Abstract
Modern granular fertilizer spreaders with variable rate application (VRA) capability can reduce fertilizer requirements and environmental impacts in Florida citrus groves by improving fertilizer application efficiency by up to 40%. A key component to the success of VRA spreaders is the real-time canopy sensing system which measures the size of the trees before the appropriate dose of fertilizer is dispensed. A strong relationship must also exist between the yield and tree age from which fertilizer recommendations are developed, and the measurable canopy size variables such as height, volume, or ground cover that could be used by VRA fertilizer spreaders to calculate fertilizer rates. In this study, the correlations between fruit yield and ground-based ultrasonically measured canopy size, or aerial photograph-based canopy dimensions and the normalized difference vegetation index (NDVI) were compared. The performances of two types of canopy sensors (ultrasonic and photoelectric) were tested using young tree targets of different sizes, representative of a citrus grove. Results showed that canopy height is an acceptable estimate of yield potential from which fertilizer rates can be calculated onthe-go using commercial canopy sensors mounted on a fertilizer spreader. Both photoelectric and ultrasonic sensor systems were capable of rapid canopy sensing and 'look-ahead' pre-compensation are required for accurate fertilizer dosing and placement. The fertilizer doses applied automatically to a range of young non-bearing reset tree sizes approximated the UF-IFAS-recommended rates. These results demonstrated that a well-tuned fertilizer spreader with high-speed sensors, control electronics, and hydraulics can precisely fertilize any tree size from resets to mature hedgerows.
