Quantifying adsorption isotherms, which measure solute affinity, is essential for understanding solute retention and transport in soils and geological media. In this study, a new general solute isotherm equation was derived for soils. The basic assumption here is that it was assumed that a soil is made up of a discrete number of constituents or site fractions each having a different strength or affinity for solute sorption on matrix surfaces. Other assumptions include the validity of linear adsorption for each site fraction or soil constituent and that the sorption capacity for each site fraction is finite. The linear assumption was based on the overwhelming evidence of observed linear isotherms for a wide range of solutes in different soils. At low concentrations where surface coverage is not limiting, this approach was capable of deriving linear two- and three-phase and nonlinear isotherms. The linear assumption was also capable of deriving the Langmuir equation. A major advantage of the new general isotherm equation is that it is valid for an arbitrary site affinity distribution (e.g., histograms) and need not be a continuous function.