PROGRAM FERTILIZATION FOR ESTABLISHMENT OF ORANGE TREES

The citrus industry recently added over 160 000 acres of new groves in south Florida. Young, densely-planted groves are typically fertilized in excess of current guidelines in an effort to accelerate fruit prod uction. The objectives of this study were to develop an efficient N fe rtilization program for maximum initial orange (Citrus sinensis L. Osb .) production, to assess the use of controlled-release N, and to ascer tain the effect of soil pH on tree growth and yield. 'Hamlin' orange t rees on Carrizo citrange rootstock were planted at 194 trees/acre in 1 989 on a sandy Alfisol, and were grown for 4 yr under four fertility l evels. In the first 3 yr, four relative rates (12.5, 25, 50, and 100%) of a complete (N-P-K) fertilizer were applied using three N sources ( ammonium nitrate [AN], a 50/50 mixture of AN and isobutylidene diurea [AN-IBDU] and a 60/40 mixture of AN and methylene urea [AN-MU]. The 10 0% rate consisted of 6, 12, and 18 lb/tree of 8-4-8 fertilizer applied in Years 1, 2, and 3, respectively. In the fourth year, N was applied at 41, 81, 163, and 325 lb/acre to trees that received the same relat ive rates the previous 3 yr. Tree canopy volume and orange yield incre ased as N rate increased. In the fourth year, canopy volume at the low est fertility level was 90% of that at the highest level (534 vs. 603 cu ft), but orange yield was only 63% as great (328 vs. 518 boxes/acre , where 1 box = 90 Ib of oranges). A quadratic-plateau model suggested that maximum yield could be obtained with about 250 Ib N/acre in the fourth year. However, 91% of maximum yield was obtained with 163 Ib N/ acre. A leaf N concentration of 2.35% or greater was associated with m aximum yield. As N increased, orange peel color was greener, juice aci d concentration increased, total soluble solids (TSS):acid ratio decre ased, and TSS yield per acre increased. The major advantage of control led-release N was lower application frequency compared with water-solu ble N. No relationship between soil pH and tree canopy volume or orang e yield was observed in the pH range of 4.6 to 8.0. Below pH 4.6, tree size and yield were depressed. Current guidelines for fertilization a ppear to be sufficient for maximum early orange yield in high-density new plantings on south Florida flatwoods soils.