Abstract: A laboratory incubation experiment was carried out to investigate effects of application of Chinese milk vetch shoot,black locust leaves,and pea stalks on acidity of an Ultisol from a tea garden,as affected by nitrogen transformation in these organic materials.Results show that the application increased soil pH to a varying extent compared with control.The extent tallied with the amount of ash alkali in the plant materials.The more the ash alkali in the plant materials,the greater the increase in soil pH.Nitrogen transformation affected the effects of these plant materials ameliorating soil acidity.Mineralization of the organic N in the plant materials consumed H+,thus raising soil pH,while nitrification of NH4+-N from mineralization of organic N released H+,thus lowering soil pH,which somewhat offset the effect of plant materials ameliorating acid soil.The application of the plant materials increased soil exchangeable base cations and decreased soil exchangeable Al significantly.