FERTIGATION FREQUENCY, WETTING PATTERNS AND NITRATE LEACHING FROM LYSIMETER-GROWN CITRUS TREES

Eighteen ‘Hamlin’ orange (Citrus sinensis [L.] Osb.) trees on ‘Swingle’ citrumelo (C. paradisi Macf. × Poncirus trifoliata [L.] Raf.) rootstock were individually transplanted from the nursery into 7.6 m3 (2000 gallon) lysimeter tanks filled with native Candler sand, irrigated similarly and grown for 5 years from 1994 to 1998. During 1997-98, all tanks were fertigated at similar annual rates of N using 360° or 180° pattern microsprinklers. To investigate management practices that might minimize leaching of NO3-N, six trees with 360° jets and six trees with 180° jets were fertigated weekly (avoiding rainy periods) or 28 times in each year. Six additional trees with 180° jets were fertigated twice as often using only half the amount of N in each application. There were no effects of wetting pattern or irrigation frequency on canopy growth, tree water usage, fruit yield or quality in either year. Root density and total root wt. in the 180°-wetted zone was greater than that in the non-wetted root zone but wetting pattern (180° vs. 360°) had little effect on the total N leached in Iysimeters. Thus, results of this study do not support the idea of reducing NO3 leaching by reducing the wetted surface area of the soil. The 180° treatment with 56 split applications per year, leached the least N, 38% and 40% of that applied in 1997 and 1998, compared to about 53% and 52% in the other treatments. These results underscore the importance of a high frequency of split applications in reducing the concentration of N that is susceptible to leaching. Using frequent fertigations, well-fertilized young citrus trees can be grown on a vulnerable soil and still maintain NO3-N concentrations in ground water that do not exceed the maximum contamination level, 10 mg N liter -1. The efficient use of fertilizer nitrogen (N) to produce crop yield while protecting the environment is an important concern in all agricultural systems. In central Florida, where citrus is the most widespread crop, sandy soils with very little organic matter and intense rainfall increase the probability of fertilizer N leaching beyond the root zone. Up to 29% of the shallow water drinking wells that were sampled in such vulnerable soils in Highlands County, had NO3-N levels that exceeded the maximum contamination level (MCL) for drinking water of 10 mg N liter -1 (Riotte, 1994). This N contamination of groundwater has been attributed to leached fertilizer N from citrus production. Minimizing N leaching losses can be a complex process due to fertilization methods, variations in soil, rainfall, and the tree uptake of N. Any increase in N uptake efficiency, however, will reduce residual soil N that is susceptible to leaching. Fertilizer management practices have been developed to sustain vigorous tree growth and fruit yield while avoiding N deficiency (Tucker et al., 1995). Citrus tree N uptake efficiency during a year can be up to 57% of the N applied to low N trees (Dasberg, 1987), and we have measured annual N uptake efficiency of 68% depending on irrigation water quality (Syvertsen et al., 1993) and rootstock (Syvertsen and Lloyd, 1994). Vigorous grapefruit trees on ‘Volkamer’ lemon rootstock took up to 83% of annual N when fertilized at low rates whereas smaller grapefruit trees on sour orange rootstock took up only 51% of the applied N during the same period (Syvertsen and Smith, 1996). Thus, availability of N can determine tree growth characteristics and subsequent N requirements. Combining liquid fertilizer with irrigation water (fertigation) provides greater flexibility in timing and distribution of fertilizer applications than conventional dry fertilizer applications. Koo (1980) emphasized the importance of a minimum coverage of 60-70% of the ground surface area by irrigation water for high efficiency of nutrient uptake. Although fertigation from a dripper can increase root density near the emitter (Alva and Syvertsen, 1991), the extent to which citrus roots can be concentrated in the fertigated zone of microsprinklers has not been described for Florida conditions. Since citrus trees store large amounts of N in leaves (Kato, 1986), N application frequency has not been a consistent determinant of tree growth and yield (Ferguson et al., 1988; Davies, 1997). Increasing N application frequency however, can increase growth of young trees on ‘Carrizo’ citrange and reduce residual nitrate levels in the soil (Willis et al., 1990). This should presumably be translated into less N leached into ground water. The purpose of these studies was to investigate N leaching losses from well-managed citrus trees which were fertigated with similar annual rates of N but with different frequencies and wetting patterns. We hypothesized that if the fertigated zone could concentrate roots, N losses to leaching could be reduced. Materials and Methods Lysimeter tanks and trees. This study was conducted in central Florida using a Candler fine sand soil (Alva and Syvertsen, 1991) having a pH of 5-6 and less than 0.8% organic matter. Nineteen polyethylene lysimeter tanks, about 3.4 m in diameter, 0.9 m deep and containing about 7.9 m 3 (ca 2000 gal) of soil, were buried leaving 10 cm above the soil surface. A drainage system of 10 cm diameter perforated plastic pipe and a sump pump were placed at the bottom of each tank (Boman and Syvertsen, 1991) to pump drainage water leachate (whenever it accumulated) into 200-liter barrels used to measure drainage water volumetrically. The sand soil was back-filled into the tanks to create a uniformly mixed profile similar to the native sand on the site. Eighteen one-year-old bare-root ‘Hamlin’ orange trees on ‘Swingle’ citrumelo rootstock, were transplanted individually into 18 tanks during the spring of 1994. At the same time. six similar trees were transplanted into soil without lysimeters (= no-tank trees). To estimate N leaching from soil without roots, one addiFlorida Agricultural Experiment Station Journal Series No. R-07047. This research was supported in part by the Florida Dept. of Agriculture and Consumer Services-NBMP Program Interagency Agreement No. 3267. Grower s Fertilizer (Lake Alfred, FL) generously provided the liquid fertilizer for this study. Reprinted from