Spatial management tools such as GPS and GIS allow agricultural resource management decisions to be made with greater precision.  It is now possible to manage variability at increasingly finer resolutions when compared with the “field-average” approach of the past.  However, site-specific fertility management is limited by the ability of existing equipment to physically apply a management strategy within the confines of a management zone.  Pneumatic and spinner disc applicators, two types of variable rate application (VRA) equipment, are susceptible to distribution and control errors that compromise their ability to effectively apply a desired fertility regiment.  Researchers at UK have attempted to quantify and identify application errors associated with spinner these applicators.  Application errors addressed include the application uniformity across the swath and rate change delays as they affect distribution patterns.  These problems contribute to application errors within management zones, along field borders, and within environmentally sensitive regions of a field.  Test regiments of fixed and variable-rate application passes were conducted for both spinner disc and pneumatic applicators using a grid of catch pans to characterize overall system performance.  Once the applicator distribution was defined, a spatial model was developed to fully assess how well actual applications matched the desired prescription.  The spatial model was applied in several fields to determine the correlation between as-applied and prescription maps.  Spinner disk applicators exhibited correlations ranging from 0.15 to 0.75 while pneumatic applicator values ranged from 0.24 to 0.81.  Recent trends in the development of agricultural machinery exploit the use of microcontrollers to performance by controlling functions on the machine with ever-increasing resolution and precision.  These control networks provide the foundation of new metering systems that will eliminate many of the problematic application errors.