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Title: NITROGEN BEST MANAGEMENT PRACTICE FOR CITRUS TREES: I. FRUIT YIELD, QUALITY, AND LEAF NUTRITIONAL STATUS

Author
item Alva, Ashok
item PARAMASIVAM, S - SAVANNAH STATE UNIV
item OBREZA, T - UNIV OF FLORIDA
item SCHUMANN, A - UNIV OF FLORIDA

Submitted to: Scientia Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/2/2005
Publication Date: 10/1/2005
Citation: Alva, A.K., Paramasivam, S., Obreza, T.A., Schumann, A.W. 2005. Nitrogen best management practice for citrus trees: i. fruit yield, quality, and leaf nutritional status. Scientia Horticultureae. 107:233-244.

Interpretive Summary: The drinking water quality evaluation of the private homeowners wells conducted in Florida in the early to mid 1990s, revealed a trend of increasing number of samples showing nitrate levels above the EPA recommended maximum contaminant limit of 10 mg/L (based on nitrate-nitrogen). The Florida legislature passed a nitrate BMP bill in 1994 which authorized the FL Department of Agriculture and Consumer Services (DACS) to work with land grant universities to fund research to develop crop specific nitrogen BMPs. This paper summarizes six years of field experiment data to develop nitrogen BMP for mature citrus trees in Florida with major attention to sandy soil production regions along central Florida. The study was conducted in a commercial grove using 24+ year old 'Hamlin' orange trees on 'Cleopatra mandarin' rootstock. Different nitrogen rates varying from 112 to 280 kg/ha (with 56 kg/ha increment), were applied either as water soluble granular (WSG, in 4 appl/year), fertigation (FRT, in 15 appl/year), 50:50 mix of WSG and FRT, or controlled release form (CRF, in 1 appl/year). Only WSG source was also evaluated at 336 kg/ha nitrogen rate. Scheduling of irrigation was improved by monitoring the soil water content at various depths and application of irrigation using tensiometer set points of either 10 cbar (January through June) or 15 cbar (July through December) at 15 and 30 cm depths. Using the regression analysis of fruit yield response to N rates, the optimum N rate appeared to be at 260 kg/ha/year. This optimum N rate was derived based on the long term study conducted in commercial grove production management conditions and with an exceptionally high fruit production of up to 80 tonnes per hectare. The combination of improved scheduling of irrigation, and timing and placement of fertilizer contributed to lowering the optimal nitrogen rate than that previously recommended.

Technical Abstract: Elevated levels of nitrate-nitrogen (NO3-N) in the surficial aquifer above the drinking water quality standard, i.e. maximum contaminant limit (MCL; 10 mg/L), have been reported in some part of central Florida citrus production regions. Soils in this region are very sandy (sand content >95%), hence are vulnerable to leaching of soluble nutrients and chemicals below the rooting depth of the trees. Poorly scheduled irrigation and/or excess rainfall contributes to increased leaching losses of water which carries the soluble nutrients. The major objective of this research was to develop N and irrigation best management practices for citrus in sandy soils to maintain optimal crop yield and quality, and to minimize potential leaching of nitrate below the root zone. Six years of field experiment was conducted using 'Hamlin' orange trees [Citrus sinensis (L.) Osbeck] on 'Cleopatra mandarin' (Citrus reticulata Blanco) rootstock grown on a well drained Tavares fine sand (hyperthermic, uncoated, Typic Quartzipsamments) in Sebring, Highland county, FL. Nitrogen rates ranged from 112 to 280 kg/ha/yr applied as fertigation (FRT), water soluble granular (WSG), a combination of FRT and WSG (WSG + FRT; 50:50), and controlled-release fertilizer (CRF). An additional N rate of 336 kg/ha/yr was also evaluated only for the WSG source. Tensiometers were used to monitor the soil moisture content at 15-, 30-, 60-, 90-, and 150-cm depth soil profile along the dripline of the trees. This information was necessary to optimize irrigation scheduling to avoid the risk of soil moisture stress on the trees and/or application of excess water to minimize leaching below the rootzone. Fruit yield, fruit quality, and fruit mineral content data were collected for 6 years. Results showed that the fruit yield response over the entire range of N rates was greater for the FRT and WSG forms of fertilizer as compared to that for the WSG+FRT or CRF sources. Fruit yield increased with increasing N rate up to 224 kg N ha/yr. The fruit yield response was marginal with further increase in N rate. Using the regression analysis of the fruit yield in relation to N rate, the optimum N rate appeared to be at 260 kg ha/yr. It should be noted that this experiment was conducted in an exceptionally high productive grove which resulted in average fruit yield of greater than 80 Mg/ha which exceeds the state average of about 60 Mg/ha for 'Hamlin' orange. Under the conditions of improved management of N and irrigation followed in this study, the N requirement for production of 10 Mg of fruit varied from 8.1 to 9.5 kg across four N sources. This provides clear evidence of improved N uptake efficiency as a result of proposed BMPs. The results of spring flush leaf tissue and soil nutrient analyses, and fate and transport of N in soil and in soil solution with application of different rates and sources of N are reported in two companion papers.