Concepedia

Soil Structure and Soil Organic Matter II. A Normalized Stability Index and the Effect of Mineralogy

Johan Six, Edward T. Elliott, Keith Paustian

Soil Science Society of America Journal · 2000 · 614 citations · 32 references

TL;DR

Soil aggregate distribution and stability measurements are used as soil quality indicators, but their interpretation is confounded by pretreatment, antecedent water content, and sand‑size distribution. The authors propose a normalized stability index (NSI) that compares aggregate distribution after slaking and rewetting, corrects for sand‑size confounding, and normalizes disruption to assess whole‑soil stability. The NSI was evaluated on four soils—three dominated by 2:1 clays and one mixed 2:1/1:1 clay—each subjected to native vegetation, no‑tillage, and conventional tillage treatments. NSI values were higher in mixed‑clay soils and declined with increasing cultivation intensity in 2:1‑clay soils, indicating that the index is sensitive to agricultural practice and that SOM loss has a smaller impact on stability in oxides/1:1‑mineral soils than in 2:1‑mineral soils.

Abstract

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.