Soil structure and soil organic matter: II. A normalized stability index and the effect of mineralogy

Soil aggregate distribution and stability measurements have been proposed as soil quality indicators. However, pretreatment of soil, antecedent water content and differences in sand size distribution among soils can confound interpretation of these measurements. We propose a normalized stability index (NSI) which (i) compares aggregate distribution after slaking and rewetting to characterize whole soil stability and eliminate confounding effects of pretreatment and antecedent water content, (ii) corrects for the confounding effect of differences in sand size distribution among soils, aggregate size classes and pretreatments, and (iii) normalizes the level of disruption imposed by slaking by using a maximum level of disruption. The NSI was tested on three soils dominated by a 2:1 clay mineralogy and one soil characterized by a mixed (2:1 and 1:1) clay mineralogy. Each site had native vegetation (NV), no-tillage (NT), and conventional tillage (CT) treatments. In soils dominated by 2:1 clays, NSI decreased with increasing cultivation intensity (NV > NT > CT). However, NSI was higher in the soil with mixed clays compared to the other soils and was not different along the cultivation gradient. These observations are hypothesized to be related to the presence of Fe- and Al-oxides and kaolinite. In conclusion, NSI appears to be a good indicator for soil structural quality since it is sensitive to changes in agricultural practices and it minimizes confounding effects. A decrease of SOM levels results in a smaller decrease of soil stability in soils dominated by oxides and 1:1 minerals compared to soils dominated by 2:1 minerals.