Inhibitory effect of acid Andosols on plants – is aluminum toxicity true for allophanic Andosols?

Abstract

Non-allophanic Andosols often show aluminum (Al) toxicity to Al-sensitive plant roots. The toxicity was evaluated by the 1 M potassium chloride (KCl)-extractable Al (AlKCl) content. In contrast, natural allophanic Andosols rarely show any Al toxicity, whereas AlKCl can appear with strong acidification. It is still not clear whether the acidic allophanic soils cause the Al toxicity, or the acid injury in Andosols is totally explained by the Al toxicity. In this study, plant cultivation experiments were performed using non-allophanic and allophanic soils over a wide-range of pH values; two typical non-allophanic soils [pH(H2O) 4.4–4.7], their limed soils [pH(H2O) 5.9–6.1], two typical allophanic soils [pH(H2O) 5.7–7.0], three acidic allophanic soils [pH(H2O) 4.6–5.4] and a commercial Kanuma pumice. We cultivated the Al-sensitive plants [burdock (Arctium lappa L.) and barley (Hordeum vulgare)] and an Al-accumulative plant [buckwheat (Fagopyrum esculentum)] in these soil samples, then measured the root lengths of the burdock and barley and determined the Al concentrations in the buckwheat. The toxic (available) Al was evaluated from the results of these cultivations. The typical non-allophanic soils showed a strong inhibition of the roots of the burdock and barley. Although the inhibition was not observed in the typical allophanic soils for the Al-sensitive plants, the acidic allophanic soils did show the inhibition as observed in the non-allophanic soils. Close negative correlations were observed between the root lengths of the Al-sensitive plants and the Al concentrations in the buckwheat (P < 0.05); the Al concentrations in the buckwheat grown in the non-allophanic soils were much higher (2.6–4.3 mg g−1) than those in the typical allophanic soils (0.4–1.4 mg g−1), and these concentrations in the buckwheat in the acidic allophanic soils were comparable (2.7–4.0 mg g−1) to those in the non-allophanic soils. Thus, it was shown that the acid injury of the Andosols is definitely caused by the Al toxicity, regardless of the type of Andosols, i.e. non-allophanic or allophanic. Possible substances controlling the Al toxicity were discussed in relation to soil properties.