Soil quality (SQ) has become a major research issue among soil scientists since the early 1990's, following the recognition of the role soils play in terms of agricultural sustainability and environmental quality. This review aims to discuss the state of the art concerning soil quality and look into the proposed indicators for its evaluation. Most part of the research work in the literature is focused on the identification of a soil attribute that would serve as a SQ indicator (SQI) underlying the evaluation of soil degradation and the estimation of research needs and funding, as well as a critical evaluation of soil management practices. Some authors have proposed criteria for the definition of a SQI. In this literature review three major approaches were clearly identified: search for soil attributes to be used as SQI; organic matter as SQI; and SQ as a result of processes in the soil-plant system. It was concluded that the understanding of the soil as a system represents the most significant advance and that therefore the identification of soil management practices to achieve SQ are more important than the effort to obtain a SQ indicator. From this point of view, soil should be treated as an open system where the non-equilibrium thermodynamics theory is applicable and the SQ emerges from the interactions of the soil-plant-biota system. The agricultural systems that enhance SQ are those where the soil is intensively cultivated with plants preferably of different species, and without tilling.
systemic approach; non-equilibrium thermodynamics; energy flow; order states