A New Definition of Soil

Abstract

Over the past years, we have entertained a lively discussion on the definition of soil. Why is this important to SSSA and society as a whole? Well, to succinctly define “soil” delineates what we study, manage, and promote; it defines who we “are” as soil scientists. Also, many official documents and statutes make reference to soil without further clarification, and SSSA's definition, therefore, has important legal implications. As our science and thinking on soil evolved, there was a need to revisit the definition. Several concerns emerged around these definitions over recent years, including: Exclusivity to the planet Earth: This has created discordance with planetary scientists as they use the term “soil” in relation to other celestial bodies (notably Mars in recent years). This also sent an implicit message that soil science (and therefore research funding) would not be relevant outside planet Earth. There was no compelling reason to limit soil to planet Earth and create unnecessary disagreement within the scientific community. Unconsolidated vs. consolidated soils: Although this distinguishes soil from other surficial material (notably hard rock), it was argued that many soils experience some level of consolidation through internal order in forming aggregates. Multi-phase system: Many regard soil as a medium where solid matter provides a skeleton for soil, but liquids, gases, and biota are also integral components. The old definition only referred to the solid phase of the soil. Soil processes: The second definition focuses on soil-forming factors. In recent years, especially with the heightened interest in soil health, the medium is increasingly described in simpler terms related to physical, chemical, and biological processes, which can also include less conventional soils (e.g., urban and disturbed soils). As a result of these concerns, the definition of “soil” was debated over the past two years, with many members contributing to the process. The SSSA Glossary working group reviewed the definition and suggested new wording. During the 2016 Annual Meeting in Phoenix, the definition was further discussed in a town hall meeting and at a session on extraterrestrial soils. A Twitter hashtag (#soildefinition) was established for further exchanges. Several interested members followed up by email and exchanged perspectives and papers. A new definition was then proposed to the SSSAJ Board at its May 2017 retreat, which resulted in further refinements and final approval at the August 2017 Board meeting. The layer(s) of generally loose mineral and/or organic material that are affected by physical, chemical, and/or biological processes at or near the planetary surface and usually hold liquids, gases, and biota and support plants. In summary, the new definition preserves mineral-organic material as the key component (skeleton) of soil but acknowledges that liquids, gases, biota, and plants are usually integral factors. It also links soil to processes at or near the planet surface, which offers a conceptual spatial extent. And, of course, we now acknowledge the existence of soils outside planet Earth. I would like to express gratitude to all who contributed to this process. Changing the definition, not unexpectedly, brought out many viewpoints. Perhaps the definition is not perfect for everyone, but we were able to build consensus on something that is inherently difficult to define.