This study was conducted to determine the properties of selected Andosols from northeastern Japan and the factors affecting the root growth of burdock, Arctium lappa cv. Takinogawa; barley, Hordeum vulgare cv. Norin-24; and orchard grass Dactylis glomerata cv. Kitamidori grown on these soils in a greenhouse. According to the clay mineralogical composition, the Andosol samples were divided into chloritized 2:1 mineral soils and allophane-imogolite1 soils. All the samples were poor in exchangeable bases, showed pH(H2O) ranging from 4.6 to 5.1, and were rich in reactive aluminum, as indicated by high pH(NaF), existence of large amounts of dithionite-citrate-soluble aluminum, and high phosphate absorption. On the other hand, a wide variation of exchange acidity Y1, determined by the titration of 125 ml of N KCl extract (soil: N KCL = 100:250) with 0.1 N NaOH, was observed; this was very large for chloritized 2:1 mineral soils and negligibly small for allophane-imogolite soils. Root growth of the test plants was reduced in chloritized 2:1 mineral soils, but not in allophane-imogolite soils. Morphological observation and aluminon test of the plant roots showed that aluminum toxicity occurred only in chloritized 2:1 mineral soils, even though all the soils contained large amounts of reactive aluminum and showed pH(H2O) range of 4.6 to 5.1. The degree of reduction of the root yields and the degree of aluminon reaction of aluminum-injured roots were in this order: burdock > barley > orchard grass. Examination of the properties of the soils sampled suggested that exchange acidity Y1 defined by Daikuhara (1914) was a realistic measure of aluminum toxicity potential in Andosols from northeastern Japan.