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Tom L. Weinert, Thomas L. Thompson, Scott A. White and Michael A. Maurer

Microsprinkler irrigation may result in increased efficiency of N and water application to citrus. However, best management practices (BMPs) have not yet been developed for microsprinkler use, particularly on newly established citrus. Experiments were conducted during 1997-98 in central Arizona to evaluate the effects of N rate and fertigation frequency on `Newhall' navel oranges (Citrus sinensis) planted in Mar. 1997. Two experiments were conducted, each with factorial combinations of N rate (0 to 204 g/tree/year) and fertigation frequency (weekly to three times per year). In one experiment, nonlabeled N fertilizer was used, and in the other 15N-labeled fertilizer was used. Trunk diameter, leaf N, and 15N partitioning in the trees were monitored. During 1997, neither trunk diameter nor leaf N were affected by N rate or fertigation frequency. No more than 6% of N applied was found in the trees. During 1998, leaf N in fertilized plots was significantly higher than in nonfertilized plots, but leaf N in all trees remained above the critical N concentration of 25 mg·g-1. During 1998, no more than 25% of the fertilizer N applied was taken up by the trees. Results suggest that N applications are not needed during the first growing season after planting for microsprinkler-irrigated citrus in Arizona. Only low rates of N (≤68 g/tree/yr) may be needed during the second growing season to maintain adequate tree N reserves.

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James L. Walworth, Scott A. White, Mary J. Comeau and Richard J. Heerema

A field study was conducted to evaluate efficacy of soil-applied zinc (Zn) fertilizer on young pecan [Carya illinoinensis (Wangenh.) K. Koch] trees growing in alkaline, calcareous soils. Chelated Zn ethylenediaminetetraacetic acid (ZnEDTA) was applied at rates of 0, 2.2, or 4.4 kg·ha−1 of Zn via injection into irrigation water (fertigation) in microsprinkler irrigated ‘Western’ and ‘Wichita’ trees. Over the 5-year duration of the study, leaf Zn levels were increased from 22 to 35 µg·g−1 in the highest rate of ZnEDTA treatment compared with 7 to 14 µg·g−1 in unfertilized trees. Zn concentrations in shoot and root tissues were also elevated in Zn-treated trees. Zn treatments largely eliminated visible Zn deficiency symptoms, and increased trunk diameter growth compared with untreated trees. Nut yield (in the third through fifth seasons) were also increased as a result of Zn fertilization. No additional benefit in terms of trunk diameter growth or nut yield was observed by adding a higher rate of Zn (4.4 kg·ha−1) vs. the lower rate (2.2 kg·ha−1). ‘Western’ and ‘Wichita’ trees responded similarly to Zn fertigation.