Soil quality evaluation and monitoring

Agricultural management systems have been historically adopted without recognizing consequences on soil conservation and environmental quality, and therefore significant decline in soil quality has occurred worldwide. Soil quality has been defined as “the capacity of a specific kind of soil to function with its surroundings, sustain plant and animal productivity, maintain or enhance soil, water and air quality and support human health and habitation” (Karlen et al., 1997). Any evaluations of the quality of the soils must consider the multiple soil uses (e.g. agricultural production, forest, rangeland, recreation or urban development). However, the most widely accepted concept of soil quality and the most significant in a global context is referred to agro-ecosystems. Using soil quality concept along with identifying key attributes or indicators among the nearly infinite list of parameters, and methods for evaluating and monitoring it with respect to the numerous soil functions is an evolving process. Unlike water and air quality, simple standards for individual soil quality indicators do not appear to be sufficient because numerous interactions and trade-offs must be considered. Soil quality evaluation or assessment tries to predict the natural ability of each soil to function, and land evaluation tries to predict land behavior for each particular use. However, land evaluation is not the same as soil quality assessment, basically because land refers not only to soil but to the combined resources of soil, terrain, water and land use. Also, the biological parameters of the soil are not considered by land evaluation. Soil physical/chemical indicators (e.g. soil-depth, texture, bulk density, infiltration, water retention, pH, carbonate content or cation exchange capacity), which are the main input soil characteristics in land evaluation, are very fix and permanent in the time. Soil biological indicators (e.g. populations of micro-organisms, microbial biomass and/or respiration, mycorrhizal association, nematode communities, enzymes or detailed characterization of organic matter) are very dynamic and exceptionally sensitive to changes in soil conditions. The biological attributes, along with the trace metals concentration, appear to be most sensitive to soil conservation/restoration and management practices such as crop rotation, conservation tillage and organic amendments. For soil quality assessment, the development of relationships between all the soil quality indicators and the numerous soil functions may be a monumental task. Within this complex context, land evaluation models may serve as a first step to develop a soil quality assessment procedure (Arshad and Martin, 2002). As shown in Figure 1, land evaluation can be an appropriate procedure for analyzing the soil physical quality from the point of view of the long-term changes. Then, a short-term evaluation and monitoring procedure can be considered mainly for the soil biological quality and metal pollution of soil. Also, according to the complex nature of the soil, the high spatial and temporal variability of its different parameters, it appears appropriate to develop soil quality assessment based exclusively on biological indicators after the traditional land evaluation using basically physical parameters.