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

Quirijn De Jong Van Lier

doi:10.1590/s0100-06832014000100001

Quirijn De Jong Van Lier

Open Access

https://doi.org/10.1590/s0100-06832014000100001

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Abstract

The 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.

Highlights

There is no single definition of soil quality
S is mathematically correlated to bulk density and total porosity
As a relative indicator of soil physical quality, the S-index has no additional value over bulk density or total porosity

Summary

Introduction

There is no single definition of soil quality. Sometimes it is defined as the ability of a specific kind of soil to perform ecological services or functions essential to people and the environment, like sustaining plant and animal productivity, maintaining or enhancing water and air quality, and supporting human health and habitation (Karlen et al, 1997). Mass and energy transfer processes in the soil and their response to soil physical quality and functioning can be evaluated by less or more complex simulation models of hydrological, meteorological, agronomical or combined origin, like WEPP (Flanagan & Nearing, 1995), DSSAT (Jones et al, 2003) , SWAT (Neistsch et al, 2005) and SWAP (Kroes et al, 2008), among several others. These models allow evaluation of the sensitivity of involved processes to boundary conditions (Cancellieri et al, 1993; Brooks et al, 2001; Pedersen et al, 2004; Castaings et al, 2009). Specific techniques for performing sensitivity analysis in complex models were presented by Drechsler (1998)

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