There is a need for a simple equation relating the adsorption of Zn to its solution concentration and pH. Such an equation was derived using the competitive Langmuir adsorption isotherm. Experimental results obtained for five different soils in batch experiments were compared with results computed for each soil according to fitted parameters. Three criteria for the adequacy of the equation were tested: an acid titration curve in the absence of Zn, Zn adsorption as a function of its solution concentration at a given pH, and Zn solution concentration at a fixed total amount of Zn as a function of pH. Close agreement between calculated and experimental results was obtained for the analyzed soils in all tests.