Soil quality can be defined as the capacity of a soil to function, whilst maintaining the environmental quality and promoting plant and animal health; it also refers to the capability of soil to function at present and in the future for an indefinite period of time. Soil quality is a basic concept in the sustainable management of any agricultural system aimed at producing, avoiding or reducing negative effects on the environment, preserving resources and saving energy on a medium or long-term basis. Its assessment might be considered a mean for the evaluation of environmental sustainability of agricultural systems. The paper presents results obtained in a 4 years study, conducted with the aim to evaluate the contribution of organic farming system introduction to the environmental sustainability of agriculture in Southern Italy. Soil quality in conventionally and organically managed citrus orchard was determined assessing inherent and dynamic soil properties by following a large scale field survey approach. In 1999, 54 soil orchards under both organic and conventional management were studied, under the same environmental conditions. Farms were selected to obtain similar pairs (27), homogeneous for cultivations and rootstock, in order to reduce effects not linked to soil management. At the beginning of the study, the requested three years conversion period foreseen by the law in force (EEC Regulation 91/2092) was completed for all organic citrus orchards included in the survey. Four years later (2003), sampling was repeated only in 26 orchards (13 pairs), since many organic citrus farms involved in the previous survey shifted back to the conventional management. For each soil, in addition to the inherent physical characteristics, chemical and biochemical parameters able to describe C and N soil dynamic generally used to assess soil quality were determined. They were: total organic carbon (TOC), total extractable carbon in alkaline environment (TEC), humic + fulvic acids carbon (CHA + FA), microbial biomass-C (Cbiom). The humification of soil organic matter was investigated by isoelectric focusing (IEF) technique on purified soil humic fraction; the sum of areas (in percentage) of IEF peaks focused at pH>4,5 was calculated and named As. The mineralization kinetics of soil organic matter was studied under laboratory conditions for each soil, determining mineralized C at 1st (C1) and at 21st (C21) day and the cumulated mineralized C after 21 days of trial (C21cum). At steady state, potentially mineralised carbon (C0) and the kinetic constant of mineralization (k) were estimated. In addition, potentially mineralised N (NPM) in anaerobic conditions was determined. Information obtained after the first sampling (1999) showed a tendency to the improvement of soil quality in organically managed fields. Furthermore, results obtained in relation to 2003 sampling demonstrated that soil quality parameters were significantly higher in organically managed citrus orchards with respect to the conventional ones. We concluded that, on long term, the introduction of organic farming management system contributes to increase the environmental sustainability of agriculture in Mediterranean areas.