MapBasic is used to develop a field GIS particularly for variable rate fertilization in precision agriculture. The GIS builds up a correlative database including soil fertility, yearly crop yields, fertilizer application and so on, using internal relation database in MapInfo. The basic functions of the GIS are data input, output, data preserving and updating, query, search, link, statistics and decision making by means of nutrient balance fertilization. The results can not only be displayed and queried, but also output to the applicator by RS 232 serial interface in a single chip controller to control the operation of the applicator.