Base-richness in ecology is the level in water or soil of chemical bases, such as calcium or magnesium ions. Many organisms are restricted to base-rich or base-poor environments. Chemical bases are alkalis, and so base-rich environments are neutral or alkaline. Because base-poor environments have few bases, they are dominated by environmental acids (usually organic acids) and so are acidic. However, the relationship between base-richness and acidity is not a rigid one – changes in the levels of acids (such as dissolved carbon dioxide) may significantly change acidity without affecting base-richness.
Base-rich terrestrial environments are characteristic of areas where the underlying rocks are limestone. Seawater is also base-rich, so maritime and marine environments are themselves base-rich.
Base-poor environments are characteristic of areas where the underlying rocks are sandstone or granite, or where the water is derived directly from rainfall (ombrotrophic).