Revisiting the S-index for soil physical quality and its use in Brazil

Abstract

SUMMARYThe S-index was introduced in 2004 in a publication by A.R. Dexter. S was proposed as an indicator of soil physical quality. A critical value delimiting soils with rich and poor physical quality was proposed. At present, Brazil is world leader in citations of Dexter´s publication. In this publication the S-theory is mathematically revisited and extended. It is shown that S is mathematically correlated to bulk density and total porosity. As an absolute indicator, the value of S alone has proven to be incapable of predicting soil physical quality. The critical value does not always hold under boundary conditions described in the literature. This is to be expected because S is a static parameter, therefore implicitly unable to describe dynamic processes. As a relative indicator of soil physical quality, the S-index has no additional value over bulk density or total porosity. Therefore, in the opinion of the author, the fact that bulk density or total porosity are much more easily determined than the water retention curve for obtaining S disqualifies S as an advantageous indicator of relative soil physical quality. Among the several equations available for the fitting of water retention curves, the Groenevelt-Grant equation is preferable for use with S since one of its parameters and S are linearly correlated. Since efforts in soil physics research have the purpose of describing dynamic processes, it is the author´s opinion that these efforts should shift towards mechanistic soil physics as opposed to the search for empirical correlations like S which, at present, represents far more than its reasonable share of soil physics in Brazil.

Pedotransfer Function to Estimate the Soil Structural “S” Index and Spatial Variability in an Oxisol Within a Livestock Farming System

“…If the organic carbon content was high, increasing it would result in an increase in the water retention, independent of the textural class (as observed in the present study). Several studies, such as those by Naderi-Boldaji & Keller (2016) and Van Lier (2014), emphasize that soil quality indices express the complexity of the system, and that the use of a single indicator (such as the S index) should be viewed with great caution along with other indicators.…”

Some questions about Yang et al. “Quantitative assessment of soil physical quality in northern China based on S-theory” [Rev Bras Cienc Solo. 2015;39:1311-21]

“…Almost two years have passed since the publication in RBCS of my critical review about the S-index and its use in Brazil (De Jong van Lier, 2014). Two main conclusions were drawn from the analysis presented in that paper.…”

Pore network of an Inceptisol under different uses and relativized S-index as an indicator of soil physical quality

“…Implicitly, the S-index assumes unimodal pore radius distribution and, therefore, it cannot be considered for soils with bimodal or multimodal pore radius distribution (Carducci et al, 2011). For van Lier (2014, the theoretical basis of the S-index is conceptually interesting, albeit it should not create expectations that the S-index, like an absolute index, is able to predict soil physical quality by itself.…”