During 2004, The soil laboratory LAPRA has conducted a review of the interpretation norms of its chemical analysis, and has developed an expert system of fertilization advice. The system considers six soils' units and sub-units: Hydric andosols, andosols, brown soils, nitisols, oxysols, and vertisols, justified by their different origin and chemical characteristics. The system also differentiate the two main crops of Guadeloupe: Sugar cane and banana. The diagnostic assessment of soil fertility relies on threshold values Sn allowing to qualify the result and to propose a fertilization advice. These threshold values have been determined according to soils characteristics, nutritional requirements of the plants, and fields results (2400 analysis). They may also consider relationships between variables. Each assessment may result from the reading of one or several variables. In this way, the diagnosis of soil acidity depends on two variables : pH and calcium, while Κ and Mg will be appraised individually and according to the ratio K/Mg. The system of interpretation and fertilization plan is automatic. Calculations include efficiencies of inputs that may vary according to soil type, and optimum quantities of calcarus amendments. It also allows to advise not to use potassium when the soil has enough reserves for the plants.