Fundamental conditions "indicators" should meet to be objective and to guide practice reliably

Abstract. Over the last 2 decades, a significant body of research has been devoted to finding parameters that could serve as “indicators” of a range of soil properties of interest, including soil health and the many functions/services of soils. Even though the topic had been the object of a sustained debate among ecologists between 2001 and 2010, no one seems to have looked so far into conditions soil indicators should satisfy to be objective, nor is it made clear to users and policy makers which ones are objective and which ones are value-laden and therefore subjective. In the present article, we identify the conditions a soil parameter A needs to satisfy in order to be an objective indicator of another property B. We argue that in addition to A and B being measurable and not merely quantifiable approximately, a testable mechanistic theory needs to exist, which causally relates A and B. In most contexts in which soil indicators are currently used, such a theory appears to be entirely lacking, leading us to call for a significant research effort focusing on theory development.