Defining soil as a surface-coupled planetary system: separating pedogenesis from habitability [Opinion]

The concept of soil is increasingly being examined within a planetary context, prompting renewed attention to the physical conditions that enable surface materials to support life. In parallel, planetary habitability has emerged as a framework for defining the minimum resources and conditions required for biological activity, independent of whether life is currently present. Here, we apply habitability principles to soils not to assert the presence of life, but to distinguish clearly between soil formation, biological viability, and functional performance. Habitability formalises a binary set of physical and chemical requirements—including accessible energy, liquid water, essential elements, and viable environmental conditions—that define whether an environment can support life in principle. By treating habitability as a state that applies to already formed soils, rather than as a defining criterion for soil existence, we resolve long-standing ambiguity around the role of biology in soil definition. This separation leads to a tightened planetary definition of soil as ‘an organised planetary surface system, comprising generally loose mineral and/or organic material, formed through sustained genesis via water-mediated surface coupling that produces internal physical, chemical, and/or biological organisation and enables system evolution through time, irrespective of the presence of extant life.’ Within this framework, soil health, soil quality, and soil security operate as continuous descriptors of function only within habitable or potentially habitable soils, while degradation is conceptualised as a contraction of habitable state space. Habitability thus provides a physically grounded foundation for interpreting soil function, selecting indicators, and differentiating soils from sediments and regolith across Earth and other planetary environments.