phosphate fertilizer experiments with non-bearing citrus trees were started at planting time in commercial groves on different soils of the flatwoods and marshes. The soils were se lected to include a wide range of properties al though all were presumed to be deficient in native soil phosphorus. None of the soils had been under cultivation nor in improved pastures prior to the development of the grove site. Some characteristics of the soils and test trees are listed in Table 1. Experiments I, II, III, and IV included both source and rate phosphate treatments. The rates were 0, 4, 8, and 12% P2O5 in an 8-X-8-4 (N-P2O5-K2O-MgO) fertilizer mixture that was applied to the individual trees at recommended rates and frequencies (7). The phosphorus source was triple superphosphate (TSP) con taining 45% available P2O5. A second source was ordinary superphosphate (OSP) containing 20% available P2O5. It was tested at 2 rates, 8 and 12% P2O5 in the 8-X-8-4 fertilizer mix ture. The 'Valencia' trees in Experiment I re ceived a total of 1 pound of mixed fertilizer in Table 1. Some characteristics of the soils a"nd test trees of. the 5 phosphate experir merits.