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.