Vol 117 (2004): Proceedings of the Florida State Horticultural Society

Evaluating variable rate granular fertilizer technologies in Florida citrus

William M. Miller
University of Florida
Published December 1, 2004
  • bmps,
  • nitrogen,
  • control,
  • geographic information system (gis),
  • global positioning system (gps)


Granular fertilizer applications through variable-rate technology (VRT) for Florida citrus can be implemented through either real-time sensing, GIS-map based control, or both. Evaluations were undertaken to establish the response of these techniques with two dry fertilizer units with different hydraulic configurations. A test program using ASAE S341.3 has been established for standardized conditions. Additionally, field trials were undertaken to ascertain fertilizer distribution under grove conditions. In these field trials, both spinner disc and pneumatic discharge units were evaluated. Fertilizer distribution under grove conditions yielded distributions quite different from the Gaussian distributions found in open-field standardized tests. Coefficients of variation in tray distribution tests across the fertilizer swath were lower; 41.0% for grove condition versus 93.9% in open field results when comparing the same spreader unit. Grove application rates for three commercial spreaders were determined by under-tree tray collection of fertilizer and varied by 7.7% from the controller set point values. For both units, delay times (on-off) was 30% less than the off-on transition.