Plant root growth in acid andosols from northeastern japan: 2. Exchange acidity Y₁as a realistic measure of aluminum toxicity potential

Chemical and greenhouse studies were conducted to show that exchange acidity Y₁, determined by titration of 125 ml of N KC1 soil extract (soil:N KC1 = 100:250) with 0.1 N NaOH, was a useful, realistic measure of aluminum toxicity potential in acid Andosols from northeastern Japan. Twenty-eight soil samples consisted of allophane-imogolite soils (11 samples; pH(H₂O) 4.6 to 5.9, Y₁0.4 to 3.1 ml/100g), chloritized 2:1 mineral soils (13 samples; pH(H₂O) 3.9 to 5.0, Y₁6.8 to 38.2 ml/100g), and allophane-imogolite-chloritized 2:1 mineral soils (4 samples; pH(H₂O) 4.6 to 5.3, Y₁3.3 to 5.0 ml/100g). Three test plants, burdock cv. Takinogawa (Arctium lappa), barley cv. Norin-24 (Hordeum vulgare), and dent corn cv. Choko-1 (Zea mays), were grown on these soils in a greenhouse, and their root length and acid-injury were observed. Root length of all the test plants had a significant relationship with exchange acidity Y₁, shown by exponential equations. Degrees of acid-injury determined by root length and root morphology were also closely related to exchange acidity Y₁. Chloritized 2:1 mineral Andosols had “critical” pH(H₂O) of about 5.0 and “critical” pH(KCl) of about 4.5, whereas allophane-imogolite Andosols did not have these “critical” pHs and did not restrict the root growth greatly even at low pH(H₂O), such as 4.6. Acid substances relating to exchange acidity Y₁were found to consist mainly of Al³⁺ions whose amount was closely correlated with total exchangeable Al. A comparative study of exchange acidity Y₁, pH(H₂O), pH(KCl), fluoride-reactive Al, etc., showed that exchange acidity Y₁was a useful realistic measure of aluminum toxicity in the acid Andosols.