Fig. 5. Radar graphs for indices of (a) soil environmental quality and (b) soil organic matter quality from the Illinois Soil Quality Initiative (ISQI) project. Each axis depicts an individual indicator. Measures farther from the center of the graph are assumed to represent better quality. Axes scales were selected to include the full range of measured values. The solid line represents the mean values of indices from no-tilled (NT) fields, the dashed line represents the values from conventionally tilled (CT) fields, and the dotted line represents the mean value from nondisturbed (ND) areas. Indices proposed for soil environmental quality include aggregate dry mean weight diameter (DMWD); aggregate wet mean weight diameter (MWWD); soil NO3 measured at 30-cm depth; percent residue measured at planting; soil organic C content in the top 15 cm; penetration resistance assessed with an impact penetrometer; macropores >1 cm2, which were not measurable in the ND soils due to the abundance of roots in those soils; and bulk density in the top 15 cm. Indices proposed for assessment of soil organic matter quality include organic matter content in the surface depth, particulate organic matter (POM, g C/kg soil) in the surface (0–5 cm) and subsurface depths (5–15 cm), MWWD, residue at planting, soil N supply potential (Min N) measured with anaerobic incubation, microbial biomass C (Bio C) chloroform fumigation extraction, and soil C/N ratios. The relevance of some of the variables included (e.g., MWWD) and the direction of their axes are hypothesized, not proved.
