December 1, 1996
Groundwater nitrate levels under a 'Valencia' (Citrus sinensis L. Osbeck) on rough lemon (C. limon L. Burm.f) grove planted in 1984 near St. Cloud, Florida, were monitored every 30 days from November 1994 to September 1996. The samples were taken from four 5 cm (2.0 in) diameter wells, 3.7 m (12 ft) deep, spaced 87 m (264 ft) apart in the center of the 600 m (1312 ft) × 70 m (230 ft) grove. The wells were jetted in with water and had 1.5 m (5 ft) of finely slotted (0.5 mm [0.02 in]) filter pipe on the lower end. Eight 30 cm (1 ft) deep shallow wells were designed to intercept seepage into drainage ditches on three sides of the grove. To the north side, the land is bordered by swampy woodland from which groundwater drained into the grove. Water in the drainage ditches was also monitored. The water table in the center of the grove was at 1.7 to 2.0 m (5.6 to 6.8 ft), with little fluctuation. The grove was irrigated with microsprinklers, but fertilization was with dry fertilizer only. The grove was fertilized uniformly until April 1995. After that, Plots 1 and 3 received approximately half the amount of N applied to Plots 2 and 4. The high N plots received 244 kg N/ha (217 lb N/ acre) in four applications, and the low N plots received 169 kg N/ha (150 lb N/acre) in three applications in 1995. In 1996, N was applied four times between January and September, 106 kg N/ha (95 lb N/acre) to the high N plots in four applications, and 70 kg N/ha (62 lb n/acre) in two applications to the low N plots. Water in the well in Plot 3 and the seepage well on its east side had between 8 and 28 mg/l nitrate N in 1995, while Well No. 2 never exceeded 12 mg/l. Water from Wells 1 and 4 was intermediate and never exceeded 10 mg/l NO[sub3]-N. All four wells were essentially nitrate-free in December 1995. In the first 8 months of 1996, none of the wells had NO[sub3]-N higher than 8 mg/l, with the highest levels in Well No. 2.