Assessing soils’ health by using soil organic carbon (SOC) content, SOC/clay ratio and mean SOC reference values: a case study on Romanian mineral soils

Abstract Soil health is essential for a resilient ecosystem. The European Union proposed a Soil Monitoring Law for a legal framework of soil health. This study proposes a way to assess the mineral soil health. A database of mineral soils containing <20% organic matter and consisting of 10 soil classes and 22 soil types was used. There were four altitudinal groups (HM [high mountains], LM [low mountains and high hills], LH [low hills], PL [low plains]), covering the vegetation/climate floors, two land uses (forestland and grassland combined and cropland) and three soil textures (coarse [CO], loamy [LO] and clayey [CL]). Both soil organic carbon (SOC)/clay ratios and observed per mean SOC (O/M SOC) ratios were calculated for 19 regions. For SOC/clay, the 1/13, 1/10 and 1/8 thresholds were used, whereas O/M SOC categories were grouped as ‘low’, ‘intermediate’, ‘high’ and ‘very high’ health. SOC/clay and O/M SOC ratios combined were used to characterize soil health. SOC sequestration depends on many factors that are specific for each pedo-climatic region and texture and so is the soil characterization as healthy or not healthy. The recommended simultaneous application of these two indicators revealed specific SOC content values as reference levels for good soil health, which decrease from the wetter climates towards the drier ones. SOC content considerably differed among pedo-climatic regions, and soil health should be compared within the same regions that have specific SOC sequestration conditions. Correlations between support points SOC values and the aridity index allow separation between ‘healthy’ and ‘non-healthy’ soil conditions for any climate, vegetation floor and land use.