Impact of Variable Rate Fertilization on Subsurface Water Contamination in Wild Blueberry Cropping System

Abstract

Abstract. Wild blueberry fields have gentle to severe topography with high proportion of bare spots, and weed patches emphasizing the need for variable rate (VR) fertilization. To evaluate the impact of VR fertilization on subsurface water contamination, two wild blueberry fields were selected in Nova Scotia, Canada. The field boundaries, weeds and bare spots were mapped using real-time kinematics global positioning system. The variation in slope was mapped and management zones i.e. steep slope (Zone-1), moderate slope (Zone-2) and low lying areas (Zone-3) were delineated. Both fields were divided into two halves (VR and uniform). The GPS-guided prescription maps were generated in ArcGIS 9.3 for VR treatment sections. Three fertilization rates 200, 150, and 100 kg ha -1 were applied in Zone-1, Zone-2 and Zone-3, respectively, with a VR fertilizer spreader. The bare spots/weeds were classified as a separate class in the prescription maps and no fertilizer was applied in those areas. For comparison, the uniform fertilizer rate of 200 kg ha -1 used by growers was applied in uniform treatment sections of the selected fields. Subsurface water samples were collected after every heavy rainfall (>20 mm) from 36 suction lysimeters placed in each field with 18 lysimeters in each treatment. The subsurface water samples were analyzed for nitrate nitrogen (NO 3 — N), and ammonium nitrogen (NH 4 + -N) concentrations under VR and uniform fertilization. Leaf samples were collected from both VR and uniform treatments and were analyzed for leaf nutrient concentrations. The results of this study suggested significantly lower NO 3 — N and NH 4 + -N loading from subsurface water samples leaving the root zone in moderate and low lying areas for VR treatment as compared to uniform treatment. The nitrogen, phosphorus, and potassium nutrient levels in leaves were found to be higher than the recommended ranges for uniform treatment as compare to VR treatment suggesting the excess uptake by plants in uniform treatment which may result in more vegetative growth and lower yield. The results indicated that VR fertilization based slope can increase input use efficiency, farm profitability and reduce subsurface water contamination.