The effects of mixed herbicides in soils may differ due to interactions. Since adsorption amounts and ratios change nonlinearly with herbicide concentrations, it is necessary to control the total number of moles of a herbicide in solution to estimate its interactions and competitive adsorption in soils. Under experimental conditions, a method was utilized based on the amount of each herbicide adsorbed when the herbicide's molecular ratio was 100%. When atrazine and linuron were applied to soils, they did not compete in terms of adsorption. © Pesticide Science Society of Japan