The unknown consequences and potential impacts of mankind's ability to destroy, alter, or manipulate ecosystems on a vast scale drives our need to better understand the earth system. A fundamental challenge for soil science in the 21st century is to understand the role of soil processes in relation to the function of the earth system. The rationale for developing a definition of soil natural capital stems from the premise that we value ‘things’ based on their perceived value to human well‐being. As a consequence, ignorance of the value of a resource, or system, may lead to its neglect and omission from decision making. Therefore, there is a need to develop a definition of soil natural capital, fitting within a broad framework, which can be used to assess soil ecosystem services that contribute to the function of the earth system. Though various definitions of soil natural capital have been proposed, mostly in the agricultural context, it still remains a nebulous and ill‐defined term. The objective of this paper is to develop an embracing definition of soil ‘natural capital’ focusing on (i) mass, (ii) energy, and (iii) organization/entropy. Mass is further subdivided into solid, liquid and gas phases, and organization into physicochemical, biotic, and spatiotemporal structure. We differentiate between two aspects of capital, the quantity and the quality. As a result of our definition, soil moisture, temperature, and structure emerge as valuable stocks, alongside the more traditionally viewed stocks such as inorganic (mineralogy, texture) and organic materials (OM content). We go on to demonstrate how natural capital fits within the ecosystem services framework, and how using integrated valuation and process based models it can be evaluated. Finally we discuss measurement and monitoring needs that fit with this vision of evaluation.