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A procedure for developing a GIS database for a Palouse wheat field is described and a case study used for an example. The database and standard fertilizer rate formulas based on potential yield were combined with an expert system to develop a fertilizer rate database for each of 200 cells in a 32 hectare field. GPS instrumentation and a variable rate fertilizer applicator were used to apply the spatially variable fertilizer rate in the field. Average fertilizer applied was reduced from the farmer's anticipated 71 kg/ha to an average of 45 kg/ha for the variable rate. The fertilizer rate on slopes steeper than 20 percent averaged 33 kg/ha or a 54 percent reduction. The fertilizer cost was reduced by $7 per acre.

This study involved refining a vehicle for computer guidance and GPS position control. The vehicle was powered by a small diesel engine coupled to a hydrostatic drive transmission adaptable to computer control. By interfacing GPS and a computer with the control portions of the vehicle, it was controlled automatically with no driver interference. The computer interface for the vehicle was built to use TTL level inputs and outputs on a standard parallel port configuration. All control aspects of the vehicle were dictated by two parallel ports with vehicle position and heading read simultaneously through two nine pin-serial connectors. This interface configuration was flexible so the vehicle’s software control could be executed by a variety of computers, not just one built specifically for the application. A hydraulic power steering valve driven by a single bidirectional DC motor controlled the electro-hydrostatic steering system.