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 a s 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 ind ex (NSI) which (i) compares aggregate distribution after slaking and rewett ing to characterize whole soil stability and eliminate confounding effects of pretreatment and antecedent water content, (ii) corrects for the confoun ding effect of differences in sand size distribution among soils, aggregate size classes and pretreatments, and (iii) normalizes the level of disrupti on imposed by slaking by using a maximum level of disruption. The NSI was t ested on three soils dominated by a 2:1 clay mineralogy and one soil charac terized by a mixed (2:1 and 1:1) clay mineralogy. Each site had native vege tation (NV), no-tillage (NT), and conventional tillage (CT) treatments. In soils dominated by 2:1 clays, NSI decreased with increasing cultivation int ensity (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 gr adient. These observations are hypothesized to be related to the presence o f 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 a gricultural practices and it minimizes confounding effects. A decrease of S OM levels results in a smaller decrease of soil stability in soils dominate d by oxides and 1:1 minerals compared to soils dominated by 2:1 minerals.