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Michael A. Maurer, Frederick S. Davies, and Donald A. Graetz

An experiment was designed to determine the effects of canal water and reclaimed wastewater on growth, yield, and fruit quality of mature (25-year-old) `Redblush' grapefruit (Citrus paradisi Macf.) trees on sour orange (C. aurantium L.) rootstock. The study was conducted from 1 Oct. 1990 to 18 Apr. 1994 at a site adjacent to the Indian River County municipal wastewater treatment facility located near Vero Beach, Fla. Treatments included canal water applied based on one-third or two-thirds soil water depletion and reclaimed wastewater applied using microsprinklers at 23.1 mm/week (low), 30.7 mm/week (moderate) and 38.6 mm/week (high). Trees receiving low and moderate levels of reclaimed wastewater had the largest canopies and trunk diameters and highest yields, even though the amount of fertilizer applied was less than that of canal water plots. Leaf nutrient levels were generally within acceptable ranges for N, P, K, Ca, Mg, and Na except in 1991 when levels were deficient due to excessive rainfall and leaching. Leaf B levels were similar for all reclaimed wastewater treatments but were lower for the canal water treatment in 1992 and 1993. Fruit growth rate, fruit and juice weight, total soluble solids (TSS), titratable acidity (TA), and TSS: TA ratio were similar for all treatments in 2 of 3 years. Peel thickness was similar for all treatments. Heavy metal concentration in the reclaimed wastewater was at low or nondetectable levels. Similarly, enteric viruses in the effluent were always <0.003 plaque forming units/liter. Reclaimed wastewater irrigation significantly increased weed growth compared to the canal water treatment.

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John D. Lea-Cox, James P. Syvertsen, and Donald A. Graetz

15Nitrogen uptake, allocation, and leaching losses from soil were quantified during spring, for 4-year-old bearing `Redblush' grapefruit (Citrus × paradisi Macf.) trees on rootstocks that impart contrasting growth rates. Nine trees on either the fast-growing `Volkamer' lemon (VL) (C. volkameriana Ten & Pasq.) or nine on the slower-growing sour orange (SO) (C. aurantium L.) rootstocks were established in drainage lysimeters filled with Candler fine sand and fertilized with 30 split applications of N, totaling 76, 140, or 336 g·year-1 per tree. A single application of double-labeled ammonium nitrate (15NH 15 4NO3, 20% enriched) was applied at each rate to replicate trees, in late April. Leaves, fibrous roots, soil, and leachates were intensively sampled from each treatment over the next 29 days, to determine the fate of the 15NH 15 4NO3 application. Newly developing spring leaves and fruit formed dominant competitive sinks for 15N, accounting for between 40% and 70% of the total 15N taken up by the various treatments. Large fruit loads intercepted up to 20% of this 15N, at the expense of spring flush development, to the detriment of overall tree N status in low-N trees. Nitrogen supply at less than the currently recommended yearly rate of 380 g/tree exceeded the requirements of 4-year-old grapefruit trees on SO rootstock; however, larger trees on VL rootstock took up the majority of 15N from this rate over the 29-day period. Nitrogen-use efficiency declined with increasing N rate, irrespective of rootstock. The residual amounts of 15N remaining in the soil profile under SO trees after this time represented a significant N leaching potential from these sandy soils. Therefore, under these conditions, present N recommendations appear adequate for rootstocks that impart relatively fast growth rates to Citrus trees, but seem excessive for trees on slower-growing rootstock species.