The chemical fertility of a forest is generally defined as the pool of plant-available nutrients in the soil which is quantified at a given time and compared to nutrient requirements established for the different tree species. This concept inherited from agronomy is often unreliable and many forest ecosystems developed on chemically poor soils (particularly in Ca, Mg, K) are highly productive. The objective of this article is to illustrate the limits of the "fertility=soil reservoir" concept and to propose the basic principles of a new concept which takes into account the specificity of the chemical fertility of forest ecosystems. To support this new concept, a comprehensive database of results acquired since the 1970s from 11 experimental sites was used. The results demonstrate that the definition of the chemical fertility of forest ecosystems should not be limited to the pool of plant- available nutrients in the soil but must also integrate the cycling and recycling of nutrients characteristic of biogeochemical cycling.