It was simulated that plant roots excreted different kinds of organic acids under phosphorus stress,and the effects of citric,malic,oxalic and tartaric acid on kinetics of phosphorus adsorption by calcareous soils have been investigated by flow technique method.Results show that all these acids can reduce phosphorus adsorption by the soils and the extent of reduction is closely Belated to the kinds of organic acids.The ability of organic acids to reduce phosphorus adsorption in calcareous soils is in the order of oxalic≥citric>malic≥tartaric acid.The effect of pH on reducing soil phosphorus adsorption by organic acids was very complicated.For example,the inhibiting effect of tartaric acid on phosphorus adsorption by soil 2,3,and 4 increased with increase in pH from 6.5 to 8.0,while the inhibiting effect of tartaric acid on phosphorus adsorption by soil 1 decreased with increase in pH.Regression analysis reveals that the first order Kinetic equation is the best one to describe the phosphorus adsorption kinetics as affected by organic acids,followed by Parabolic diffusion equation and Two-constant rate equation,and then by Elovich equation.According to the research,the influence of low-molecular weight organic acids on kinetics of phosphorus adsorption by the soils is a first order reaction.Organic acids inhibit adsorption mainly by eliminating the phosphorus sorption sites in the soils.Compared with control,All the four kinds of organic acids tested can dissolve a large amount of CaCO₃ in calcareous soils,thereby considerably reducing phosphorus adsorption by soils.