FERTILIZER EFFECTS ON EARLY GROWTH AND YIELD OF 'HAMLIN' ORANGE TREES

The growth response of young 'Hamlin' orange (Citrus sinensis L. Osbeck) on Carrizo citrange (C. sinensis × Poncirus trifoliatu L. Raf.) trees to N-P-K fertilizer rates under field conditions in southwestern Florida was studied to determine the minimum fertilizer required to bring trees into maximum early production. The highest 8N-1.8P- 6.6K fertilizer rate was 2.72,5.45, and 8.17 kg/tree in 1989,1990, and 1991, respectively. Additional fertilizer treatments equaled 50%, 25%, or 13% of the maximum rate. Fertilizer sources contained either all water-soluble N (applied more frequently) or 40% to 50% controlled-release N (applied less frequently), and they did not affect fruit yield or quality. The response of trunk cross-sectional area, tree canopy volume, and fruit yield to fertilizer rate was described by a linear plateau model. The model predicted a fruit yield of 22.6 kg/tree at the estimated critical fertilizer rate of 48% of maximum. Fruit yield at the 50% maximum rate averaged 21.2 kg/tree. As fertilizer rate increased, total soluble solids concentration (TSS) in juice and the TSS : acid ratio decreased, but weight per fruit and TSS per tree increased. A fruit yield >21 kg/31-month-old tree indicated vigorous growth. More than 48,000 ha of new citrus groves has been established in Florida since 1988 (Florida Dept. of Agr. and Consumer Serv., 1990). Land used for citrus production in southwestern Florida increased from 21,000 ha in 1980 to 51,000 ha in 1990. New plantings are expected to continue through the 1990s, most in Florida's new citrus-producing re- gion. The effect of agricultural practices on the environment is a concern associated with this growth. The mobility of N fertilizer in sandy Florida soils presents the potential for ground- water contamination. Adopting management practices to increase fertilizer efficiency (de- fined as the proportion of applied fertilizer taken up by the plant) should minimize envi- ronmental effects andreduce production costs. An example of such practices is using fertilizer rates near the minimum required by the plant for maximum growth under an application strategy that decreases leaching. Milder winter temperatures in southwest- em Florida allow a longer growing season than in central Florida. To produce large trees quickly and accelerate fruit production, grow- ers have fertilized young trees at rates exceed- ing current N-P-K recommendations (Koo et al., 1984). These recommendations do not differentiate between central and southern Florida with respect to fertilizer rates. Most citrus fertilization studies in Florida have involved trees established in the field for