Submitted to: Journal of Florida State Horticulture Society
Publication Type: Proceedings
Publication Acceptance Date: December 20, 1996
Publication Date: N/A
Interpretive Summary: Groundwater nitrate under a 7-acre citrus grove divided into four plots, each with three sampling wells, varied strongly even when all plots were fertilized uniformly. Cutting nitrogen application in two plots by half brought the high nitrate level down after 12 months, but even from then on, nitrate was higher in a small area of the grove. Factors other than the amount of nitrogen applied seemed to govern the amount of nitrate in the groundwater under the trees. Only one of eight wells intercepting water draining into ditches on three sides of the grove showed consistent high nitrate levels, but the drainage water in the ditches contained very little or no nitrate in the 20-month observation period. It appears that in the East Coast citrus producing area nitrate levels in excess of 10 ppm are confined to the area under the trees and very little nitrate enters the surrounding environment.
Groundwater nitrate and nitrate levels in the drainage water in and around a 'Valencia' on rough lemon grove planted in 1984 near St. Cloud, Florida, were monitored every 30 days from November 1994 to September 1996. On the north side, the land was attached to swampy woodland from which the groundwater drained into the grove. 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 NO3-N. All four wells were essentially nitrate free in December 1995. In the first 8 months of 1996, none of the wells had NO3-N higher than 8 mg/l, with the highest levels in Well No. 2.