Olsen phosphorus, exchangeable cations and salinity in two long-term experiments of north-eastern Italy and assessment of soil quality evolution

Due to its food production and environmental buffering functions, soil is considered a strategic target by the European Union and its quality evaluation could be used as an effective index of sustainability. The evolution of some soil chemical parameters has been studied in two long-term trials established in the early 1960s in north-eastern Italy: one (SF) comparing nutrient management treatments (i.e. organic, mineral and mixed fertilizers) in lysimeters containing widely contrasting soil types (i.e. sand, clay and peat) and the other (CR) involving a field study with crop rotation, nutrient (organic vs. mineral fertilizers), and management intensity variables. Soil was analysed for Olsen phosphorus (Pav), exchangeable cations (Kex, Naex, Mgex and Caex) and salinity, evaluated by measuring the electrical conductivity (EC). A bi-factorial quality index based on fuzzy logic was then tested and applied to assess the overall soil quality and its evolution in the cropping systems. A minimum dataset of chemical (pH, organic carbon, cation exchange capacity, Pav and Kex) and physical parameters (available water content and water-filled porosity) was used to elaborate the index. In general, the use of organic fertilizer resulted in higher Pav, Kex and Mgex concentrations than the mineral treatments at the same levels, whereas no differences for Naex and Caex were observed. Salinity level was not influenced by the organic treatments probably because the consistent water drainage in the area prevented salt accumulation. The soil quality index represented the soil productivity function, explaining up to 74% of yield variability. Index comparison highlighted the positive role of organic and mixed fertilizations in increasing and maintaining the soil quality. The bi-factorial index of soil quality based on a minimum dataset is a good tool for the policy maker to evaluate the effects of management practices. However, standardization and accuracy of the soil analyses are important to reduce sources of variability that could have a strong influence on the soil quality evaluation.