What is soil?

What is soil, beyond an assemblage of measurable attributes? Soil is commonly defined by its composition, formation processes, and functional roles, but these descriptions do not always identify the physical principles that govern how soil forms, persists, and degrades. Here, we frame soil as an open, non-equilibrium transformation system in which external energy and matter fluxes are transformed through interacting physical, chemical, and biological processes. Solar energy is a major energy source to this system through direct and indirect pathways. Within this view, soil formation, persistence, degradation, and function represent different states and trajectories of the same system. We develop a conceptual and mathematical framework in which soil organisation emerges from the development, balance, and coupling of physical, chemical, and biological domains. Soil organisation ( Ω ) is introduced as a system-level state variable that links these domains and provides a basis for interpreting soil function, health, resilience, and response to environmental change. Sustained increases in organisation require entropy export to exceed internal entropy production, providing a thermodynamic constraint on the maintenance of the organised soil state. A first-order operational formulation links Ω to measurable soil properties and cross-domain coupling, allowing the framework to be tested and refined with field observations. This system-based definition complements existing descriptions of soil by providing a physical basis for understanding how soil organisation develops, changes, and is maintained through time.