PLANT ROOT GROWTH IN ACID ANDOSOLS FROM NORTHEASTERN JAPAN

Abstract

Chemical and greenhouse studies were conducted to show that exchange acidity Y1, determined by titration of 125 ml of N KCl soil extract (soil:N KCl = 100:250) with 0.1 N NaOH, was a useful, realistic measure of Al toxicity potential in acid Andosols. Twenty-eight soil samples consisted of allophane-imogolite soils (11 samples; pH(H2O) 4.6-5.9, Y1 0.4-3.1 ml/100g), chloritized 2:1 mineral soils (13 samples; pH(H2O) 3.9-5.0, Y1 6.8-38.2 ml/100g), and allophane-imogolite-chloritized 2:1 mineral soils (4 samples; pH(H2O) 4.6-5.3, Y1 3.3-5.0 ml/100g). Three test plants, burdock cv. Takinogawa (Arctium lappa), barley cv. Norin-2H (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 Y1 were found to consist mainly of Al2+ 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 Y1 was a useful realistic measure of Al toxicity in the acid Andosols.